Emerging pathogens that cause pneumonia in children

Emerging Pathogens That Cause Pneumonia in Children Peter N. Wenger,MD This article reviews several emerging pathogens responsible for pneumonia in children. Streptococcus pneumoniae has long been known for its ability to cause disease in humans; however, the recant development of multidrug resistance with respect to this organism qualifies it as an emerging pathogen. Spneumoniaethatare resistant to penicillin and caphalosporins are distributed widely both nationally and internationally. The clinical presentation and course of infection caused by multidrug-resistant organisms are identical to those caused by penicillin-susceptible $ pneumoniae. Chlamydiapneumoniae also is not a new respiratory pathogen, but it is rather newly recognized as a cause of atypical pneumonia in older children and adults. The hantaviruses, which are responsible for the hantavirus pulmonary syndrome, cause disease primarily in young adults with heavy exposure to rodents. This dramatic pulmonary syndrome is managed with aggressive and meticulous supportive therapy.

Copyright9 1998by W.B.SaundersCompany

he concept of "epidemiological transition" anticipates that, as countries develop, infectious diseases will fade in significance as a public health problem because of the changing age distribution of the population and decreasing infection rates. As recently as the 1960s, this principle was considered to apply to the United States. Successful initiatives in public health practices, such as community sanitation and immunization programs, and the wide availability of antimicrobial agents seemed to significantly diminish the public health risks attributable to infectious diseases. However, numerous developments over the past 20 years dearly have shown that infectious diseases remain a critically important public health concern for both the developed and developing worlds. In February 1991, the Institute of Medicine convened an expert, multidisciplinary committee to conduct a study of emerging infectious pathogens of significance. In October 1992, the committee issued a report, Emerging Infections: Microbial Threats to Health in the United States, that provides a framework of thought and specific recommendations for this problem. Emerging infections were defined as "new, reemerging or drugresistant infections whose incidence in humans has increased within the past two decades or whose incidence threatens to increase in the near future. ''1 The report identifies and discusses

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FromtheDivisionofPulmona~y,Allergy,Immunology,andInfectiousDiseases, DepartmentofPediatrics, UniversiO~ofMedicineandDentist~yofNewJcrsey,New JerseyMedicalSchool,Newark,NJ. Address correspondenceto Peter N. Wenger,MD, Division of Pulmonary, Allergy, Immunology, and I@ctious Diseases, UMDNJ/NJMS, 185 South OrangeAve,MSB, F-570A,Newark,NJ 07103-2714. Copyright9 1998by W.B. SaundersCompany 1045-1870/98/0903-000358.00/0

the major factors influencing the emergence of infectious diseases. The following factors are discussed: 1. Human demographics and behavior (eg, the recent trend in the United States of increasing mortality caused by pneumonia and influenza, reflecting the increase in the proportion of people in older age groups), ~ 2. Technology and industry (eg, the large-scale use of antimicrobials to enhance growth in domestic animals, thereby potentiating the development and dissemination of antimicrobial resistance in bacteria) 3. Economic development and land use (eg, reforestation of the eastern United States, abetting the emergence of Lyme disease) 4. International travel and commerce (eg, worldwide spread of H1V disease in the last 20 years) 5. Microbial adaptation and change (eg, the development of multidrug resistance in infectious agents caused by the selective pressure exerted by antimicrobial use) and 6. The breakdown of public health measures (eg, reemergence of tuberculosis in the United States in the 1980s). Pneumonia is defined as inflammation and consolidation of lung tissue caused by an infectious agent. The development of pneumonia requires one or more of the following: (1) a defect in host defenses, (2) invasion by a virulent pathogen, or (3) an overwhelming inoculation. Pathogens gain entry into the lower respiratory tract via aspiration of upper airway flora or inhalation of aerosolized particles. Occasionally, pneumonia may result from hematogenous spread from another focus of infection or site of colonization. Pneumonia is a common condition, and its highest incidence rates are found in the very old and the very young. The highest mortality rates are found in developing countries. The mortality rate has declined markedly among

Seminars in PediatricInfectious Diseases, Vol 9, No 3 (July), 1998.'pp 181-190

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young children and infants in the developed world; however, it remains a significant cause of morbidity. Annually, approximately 25 percent of children younger than 1 year old and 18 percent of children 1 to 4 years old develop pneumonia) The decreasing risk of developing pneumonia with increasing age in children probably is due in part to the acquisition of specific immunity to an increasingly large array of pathogens and to changing patterns of exposure) Risk factors for pneumonia in children, regardless of cause, include attendance in child care facilities, crowding in the household, low socioeconomic status, low parental educational status, early history of lower respiratory tract infection, young maternal age, exposure to secondhand tobacco smoke (especially maternal smoking), and exposure to outdoor air pollution (eg, suspended particulates, suspended sulfates, and sulfur dioxide).3,4 Prospective studies of community-acquired pneumonia suggest that a specific cause is identified in only approximately 50 percent of cases,5 reflecting the lack of sensitive diagnostic tests for some known pathogens and the occurrence of respiratory disease caused by new or previously uncharacterized (emerging) pathogens. Several of these emerging pathogens responsible for pneumonia in children are discussed in this article.

Multidrug-resistant Streptococcus

pneumoniae

S pneumoniae is a gram-positive, catalase-negative, facultatively anaerobic organism that grows as a single coccus or diplococci, or in chains of variable length. Colonies show alpha-hemolysis on blood agar and can be identified presumptively by their susceptibility to optochin or with a latex agglutination test. Growth is enhanced in 5 percent carbon dioxide or anaerobic conditions. The cell wall is surrounded by a polysaccharide capsule. The capsular polysaccharides are responsible for the organism's virulence and for stimulating the protective humoral response in the host. These complex polysaccharides constitute the basis for classifying the pneumococcus by serotypes. Although 90 serotypes have been described, most disease is caused by a few serotypes, 6 which vary depending on geographic locale, and differ between adults and children. For example, serotypes 6, 14, 18, and 19 were responsible for 62.6 percent of invasive pneumococcal disease in the United States, but accounted for only 20.3 percent of invasive disease in Papau, New Guinea.7 Immunity with respect to invasive pneumococcal disease is serotype-specific, which has important implications for the successful development of an effective pneumococcal vaccine. No difference in virulence between multidrug-resistant Spneumoniae and drug-susceptible (penicillin-susceptible) organisms is apparent. The clinical and radiographic presentations of pneumonia caused by multidrug-resistant or drug-susceptible pneumococcus are similar; therefore, the reader is referred to standard texts for a clinical description of pneumococcal pneumonia.

Epidemiology Infection with S pneumoniae is one of the leading causes of morbidity and mortality in young children, the elderly, and persons with serious underlying medical conditions throughout the world. It is a common cause of mucosal infections, such as

otitis media and sinusitis, as well as serious systemic infections, including sepsis, pneumonia, and meningitis. An estimated 500,000 cases of pneumococcal pneumonia occur each year in the United States. 8 Increased rates of pneumococcal infection are observed in African-Americans (including those without sickle cell disease), Native Americans, and Alaskan Eskimos. An increased incidence of disease also is found in persons with certain immune system deficiencies, including congenital or acquired antibody deficiency, HIV infection, disorders of the complement system (C3 or C9), asplenia or dysfunctional spleen (sickle cell disease, surgical splenectomy), malignancy(Hodgkins disease), and nephrotic syndrome. 9 Incidence is seasonal, with the most common occurrences in winter and spring months, paralleling the period of peak incidence for viral respiratory tract infections. S pneumoniae is a part of the normal flora of the upper respiratory tract in humans.6 Rates of nasopharyngeal colonization have been reported as high as 60 percent in preschool children, 35 percent in grammar school students, and 28 percent in high school students.9A viral respiratory tract infection often precedes the onset of pneumococcal pneumonia. Many experts feel that some prior insult to the respiratory tract epithelium (eg, viral respiratory tract infection) is necessary for pneumococcal pneumonia to develop.

Antimicrobial Resistance According to the standards adopted by the National Committee for Clinical Laboratory Standards,6 antimicrobial resistance is defined in terms of the minimal inhibitoryconcentration (MIC). In vitro susceptibility to penicillin G is defined as an MIC less than or equal to 0.06/~g/mL; intermediate resistance is defined as an MIC of 0.12/.~g/mL to 1.0/~g/mL; and high resistance is defined as an MIC greater than or equal to 2.0 /xg/mL. Susceptibility testing must adhere to the standards of inoculum size (5 • 105 cfu/mL) and growth media (Mueller-Hinton agar with sheep blood or lysed-horse red blood cells). Increasing resistance to penicillinusually corresponds directly to increasing resistance to cephalosporins. Antimicrobial resistance in the pneumococcus was reported as early as 1943.l~ In the 1960s and 1970s, reports from Australia11and New Guinea12documented pneumococcal resistance to tetracycline and intermediate resistance to penicillin. In the United States, reports appeared in the 1970s documenting sporadic episodes of penicillin-resistantpneumococcal infections in chronically-ill children. 13,14By the late 1970s, high-level resistance to penicillin and other antimicrobials was reported in organisms isolated from hospitalized children in South Africa.6 By the early 1980s, multidrug-resistant pneumococcus was reported in community-acquired disease in both adults and children in South Africa, 15 and by the end of the decade, reports of infections caused by drug-resistant pneumococci were appearing worldwide. Worldwide dissemination of drugresistant Spneumoniae took approximately two decades. In the United States, the prevalence of multidrug-resistant pneumococci is increasing rapidly. A survey by the Centers for Disease Control and Prevention (CDC) 16 showed that 16.4 percent of isolates were resistant to at least one of the following drugs or drug classes: penicillin, cephalosporins, macrolides, trimethoprim-sulfamethoxazole (TMP-SMX), or chlorampheni-

EmergingPathogensCausingPneumonia col. Prevalence of drug-resistant pneumococci varies widely in different regions in the United States. Surveillance studies by the CDC showed very high prevalence rates of drug-resistant pneumococcal isolates in Kentucky, Tennessee,17and Connecticut.18 In Kentucky, nasopharyngeal swab cultures from children attending child care centers and those visiting the county health department revealed penicillin resistance in 61 percent and 33 percent of isolates, respectively. Of the penicillin-resistant isolates, 65 percent were highly resistant and 27 percent showed high-level resistance to cefotaxime (MIC --> 2.0/zg/mL). Fortythree percent of all isolates were resistant to penicillin, erythromycin, and TMP-SMX. In some areas, the prevalence of nasopharyngeal carriage of penicillin-resistant pneumococci is comparable to the incidence of invasive pneumococcal disease caused by penicillin-resistant organisms in the same populationL9; in other areas, the rate of nasopharyngeal carriage of resistant organisms is two to three times higher than the rate of invasive disease caused by these organisms. Penicillin resistance in S pneumoniae is mediated by alterations in the structural genes encoding for the penicillinbinding proteins (PBPs) found in the cell wall of the organism, which results in decreased binding affinity of the PBPs for [3-1actam antimicrobials. [3-1actamresistance is chromosomally mediated and most likely the result of chromosomal exchange with other species of streptococci and other bacteria. Resistance to multiple antimicrobials is transferred via conjugation among bacteria?~ Chtoramphenicol resistance is mediated by the production of chloramphenicol acetyltransferase, which converts the drug to either the monoaeetate or diacetate form. Neither of these derivatives is able to bind to the bacterial 50S ribosomal subunit, its site of action. 21 Production of adenine dimethylase causes decreased affinity of the 23S ribosomal ribonucleic acid for erythromycin, resulting in macrolide (erythromycin, clarithromycin, and azithromycin) resistance. Cross-resistance to clindamycin sometimes is present. Because many strains of penicillin-resistantpneumococci show resistance to multiple antimicrobials, penicillin-resistantstrains must be tested for susceptibility to ceftriaxone, cefotaxime, erythromycin, chloramphenicol, clindamycin, rifampin, meropenem, imipenem, and TMP-SMX. Selective pressure encouraged by the large-scale use of antimicrobials has contributed to the development of drug resistance in Spneumoniae. This point is supported by data that document increased risk for colonization or infection with penicillin-resistantpneumococci after use of[3-1actamantimicrobials (for prophylaxis or treatment). In addition, intercontinental travel has played an important role in the dissemination of infectious agents throughout the world. For example, a multiresistant 23F serotype was isolated and identified as the agent responsible for invasive disease in northeast Ohio; this isolate was found to have an identical PBP profile to a 23F serotype found in Spain. 22

Diagnosis Identifying the causative agent in pneumonia often is difficult. Sputum collection for culture is difficult in infants and young children because of their inability to produce a reliable lower tract specimen. Even in older children, adolescents, and adults, induced or "deep" sputum collection often is contaminated with

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upper respiratory tract flora, of which pneumococci may be a normal constituent. Deep tracheal suctioningand bronchoalveolar lavage also may be contaminated with upper respiratory tract flora, which can confuse interpretation of cultures. The most reliable diagnostic tests are needle aspiration of infected lung tissue or an open lung biopsy; however, neither invasive technique would be justifiable in cases of uncomplicated pneumonia. Blood cultures in adults with pneumococcal pneumonia have been found to be positive in approximately 20 to 25 percent of cases? A small study in Nigerian children with pneumococcal pneumonia found positive blood cultures in 24 percent of affected children. 23Accordingly, treating a patient with pneumonia empirically without documented evidence of the causative agent or its antimicrobial susceptibility pattern often is necessary. In order for a clinicianto prescribe the appropriate therapy in these cases, knowledge of the antimicrobial resistance patterns of the local Spneumoniae serotypes is required. When S pneumoniae is isolated on culture, it should be assessed for penicillin sensitivity, initially using a sensitive agar-diffusion screening test using a 1-#g oxacillin disc. After identification of an oxacillin-resistant strain, precise MICs for penicillin G and other candidate antimicrobial agents should be determined. Broth dilution assays may be used; however, the gradient diffusion or E test (AB Biodisk NA, Piscataway, NJ) is recommended as a simple and accurate method to determine the MIC for penicillinG, ceftriaxone, cefotaxime, chloramphenicol, erythromycin, tetracycline, and amoxicillin.24,25The E test consists of a paper strip impregnated with graded concentrations of an antimicrobial that is placed on an agar plate inoculated with the test organism. An elliptical zone of inhibition around the paper strip is produced after incubation. The MIC is read off the paper strip where the zone of inhibition intersects the strip. Disc diffusion methods may be used to accurately determine the MICs of erythromycin, tetracycline, chloramphenicol, and TMP-SMX. 6 When using the tests described previously, susceptibility results usually are available within 24 to 48 hours after inoculation.

Treatment Treatment decisions for invasive pneumococcal disease depend on the anatomic location of the infection and the specific antimicrobial susceptibility profile of the responsible isolate. This article will limit its discussion to therapy for drug-resistant pneumococcal pneumonia. The serum concentration of penicillin G and other [3-1actam antimicrobials achieved with intravenous (iv) administration exceed by many fold the MIC for intermediate and, often, highly penicillin-resistant strains. Therefore, high-dose iv penicillinG (250,000-400,000 U/kg/d every 4-6 hours) or a highly active cephalosporin such as iv cefuroxime (150 mg/kg/d + every 8 hours) should suffice. Some experts would use ceftriaxone (80-100 mg/kg/d) or cefotaxime (180 mg/kg/d + every 6-8 hours) as first-line empiric therapy because of higher drug levels achieved than those with cefuroxime. Ifa highly-resistant organism is isolated and the initial therapeutic response is suboptimal, treatment with ceftriaxone or cefotaxime should be started. If the isolate shows high-level resistance to ceftriaxone and cefotaxime, then vancomycin (40 mg/kg/d + every 6 hours) should be used. Imipenen (40-60 mg/kg/d + every 6 hours; not formally approved for children)

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or meropenem (60-120 mg/kg/d + every 8 hours) may be considered as well. In children with severe hypersensitivity to [3-1actam antimicrobials, initial management of presumed pneumococcal pneumonia should include clindamycin (25-40 mg/kg/ d + every 6-8 hours) or vancomycin. Vancomycin should be discontinued if the organism proves to be susceptible to other non~-lactam antimicrobials to which the patient is not allergic. Recommended duration of therapy with an active and effective agent is 10 days. If highly-resistant organisms are encountered, consultation with an infectious diseases specialist is strongly recommended.

Prevention The high rates of severe morbidity and mortality associated with invasive pneumococcal disease and the rapidly increasing dissemination of multidrug-resistant organisms underscore the critical need for a truly effective pneumococcal vaccine. The current polyvalent pneumococcal polysaccharide vaccine is composed of purified capsular polysaccharide antigens of 23 serotypes, which include the vast majority of serotypes that cause illness in humans. However, polysaccharide antigens do not reliably induce a protective humoral response in children younger than 2 years of age (the age group that suffers the highest incidence of severe invasive pneumococcal disease) nor in other vulnerable hosts. The immune responses to polysaccharide antigens are thymus-independent (T-cell independent) and, therefore, do not produce high-level, high-affinity antibodies or induce the T-cell memory required for a booster response. 26,27Coupling polysaccharide antigens to protein carriers elicits a T-cell dependent immune response (even in vaccinees younger than 2 years old), which permits the development of immunologic memory and production of a high-affinity antibody response. However, applying this technology to the development of a conjugate pneumococcal vaccine has been complicated. Nonetheless, recent studies have shown antibody induction, immunologic memory response, and reduction of nasopharyngeal carriage in children who received a conjugated 4-valent pneumococcal vaccine (conjugated to either tetanus or diphtheria toxid) at 2, 4, and 6 months of age, followed by the unconjugated 23-valent polysaccharide vaccine administered as a booster at 12 months of age28,29 Other pneumocoecal vaccines under investigation include those using noncapsular virulence factors, such as enzymes and toxins, that are excreted or released after autolysis of the organism and surface proteins, whose exact functions are unclear but may be present on several different serotypes. 3~If an effective vaccine using pneumococcal antigens that occur in all, many, or most serotypes can be developed, specific serotypic immunity would become an issue of less importance.

Control As the next century approaches, the control and prevention of multidrug-resistance in Spneumoniae and other bacteria is one of the most important public health challenges that society will face. A deliberate and comprehensive reexamination of antimicrobial use both in the United States and abroad is necessary. The indiscriminate use of antimicrobial agents in medicine and

agriculture must be curtailed. Policies governing the judicious use of antimicrobials in the management of communityacquired upper respiratory tract infections, as well as strict adherence to infection control policies and practices in hospitals, nursing homes, and child care facilities, will decrease the risk of the development and spread of drug-resistant organisms.

Chlamydia pneumoniae Chlamydiatrachomatis,Chlamydiapsittaci, Chlamydiapneumoniae, and Chlamydia pecorum are the four known species of the genus Chlamydiae. Cpecorum, the most recently recognized species, is a pathogen of ruminants.31 The other three species are responsible for a wide spectrum of human disease. Of these three, Cpneumoniae, or TWAR, is the most recently recognized species. In 1965, the prototypic isolate of C pneumoniae, TW-183, was isolated from the conjunctiva of a child in Taiwan. 32 The first isolate (AR-39) from a patient with acute respiratory disease was obtained from a university student in Seattle in 1983.33 Initially considered a strain ofC trachomatisand then a strain of C psittaci, it was recognized as a distinct species in 1989. Although Cpneumoniae can be the causative agent in sinusitis, pharyngitis, and bronchitis, it derived its name from the disease with which it is most frequently associated, pneumonia.34 Before it was recognized as a distinct species, the organism was called TWAR; that name was derived from the designations given to the first two isolates from humans, TW-183 and AR-39.33 Studies on sera available from serum banks show that C pneumoniae infection occurred as frequently in 1963 as it does today34 and was prevalent in Finland as early as 1958.35Thus, Cpneumoniae is not a new, but rather a newly recognized, infectious agent. The chlamydiae are obligate intracellular organisms that are classified as bacteria because of their cell wall composition and binary fission reproduction. They have a unique biphasic developmental cycle consisting of two morphologically distinct forms. The smaller extracellular form, the elementary body, is the infectious form, and the larger intracellular form, the reticulate body, is the replicating form. They can grow only intracellularly, using the host's adenosine triphosphate for synthesis of their own proteins. Molecular analysis of the organism and sequencing of the major outer membrane proteins of C pneumoniae isolates from different continents have found them all to be identical.34 The dominant immunogenic antigens associated with Cpneumoniae infection are not located on the major outer membrane proteins, but they are found elsewhere on the outer membrane. 34

Epidemiology C pneumoniae is a common cause of acute respiratory tract infection and accounts for 6 to 19 percent of communityacquired pneumonia, depending on age group and geographic location.36 Because infection often produces mild disease and frequently is asymptomatic, determination of prevalence has relied on serological studies. The serologic test most commonly used is the microimmunofluorescence (MIF) test. Its sensitivity and specificity are controversial, and it may be difficult to interpret without proper experience. In addition, there may be significant intersubject variability with respect to the humoral

EmergingPathogensCausingPneumonia response to infection. 37These concerns notwithstanding, several studies have shown that C pneumoniae is the most common chlamydial species infecting humans,as Seroepidemiological data suggest that infection with Cpneumoniae is rare in preschool children and that primary infection occurs most commonly in children between the ages of 5 and 15 years) 9,4~It shows no gender preference until approximately 15 years of age, after which males are affected slightly more often than are females. Serological data show continued acquisition of antibody throughout adolescence and adulthood, culminating in approximately 70 to 80 percent seropositivity among the elderly. A higher prevalence of infection is apparent in populations that reside in tropical climates than occurs among those who live in the cooler northern regions (eg, Canada, Denmark, Norway).4! Although data are limited, they indicate that greater antibody prevalence in a population is associated with more severe disease, at least in children younger than 5 years of age.42 The incidence of Cpneumoniaepneumonia is difficult to assess and probably is underestimated, in part because it often fails to elicit medical attention because of its generally mild clinical presentation, especially in young people. Grayston et al34,39,40 monitored the incidence of pneumonia in a prepaid cooperative medical plan. Paired sera from 2,000 patients with clinically apparent pneumonia were evaluated retrospectively for evidence of Cpneumoniae infection, as suggested by a greater than or equal to fourfold increase in antibody titer. Approximately 10 percent of the 2,000 patients (60% of whom were between the ages of 4 months and 14 years) had evidence of acute infection, with the highest frequency being among the elderly. Evidence of acute Cpneumoniaepneumonia was shown in less than 1 percent of children 7 months to 4 years of age and in 6 percent of 5- to 14year-olds. Mycoplasmapneumoniaewas implicated more often than was Cpneumoniaeas the cause of clinically apparent pneumonia in children up to 14 years of age. Transmission of Infection

Though data are limited, Cpneumoniaeseems to be transmitted from person to person via respiratory tract secretions. In vitro studies show that Cpneumoniaesurvives small particle aerosolization, and infection by droplet aerosol also has been documented.43 The organism has been shown to remain viable on environmental surfaces for 20 to 30 hours and on tissue paper for 12 hours, thus raising the possibility of contact or fomite transmission.44 The estimated incubation period is 3 weeks. Epidemics or periods of increased incidence usually occur among dosed populations (eg, those attending child care facilities or schools or military recruits) and may be prolonged (lasting up to 2 to 3 years).45 They are superimposed on periods of endemicity (lasting approximately 3 years). 34 The slow progression of infection through dosed populations46,47and data documentingfew or no secondary infections in the households of index cases34,48 indicate that person-to-person transmission of infection is inefficient. Shedding of the organism in respiratory secretions can be prolonged in both symptomatic and asymptomatic infection. Cpneumoniaehas been isolated from individuals as long as a year after symptomatic infection. It is not clear if symptomatic infection influences transmission or what role asymptomatic carriers play in the spread of infection.

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Clinical Presentation

Pneumonia and bronchitis are the two most common clinical syndromes associated with Cpneumoniaeinfection. Symptomatic disease is more likely to be found in adults than in children. No signs and symptoms are unique to Cpneumoniaepneumonia. The clinical presentation is very similar to that observed in "atypical" pneumonia (ie, pneumonia caused by M pneumoniae and viral respiratory pathogens). Illness tends to have a subacute onset. Upper respiratory tract symptoms, most commonly sore throat and hoarseness often associated with fever, are the usual initial clinical features. Almost all patients with Cpneumoniaepneumonia develop a cough, although it may not be present for several days or even a week or more after onset of initial complaints. By the time the cough develops, initial symptoms may have resolved, giving the impression ofa biphasic illness. On auscultation, crackles and rhonchi are appreciated frequently, even with mild disease. The erythrocyte sedimentation rate usually is elevated, and the white blood count usually is not elevated, although occasionally neutrophilia may be present. The chest radiograph usually shows a localized unilateral parenchymal infiltrate; however, more extensive bilateral infiltrates and pleural effusion have been reported in more serious disease. Although the clinical presentation almost always is mild, especially in young people, complete recovery often is slow, even with appropriate therapy. Cough and malaise may persist for months after resolution of other symptoms. The reports of serious systemic illness in children are rare. 34 Cpneumoniae impairs the host's respiratory defense mechanisms by inhibiting the ciliary motion of bronchial epithelial cells. Coinfections with other bacteria, Mpneumoniae, or respiratory viruses have been reported. In these cases, Cpneumoniaemay not be the cause of the symptomatic respiratory illness, but may have allowed other pathogens entrance into the lungs by disrupting the normal respiratory clearance mechanisms.36 Cpneumoniae has been associated with the onset and exacerbation of asthma. In addition, an association has been identified between Cpneumoniaeinfection and the development of atherosclerosis and coronary heart disease. Diagnosis

No accurate rapid diagnostic tests are currently available commercially for the diagnosis of C pneumoniae. Growing the organism in culture is labor-intensive, requires prolonged incubation, and demands stringent conditions for maintainingand transporting specimens. Antigen tests often lack sufficient sensitivity and specificity, and serological tests yield results only belatedly. Polymerase chain reaction (PCR) holds great promise, but it is not commercially available at this time. Optimal specimens for culture are obtained from the nasopharynx rather than from the throat) 6 The nasopharynx should be sampled with a dacron-tipped, wire-shafted swab. The specimen should be placed in the appropriate transport media (containing sucrose-phosphate buffer with antibiotics and fetal calf serum) and stored immediately at 4~ If not processed within 24 hours, the specimen must be maintained at -70~ until inoculation of the appropriate cell line is performed. After 72 hours incubation, identification can be made with specific antibody)6

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The complement fixation test is genus-specific and cannot distinguish between Cpsittacosis and Cpneumoniae infection. The MIF test, originally developed for C trachomatis,49 now is considered the most sensitive method for diagnosing C pneumoniae infection. 34 Unfortunately, after primary infection with Cpneumoniae, young children may develop antibodies slowly and at low titers, thereby decreasing the sensitivity of the MIF test for this population. 5~The use oflgM- and IgG-specific conjugates allows the MIF test to distinguish between current and past infections; however, there is no IgM response with reinfection. Many laboratories use the criteria for serological diagnosis proposed by Grayston et al.5I Acute infection is indicated by a single IgM titer greater than or equal to 1:16, a single IgG titer greater than or equal to 1:512, or a greater than or equal to fourfold increase between acute and convalescent IgG titers (with an interval between titer determinations of 30 to 45 days). Past infection is shown by an IgG titer greater than or equal to 1:16 but less than 1:512.An IgG response may occur earlier (within 1 to 2 weeks) with reinfection than is observed in primary infection. Antigen detection tests, such as enzyme immunoassays and direct immunofluorsecence assays, and molecular detection methods, such as PCR, provide the greatest hope for the development of tests that will provide rapid diagnosis and eliminate the stringent transport requirements necessary for culture of specimens.

Treatment Very few data derived from controlled, prospective, clinical trials on the effectiveness of different antimicrobial therapies for C pneumoniae pneumonia exist. In vitro studies in cell culture have shown susceptibility to tetracyclines, erythromycin, clarithromycin, azithromycin, and ofloxacin.36 Cpneumoniae is resistant to sulfa drugs. 52Anecdotal treatment failures with tetracycline and erythromycin have been reported, especially with therapy lasting less than 2 weeks.53'54The current treatment recommended for children is orally-administered erythromycin at a dose of 40 mg/kg/d + every 6 hours for 14 to 21 days. For adolescents, alternatives to erythromycin include tetracycline at a dose of 25 to 50 mg/kg/d + every 6 hours (maximum daily adult dose 1-2 g) for 14 to 21 days or doxycycline at a dose of 2 to 4 mg/kg + every 12 hours (maximum adult daily dose 100-200 mg) for 14 to 21 days.55Although clarithromycin and azithromycin seem very promising, clinical evidence is insufficient to recommend them formally at this time.

Hantavirus Pulmonary Syndrome On May 14, 1993, the New Mexico Department of Health was notified that two young people who shared a household had died within 5 days of one another. Their illnesses both were characterized by abrupt onset of fever, myalagias, headache, and cough followed by rapid deterioration of their respiratory status, respiratory failure, and death. 56 By May 17th, several Indian Health Service physicians from the Four Corners region reported five deaths in individuals with similar clinical presentations. The principle common feature of these lethal episodes was the rapid development ofnoncardiogenicpulmonary edema,

leading to respiratory failure, shock, and death in young, previously healthy people. Surveillance was initiated for an influenza-like prodromal illness followed by rapid onset of respiratory failure. CDC investigators got their first breakthrough when they identified cross-reactive antibodies to known hantaviruses in the serum of affected patients.57 Within weeks, the observation was confirmed with the identificationof hantavirus antigens and nucleic acid sequences in autopsy samples; the causative agent was isolated in the deer mouse (Peromyscusmaniculatus). The agent was found to be a previously unrecognized member of the genus hantaviruses, subsequently named the Sin Nombre virus (SNV). The disease now is called the hantavirus pulmonary syndrome (HPS). Since the discovery of SNV, two other geneticallydistinct hantaviruses (Black Creek Canal virus and Bayouvirus) have been identified and associated with HPS in the United States. The hantaviruses are one of the 160-member Bunyaviridae family, of which only four genera cause recognized human disease: the bunyavirus (California encephalitis), phelboviruses (Rift Valley fever), nairoviruses (Congo-Crimean Hemorrhagic fever), and the hantaviruses. Hantavirus is a negative-sense ribonucleic acid virus that contains three single-stranded elements in a helical capsid. The helical capsid is assembled in the cytoplasm, and the virus is coated in a lipid envelope.5s Hantaviruses are responsible for several distinct clinical syndromes: hemorrhagic fever with renal syndrome, nephropathia epidemica (self-limited flu-like febrile illness with proteinuria, hematuria, azotemia, and thrombocytopenia), and the newly recognized HPS.

Epidemiology All HPS cases included in the CDC registry have been confirmed by one or more antibody-antigen detection or PCR techniques.59As of May 1997, the registry included 160 cases of HPS from 26 states. SNV has been identified as the causative agent in 154 cases. The average age of patients is 36 years (range, 11 to 69 years), and 60 percent of the cases are male. Seventy-three percent of the cases are Caucasian, 24 percent Native American, 2 percent African-American, and 1 percent Asian. In 1994, 56 percent (18/32) of cases involved people from the Four Corner states, but since then, 19 percent (9/48) are from that region, whereas 31 percent (15/48) are from Oregon and Washington. Case reports of HPS from Canada, Central, and South America show the widespread geographical distribution of this disease. A 4-year-old Native American boy from rural New Mexico was identified in June 1993 with a mild respiratory disease that did not meet the CDC surveillance case definition of HPS, bnt was associated with positive IgM serology for SNV.6~ Between May and June 1993, 57 percent (16/28) of cases died, whereas in 1996, the case fatality rate fell to 30 percent (6/20). The decline in mortality may be because of (1) increased awareness of the disease, (2) earlier onset of appropriate supportive therapy, (3) increased clinical experience in disease management, and (4) enhanced surveillance and detection of mild or atypical presentations.59 Data from investigations examining the seroprevalence of hantavirus antibodies in people from the region of the 1993 Four Corners outbreak suggest that asymptomatic infection exists

Emerging Pathogens CausingPneumonia but is uncommon. Seroprevalance rates ranged from 1.0 to 1.3 percent in people who experienced either no or mild respiratory symptoms. 61To date, the vast majority of liPS has been found in the 20- to 50-year-old age group. The reason for the lack of disease in younger populations is unclear. It may reflect agespecific activities associated with rodent exposure 62 or the role that the immune response plays in the pathophysiology of HPS.6a Further studies are necessaI2r to more precisely define the prevalence of infection (asymptomatic and symptomatic) and to clarify the pathophysiological mechanisms involved in the development of liPS.

Transmission of Infection Transmission of infection is caused primarily by inhalation of aerosolized rodent urine, excreta, and/or saliva. Direct inoculation of infected material through broken skin, animal bites, and conjunctiva also has been shown to cause infection. 58 Unlike other Bunyaviridae-associated disease, no arthropod vector exists, and human-to-human transmission has not been reported. Twenty-seven percent of Peromyscus rodents captured in the outbreak area during the 1993 Four Corners epidemic were found to be seropositive for hantavirus antibodies. 63 The geographic habitat of Peromyscus rodents includes most of the continental United States except for some states in the Southeast and Northeast. Because of the high hantavirus infection rate found in these rodents (as reflected by seroprevalence data) and their widespread geographic distribution, a reasonable expectation is that hantavirus infection will remain an endemic problem in the United States. Risk factors for infection include activities and living conditions associated with increased exposure to rodents. The predilection for disease in males probably is because of their increased occupational risk of exposure to rodents. Native Americans are overrepresented in the disease registry, most likely as a consequence of the 1993 Four Corners outbreak and not because of any intrinsic biological predisposition. Since the 1993 outbreak, the overwhelming majority of reported cases have occurred in Caucasians. The incubation period for HPS is estimated to be 1 to 3 weeks.64 The weather may have been a factor in the 1993 outbreak. The Four Corners region experienced unusuallyheavy rainfall that season, causing an abundant harvest of pinon nut, a favorite food of the deer mouse. The abundant food supply was associated with a large increase in the deer mouse population, resulting in increased contact between humans and mice.

Clinical P r e s e n t a t i o n Although the acute presentation of a febrile illness that rapidly progresses to respiratory failure (caused by noncardiogenic pulmonary edema or adult respiratory distress syndrome) in a previously healthy young adult is uncommon, no clinical features can reliably distinguish HPS from other causes of this unusual syndrome. The differential diagnosis includes sepsis and pneumonia caused by group A streptococci, S pneumoniae, Legionellapneumoniae, influenzaA, adenoviruses, and Yersinapestis. Other potential diagnostic considerations include leptospirosis, Q fever, tularemia, ehrlichiosis, coccidioidomycosis,histoplasmosis, and Goodpasture syndrome.

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HPS usually presents with a nonspecific 3- to 6-day prodome; fever and myalgia are the most common features. Headache, dizziness, weakness, and fatigue are observed; nausea, emesis, or abdominal pain are found in 43 percent of patients. Respiratory symptoms often are absent in the prodromal phase, accounting for frequent misdiagnoses early in the course of HPS. Findings on physical examination, laboratory testing, and chest radiographs may be completely normal during the early prodromal phase. The prodromal phase of ItPS is followed by a cardiopulmonary phase, onset of which generally is marked by the development of a progressive, usually nonproductive cough and shortness of breath. Commonly associated signs include tachypnea, tachycardia, fever, and hypotension. Progressive pulmonary edema and hypoxia develop rapidly, often requiring intubation and mechanical ventilation. In the 1993 Four Corners outbreak, mortality was associated with pulmonary edema accompanied by severe hypotension. 65 Dysrhythmias such as bardycardia, electromechanical disassociation, or ventricular fibrillation or tachycardia were common terminal events. Hypoxia was not solely responsible for hypotension; several patients with adequate oxygenation were severely hypotensive. Poor prognostic markers included lactate concentration of greater than or equal to 36 mg/dL and a cardiac index of less than 2.2.59 The convalescent phase of liPS features rapid improvement of oxygenation and hemodynamic function. Even severely ill patients are extubated within a few days and discontinue use of vasoactive medications. Diuresis often is observed during this period. Apparently, recovery is complete, but data on long-term outcomes are wanting at this time. Patients often present to medical practitioners early in the prodromal phase of the disease and frequently are sent home with the diagnosis of the "flu" because of the nonspecific nature of their symptoms. They often return within a few days with much more severe disease, requiring hospitalization. Progression from the onset of symptoms to severe disease and death is rapid. Convalescence seems to be just as rapid. Duchin et al,65 analysing the 17 patients from the 1993 Four Corners outbreak, reported that the mean duration of symptoms before hospitalization was 5.4 days (median, 4; range, 2 to 15); no association was found between survival and duration of symptoms before hospitalization. The same study showed that 88 percent (15/17) of the patients required intubation and mechanical ventilation during the first 24 hours after admission. In those who died, the mean number of days from onset of symptoms to death was 8 (median, 7; range, 2 to 16). In the appropriate clinical setting, the following nonspecific laboratory findings can support a diagnosis of HPS: hemoconcentration, thrombocytopenia, and leukocytosis with a predominance of neutrophils and circulating immunoblasts. Hypoalbuminemia, elevated serum transaminases, creatinine phosphokinase, amylase, and lactate dehydrogenase also have been reported. Metabolic acidosis with a depressed bicarbonate level and lactic acidemia is observed in severely ill patients. Although prolongation of prothrombin and partial thromboplastin times is observed, fibrinogen levels generally are normal and disseminated intravascular coagulation is rarely observed. Mild to moderate proteinuria often is present; however, renal insufficiency is not a regular feature of HPS caused by SNV. Mild

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elevation in creatinine levels (2.5 mg/dL) is observed in only severe cases. Radiographic findings can be normal on initial presentation; however, most patients present with some abnormalities on chest radiograph. Initial findings include interstitial edema manifest by Kerley's B-lines, hilar indistinctness, or peribronchial cuffing. Radiographs of all patients will show these findings within 48 hours of admission. The majority of chest radiographs also will show extensive bibasilar or perihilar airspace disease, often accompanied by pleural effusions.66 A 4-year-old child with suspected mild hantaviral respiratory disease 6~ presented with a 2-day history of fever, cough, and earache. Temperature on presentation was 100.6~F, and pulmonaP/ auscultation showed bilateral rhonchi, hut respiratory distress was not noted. Throat and blood cultures were negative, and laboratory studies showed a mild anemia without leukocytosis or thrombocytopenia. The patient was diagnosed with bronchitis and bilateral otitis media, for which he received antimicrobials and fully recovered. Ten days after this patient presented to the clinic, his mother became seriously ill and died from what subsequently proved to he HPS. The child was found to be IgM-positive for SNV from a serum sample collected 23 days after onset of his symptoms. It seems that SNV was responsible for this mild respiratory illness; however, possibly the respiratoP/symptoms, due to a different cause, occurred concurrently with asymptomatic hantavirus infection. Evidence suggests that increased permeability of the pulmonary capillaries is responsible for pulmonary edema, the central feature of HPS. The initial pulmonary artery wedge pressures of less than 15 mm Hg found in patients with HPS and the ratio of total protein in the edema fluid versus that in the serum of greater than 60 percent support a noncardiac origin of the pulmonary edema. Whether the pulmonary manifestations of HPS are a result of a direct cellular effect of viral infection, viral antigen in the pulmonary-capillary endothelium, or a viralinduced immune-mediated response is unclear. Histopathology shows an interstitial pneumonitis of varying extent among patients. Variable degrees of congestion, edema, and infiltration with a mixture of small and enlarged mononuclear cells that resemble immunoblasts are observed. Extensive intraalveolar edema, including fibrin, focal hyaline membranes, and inflammatory cells, is found. Immunoblasts may be observed in the spleen, lymph nodes, and peripheral blood.67Immunohistochemical staining with monoclonal antibodies shows hantavirus antigen in the capillary endothelium of most systems, although significant edema is present only in the lungs.

Diagnosis Detection of the presence of hantavirus either by antigen or seroconversion is required for definitive diagnosis. Enzymelinked immunosorbent assays, using native or recombinant antigens, showing IgM or rising IgG titers in paired sera are the diagnostic method of choice. IgM and IgG usually are detectable at the time of onset of symptoms or shortly thereafter. Neutralizing antibody tests provide greater virus-strain specificity. A Western blot assay using Sin Nombre recombinant antigens and isotype-specific conjugates for IgM-IgG differentiation has been developed.59 Immunofluorescent and complement-fixing antibody tests can be used for diagnosis. Immunohistochemistry,by

means of specific mono- and polyclonal antibodies, is used in the detection of hantavirus antigens in formalin-fixed tissues. This technique is used most often in postmortem evaluations and in retrospective assessments of possible hantaviral infections, although it may be used for acute diagnosis if biopsy material is available. PCR can be used to detect hantavirus ribonucleic acid on fresh/frozen tissue, blood clots, and buffy coats; however, it is not routinely available. Recovery of hantavirus from clinical specimens is difficult and often unsuccessful; therefore, culture techniques are not used routinely for diagnosis.

Treatment The main principle of HPS treatment is early interventionwith provision of supportive care. The clinical status in HPS can deteriorate rapidly; therefore, early initiation of intensive monitoring in recognized or suspected cases is mandatory. Close monitoring of oxygenation is essential so that timely intubation and mechanical ventilation can be provided if needed. Placement of a flow-directed pulmonary artery catheter is highly recommended to closely monitor pulmonary capillary wedge pressure and cardiac indices. Avoiding overhydration when treating hypotension and shock caused by extreme pulmonary capillary leak is extremely important. Administrationof crystalloid fluids actually may exacerbate the situation; therefore, keeping the capillary wedge pressure as low as is compatible with satisfactory cardiac indices is essential. Use of inotropic agents, such as dopamine, dobutamine, and norepinephrine, as early as possible is recommended in order to minimize the need for fluid resuscitation. Decreased morbidity and mortality have been shown in hemorrhagic fever with renal syndrome treated with intravenous ribavirin,6a although an initial open-label trial failed to show any decrease in mortality in ribavirin-treated patients with HPS. The National Institute of Allergy and Infectious Diseases currently is sponsoring a double-blind randomized trial of intravenous ribavirin at selected centers for treatment of HPS. However, even in the absence of specific antiviral therapy, early suspicion or recognition of HPS with early initiation of intensive supportive care has improved survival since the time when this syndrome first was identified.

Prevention The primary mode of transmission of hantavirus to humans is via inhalation of urine, saliva, or excreta from infected rodents. Therefore, prevention is premised on avoiding or reducing human exposure to rodents. Total eradication of rodents is not possible, but control of rodents in the household is achievable. General precautions69 include (1) eliminating rodents and reducing the availability of food sources and nesting sites used by rodents, (2) preventing rodents from entering the home, and (3) reducing rodent food and shelter within 100 feet of the home. While camping, one is advised to do the following: 1. Avoid contact with rodents, rodent burrows, or dens 2. Refrain from using rodent-infested cabins or enclosed shelters unless adequately cleaned and disinfected 3. Avoid pitching tents or placing sleeping bags near rodent shelters, burrows, or feces

Emerging Pathogens Causing Pneumonia 4. Use cots or sleeping surfaces placed more than 12" above the bare ground, or use tents with floors 5. Keep food in rodent-proof containers and bury or burn all garbage immediately, and 6. Use disinfected or bottled water. 69 Hantaviruses are inactivated easily by chlorine bleach and other household disinfectants (eg, 70% ethyl alcohol). Survival of the virus after shedding is unknown; therefore, all rodentinfested areas should be wetted down with disinfectant before cleaning to minimize inhaling aerosols. If the area is infested heavily by rodents, respirators with high-efficiency particulate air filters should be worn. Anyone working in a rodent-infested area who develops respiratory symptoms or fever within 45 days of exposure should seek medical attention immediately. Although human-to-human transmission has not been reported in the United States, universal precautions should be maintained when working with patients or with clinical specimens from patients with HPS. Additional information on prevention of HPS and other hantaviral diseases is available from the CDC (telephone 1-800-532-9929).

Acknowledgment I would like to thank Karen M.Jones for her valuable help in preparing this manuscript for publication.

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