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Immunocompromised children: conditions and infectious agents
PAEDIATRIC RESPIRATORY REVIEWS (2007) 8, 231–239
MINI-SYMPOSIUM: MICROBIOLOGICAL DIAGNOSTIC PROCEDURES IN RESPIRATORY INFECTIONS
Immunocompromised children: conditions and infectious agents Alison M. Kesson1,* and Alyson Kakakios2 1
Department of Infectious Diseases and Microbiology, The Children’s Hospital at Westmead, Discipline of Paediatrics and Child Health, University of Sydney, LMB 4001, Westmead NSW 2145, Australia; 2 Departments of Allergy and Immunology, The Children’s Hospital at Westmead, LMB 4001, Westmead NSW 2145, Australia KEYWORDS immunodeficiency; opportunistic pathogen; respiratory infection
Summary Individuals with immunodeficiency, either primary or acquired, are increasingly common. These individuals have increased susceptibility to a range of infections which are uncommon in the normal host. An understanding of the individual’s immune defect provides important information about the range of organisms that this individual may be susceptible to. As a corollary, identification of an ‘opportunistic pathogen’ may indicate the patient’s type of underlying immune defect. ß 2007 Elsevier Ltd. All rights reserved.
IMMUNITY Normal healthy individuals protect themselves against invading micro-organisms by means of many interrelated but different mechanisms, including physicochemical barriers, circulating molecules and cells and their soluble mediators.
Innate immunity Defence mechanisms which are present prior to exposure to micro-organisms, and which are not enhanced by exposure to most of these, constitute the innate immune system. The innate immune system consists of phagocytes (macrophages, neutrophils and natural killer cells) and complement.
Acquired immunity In contrast to the innate immune system, the acquired immune system is induced or stimulated by exposure to * Corresponding author. Department of Virology, The Children’s Hospital at Westmead, LMB 4001, Westmead NSW 2145, Australia. Tel.: +61 2 98453823; Fax: +61 2 98453291. E-mail address: [email protected] (A.M. Kesson). 1526-0542/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.prrv.2007.07.005
micro-organisms with exquisite specificity and is amplified with each exposure. The acquired immune system consists of lymphocytes, both T cells and B cells, and the secreted products of B cells, antibodies. Foreign substances which induce specific immunity are called antigens. In addition, the acquired immune system has memory so that subsequent encounters with the same antigen stimulate increasingly strong and rapid responses. The acquired immune system constantly surveys the body for foreign antigens and pathogens. Antibodies respond to antigens in their native conformational state in the extracellular environment while T cells respond to degraded antigens ‘presented’ on the surface of cells and thus respond to intracellular pathogens. Intracellular pathogens, e.g. viruses which infect cells, are degraded and peptides from the viruses (foreign peptides) bind to the major histocompatibility molecules (MHC), called human leukocyte antigens (HLA) in humans, which are then displayed on the cell surface for recognition by T cells via the T-cell receptor. This response has dual specificity; it is antigen specific and restricted by a specific allele of the MHC (MHC restriction). This conceptual framework for understanding the broad functions of host defence provides an approach to infec-
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KEY POINTS Intracellular pathogens, e.g. viruses, fungi and mycobacteria cause opportunistic infections in patients with T-cell deficiency. Bacterial pathogens cause infections in patients with neutrophil or immunoglobulin deficiency. Recurrent sino-pulmonary infections may indicate an immunodeficiency state.
tions in immunocompromised individuals. An understanding of the type of infection may indicate which arm of the immune system is deficient or, alternatively, knowledge of the immune deficiency will provide insight into the types of infections a patient may experience. PRIMARY IMMUNODEFICIENCES
Disorders of physical barriers Primary ciliary dyskinesia is a rare problem and is phenotypically and genetically heterogenous. Affected individuals have recurrent or chronic bacterial infections causing rhinitis, sinusitis, otitis media, pneumonia and/or bronchiectasis.
Disorders of antibodies
A. M. KESSON, AND A. KAKAKIOS
sinusitis. Recurrent pneumonia can occur and is usually more common in the lower lobes, often with incomplete or slow resolution. There may be persistent collapse and infection in affected lobes, and with delayed diagnosis and treatment of infectious episodes bronchiectasis may develop. Gastrointestinal tract infection with Giardia lamblia is another common infection. Diagnosis rests on the demonstration of reduced levels of IgG, IgA and IgM, as well as the absence of B cells in the peripheral circulation. Transient neutropenia may be associated with acute infections. When appropriately treated with replacement immunoglobulin, the incidence of serious pulmonary infections with complications in XLA is less than in patients with common variable immunodeficiency, presumably because of the T-cell defects found in the latter condition.
Hypogammaglobulinemia with elevated IgM Hypogammaglobulinemia associated with elevated IgM (hyper IgM) results from the inability of B cells to switch from IgM to IgG and IgA production. Children with this disorder usually present with suppurative infections of the upper and lower respiratory tract within the first 2 years of life. Unlike other causes of hypogammaglobulinemia, Pneumocystis jirovecii (previously Pneumocystis carinii) pneumonia is a common presentation. This disorder is also associated with neutropenia which may be either cyclical or persistent.
IgA deficiency IgA deficiency has been defined as a serum IgA level less than 0.05 g/L with normal IgG and IgM levels. The incidence of selective IgA deficiency is approximately 1 in 500 people. IgA deficiency may be transient or may be associated with the use of some drugs, e.g. phenytoin. Approximately one third of individuals with selective IgA deficiency develop symptoms of recurrent respiratory tract infection, usually bronchitis and otitis media. More severe lower respiratory tract infections such as pneumonia are uncommon and the frequency and severity of infections usually improve over 5–10 years. Lower respiratory tract suppuration with bronchiectasis is more common in older individuals when the deficiency may be associated with abnormalities of other IgG subclasses.
X-linked agammaglobulinemia In X-linked agammaglobulinemia (XLA) there is a failure of pre-B cells to develop into mature circulating B cells. Boys with this disorder are usually quite well for the first 4–6 months of life after which the normal decline in maternally transferred immunoglobulin results in susceptibility to upper and lower respiratory tract infections, usually with Streptococcus pneumoniae, Haemophilus influenzae type B and Staphylococcus aureus. Commonly seen infectious manifestations include chronic otitis media with discharge and
Common variable immunodeficiency Common variable immunodeficiency (CVID) usually has a later onset than XLA and symptoms of infection may be more insidious. The spectrum of respiratory infections is similar to that observed in XLA with both S. pneumoniae and H. influenzae pneumonia being common. Rarely, pneumonia may be associated with Pseudomonas aeruginosa or P. jirovecii. Another complication is lymphoid interstitial pneumonitis more prominent in lung bases. Gastrointestinal tract infection must be distinguished from the chronic inflammatory ‘sprue-like’ bowel disease which is a feature of CVID.
IgG subclass deficiency Four subclasses of IgG contribute to the total IgG level. IgG1 and IgG3 are immunoglobulins that develop against protein antigens and bind complement. IgG2 is predominantly active against polysaccharide antigens but does not bind complement effectively. The role of IgG4 is unclear. Only IgG1 and IgG3 cross the placenta; thus IgG2 and IgG4 are not present in the newborn. Identification of the specific IgG subclass deficiency is difficult due to the variable levels of IgG subclasses found in children, the technical difficulties with measuring IgG subclasses and the lack of clear association with significant infections. When
IMMUNOCOMPROMISED CHILDREN: CONDITIONS AND INFECTIOUS AGENTS
attempting to establish whether a low subclass IgG is associated with respiratory tract infections often caused by pathogens with a polysaccharide capsule, e.g. S. pneumoniae pneumonia, it should be remembered that young children under 2 years old have defective antibody production to polysaccharide antigens, making clinical diagnosis difficult. Symptoms which may result from subclass deficiency are recurrent otitis media, recurrent sinusitis, mastoiditis and suppurative lower respiratory tract disease with bronchitis and bronchiectasis. A history of recurrent bronchitis, particularly if associated with purulent sputum, recurrent pneumonia and persisting chest X-ray abnormalities suggests the possibility of subclass deficiency. Bronchiectasis has been detected in subclass deficient older individuals. There is an association between IgA deficiency and IgG2 subclass deficiency.
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lung infiltrates and pleural effusions often noted on X-ray. In addition, an abnormal oligoclonal T-cell proliferation with hepatosplenomegaly and interstitial pulmonary disease has also been described.
Velocardiofacial syndrome
This usually presents in the second 6 months of life and most children do not have a significant increase in serious infection. IgG levels return to normal within the first 2 years of life.
The velocardiofacial syndrome consists of thymic and parathyroid hypoplasia with cardiac outflow abnormalities. Affected infants usually present because of the cardiac problems and the Di George syndrome is diagnosed during investigation of the cardiac lesion. The immunological defect is variable with more severe cases having marked reductions in T-cell numbers and associated infectious problems. Less severe degrees of immunological abnormality are usually not associated with any noticeable clinical problems. The more severely affected individuals have an increased susceptibility to viral infections of the respiratory tract, including RSV and influenza. P. jirovecii may also occur. A complicating factor is the occurrence of tracheo- or broncho-malacia, often caused by compression by abnormal vessels and this may predispose to more severe lower respiratory tract symptoms.
Disorders of T cells
Disorders of phagocytes
Severe combined immunodeficiency
Chronic granulomatous disease
Severe combined immunodeficiency (SCID) results in a deficiency of both T- and B-cell number and function. There are several causes including X-linked SCID which is associated with mutations in the gene for the interleukin2 receptor, autosomal recessive SCID, caused by adenosine deaminase deficiency, and bare lymphocyte syndrome associated with defective expression of class 1 and/or class 2 HLA antigens. In these conditions there is a profound deficiency of T cells and lack of immunoglobulins, with patients usually presenting within the first 6 months of life with diarrhoea, lower respiratory tract infection and failure to thrive. The less severe forms (e.g. Nezelof syndrome) may present later in childhood but often have a history of symptoms starting in the first year of life. Most children present with P. jirovecii pneumonia which may be insidious in onset with the history of cough progressing over several weeks to marked pulmonary involvement. In addition, many of these children have cytomegalovirus (CMV) pneumonitis. Recurrent pneumonias can occur with CMV, respiratory syncytial virus (RSV), P. jirovecii and Gram positive and Gram negative bacteria. Prophylactic therapy against P. jirovecii must be given to all patients following diagnosis. Live virus vaccines must be avoided and children should be isolated to prevent infections with measles, chicken pox or influenza, all of which can cause life-threatening pneumonitis. In addition, Epstein–Barr virus (EBV) can cause lymphoproliferative disease in these patients in whom there is abnormal lymphocyte proliferation of B-cell origin often associated with abnormal paraproteins, with
Chronic granulomatous disease is a disorder of oxidative metabolism of phagocytic cells. In this condition phagocytes can ingest micro-organisms normally, but killing is defective and patients develop infections with catalasepositive organisms, most notably staphylococci, Gram negative enteric bacteria, fungi especially Aspergillus species, and unusual pathogens such as Pseudomonas cepacia and Nocardia species. The lung is the most common site of infection and pulmonary lesions can vary from a patchy bronchopneumonia to extensive consolidation of a lobe or an entire lung. Pneumonias may begin as unilateral or bilateral hilar consolidation and often progress slowly with increasing involvement of the lobes despite antibiotic treatment. Often lobectomy or segmentectomy is required for resolution of infection and hospitalization may be prolonged because of delayed healing and wound breakdown and infection. Lung infections may also lead to invasion of the chest wall with resultant osteomyelitis of the ribs and vertebral bodies often due to Aspergillus species. Lung abscesses, empyema and pleural effusion are uncommon. Another pulmonary manifestation of chronic granulomatous disease is a widespread nodular infiltrate due to multiple small granulomata and which is often initially asymptomatic but can progress to pulmonary insufficiency and death. No organism has been identified in this condition. Long-term prophylactic therapy with trimethoprim–sulfamethoxazole has been used to decrease the incidence of infections but it is unclear whether it reduces the incidence of lower respiratory
Transient hypogammaglobulinemia of infancy
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tract infections. Prophylactic gamma-interferon has reduced the incidence of lower respiratory tract infections including fungal disease in some patients. Prophylaxis for Aspergillus species infection using itraconazole should be considered.
Disorders of natural killer cells Several patients with a defect in natural killer (NK) cell activity have been described. These patients suffered from severe overwhelming infections from Herpesviridae, most notably overwhelming varicella-zoster virus infection with lung involvement and occasional secondary bacterial sepsis. CMV interstitial pneumonitis has also been described.
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Ataxia telangiectasia Ataxia telangiectasia is a complex, autosomal recessive, multisystem disorder. The majority of patients develop significant sinusitis and lower respiratory tract suppurative infections which may progress to bronchiectasis. Approximately half of the patients have an associated IgA deficiency with a concomitant IgG2 deficiency. Those with more severe neurological disease often have episodes of aspiration and more severe respiratory tract disease which may progress to pneumonia and death. SECONDARY IMMUNODEFICIENCIES
Asplenia Miscellaneous disorders Leukocyte adhesion molecule deficiency These subjects have deficiency of the beta-chain subunit of the cell adhesion molecules that occur on lymphocytes, NK cells and haematopoietic cells. Affected children often have a history of delayed separation of the umbilical cord and present with marked gingivitis and otitis media. Viral respiratory tract infections may lead to severe secondary bacterial infections of the trachea and bronchial tree. Pneumonia has been reported in only a minority of subjects.
Hyper IgE syndrome Hyper IgE syndrome is associated with subcutaneous cold abscesses and lower respiratory tract suppurative disease caused by organisms such as S. aureus, H. influenzae and Pseudomonas species. Pneumonia and empyema develop in the early years of life. Pneumatoceles, pulmonary abscesses, bronchopleural fistulae and pneumothoraces are common.
Asplenia may be congenital or acquired and predisposes the patient to serious disseminated and overwhelming infection due to encapsulated organisms such as S. pneumoniae and H. influenza type B and Neisseria meningitidis. Pneumonia may accompany these infections or be the presenting illness.
Corticosteroid therapy Steroids cause significant decrease in T-cell numbers and potential increase in the risk of serious viral infections in individuals receiving >1 mg/kg of prednisone or equivalent. Chronic corticosteroid therapy of connective tissue disorders such as systemic lupus erythematosis may be complicated by infections with S. pneumoniae.
Post solid-organ transplantation Immunosuppression given to patients following solid organ transplantation primarily suppresses T-cell function and predisposes patients to serious viral infection, particularly cytomegalovirus (CMV) and other Herpesviridae.
Mucocutaneous candidiasis Mucocutaneous candidiasis is characterized by Candida species infections of the skin, mucous membranes and nails. Some patients have an increased susceptibility to bacterial infections and develop pneumonia caused by S. aureus, S. pneumoniae or H. influenzae type B. Fungal or viral pneumonias are much less common.
Wiskott–Aldrich syndrome This is an X-linked disorder associated with defective expression of the sialoglycoprotein, CD43, on circulating leukocytes and platelets. All patients are defective in an antibody response to polysaccharide antigens and infection by encapsulated bacteria with associated otitis media or pneumonia is common, particularly in the first year of life.
Post bone-marrow transplantation The type of organism providing the greatest risk to these patients changes over time as a function of immune reconstitution following bone-marrow transplantation. During the initial neutropenic and lymphopenic phase the patients are at significant risk of bacterial, HSV and Aspergillus species infections. After the initial recovery of neutrophils, patients still have significant immunosuppression due to depressed lymphocyte numbers and function and are at significant risk of viral infections, fungal infections most commonly Aspergillus species and Pneumocystis jirovecii. Ongoing deficiency in B cell numbers and antibody production leads to increased risk of infection with encapsulated bacteria such as S. pneumoniae, H. influenzae B and N. meningitidis.
IMMUNOCOMPROMISED CHILDREN: CONDITIONS AND INFECTIOUS AGENTS
Human immunodeficiency virus infection Unlike adults, children with perinatally acquired human immunodeficiency virus (HIV) infection have a greater risk of serious bacterial infections, including sinusitis and pneumonia. They are also at significant risk of P. jirovecii, especially during the first 2 years of life and later on when they develop significant lymphopenia. Lymphoid interstitial pneumonitis is also seen in these patients and, although no definitive aetiology has been defined, EBV has been implicated. INFECTIOUS AGENTS Immunocompromised patients often have attenuation of physical symptoms and signs of respiratory infection and an apparent trivial complaint such as a persistent dry cough may prove to be an early sign of progressive lower respiratory tract disease. In immunodeficient patients, not only do the more common respiratory pathogens cause disease, but so do organisms of lower virulence which rarely cause disease in normal hosts. After a careful physical examination, imaging with chest X-ray and/or with CT scanning (which is more sensitive than X-ray for detecting pulmonary infiltrates) may be indicated. Bronchoalveolar lavage (BAL) specimens from patients who are immunodeficient and have pulmonary infiltrates should be tested for a wide range of potential pathogens including P. jirovecii, acid-fast bacilli, Nocardia species, Legionella species, bacteria and moulds. Investigation for respiratory viruses should include influenza, parainfluenza, adenovirus, RSV and CMV. Investigation for other viral infections may be indicated depending on the clinical syndrome, including human metapneumovirus, measles virus or varicella-zoster virus. Enzyme linked assays or direct fluorescent antibodies may aid rapid identification of a causative virus. Nasopharyngeal swabs or aspirates are useful for culturing respiratory viruses. CYTOMEGALOVIRUS CMV is a member of the Herpesviridae and in immunocompromised patients can cause constitutional symptoms, most notably fever, and a very wide spectrum of disease ranging from asymptomatic to life threatening and involving many different organs. Diffuse CMV pneumonitis is associated with high morbidity and mortality in immunocompromised individuals. Pneumonitis with interstitial infiltrates may occur in recipients of solid organ allografts and bone marrow grafts. It is more common in situations where the donor is CMV positive and the recipient is CMV negative but can occur if both are positive. In solid organ allografts it is especially common in recipients of lung or heart–lung transplant where, despite prolonged anti-CMV treatment, obliterative bronchiolitis may supervene, indicating that CMV plays an important role in the aetiology of chronic
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rejection.1 Pneumonitis in bone marrow allografts recipients is a major life-threatening illness with very high mortality. In addition, CMV may delay bone marrow engraftment and CMV disease, especially pneumonitis, is statistically associated with graft versus host disease (GVHD).2 CMV pneumonitis is very uncommon in patients with acquired immune deficiency syndrome (AIDS); however, CMV infection remains a significant cause of morbidity mainly due to retinitis.
Laboratory diagnosis Detection of virus The definitive diagnosis of CMV pneumonitis is the detection of CMV in lung tissue in a patient with symptoms and signs consistent with pneumonitis. Lung biopsy histopathology characteristically demonstrates intranuclear inclusions which have a surrounding halo and marginated chromatin (owl’s eyes). While histopathology provides a very specific diagnosis, it is insensitive and can only be used for diagnosis when the disease is established.3 Furthermore, open lung biopsy is very invasive and many patients may not be clinically stable enough to warrant the inherent risks of the procedure. For many years it has been the practice to monitor CMV replication in transplant patients, initially using urine and saliva samples but more recently blood, and to predict a high risk for development of CMV pneumonitis by demonstrating increasing replication.4 The detection of CMV in blood has a higher predictive relative risk of CMV disease than the detection of CMV in urine. Pre-emptive antiviral therapy to try to prevent CMV pneumonitis and other CMV disease can be commenced when CMV is detected in the blood and before significant disease has developed. This approach has resulted in a significant improvement in prognosis due to decreased mortality.5 There are several laboratory assays which can be used to detect CMV, including cell culture, detection of antigenemia and polymerase chain reaction (PCR) amplification of CMV DNA. CMV culture was the ‘gold’ standard for detection of CMV; however, it usually takes 12–14 days for detection of cytopathic effect in cell cultures and cultures are not declared negative until at least 21 days. More recently, rapid cultures using immunofluorescence to detect CMV proteins rather than cell cytopathic effects can be performed on infected cell monolayers stained with a monoclonal antibody directed against a CMV antigen, pp65, 16 and 48 h after inoculation. This method of detection is not as sensitive as conventional cell culture but is highly specific and, as results are available within 48 h, it has much better clinical utility.6 Antigenemia is a test done to detect CMV antigens directly in leukocytes from the blood of immunocompromised patients. The monoclonal antibodies react with pp65, hence labelling CMV-positive polymorphonuclear cells and monocytes from the peripheral circulation. This technique has the
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advantage that it takes only a few hours; however, it is an operator-dependent test and samples can deteriorate rapidly if they are not processed within 6 h of collection. Using this assay it is possible to count the number of labelled cells and provide a semi-quantitative assessment of viral load. PCR amplification is now being used more frequently for detection and quantification of CMV in blood. This CMV quantitation is used to predict the risk of onset of CMV disease or may be used as the diagnostic test in some clinical situations.7 CMV remains latent in CMV infected individuals and is never cleared post-infection. CMV pneumonitis can evolve from a primary CMV infection, a secondary CMV infection with a different strain or reactivation of the patient’s own CMV strain. One concern regarding CMV PCR testing of blood is the possibility of detecting this latent or non-replicating virus in a patient who is CMV positive from previous infection but where the detection of CMV has no clinical implications. Several techniques have been used to overcome this potential problem, including the use of a sensitive assay combined with a very small volume of blood, thus minimizing the chance of detecting latent virus. Another approach has been the detection of CMV DNA in plasma which would reflect the active infection as latent CMV is in cells and no free CMV would be expected in the plasma. A third approach is the detection of CMV RNA by a reverse transcription PCR or nucleic acid-based sequence amplification (NASBA) assay indicating that CMV is replicating.8 It has been demonstrated that CMV load is an important predictor of development of diseases and detecting patients with high CMV loads is used to determine those patients at significant risk of developing CMV disease. A qualitative PCR can be used for screening patients if the level of detection of the assay is set at a value which does not correlate with future CMV disease; however, this is very difficult to determine in practice as there are no data indicating what level of CMV replication correlates with disease incidence. In practice, clinicians often consider the presence of two consecutive positive qualitative PCR results to identify high-risk patients and use this as an indicator to start pre-emptive therapy. With the introduction of real-time PCR it is now possible to quantify CMV viral load rapidly, and with the increased availability of these tests it is predicted that quantitative real-time PCR assays will be of significant assistance for clinical care. The rate of increase in viral load can also be estimated by back projecting from the initial positive viral load to the last available PCR negative sample from that patient. Both initial viral load and rate of increase can be used as independent parameters to predict an individual risk of CMV disease.8 Serological assays for CMV infection include detection of IgG antibody against CMV which is indicative of an infection some time in the past. However, immunocom-
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promised patients often have a delayed or absent response. IgM assays are useful in immunocompetent patients for detecting acute CMV infection or reactivations but lack sensitivity for detection of CMV disease in the immunocompromised and are not recommended. CMV IgM is positive in approximately 30% of CMV reactivations in immunocompetent people.5 VARICELLA ZOSTER VIRUS Primary varicella zoster virus (VZV) infection, chickenpox, can be severe and even fatal in immunocompromised patients, i.e. those with lymphoid malignancy or congenitally acquired defects in cell-mediated immunity (particularly Tcell defects, organ transplantation, HIV infection or highdose corticosteroids). Reactivation of VZV, shingles, can cause severe disease in the immunocompromised but the mortality is much less than in primary infection. Children with isolated hypogammaglobulinemia are not at risk for these complications. Primary varicella pneumonitis accounts for many of the fatalities from varicella and it usually occurs within several days after onset of rash, but sometimes the interval is longer. Symptoms include fever, cough and dyspnoea.
Diagnosis The diagnosis of chickenpox is usually made clinically but laboratory diagnosis may be necessary in unusual cases. Diagnosis is best made by demonstration of VZV-specific viral antigens in cell scrapings from skin lesions or by isolation of VZV from skin vesicles. VZV antigens may be demonstrated by immunofluorescence which is a highly sensitive, specific, rapid diagnostic test. Culturing vesicular fluid for virus should be performed very early in the illness and the fluid or swabs transported rapidly to the laboratory at 4 8C as varicella is quite labile and is often quite difficult to isolate. PCR has been successful in the diagnosis of varicella virus infection in vesicular fluid, skin biopsies and respiratory secretions. VZV antibodies can be detected by several methods including fluorescence antibody to membrane antigen,9 latex agglutination and enzyme linked immunofluorescence assay. The presence of specific IgM suggests recent VZV infection but may also be detected in a significant proportion (up to 70%) of immunocompetent patients with herpes zoster (shingles). It should be noted that current serological methods may not detect immunity in individuals who have been immunized with live attenuated VZV vaccine. HERPES SIMPLEX VIRUS Herpes simplex virus (HSV) infection has been documented to extend into the lungs and cause disease in severely immunocompromised individuals.
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Diagnosis Diagnosis can be achieved by clinical suspicion and detection of viral antigen from ulcerating lesions, growth of HSV in culture or HSV DNA using PCR in respiratory secretions. Careful clinical correlation is needed as HSV is often shed orally in immunocompromised patients without lesions. HSV in culture grows rapidly, usually within 2 or 3 days. Serological assays have little clinical value in this setting. MEASLES Measles is a highly infectious disease which usually occurs in the first decade of life. It is characterised by a prodrome of fever, cough, coryza and conjunctivitis followed by a generalized maculopapular rash. Infection with measles confers life-long immunity, and a live attenuated viral vaccine against measles is available and is very efficacious at preventing measles. However, measles is still very prevalent globally and accounts for a large amount of morbidity and millions of deaths in children in third world countries.10 Children with cell-mediated immune deficiency who develop measles often do not develop the typical prodromal illness or rash and usually have severe, progressive and often fatal disease frequently due to giant cell pneumonia. Congenital measles in which the rash is present at birth or within 10 days varies from a mild illness to rapidly fatal disease, with higher mortality in preterm infants or in infants who do not develop a rash and the majority of deaths due to pneumonia.11
Diagnosis Diagnosis of measles is usually made clinically due to history of a prodromal coryzal illness, Koplick’s spots and a typical rash. However, the typical syndrome is often not apparent in severely immunocompromised individuals and a high index of clinical suspicion and appropriate testing is required to make the diagnosis. Laboratory diagnosis can be made by detection of measles antigen using monoclonal antibodies on nasopharyngeal aspirates. Lung biopsy may reveal characteristic multinucleated epithelial giant cells. Serology for measles IgM is helpful if positive but an immunocompromised patient may have a poor response and the sensitivity of the test in these situations is not known. Reverse transcriptase (RT)-PCR for measles virus RNA is also possible as a diagnostic test but is unlikely to be available except in reference laboratories.
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Patients at risk include those with organ transplantation, leukaemia and occasionally patients with AIDS. Patients with chronic pulmonary disorders such as pulmonary alveolar proteinosis or patients requiring long-term corticosteroid use are also at risk. Pulmonary disease is the main clinical presentation with the majority of infections caused by the Nocardia asteroides complex.12 Pulmonary nocardiosis may present as endobronchial inflammatory masses, pneumonia, lung abscesses and cavitary disease with extension to surface or deep structures, and it may cause effusions and empyema.
Diagnosis Diagnosis of Nocardia species is performed by observing thin branching Gram positive rods on Gram stains of sputum, BALs and lung biopsies and/or by culture of the organism in the laboratory. Cultures for Nocardia species usually take 5 days to become positive but all cultures are held for 14 days until declared negative. Nocardia species have previously been identified by biochemical reactions in the laboratory but, more recently, new molecular methods using 16S ribosomal RNA (rRNA) gene sequencing has greatly expand the spectrum of Nocardia species with more than 30 now described.13 Most isolates of Nocardia species are positive with modified acid-fast staining using the Kinyoun technique. Restriction endonuclease analysis following PCR has also been used to differentiate Nocardia species.14 Speciation of Nocardia is an important guide to therapy as antimicrobial susceptibility varies among the species. FUNGAL INFECTIONS It is often very difficult to assess the clinical significance of the isolation of fungi, apart from the dimorphic moulds Coccidioides immitis, Histoplasma capsulatum and Blastomyces dermatitidis, from respiratory specimens. Determining the clinical significance of the isolation of Candida species from specimens other than a lung biopsy is often impossible as Candida is part of the normal flora of the respiratory tract. Occasionally Cryptococcus neoformans can be a colonizer and not a pathogen. There are no guidelines to assist in interpretation but the following may assist. If the organism was seen on Gram stain and a reasonable quantity of organism was grown, it is more likely to represent an invasive infection. The nature of the particular genus or species isolated may assist, e.g. Aspergillus fumigatus is more likely to be a pathogen than Penicillium species other than P. marneffei.
NOCARDIA SPECIES Nocardia is a genus of aerobic actinomycetes which are a diverse group of Gram positive bacteria with branching filamentous cells. Immunocompromised patients are at risk for nocardiosis which is considered an opportunistic pathogen that can cause serious and disseminated disease.
PNEUMOCYSTIS JIROVECII P. jirovecii pneumonia needs to be considered in any patient with defective cell-mediated immunity who develops a persistent cough and chest X-ray changes with a reticular
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or granular infiltrative pattern. The chest is often clear to auscultation despite the CXR changes. Pneumocystis is classically diagnosed by microscopy of respiratory specimens using a variety of stains. The sensitivity of this approach is dependent upon the quality of the specimen and the experience of the microscopist.15 Collection of specimens which accurately reflect the disease process in the lungs is essential, with BAL or lung biopsy being the best. Pneumocystis is rarely detected in expectorated sputum but may be frequently detected (50–90% of the time) in sputum induced by inhalation of nebulized 3% saline. The development of DNA detection using PCR has increased sensitivity for detection of Pneumocystis but false positives do occur in paediatric patients and further studies of this diagnostic approach need to be performed before it can be universally recommended.16
HIV positive patients Fungi other than P. jirovecii may occasionally cause lung infection in HIV positive patients. The most common causes are C. neoformans, and endemic dimorphic fungi such as C. immitis, H. capsulatum and B. dermatitidis, and the mould Aspergillus fumigatus. HIV positive patients are rarely infected unless they are severely immunosuppressed with T cell counts below 100 cells/mm3. While the lungs act as a portal of entry and may be the initial site of infection, haematogenous dissemination commonly occurs to other sites such as the central nervous system and skin. The usual method of diagnosis is by culture of BAL fluid or biopsy of radiologically defined lung lesions. In cryptococcal disease measurement of serum cryptococcal antigen is a very useful sensitive, highly specific, noninvasive test. Infections with histoplasmosis, coccidiomycosis and blastomycosis are generally limited to the geographical regions where these fungi are endemic. Histoplasmosis can be diagnosed by detection of the polysaccharide antigen in urine or blood (sensitivity and specificity both >90%). Blood cultures may also be positive in patients with histoplasmosis or cryptococcus. C. immitis is usually diagnosed by culture and if this fungus is suspected, the laboratory should be advised as it is an extremely dangerous pathogen within the laboratory and special containment precautions (PC3) for staff safety need to be employed. C. immitis can also be isolated in blood cultures. Aspergillus species infection in HIV positive patients can present with one of two clinical syndromes: a semiinvasive pseudo-membranous tracheitis or an invasive pneumonitis. DIAGNOSIS OF MOULDS Definitive diagnosis of filamentous fungi, moulds, most commonly Aspergillus fumigatus or Aspergillus niger, is achieved by culture of lung biopsy specimens or BAL fluid. Fungal pneumonia is an infrequent but serious infection in highly immunosuppressed patients, particularly
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those with prolonged neutropenia and/or haematopoietic stem cell transplantation. Detection of lesions consistent with fungal infection on high resolution CT scanning is not in itself diagnostic of any particular fungus and is not specific. Definitive diagnosis is dependent on culture of sputum or BAL fluid or lung biopsy. Early in the clinical course sputum cultures lack sensitivity and may also lack specificity.17 New methods for detection of Aspergillus species are being developed and detection of Aspergillus DNA or 1-3-beta-D-glucan or galactomannan in the blood may prove helpful in early diagnosis.18 The clinical utility of these recent serological tests is still being ascertained. Other moulds such as Scedosporium species, Fusarium species and the agents of mucormycosis (Rhizopus, Rhizomucor and Cunninghamella species being the most frequent) are an increasing problem in neutropenic patients and bone marrow transplant recipients. Detection of fungal hyphae in histological specimen of lung tissue is diagnostic of fungal infection but this cannot be used to speciate the fungus. Confirmation by culture should be pursued. However, attempts to culture the agents of mucormycosis from infected tissue are often unsuccessful as the dissociation of the tissue in the microbiology laboratory for culture often leads to inadvertent destruction of the very broad hyphae of this group of fungi. STRONGYLOIDIASIS Strongyloides stercoralis is a globally prevalent round worm infection. Patients with defects of cell-mediated immunity, especially those with corticosteroid therapy, immunosuppressive drugs, haematological malignancy or infection with HIV are at significant risk of hyperinfection or increased susceptibility to infection and they are often refractory to treatment. With hyperinfection increased numbers of larvae are found in many organs, including the lungs, and they may produce pneumonitis with cough, haemoptysis and respiratory failure. Interstitial infiltrates or consolidation may be seen on chest X-ray.
Diagnosis Strongyloidiasis is diagnosed by finding larvae in microscopic examination of stool specimens. Repeated stool examinations may be required because sensitivity can be low. Hyperinfection can be diagnosed by detection of larvae in sputum or lung tissue, which typically contain a high number of filariform larvae. If tests are negative but there is a high index of suspicion therapy is recommended. Care must be exercised when collecting specimens from potentially infected patients as contact with the skin can lead to infection of health care workers. Serological tests which provide evidence of infection are also available with a sensitivity of approximately 85%; however, cross-reactions with filarial infections limit specificity.19
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REFERENCES 1. Duncan SR, Grgurich WF, Iacono AT et al. A comparison of ganciclovir and acyclovir to prevent cytomegalovirus after lung transplantation. Am J Respir Crit Care Med 1994; 150: 146–152. 2. Bostrom L, Ringden O, Sundberg B, Ljungman P, Linde A, Nilsson B. Pretransplant herpes virus serology and chronic graft-versus-host disease. Bone Marrow Transplant 1989; 4: 547–552. 3. Mattes FM, McLaughlin JE, Emery VC, Clark DA, Griffiths PD. Histopathological detection of owl’s eye inclusions is still specific for cytomegalovirus in the era of human herpes viruses 6 and 7. J Clin Pathol 2000; 53: 612–614. 4. Pillay D, Ali AA, Liu SF, Kops E, Sweeny P, Griffiths PD. The prognostic significance of positive CMV cultures during surveillance of renal transplant recipients. Transplantation 1993; 56: 103–108. 5. Sia IG, Patel R. New strategies for prevention and therapy of cytomegalovirus infection and disease in solid-organ transplant recipients. Clin Microbiol Rev 2000; 13: 83–121. 6. Chou S. Newer methods for diagnosis of cytomegalovirus infection. Rev Infect Dis 1990; 12(Suppl. 7): S727–S736. 7. Meijer E, Boland GJ, Verdonck LF. Prevention of cytomegalovirus disease in recipients of allogeneic stem cell transplants. Clin Microbiol Rev 2003; 4: 647–657. 8. Emery VC, Sabin CA, Cope AV, Gor D, Hassan-Walker AF, Griffiths PD. Application of viral-load kinetics to identify patients who develop cytomegalovirus disease after transplantation. Lancet 2000; 355: 2032–2036. 9. Williams V, Gershon A, Brunell P. Serologic response to varicellazoster membrane antigens measured by indirect immunofluorescence. J Infect Dis 1974; 130: 669–672.
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10. Ornstein WA, Strebel PM, Papania M, Sutter RM, Bellini WJ, Cochi SL. Measles eradication: is it in our future? Am J Public Health 2000; 90: 1521–1525. 11. Gershon AA. Chickenpox, measles and mumps. In: Remington JS, Klein JO, eds. Infectious Diseases of the Fetus and Newborn Infant. 5th edition. Philadelphia: WB Saunders, 2001; pp. 683–732. 12. Beaman BM, Beaman L. Norcardia species: host–parasite relationship. Clin Microbiol Rev 1994; 7: 213–264. 13. Wellinghausen N, Pietzcker T, Kern WV et al. Expanded spectrum Norcadia species causing clinical norcardiosis detected by molecular methods. Int J Med Microbiol 2002; 292: 277–282. 14. Steingrube VA, Brown BA, Gibson JL et al. DNA amplification and restriction endonuclease analysis for differentiation of 12 species and taxa of Nocardia, including recognition of four new taxa within the Nocardia asteroides complex. J Clin Microbiol 1995; 33: 3096–3101. 15. Aslanzadeh J, Stelmach PS. Detection of Pneumocystis carinii with direct fluorescence antibody and calcofluor white stain. Infection 1996; 24: 248–250. 16. Olsson M, Elvin K, Lidman C, Lo¨fdahl S, Linder E. A rapid and simple nested PCR assay for the detection of Pneumocystis carinii in sputum samples. Scand J Infect Dis 1996; 28: 597–600. 17. Bodey G, Bueltmann B, Duguid W et al. Fungal infections in cancer patients: An international autopsy survey. Eur J Clin Microbiol Infect Dis 1992; 11: 99–109. 18. Maertens J, Van Eldere J, Verhaegen J, Verbeken E, Verschakelen J, Boogaerts M. Use of circulating galactomannan for early diagnosis of invasive aspergillosis in stem cell transplant recipients. J Infect Dis 2002; 186: 1297–1306. 19. Siddiqui AA, Berk LL. Diagnosis of Strongyloides stercoralis infection. Clin Infect Dis 2001; 33: 1040–1047.
MINI-SYMPOSIUM: MICROBIOLOGICAL DIAGNOSTIC PROCEDURES IN RESPIRATORY INFECTIONS
Immunocompromised children: conditions and infectious agents Alison M. Kesson1,* and Alyson Kakakios2 1
Department of Infectious Diseases and Microbiology, The Children’s Hospital at Westmead, Discipline of Paediatrics and Child Health, University of Sydney, LMB 4001, Westmead NSW 2145, Australia; 2 Departments of Allergy and Immunology, The Children’s Hospital at Westmead, LMB 4001, Westmead NSW 2145, Australia KEYWORDS immunodeficiency; opportunistic pathogen; respiratory infection
Summary Individuals with immunodeficiency, either primary or acquired, are increasingly common. These individuals have increased susceptibility to a range of infections which are uncommon in the normal host. An understanding of the individual’s immune defect provides important information about the range of organisms that this individual may be susceptible to. As a corollary, identification of an ‘opportunistic pathogen’ may indicate the patient’s type of underlying immune defect. ß 2007 Elsevier Ltd. All rights reserved.
IMMUNITY Normal healthy individuals protect themselves against invading micro-organisms by means of many interrelated but different mechanisms, including physicochemical barriers, circulating molecules and cells and their soluble mediators.
Innate immunity Defence mechanisms which are present prior to exposure to micro-organisms, and which are not enhanced by exposure to most of these, constitute the innate immune system. The innate immune system consists of phagocytes (macrophages, neutrophils and natural killer cells) and complement.
Acquired immunity In contrast to the innate immune system, the acquired immune system is induced or stimulated by exposure to * Corresponding author. Department of Virology, The Children’s Hospital at Westmead, LMB 4001, Westmead NSW 2145, Australia. Tel.: +61 2 98453823; Fax: +61 2 98453291. E-mail address: [email protected] (A.M. Kesson). 1526-0542/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.prrv.2007.07.005
micro-organisms with exquisite specificity and is amplified with each exposure. The acquired immune system consists of lymphocytes, both T cells and B cells, and the secreted products of B cells, antibodies. Foreign substances which induce specific immunity are called antigens. In addition, the acquired immune system has memory so that subsequent encounters with the same antigen stimulate increasingly strong and rapid responses. The acquired immune system constantly surveys the body for foreign antigens and pathogens. Antibodies respond to antigens in their native conformational state in the extracellular environment while T cells respond to degraded antigens ‘presented’ on the surface of cells and thus respond to intracellular pathogens. Intracellular pathogens, e.g. viruses which infect cells, are degraded and peptides from the viruses (foreign peptides) bind to the major histocompatibility molecules (MHC), called human leukocyte antigens (HLA) in humans, which are then displayed on the cell surface for recognition by T cells via the T-cell receptor. This response has dual specificity; it is antigen specific and restricted by a specific allele of the MHC (MHC restriction). This conceptual framework for understanding the broad functions of host defence provides an approach to infec-
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KEY POINTS Intracellular pathogens, e.g. viruses, fungi and mycobacteria cause opportunistic infections in patients with T-cell deficiency. Bacterial pathogens cause infections in patients with neutrophil or immunoglobulin deficiency. Recurrent sino-pulmonary infections may indicate an immunodeficiency state.
tions in immunocompromised individuals. An understanding of the type of infection may indicate which arm of the immune system is deficient or, alternatively, knowledge of the immune deficiency will provide insight into the types of infections a patient may experience. PRIMARY IMMUNODEFICIENCES
Disorders of physical barriers Primary ciliary dyskinesia is a rare problem and is phenotypically and genetically heterogenous. Affected individuals have recurrent or chronic bacterial infections causing rhinitis, sinusitis, otitis media, pneumonia and/or bronchiectasis.
Disorders of antibodies
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sinusitis. Recurrent pneumonia can occur and is usually more common in the lower lobes, often with incomplete or slow resolution. There may be persistent collapse and infection in affected lobes, and with delayed diagnosis and treatment of infectious episodes bronchiectasis may develop. Gastrointestinal tract infection with Giardia lamblia is another common infection. Diagnosis rests on the demonstration of reduced levels of IgG, IgA and IgM, as well as the absence of B cells in the peripheral circulation. Transient neutropenia may be associated with acute infections. When appropriately treated with replacement immunoglobulin, the incidence of serious pulmonary infections with complications in XLA is less than in patients with common variable immunodeficiency, presumably because of the T-cell defects found in the latter condition.
Hypogammaglobulinemia with elevated IgM Hypogammaglobulinemia associated with elevated IgM (hyper IgM) results from the inability of B cells to switch from IgM to IgG and IgA production. Children with this disorder usually present with suppurative infections of the upper and lower respiratory tract within the first 2 years of life. Unlike other causes of hypogammaglobulinemia, Pneumocystis jirovecii (previously Pneumocystis carinii) pneumonia is a common presentation. This disorder is also associated with neutropenia which may be either cyclical or persistent.
IgA deficiency IgA deficiency has been defined as a serum IgA level less than 0.05 g/L with normal IgG and IgM levels. The incidence of selective IgA deficiency is approximately 1 in 500 people. IgA deficiency may be transient or may be associated with the use of some drugs, e.g. phenytoin. Approximately one third of individuals with selective IgA deficiency develop symptoms of recurrent respiratory tract infection, usually bronchitis and otitis media. More severe lower respiratory tract infections such as pneumonia are uncommon and the frequency and severity of infections usually improve over 5–10 years. Lower respiratory tract suppuration with bronchiectasis is more common in older individuals when the deficiency may be associated with abnormalities of other IgG subclasses.
X-linked agammaglobulinemia In X-linked agammaglobulinemia (XLA) there is a failure of pre-B cells to develop into mature circulating B cells. Boys with this disorder are usually quite well for the first 4–6 months of life after which the normal decline in maternally transferred immunoglobulin results in susceptibility to upper and lower respiratory tract infections, usually with Streptococcus pneumoniae, Haemophilus influenzae type B and Staphylococcus aureus. Commonly seen infectious manifestations include chronic otitis media with discharge and
Common variable immunodeficiency Common variable immunodeficiency (CVID) usually has a later onset than XLA and symptoms of infection may be more insidious. The spectrum of respiratory infections is similar to that observed in XLA with both S. pneumoniae and H. influenzae pneumonia being common. Rarely, pneumonia may be associated with Pseudomonas aeruginosa or P. jirovecii. Another complication is lymphoid interstitial pneumonitis more prominent in lung bases. Gastrointestinal tract infection must be distinguished from the chronic inflammatory ‘sprue-like’ bowel disease which is a feature of CVID.
IgG subclass deficiency Four subclasses of IgG contribute to the total IgG level. IgG1 and IgG3 are immunoglobulins that develop against protein antigens and bind complement. IgG2 is predominantly active against polysaccharide antigens but does not bind complement effectively. The role of IgG4 is unclear. Only IgG1 and IgG3 cross the placenta; thus IgG2 and IgG4 are not present in the newborn. Identification of the specific IgG subclass deficiency is difficult due to the variable levels of IgG subclasses found in children, the technical difficulties with measuring IgG subclasses and the lack of clear association with significant infections. When
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attempting to establish whether a low subclass IgG is associated with respiratory tract infections often caused by pathogens with a polysaccharide capsule, e.g. S. pneumoniae pneumonia, it should be remembered that young children under 2 years old have defective antibody production to polysaccharide antigens, making clinical diagnosis difficult. Symptoms which may result from subclass deficiency are recurrent otitis media, recurrent sinusitis, mastoiditis and suppurative lower respiratory tract disease with bronchitis and bronchiectasis. A history of recurrent bronchitis, particularly if associated with purulent sputum, recurrent pneumonia and persisting chest X-ray abnormalities suggests the possibility of subclass deficiency. Bronchiectasis has been detected in subclass deficient older individuals. There is an association between IgA deficiency and IgG2 subclass deficiency.
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lung infiltrates and pleural effusions often noted on X-ray. In addition, an abnormal oligoclonal T-cell proliferation with hepatosplenomegaly and interstitial pulmonary disease has also been described.
Velocardiofacial syndrome
This usually presents in the second 6 months of life and most children do not have a significant increase in serious infection. IgG levels return to normal within the first 2 years of life.
The velocardiofacial syndrome consists of thymic and parathyroid hypoplasia with cardiac outflow abnormalities. Affected infants usually present because of the cardiac problems and the Di George syndrome is diagnosed during investigation of the cardiac lesion. The immunological defect is variable with more severe cases having marked reductions in T-cell numbers and associated infectious problems. Less severe degrees of immunological abnormality are usually not associated with any noticeable clinical problems. The more severely affected individuals have an increased susceptibility to viral infections of the respiratory tract, including RSV and influenza. P. jirovecii may also occur. A complicating factor is the occurrence of tracheo- or broncho-malacia, often caused by compression by abnormal vessels and this may predispose to more severe lower respiratory tract symptoms.
Disorders of T cells
Disorders of phagocytes
Severe combined immunodeficiency
Chronic granulomatous disease
Severe combined immunodeficiency (SCID) results in a deficiency of both T- and B-cell number and function. There are several causes including X-linked SCID which is associated with mutations in the gene for the interleukin2 receptor, autosomal recessive SCID, caused by adenosine deaminase deficiency, and bare lymphocyte syndrome associated with defective expression of class 1 and/or class 2 HLA antigens. In these conditions there is a profound deficiency of T cells and lack of immunoglobulins, with patients usually presenting within the first 6 months of life with diarrhoea, lower respiratory tract infection and failure to thrive. The less severe forms (e.g. Nezelof syndrome) may present later in childhood but often have a history of symptoms starting in the first year of life. Most children present with P. jirovecii pneumonia which may be insidious in onset with the history of cough progressing over several weeks to marked pulmonary involvement. In addition, many of these children have cytomegalovirus (CMV) pneumonitis. Recurrent pneumonias can occur with CMV, respiratory syncytial virus (RSV), P. jirovecii and Gram positive and Gram negative bacteria. Prophylactic therapy against P. jirovecii must be given to all patients following diagnosis. Live virus vaccines must be avoided and children should be isolated to prevent infections with measles, chicken pox or influenza, all of which can cause life-threatening pneumonitis. In addition, Epstein–Barr virus (EBV) can cause lymphoproliferative disease in these patients in whom there is abnormal lymphocyte proliferation of B-cell origin often associated with abnormal paraproteins, with
Chronic granulomatous disease is a disorder of oxidative metabolism of phagocytic cells. In this condition phagocytes can ingest micro-organisms normally, but killing is defective and patients develop infections with catalasepositive organisms, most notably staphylococci, Gram negative enteric bacteria, fungi especially Aspergillus species, and unusual pathogens such as Pseudomonas cepacia and Nocardia species. The lung is the most common site of infection and pulmonary lesions can vary from a patchy bronchopneumonia to extensive consolidation of a lobe or an entire lung. Pneumonias may begin as unilateral or bilateral hilar consolidation and often progress slowly with increasing involvement of the lobes despite antibiotic treatment. Often lobectomy or segmentectomy is required for resolution of infection and hospitalization may be prolonged because of delayed healing and wound breakdown and infection. Lung infections may also lead to invasion of the chest wall with resultant osteomyelitis of the ribs and vertebral bodies often due to Aspergillus species. Lung abscesses, empyema and pleural effusion are uncommon. Another pulmonary manifestation of chronic granulomatous disease is a widespread nodular infiltrate due to multiple small granulomata and which is often initially asymptomatic but can progress to pulmonary insufficiency and death. No organism has been identified in this condition. Long-term prophylactic therapy with trimethoprim–sulfamethoxazole has been used to decrease the incidence of infections but it is unclear whether it reduces the incidence of lower respiratory
Transient hypogammaglobulinemia of infancy
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tract infections. Prophylactic gamma-interferon has reduced the incidence of lower respiratory tract infections including fungal disease in some patients. Prophylaxis for Aspergillus species infection using itraconazole should be considered.
Disorders of natural killer cells Several patients with a defect in natural killer (NK) cell activity have been described. These patients suffered from severe overwhelming infections from Herpesviridae, most notably overwhelming varicella-zoster virus infection with lung involvement and occasional secondary bacterial sepsis. CMV interstitial pneumonitis has also been described.
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Ataxia telangiectasia Ataxia telangiectasia is a complex, autosomal recessive, multisystem disorder. The majority of patients develop significant sinusitis and lower respiratory tract suppurative infections which may progress to bronchiectasis. Approximately half of the patients have an associated IgA deficiency with a concomitant IgG2 deficiency. Those with more severe neurological disease often have episodes of aspiration and more severe respiratory tract disease which may progress to pneumonia and death. SECONDARY IMMUNODEFICIENCIES
Asplenia Miscellaneous disorders Leukocyte adhesion molecule deficiency These subjects have deficiency of the beta-chain subunit of the cell adhesion molecules that occur on lymphocytes, NK cells and haematopoietic cells. Affected children often have a history of delayed separation of the umbilical cord and present with marked gingivitis and otitis media. Viral respiratory tract infections may lead to severe secondary bacterial infections of the trachea and bronchial tree. Pneumonia has been reported in only a minority of subjects.
Hyper IgE syndrome Hyper IgE syndrome is associated with subcutaneous cold abscesses and lower respiratory tract suppurative disease caused by organisms such as S. aureus, H. influenzae and Pseudomonas species. Pneumonia and empyema develop in the early years of life. Pneumatoceles, pulmonary abscesses, bronchopleural fistulae and pneumothoraces are common.
Asplenia may be congenital or acquired and predisposes the patient to serious disseminated and overwhelming infection due to encapsulated organisms such as S. pneumoniae and H. influenza type B and Neisseria meningitidis. Pneumonia may accompany these infections or be the presenting illness.
Corticosteroid therapy Steroids cause significant decrease in T-cell numbers and potential increase in the risk of serious viral infections in individuals receiving >1 mg/kg of prednisone or equivalent. Chronic corticosteroid therapy of connective tissue disorders such as systemic lupus erythematosis may be complicated by infections with S. pneumoniae.
Post solid-organ transplantation Immunosuppression given to patients following solid organ transplantation primarily suppresses T-cell function and predisposes patients to serious viral infection, particularly cytomegalovirus (CMV) and other Herpesviridae.
Mucocutaneous candidiasis Mucocutaneous candidiasis is characterized by Candida species infections of the skin, mucous membranes and nails. Some patients have an increased susceptibility to bacterial infections and develop pneumonia caused by S. aureus, S. pneumoniae or H. influenzae type B. Fungal or viral pneumonias are much less common.
Wiskott–Aldrich syndrome This is an X-linked disorder associated with defective expression of the sialoglycoprotein, CD43, on circulating leukocytes and platelets. All patients are defective in an antibody response to polysaccharide antigens and infection by encapsulated bacteria with associated otitis media or pneumonia is common, particularly in the first year of life.
Post bone-marrow transplantation The type of organism providing the greatest risk to these patients changes over time as a function of immune reconstitution following bone-marrow transplantation. During the initial neutropenic and lymphopenic phase the patients are at significant risk of bacterial, HSV and Aspergillus species infections. After the initial recovery of neutrophils, patients still have significant immunosuppression due to depressed lymphocyte numbers and function and are at significant risk of viral infections, fungal infections most commonly Aspergillus species and Pneumocystis jirovecii. Ongoing deficiency in B cell numbers and antibody production leads to increased risk of infection with encapsulated bacteria such as S. pneumoniae, H. influenzae B and N. meningitidis.
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Human immunodeficiency virus infection Unlike adults, children with perinatally acquired human immunodeficiency virus (HIV) infection have a greater risk of serious bacterial infections, including sinusitis and pneumonia. They are also at significant risk of P. jirovecii, especially during the first 2 years of life and later on when they develop significant lymphopenia. Lymphoid interstitial pneumonitis is also seen in these patients and, although no definitive aetiology has been defined, EBV has been implicated. INFECTIOUS AGENTS Immunocompromised patients often have attenuation of physical symptoms and signs of respiratory infection and an apparent trivial complaint such as a persistent dry cough may prove to be an early sign of progressive lower respiratory tract disease. In immunodeficient patients, not only do the more common respiratory pathogens cause disease, but so do organisms of lower virulence which rarely cause disease in normal hosts. After a careful physical examination, imaging with chest X-ray and/or with CT scanning (which is more sensitive than X-ray for detecting pulmonary infiltrates) may be indicated. Bronchoalveolar lavage (BAL) specimens from patients who are immunodeficient and have pulmonary infiltrates should be tested for a wide range of potential pathogens including P. jirovecii, acid-fast bacilli, Nocardia species, Legionella species, bacteria and moulds. Investigation for respiratory viruses should include influenza, parainfluenza, adenovirus, RSV and CMV. Investigation for other viral infections may be indicated depending on the clinical syndrome, including human metapneumovirus, measles virus or varicella-zoster virus. Enzyme linked assays or direct fluorescent antibodies may aid rapid identification of a causative virus. Nasopharyngeal swabs or aspirates are useful for culturing respiratory viruses. CYTOMEGALOVIRUS CMV is a member of the Herpesviridae and in immunocompromised patients can cause constitutional symptoms, most notably fever, and a very wide spectrum of disease ranging from asymptomatic to life threatening and involving many different organs. Diffuse CMV pneumonitis is associated with high morbidity and mortality in immunocompromised individuals. Pneumonitis with interstitial infiltrates may occur in recipients of solid organ allografts and bone marrow grafts. It is more common in situations where the donor is CMV positive and the recipient is CMV negative but can occur if both are positive. In solid organ allografts it is especially common in recipients of lung or heart–lung transplant where, despite prolonged anti-CMV treatment, obliterative bronchiolitis may supervene, indicating that CMV plays an important role in the aetiology of chronic
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rejection.1 Pneumonitis in bone marrow allografts recipients is a major life-threatening illness with very high mortality. In addition, CMV may delay bone marrow engraftment and CMV disease, especially pneumonitis, is statistically associated with graft versus host disease (GVHD).2 CMV pneumonitis is very uncommon in patients with acquired immune deficiency syndrome (AIDS); however, CMV infection remains a significant cause of morbidity mainly due to retinitis.
Laboratory diagnosis Detection of virus The definitive diagnosis of CMV pneumonitis is the detection of CMV in lung tissue in a patient with symptoms and signs consistent with pneumonitis. Lung biopsy histopathology characteristically demonstrates intranuclear inclusions which have a surrounding halo and marginated chromatin (owl’s eyes). While histopathology provides a very specific diagnosis, it is insensitive and can only be used for diagnosis when the disease is established.3 Furthermore, open lung biopsy is very invasive and many patients may not be clinically stable enough to warrant the inherent risks of the procedure. For many years it has been the practice to monitor CMV replication in transplant patients, initially using urine and saliva samples but more recently blood, and to predict a high risk for development of CMV pneumonitis by demonstrating increasing replication.4 The detection of CMV in blood has a higher predictive relative risk of CMV disease than the detection of CMV in urine. Pre-emptive antiviral therapy to try to prevent CMV pneumonitis and other CMV disease can be commenced when CMV is detected in the blood and before significant disease has developed. This approach has resulted in a significant improvement in prognosis due to decreased mortality.5 There are several laboratory assays which can be used to detect CMV, including cell culture, detection of antigenemia and polymerase chain reaction (PCR) amplification of CMV DNA. CMV culture was the ‘gold’ standard for detection of CMV; however, it usually takes 12–14 days for detection of cytopathic effect in cell cultures and cultures are not declared negative until at least 21 days. More recently, rapid cultures using immunofluorescence to detect CMV proteins rather than cell cytopathic effects can be performed on infected cell monolayers stained with a monoclonal antibody directed against a CMV antigen, pp65, 16 and 48 h after inoculation. This method of detection is not as sensitive as conventional cell culture but is highly specific and, as results are available within 48 h, it has much better clinical utility.6 Antigenemia is a test done to detect CMV antigens directly in leukocytes from the blood of immunocompromised patients. The monoclonal antibodies react with pp65, hence labelling CMV-positive polymorphonuclear cells and monocytes from the peripheral circulation. This technique has the
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advantage that it takes only a few hours; however, it is an operator-dependent test and samples can deteriorate rapidly if they are not processed within 6 h of collection. Using this assay it is possible to count the number of labelled cells and provide a semi-quantitative assessment of viral load. PCR amplification is now being used more frequently for detection and quantification of CMV in blood. This CMV quantitation is used to predict the risk of onset of CMV disease or may be used as the diagnostic test in some clinical situations.7 CMV remains latent in CMV infected individuals and is never cleared post-infection. CMV pneumonitis can evolve from a primary CMV infection, a secondary CMV infection with a different strain or reactivation of the patient’s own CMV strain. One concern regarding CMV PCR testing of blood is the possibility of detecting this latent or non-replicating virus in a patient who is CMV positive from previous infection but where the detection of CMV has no clinical implications. Several techniques have been used to overcome this potential problem, including the use of a sensitive assay combined with a very small volume of blood, thus minimizing the chance of detecting latent virus. Another approach has been the detection of CMV DNA in plasma which would reflect the active infection as latent CMV is in cells and no free CMV would be expected in the plasma. A third approach is the detection of CMV RNA by a reverse transcription PCR or nucleic acid-based sequence amplification (NASBA) assay indicating that CMV is replicating.8 It has been demonstrated that CMV load is an important predictor of development of diseases and detecting patients with high CMV loads is used to determine those patients at significant risk of developing CMV disease. A qualitative PCR can be used for screening patients if the level of detection of the assay is set at a value which does not correlate with future CMV disease; however, this is very difficult to determine in practice as there are no data indicating what level of CMV replication correlates with disease incidence. In practice, clinicians often consider the presence of two consecutive positive qualitative PCR results to identify high-risk patients and use this as an indicator to start pre-emptive therapy. With the introduction of real-time PCR it is now possible to quantify CMV viral load rapidly, and with the increased availability of these tests it is predicted that quantitative real-time PCR assays will be of significant assistance for clinical care. The rate of increase in viral load can also be estimated by back projecting from the initial positive viral load to the last available PCR negative sample from that patient. Both initial viral load and rate of increase can be used as independent parameters to predict an individual risk of CMV disease.8 Serological assays for CMV infection include detection of IgG antibody against CMV which is indicative of an infection some time in the past. However, immunocom-
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promised patients often have a delayed or absent response. IgM assays are useful in immunocompetent patients for detecting acute CMV infection or reactivations but lack sensitivity for detection of CMV disease in the immunocompromised and are not recommended. CMV IgM is positive in approximately 30% of CMV reactivations in immunocompetent people.5 VARICELLA ZOSTER VIRUS Primary varicella zoster virus (VZV) infection, chickenpox, can be severe and even fatal in immunocompromised patients, i.e. those with lymphoid malignancy or congenitally acquired defects in cell-mediated immunity (particularly Tcell defects, organ transplantation, HIV infection or highdose corticosteroids). Reactivation of VZV, shingles, can cause severe disease in the immunocompromised but the mortality is much less than in primary infection. Children with isolated hypogammaglobulinemia are not at risk for these complications. Primary varicella pneumonitis accounts for many of the fatalities from varicella and it usually occurs within several days after onset of rash, but sometimes the interval is longer. Symptoms include fever, cough and dyspnoea.
Diagnosis The diagnosis of chickenpox is usually made clinically but laboratory diagnosis may be necessary in unusual cases. Diagnosis is best made by demonstration of VZV-specific viral antigens in cell scrapings from skin lesions or by isolation of VZV from skin vesicles. VZV antigens may be demonstrated by immunofluorescence which is a highly sensitive, specific, rapid diagnostic test. Culturing vesicular fluid for virus should be performed very early in the illness and the fluid or swabs transported rapidly to the laboratory at 4 8C as varicella is quite labile and is often quite difficult to isolate. PCR has been successful in the diagnosis of varicella virus infection in vesicular fluid, skin biopsies and respiratory secretions. VZV antibodies can be detected by several methods including fluorescence antibody to membrane antigen,9 latex agglutination and enzyme linked immunofluorescence assay. The presence of specific IgM suggests recent VZV infection but may also be detected in a significant proportion (up to 70%) of immunocompetent patients with herpes zoster (shingles). It should be noted that current serological methods may not detect immunity in individuals who have been immunized with live attenuated VZV vaccine. HERPES SIMPLEX VIRUS Herpes simplex virus (HSV) infection has been documented to extend into the lungs and cause disease in severely immunocompromised individuals.
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Diagnosis Diagnosis can be achieved by clinical suspicion and detection of viral antigen from ulcerating lesions, growth of HSV in culture or HSV DNA using PCR in respiratory secretions. Careful clinical correlation is needed as HSV is often shed orally in immunocompromised patients without lesions. HSV in culture grows rapidly, usually within 2 or 3 days. Serological assays have little clinical value in this setting. MEASLES Measles is a highly infectious disease which usually occurs in the first decade of life. It is characterised by a prodrome of fever, cough, coryza and conjunctivitis followed by a generalized maculopapular rash. Infection with measles confers life-long immunity, and a live attenuated viral vaccine against measles is available and is very efficacious at preventing measles. However, measles is still very prevalent globally and accounts for a large amount of morbidity and millions of deaths in children in third world countries.10 Children with cell-mediated immune deficiency who develop measles often do not develop the typical prodromal illness or rash and usually have severe, progressive and often fatal disease frequently due to giant cell pneumonia. Congenital measles in which the rash is present at birth or within 10 days varies from a mild illness to rapidly fatal disease, with higher mortality in preterm infants or in infants who do not develop a rash and the majority of deaths due to pneumonia.11
Diagnosis Diagnosis of measles is usually made clinically due to history of a prodromal coryzal illness, Koplick’s spots and a typical rash. However, the typical syndrome is often not apparent in severely immunocompromised individuals and a high index of clinical suspicion and appropriate testing is required to make the diagnosis. Laboratory diagnosis can be made by detection of measles antigen using monoclonal antibodies on nasopharyngeal aspirates. Lung biopsy may reveal characteristic multinucleated epithelial giant cells. Serology for measles IgM is helpful if positive but an immunocompromised patient may have a poor response and the sensitivity of the test in these situations is not known. Reverse transcriptase (RT)-PCR for measles virus RNA is also possible as a diagnostic test but is unlikely to be available except in reference laboratories.
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Patients at risk include those with organ transplantation, leukaemia and occasionally patients with AIDS. Patients with chronic pulmonary disorders such as pulmonary alveolar proteinosis or patients requiring long-term corticosteroid use are also at risk. Pulmonary disease is the main clinical presentation with the majority of infections caused by the Nocardia asteroides complex.12 Pulmonary nocardiosis may present as endobronchial inflammatory masses, pneumonia, lung abscesses and cavitary disease with extension to surface or deep structures, and it may cause effusions and empyema.
Diagnosis Diagnosis of Nocardia species is performed by observing thin branching Gram positive rods on Gram stains of sputum, BALs and lung biopsies and/or by culture of the organism in the laboratory. Cultures for Nocardia species usually take 5 days to become positive but all cultures are held for 14 days until declared negative. Nocardia species have previously been identified by biochemical reactions in the laboratory but, more recently, new molecular methods using 16S ribosomal RNA (rRNA) gene sequencing has greatly expand the spectrum of Nocardia species with more than 30 now described.13 Most isolates of Nocardia species are positive with modified acid-fast staining using the Kinyoun technique. Restriction endonuclease analysis following PCR has also been used to differentiate Nocardia species.14 Speciation of Nocardia is an important guide to therapy as antimicrobial susceptibility varies among the species. FUNGAL INFECTIONS It is often very difficult to assess the clinical significance of the isolation of fungi, apart from the dimorphic moulds Coccidioides immitis, Histoplasma capsulatum and Blastomyces dermatitidis, from respiratory specimens. Determining the clinical significance of the isolation of Candida species from specimens other than a lung biopsy is often impossible as Candida is part of the normal flora of the respiratory tract. Occasionally Cryptococcus neoformans can be a colonizer and not a pathogen. There are no guidelines to assist in interpretation but the following may assist. If the organism was seen on Gram stain and a reasonable quantity of organism was grown, it is more likely to represent an invasive infection. The nature of the particular genus or species isolated may assist, e.g. Aspergillus fumigatus is more likely to be a pathogen than Penicillium species other than P. marneffei.
NOCARDIA SPECIES Nocardia is a genus of aerobic actinomycetes which are a diverse group of Gram positive bacteria with branching filamentous cells. Immunocompromised patients are at risk for nocardiosis which is considered an opportunistic pathogen that can cause serious and disseminated disease.
PNEUMOCYSTIS JIROVECII P. jirovecii pneumonia needs to be considered in any patient with defective cell-mediated immunity who develops a persistent cough and chest X-ray changes with a reticular
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or granular infiltrative pattern. The chest is often clear to auscultation despite the CXR changes. Pneumocystis is classically diagnosed by microscopy of respiratory specimens using a variety of stains. The sensitivity of this approach is dependent upon the quality of the specimen and the experience of the microscopist.15 Collection of specimens which accurately reflect the disease process in the lungs is essential, with BAL or lung biopsy being the best. Pneumocystis is rarely detected in expectorated sputum but may be frequently detected (50–90% of the time) in sputum induced by inhalation of nebulized 3% saline. The development of DNA detection using PCR has increased sensitivity for detection of Pneumocystis but false positives do occur in paediatric patients and further studies of this diagnostic approach need to be performed before it can be universally recommended.16
HIV positive patients Fungi other than P. jirovecii may occasionally cause lung infection in HIV positive patients. The most common causes are C. neoformans, and endemic dimorphic fungi such as C. immitis, H. capsulatum and B. dermatitidis, and the mould Aspergillus fumigatus. HIV positive patients are rarely infected unless they are severely immunosuppressed with T cell counts below 100 cells/mm3. While the lungs act as a portal of entry and may be the initial site of infection, haematogenous dissemination commonly occurs to other sites such as the central nervous system and skin. The usual method of diagnosis is by culture of BAL fluid or biopsy of radiologically defined lung lesions. In cryptococcal disease measurement of serum cryptococcal antigen is a very useful sensitive, highly specific, noninvasive test. Infections with histoplasmosis, coccidiomycosis and blastomycosis are generally limited to the geographical regions where these fungi are endemic. Histoplasmosis can be diagnosed by detection of the polysaccharide antigen in urine or blood (sensitivity and specificity both >90%). Blood cultures may also be positive in patients with histoplasmosis or cryptococcus. C. immitis is usually diagnosed by culture and if this fungus is suspected, the laboratory should be advised as it is an extremely dangerous pathogen within the laboratory and special containment precautions (PC3) for staff safety need to be employed. C. immitis can also be isolated in blood cultures. Aspergillus species infection in HIV positive patients can present with one of two clinical syndromes: a semiinvasive pseudo-membranous tracheitis or an invasive pneumonitis. DIAGNOSIS OF MOULDS Definitive diagnosis of filamentous fungi, moulds, most commonly Aspergillus fumigatus or Aspergillus niger, is achieved by culture of lung biopsy specimens or BAL fluid. Fungal pneumonia is an infrequent but serious infection in highly immunosuppressed patients, particularly
A. M. KESSON, AND A. KAKAKIOS
those with prolonged neutropenia and/or haematopoietic stem cell transplantation. Detection of lesions consistent with fungal infection on high resolution CT scanning is not in itself diagnostic of any particular fungus and is not specific. Definitive diagnosis is dependent on culture of sputum or BAL fluid or lung biopsy. Early in the clinical course sputum cultures lack sensitivity and may also lack specificity.17 New methods for detection of Aspergillus species are being developed and detection of Aspergillus DNA or 1-3-beta-D-glucan or galactomannan in the blood may prove helpful in early diagnosis.18 The clinical utility of these recent serological tests is still being ascertained. Other moulds such as Scedosporium species, Fusarium species and the agents of mucormycosis (Rhizopus, Rhizomucor and Cunninghamella species being the most frequent) are an increasing problem in neutropenic patients and bone marrow transplant recipients. Detection of fungal hyphae in histological specimen of lung tissue is diagnostic of fungal infection but this cannot be used to speciate the fungus. Confirmation by culture should be pursued. However, attempts to culture the agents of mucormycosis from infected tissue are often unsuccessful as the dissociation of the tissue in the microbiology laboratory for culture often leads to inadvertent destruction of the very broad hyphae of this group of fungi. STRONGYLOIDIASIS Strongyloides stercoralis is a globally prevalent round worm infection. Patients with defects of cell-mediated immunity, especially those with corticosteroid therapy, immunosuppressive drugs, haematological malignancy or infection with HIV are at significant risk of hyperinfection or increased susceptibility to infection and they are often refractory to treatment. With hyperinfection increased numbers of larvae are found in many organs, including the lungs, and they may produce pneumonitis with cough, haemoptysis and respiratory failure. Interstitial infiltrates or consolidation may be seen on chest X-ray.
Diagnosis Strongyloidiasis is diagnosed by finding larvae in microscopic examination of stool specimens. Repeated stool examinations may be required because sensitivity can be low. Hyperinfection can be diagnosed by detection of larvae in sputum or lung tissue, which typically contain a high number of filariform larvae. If tests are negative but there is a high index of suspicion therapy is recommended. Care must be exercised when collecting specimens from potentially infected patients as contact with the skin can lead to infection of health care workers. Serological tests which provide evidence of infection are also available with a sensitivity of approximately 85%; however, cross-reactions with filarial infections limit specificity.19
IMMUNOCOMPROMISED CHILDREN: CONDITIONS AND INFECTIOUS AGENTS
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