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Melioidosis
SECTION II: PATHOGENS
PART A: Bacterial and Mycobacterial Infections
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CHAPTER 33 Melioidosis Sharon J. Peacock • David A.B. Dance
INTRODUCTION Melioidosis is an infection caused by the Gram-negative bacillus Burkholderia pseudomallei. This disease has emerged over the past 25 years as an important cause of morbidity and mortality in Southeast Asia and northern Australia, and is also endemic in other tropical regions. B. pseudomallei has been classified as a “Category B” agent by the US Centers for Disease Control and Prevention.1
THE AGENT Melioidosis was first recognized as a fatal “glanders-like” illness in Burma in 1911 by Alfred Whitmore.2 The genetic similarity of the causative organism to that of glanders, B. mallei, has since been confirmed by multilocus sequence typing and whole genome sequencing.3–5 Animal and human infections with Whitmore’s bacillus were later recognized by Stanton and Fletcher in the Federated Malay States.6 They considered the disease to be a zoonosis and coined the term melioidosis. Workers in French Indochina subsequently showed that the organism was really an environmental saprophyte.7 B. pseudomallei was previously classified in various genera (e.g., Malleomyces, Pfeifferella, Actinobacillus, Bacillus, Pseudomonas) but, along with other members of ribosomal RNA (rRNA) homology group II of the genus Pseudomonas (e.g., P. cepacia), it was assigned to the new genus, Burkholderia, by Yabuuchi in 1992.8
EPIDEMIOLOGY Geographic Distribution The main areas of melioidosis endemicity are Southeast Asia and northern Australia. An estimated 2000–5000 cases occur each year in Thailand,9 while up to 50 cases are diagnosed annually in both Singapore and Australia. In recent years the known distribution of areas endemic for melioidosis has expanded to include Cambodia, Laos, Vietnam, Indonesia, southern China, the Indian subcontinent, Hong Kong, and Taiwan.10,11 Sporadic cases have been reported from the Pacific islands, central Africa, Central and South America, and the Caribbean.10,11 The disease is probably underdiagnosed in many of these regions, because relatively sophisticated laboratory facilities are necessary to confirm the diagnosis. Reports of cases in Iran and an epizootic that occurred in France during the 1970s12 suggest that infection may be transmitted in nontropical regions.
Reservoirs and Transmission B. pseudomallei is readily isolated from soil and surface water in endemic areas.13,14 Humans and a wide range of animals are thought usually to become infected by inoculation or contamination of wounds or mucosae with soil or surface water, although a specific exposural incident is only
identified in 6–25% of cases.15,16 Infections occurred in a disproportionate number of helicopter crewmen during the Vietnam War, possibly through inhalation of aerosols generated by the rotors.17 Two recent outbreaks in Australia have been traced to potable water supplies, although the mode of transmission was uncertain.18,19 There is no evidence that insect vectors play a role in transmission. Melioidosis occurs in a wide range of animal species (including rodents, primates, sheep and goats, pigs, cattle, horses, deer, dogs and cats, dolphins, koalas, kangaroos, camels, crocodiles, and birds). Transmission from animals to humans has rarely been reported,20 and person-to-person spread is also extremely uncommon.21 Iatrogenic infection from contaminated injections and laboratory-acquired infections have also been reported occasionally, resulting in the classification of B. pseudomallei as a Hazard Group 3 pathogen.
Descriptive Epidemiology Melioidosis predominantly affects people in regular contact with soil and water. In northeast Thailand, melioidosis accounts for 18% of communityacquired septicemias,22 and in the Northern Territory of Australia it is the commonest cause of fatal community-acquired sepsis.16 The annual incidence of human melioidosis in Ubon Ratchathani province in northeast Thailand in 2006 was 21.3 per 100 000 population (unpublished data, D. Limmathurotsakul and S. Peacock), which is comparable to figures reported from northern Australia and Papua New Guinea (19.6 and 20.0 per 100 000 population per year, respectively).23,24 All age groups can develop melioidosis, but incidence peaks between the ages of 40 and 60 years. The male : female ratio is 3 : 2 in Thailand but is higher in Australia and Singapore, probably because of differences in exposure to soil during rice farming. Melioidosis is markedly seasonal in most settings (an exception being Singapore),25 with approximately 75% of cases presenting during the rainy season and the highest incidence occurring during especially heavy monsoons.16 Most cases of melioidosis appear to be recently acquired.26 Risk factors for melioidosis include the presence of diabetes mellitus, chronic renal failure, immunosuppressive treatments including steroids, thalassemia, chronic liver disease, chronic lung disease (including cystic fibrosis), and kava consumption, one or more of which are found in 60–80% of cases.15,16,27,28 The association with diabetes mellitus is particularly strong and may increase the relative risk of infection by up to 100-fold.15 There is no evidence that human immunodeficiency virus infection predisposes to melioidosis.29
THE DISEASE The clinical spectrum of B. pseudomallei infection is extremely broad, and none of the clinical classifications of melioidosis is entirely satisfactory. Infections may be acute or chronic and localized or disseminated, but one form of the disease may progress to another and individual patients are often difficult to categorize. Several reviews have summarized the clinical manifestations of melioidosis.2,6,10,16,17,22,30
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Section II
Melioidosis. In: Cook G, Zumla A, eds. Manson’s Tropical Diseases, 22nd ed. London: WB Saunders; 2009.)
A
B
PART A: Bacterial and Mycobacterial Infections
PATHOGENS
Figure 33.1 Chest radiographs of two patients with melioidosis. (A) Right upper lobe consolidation in a patient with bacteremia, pneumonia, pyelonephritis, and subcutaneous abscesses. (B) Widespread bilateral shadowing in a patient with bacteremia, pneumonia, and multiple abscesses in the liver and spleen. (From Dance DAB.
Mild and Subclinical Infections In endemic areas such as northeast Thailand, 80% of children have antibodies to B. pseudomallei by the time they are 4 years old.31 Seroconversion provides evidence of exposure to B. pseudomallei, but the clinical consequences are usually asymptomatic or sufficiently trivial not to reach medical attention.
Latent Infections Long periods of latency have been observed between exposure to B. pseudomallei and the onset of clinical features of infection, the maximum recorded being 62 years.32 The development of infection after prolonged latency usually occurs at times of intercurrent stress (e.g., other acute infections, burns or trauma, malignancies, diabetes mellitus), when cellular immunity is likely to be suppressed. The sites and mechanisms of persistence are unknown, although clinically silent, chronic, localized foci of melioidosis in the lung, liver, or spleen have been reported in animals. The proportion of seropositive patients who harbor latent infection is unknown.
Figure 33.2 Abdominal ultrasound showing multiple liver abscesses in a patient with septicemic melioidosis (“Swiss cheese liver”). A similar picture may be seen in the spleen.
Septicemic Melioidosis
220
Approximately 50–60% of cases of culture-positive melioidosis have positive blood cultures.15,16 Most of these present clinically as communityacquired sepsis syndrome, with a short history (median, 6 days; range, 1 day to 2 months) of high fever and rigors, although some have a less acute, typhoidal illness with a swinging fever, often associated with profound weight loss.22 Patients with multiple, noncontiguous foci of infection, which probably reflect bacteremia at some stage, behave similarly. Only half have evidence of a primary focus of infection, usually in the lung or skin and subcutaneous tissues. Confusion, stupor, jaundice, and diarrhea may also be prominent features. Initial investigations usually reveal anemia, a neutrophil leukocytosis, coagulopathy, and evidence of renal and hepatic impairment. Such patients often deteriorate rapidly, developing widespread metastatic abscesses, particularly in the lungs, liver, and spleen, and metabolic acidosis with Kussmaul’s breathing. Once septic shock has supervened, the untreated mortality approaches 95%, with many patients dying within 48 hours of hospital admission. Other poor prognostic features include absence of fever, leukopenia, azotemia, and abnormal liver function tests.22 A high level of bacteremia (>50 CFU/ mL) is associated with a fatal outcome.33 If the patient survives this acute phase, the manifestations of the multiple septic foci that result from bacteremic dissemination become prominent. Any site or tissue may be involved, but the most common foci are the lungs, liver, spleen, prostate, and skin and soft tissues. An abnormal chest radiograph is found in 60–80% of patients, the most common pattern being widespread shadowing34 (Fig. 33.1). Multiple liver and splenic abscesses are also common (Fig. 33.2). Cutaneous pustules or subcutaneous abscesses occur in 10–20% of cases.22 Secondary lesions
Figure 33.3 Child with suppurative parotitis. In this case the overlying skin is involved, and there is facial nerve palsy.
may occur in any other tissue or organ (e.g., kidneys, bones and joints, brain). “Neurological melioidosis,” characterized by peripheral motor weakness, brainstem encephalitis, aseptic meningitis, and respiratory failure, is now thought to reflect direct invasion of the central nervous system rather than being toxin-mediated.16,35
Localized Melioidosis Localized melioidosis in the lung can be confused with tuberculosis, with cavitating pneumonia accompanied by profound weight loss.36 Relative sparing of the apices and the infrequency of hilar adenopathy may help to distinguish the two.34 There is a predilection for the upper lobes, although any lung zone may be affected. Complications include pneumothorax, empyema, and purulent pericarditis, and ultimately progression to septicemia. Acute suppurative parotitis (Fig. 33.3) is a characteristic manifestation of melioidosis in Thai children, accounting for approximately
Pathology Although B. pseudomallei is a pyogenic organism, the pathological features of lesions of melioidosis may vary from an acute, necrotizing inflammation with abscess formation to a chronic granulomatous inflammation, depending on the duration of the infection, and sometimes a mixed picture is seen.38 It is difficult to make a specific histopathologic diagnosis, but features that may be helpful include the presence of intracellular “globi” of Gram-negative bacilli combined with giant cells against a background of acute necrotizing inflammation.39
PATHOGENESIS AND IMMUNITY Bacterial Virulence Factors The organism possesses several potential virulence determinants and strains of the organism undoubtedly vary in virulence.40 Lipopolysaccharide (LPS) is presumably important during septicemia, although the biologic effects of B. pseudomallei LPS differ from those of enterobacterial LPS.41 Polysaccharide capsule is required for virulence in experimental animal models.42,43 Capsule expression is induced in the presence of serum and appears to interfere with deposition of complement factor C3b on the bacterial cell surface.43 Other putative virulence determinants include a heat-labile, lethal exotoxin with a molecular weight of approximately 31 kDa,44 various other toxins and enzymes (e.g., hemolysin, lecithinase, lipase, and proteases),45 a siderophore (malleobactin),46 flagella,47 type IV pili,48 and a type III secretion system.49 Six type VI secretion systems have been noted in the B. pseudomallei genome,50 although the investigation of their role in disease pathogenesis has not yet been published. Quorum sensing, a cell-density dependent communication system involved in coordination of gene regulation via N-acyl-homoserine lactones, has been described for B. pseudomallei, disruption of which leads to reduced bacterial virulence in an animal model.51 B. pseudomallei is able to survive intracellularly,52 which probably contributes to the recalcitrant nature of melioidosis, its potential for long periods of latency, and its tendency to relapse.53 Spread between cells occurs via a process involving actin rearrangement into a comet tail appearance within membrane protrusions.54 A type III secretion system (TTSS3) that shares homology with the inv/ spa/prg TTSS of Salmonella enterica serovar Typhimurium and the ipa/mxi/ spa TTSS cluster of Shigella flexneri appears to play a central role in this process.49,55
Host Defense The innate and adaptive immune response to B. pseudomallei has been reviewed in detail elsewhere.56,57 Evidence from studies in experimental animals suggests that antibodies to exotoxin, LPS, flagellin, flagellin–LPS conjugates, and capsular polysaccharides may all have some protective effect. In human infections, the level of antibody to lipopolysaccharide but not capsular polysaccharide was significantly higher in patients who survived melioidosis than in those who died.58 Production of interferon-γ appears to be particularly important in protection in some mouse models.59 The fact that C57BL/6 mice are considerably more resistant to infection by B. pseudomallei than BALB/c mice also suggests a role for cellular immunity in host defense.60 On the other hand, an overaggressive host response may actually contribute to pathogenesis, and raised levels
DIAGNOSIS Melioidosis is difficult to diagnose on clinical grounds alone, so where possible laboratory confirmation by detection of B. pseudomallei should be sought. This requires relatively sophisticated facilities, which are not available in many endemic areas. Because of the potential for latency, the diagnosis should be considered in any patient who has ever visited an endemic area who presents with septicemia or abscesses, particularly if there is evidence of an underlying disease such as diabetes mellitus.
Chapter 33
of several proinflammatory cytokines have been found to be associated with a poor outcome in human melioidosis.61–63
Melioidosis
one-third of pediatric cases,37 but has rarely been reported elsewhere. This strong age–site association probably reflects ascending infection after oral contamination with muddy water. Most cases are unilateral and result in parotid abscesses that require surgical drainage, although they may rupture spontaneously into the auditory canal. Facial nerve palsy and septicemia are rare complications. Other sites include cutaneous and subcutaneous abscesses, lymphadenitis, osteomyelitis and septic arthritis, liver or splenic abscesses, cystitis, pyelonephritis, prostatic abscesses, epididymo-orchitis, keratitis, and brain abscesses.
Microscopy and Culture The organism should be sought in blood, pus, throat swab, sputum, urine, or any other specimen appropriate to the clinical presentation. Microscopy of a Gram-stained smear may reveal bipolar or unevenly staining Gram-negative rods, but this has a low specificity and sensitivity. Direct immunofluorescent microscopy may be helpful in endemic areas, but is not widely available.64 Isolation and identification of B. pseudomallei is diagnostic, since asymptomatic carriage has never been reported. The organism grows readily on most laboratory media, although it may take 48 hours or more to develop characteristic colonial morphology. The sensitivity of culture may be increased by the use of selective media.65 Culture of a throat swab alone using these selective techniques has an overall sensitivity of 36% for the diagnosis of melioidosis (79% in sputumpositive patients), which is particularly useful in children or others who cannot produce sputum.66 Identification of cultures may be conducted using conventional biochemical tests, commercial kits, or latex agglutination tests, but may be delayed or incorrect because many microbiologists are not familiar with the characteristics of the organism. Consequently, if melioidosis is suspected, the laboratory should always be warned to ensure that appropriate techniques and containment measures are employed.
Detection of Antigens and Nucleic Acids Several rapid diagnostic techniques for the detection of B. pseudomallei antigens have been developed, but most of these currently have suboptimal sensitivity when used directly on clinical samples and/or are not widely available.67,68 A conventional polymerase chain reaction (PCR) assay targeting the 16S rRNA gene for the detection of B. pseudomallei in clinical specimens was reported to have a high diagnostic sensitivity but lacked specificity.69 Several real-time PCR assays have been developed,70–74 and two of these have undergone clinical evaluation for the detection of B. pseudomallei in clinical specimens.73,74 One assay had a diagnostic sensitivity of 91%,73 but the second had a much lower sensitivity of 61%74; the reasons for this disparity are unclear. The diagnostic sensitivity of PCR is highest for pus samples but is low for blood.73,74 Loop-mediated isothermal amplification (LAMP) has been developed for the detection of B. pseudomallei, which on clinical evaluation was shown to have a diagnostic sensitivity of 44%.75 At the present time, molecular diagnostics are not sufficiently sensitive to replace culture for the diagnosis of melioidosis.
Detection of Antibodies The serodiagnostic test most widely used in endemic areas is an indirect hemagglutination (IHA) test, which detects antibodies to a mixture of crude heat-stable antigens.76 The test is poorly standardized, and there is considerable interlaboratory variation between the titers regarded as positive. The IHA and other currently available serologic tests are not useful in regions where melioidosis is endemic because the healthy indigenous population is often seropositive.31 Serologic testing may have a greater diagnostic utility in persons who do not normally reside in regions
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Section II PATHOGENS
TREATMENT AND PROGNOSIS
PART A: Bacterial and Mycobacterial Infections
endemic for melioidosis, including returning travelers and laboratory workers following accidental laboratory exposure to B. pseudomallei.77 Different IHA titers have been used to infer a positive result, but a single raised titer (>1 : 40) in someone from a non-endemic area, or a rising titer, is suggestive of exposure to B. pseudomallei. A negative serologic test does not rule out exposure or infection since some patients with cultureproven melioidosis do not have detectable antibodies.78
Supportive Treatment Patients with septicemic melioidosis usually require aggressive supportive treatment and should ideally be managed in an intensive care unit. Particular attention should be paid to correction of volume depletion and septic shock, respiratory and renal failure, and hyperglycemia or ketoacidosis. Abscesses should be drained whenever possible.
Specific Treatment B. pseudomallei is intrinsically resistant to many antibiotics, including aminoglycosides and early β-lactams. Until the mid-1980s, various empirical combination regimens were used to treat melioidosis, usually including a tetracycline, chloramphenicol, and co-trimoxazole. Several more recent studies, which have been well summarized elsewhere,10,30,79 have provided a sound evidence base for the treatment of severe meli oidosis. Treatment comprises two phases: an acute phase, the aim of which is to reduce mortality, and an eradication phase, the aim of which is to reduce the risk of relapse. The mortality of acute severe melioidosis has been substantially reduced by the use of ceftazidime, imipenem, or cefoperazone-sulbactam.80–85 A striking reduction in mortality from 95% to 10% in patients with melioidosis shock associated with the use of meropenem plus co-trimoxazole with adjunctive granulocyte colony-stimulating factor (G-CSF) was reported from the Royal Darwin Hospital, northern Australia in 2004.86 A subsequent randomized, placebo-controlled trial of G-CSF in ceftazidime-treated patients with severe sepsis caused by suspected melioidosis conducted in northeast Thailand did not show a mortality benefit associated with G-CSF therapy.87 Ceftazidime 50 mg/kg per dose (up to 2 g) every 6–8 hours, or meropenem 25 mg/kg per dose (up to 1 g) every 8 hours is currently the treatment of choice for melioidosis and should be given for a minimum of 10–14 days, and longer (4–8 weeks) for deep-seated infection according to the clinical response. Trial data do not support the addition of trimethoprim-sulfamethoxazole (TMP-SMX) during this initial phase, although some clinicians elect on grounds of drug penetration to add TMP-SMX 8/40 mg/kg (up to 320/1600 mg) every 12 hours for treatment of patients with neurologic, prostatic, bone, or joint melioidosis. Amoxicillin-clavulanate therapy is an alternative but is associated with a higher failure rate,88 and ceftriaxone and cefotaxime should not be used.89
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222
Following parenteral treatment, prolonged oral antibiotics are needed to prevent relapse, which occurs in up to 23% of patients,26,53 and is more common in patients with more severe disease. This can be reduced to less than 10% if antibiotics are given for a total of 20 weeks.90 The current treatment recommendation for oral eradication therapy is TMP-SMX plus doxycycline.91 TMP-SMX is given every 12 hours using a dose based on weight (2 × 160–800 mg (960 mg) tablets if more than 60 kg; 3 × 80– 400 (480 mg) tablets if 40–60 kg; and 1 × 160–800 mg (960 mg) or 2 × 80–400 (480 mg) tablets if adult less than 40 kg). Dosing of doxycycline is 2.5 mg/kg per dose up to 100 mg orally every 12 hours. A clinical trial being conducted in Thailand to determine the equivalence of TMP-SMX with or without doxycycline is nearing completion. Amoxicillin-clavulanate therapy is preferable for oral eradication treatment in children and in pregnant or lactating women, but is associated with a higher rate of relapse compared with TMP-SMX plus doxycycline.90,92 Consensus guidelines have been developed for dosing of amoxicillin-clavulanate in melioidosis, which in the oral eradication phase should be used at a dose of 20/5 mg/kg three times a day with a maximum dose of 1500/375 mg three times a day for patients >60 kg, and a lower maximum dose for patients <60 kg of 1000/250 mg three times per day.93 The fluoroquinolones, with or without azithromycin, and doxycycline alone are all associated with unacceptable relapse rates.92,94–96 In patients with mild localized disease, the preceding oral regimens may be used, although the optimal agents and duration of treatment remain to be defined.
Outcome and Follow-up Even with optimal treatment, the mortality from acute severe melioidosis is high (30–47%). In patients who survive, there is often chronic morbidity resulting from both the disease itself and the underlying conditions. Patients require long-term follow-up to detect relapse. Susceptibility tests should be carried out on isolates obtained during or after treatment, since resistance may emerge in 5–10% of cases.97
PREVENTION AND CONTROL B. pseudomallei is ubiquitous in the environment in endemic areas, so it is difficult for those whose occupations involve soil and water contact to avoid exposure. Common sense would suggest that those particularly at risk (e.g., diabetic patients) should avoid regular contact with contaminated environments or use hand and foot protection, although the effectiveness of this has not been evaluated. Elimination of the organism from soil using disinfectants was attempted in the French outbreak,12 but is probably futile. There is no B. pseudomallei vaccine licensed for human use, although experimental vaccines are under development and have been used in animals.98 The organism should be handled in containment level 3 facilities in the laboratory. Patients should ideally be nursed in standard isolation, although person-to-person spread is very rare.
PART A: Bacterial and Mycobacterial Infections
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CHAPTER 33 Melioidosis Sharon J. Peacock • David A.B. Dance
INTRODUCTION Melioidosis is an infection caused by the Gram-negative bacillus Burkholderia pseudomallei. This disease has emerged over the past 25 years as an important cause of morbidity and mortality in Southeast Asia and northern Australia, and is also endemic in other tropical regions. B. pseudomallei has been classified as a “Category B” agent by the US Centers for Disease Control and Prevention.1
THE AGENT Melioidosis was first recognized as a fatal “glanders-like” illness in Burma in 1911 by Alfred Whitmore.2 The genetic similarity of the causative organism to that of glanders, B. mallei, has since been confirmed by multilocus sequence typing and whole genome sequencing.3–5 Animal and human infections with Whitmore’s bacillus were later recognized by Stanton and Fletcher in the Federated Malay States.6 They considered the disease to be a zoonosis and coined the term melioidosis. Workers in French Indochina subsequently showed that the organism was really an environmental saprophyte.7 B. pseudomallei was previously classified in various genera (e.g., Malleomyces, Pfeifferella, Actinobacillus, Bacillus, Pseudomonas) but, along with other members of ribosomal RNA (rRNA) homology group II of the genus Pseudomonas (e.g., P. cepacia), it was assigned to the new genus, Burkholderia, by Yabuuchi in 1992.8
EPIDEMIOLOGY Geographic Distribution The main areas of melioidosis endemicity are Southeast Asia and northern Australia. An estimated 2000–5000 cases occur each year in Thailand,9 while up to 50 cases are diagnosed annually in both Singapore and Australia. In recent years the known distribution of areas endemic for melioidosis has expanded to include Cambodia, Laos, Vietnam, Indonesia, southern China, the Indian subcontinent, Hong Kong, and Taiwan.10,11 Sporadic cases have been reported from the Pacific islands, central Africa, Central and South America, and the Caribbean.10,11 The disease is probably underdiagnosed in many of these regions, because relatively sophisticated laboratory facilities are necessary to confirm the diagnosis. Reports of cases in Iran and an epizootic that occurred in France during the 1970s12 suggest that infection may be transmitted in nontropical regions.
Reservoirs and Transmission B. pseudomallei is readily isolated from soil and surface water in endemic areas.13,14 Humans and a wide range of animals are thought usually to become infected by inoculation or contamination of wounds or mucosae with soil or surface water, although a specific exposural incident is only
identified in 6–25% of cases.15,16 Infections occurred in a disproportionate number of helicopter crewmen during the Vietnam War, possibly through inhalation of aerosols generated by the rotors.17 Two recent outbreaks in Australia have been traced to potable water supplies, although the mode of transmission was uncertain.18,19 There is no evidence that insect vectors play a role in transmission. Melioidosis occurs in a wide range of animal species (including rodents, primates, sheep and goats, pigs, cattle, horses, deer, dogs and cats, dolphins, koalas, kangaroos, camels, crocodiles, and birds). Transmission from animals to humans has rarely been reported,20 and person-to-person spread is also extremely uncommon.21 Iatrogenic infection from contaminated injections and laboratory-acquired infections have also been reported occasionally, resulting in the classification of B. pseudomallei as a Hazard Group 3 pathogen.
Descriptive Epidemiology Melioidosis predominantly affects people in regular contact with soil and water. In northeast Thailand, melioidosis accounts for 18% of communityacquired septicemias,22 and in the Northern Territory of Australia it is the commonest cause of fatal community-acquired sepsis.16 The annual incidence of human melioidosis in Ubon Ratchathani province in northeast Thailand in 2006 was 21.3 per 100 000 population (unpublished data, D. Limmathurotsakul and S. Peacock), which is comparable to figures reported from northern Australia and Papua New Guinea (19.6 and 20.0 per 100 000 population per year, respectively).23,24 All age groups can develop melioidosis, but incidence peaks between the ages of 40 and 60 years. The male : female ratio is 3 : 2 in Thailand but is higher in Australia and Singapore, probably because of differences in exposure to soil during rice farming. Melioidosis is markedly seasonal in most settings (an exception being Singapore),25 with approximately 75% of cases presenting during the rainy season and the highest incidence occurring during especially heavy monsoons.16 Most cases of melioidosis appear to be recently acquired.26 Risk factors for melioidosis include the presence of diabetes mellitus, chronic renal failure, immunosuppressive treatments including steroids, thalassemia, chronic liver disease, chronic lung disease (including cystic fibrosis), and kava consumption, one or more of which are found in 60–80% of cases.15,16,27,28 The association with diabetes mellitus is particularly strong and may increase the relative risk of infection by up to 100-fold.15 There is no evidence that human immunodeficiency virus infection predisposes to melioidosis.29
THE DISEASE The clinical spectrum of B. pseudomallei infection is extremely broad, and none of the clinical classifications of melioidosis is entirely satisfactory. Infections may be acute or chronic and localized or disseminated, but one form of the disease may progress to another and individual patients are often difficult to categorize. Several reviews have summarized the clinical manifestations of melioidosis.2,6,10,16,17,22,30
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Section II
Melioidosis. In: Cook G, Zumla A, eds. Manson’s Tropical Diseases, 22nd ed. London: WB Saunders; 2009.)
A
B
PART A: Bacterial and Mycobacterial Infections
PATHOGENS
Figure 33.1 Chest radiographs of two patients with melioidosis. (A) Right upper lobe consolidation in a patient with bacteremia, pneumonia, pyelonephritis, and subcutaneous abscesses. (B) Widespread bilateral shadowing in a patient with bacteremia, pneumonia, and multiple abscesses in the liver and spleen. (From Dance DAB.
Mild and Subclinical Infections In endemic areas such as northeast Thailand, 80% of children have antibodies to B. pseudomallei by the time they are 4 years old.31 Seroconversion provides evidence of exposure to B. pseudomallei, but the clinical consequences are usually asymptomatic or sufficiently trivial not to reach medical attention.
Latent Infections Long periods of latency have been observed between exposure to B. pseudomallei and the onset of clinical features of infection, the maximum recorded being 62 years.32 The development of infection after prolonged latency usually occurs at times of intercurrent stress (e.g., other acute infections, burns or trauma, malignancies, diabetes mellitus), when cellular immunity is likely to be suppressed. The sites and mechanisms of persistence are unknown, although clinically silent, chronic, localized foci of melioidosis in the lung, liver, or spleen have been reported in animals. The proportion of seropositive patients who harbor latent infection is unknown.
Figure 33.2 Abdominal ultrasound showing multiple liver abscesses in a patient with septicemic melioidosis (“Swiss cheese liver”). A similar picture may be seen in the spleen.
Septicemic Melioidosis
220
Approximately 50–60% of cases of culture-positive melioidosis have positive blood cultures.15,16 Most of these present clinically as communityacquired sepsis syndrome, with a short history (median, 6 days; range, 1 day to 2 months) of high fever and rigors, although some have a less acute, typhoidal illness with a swinging fever, often associated with profound weight loss.22 Patients with multiple, noncontiguous foci of infection, which probably reflect bacteremia at some stage, behave similarly. Only half have evidence of a primary focus of infection, usually in the lung or skin and subcutaneous tissues. Confusion, stupor, jaundice, and diarrhea may also be prominent features. Initial investigations usually reveal anemia, a neutrophil leukocytosis, coagulopathy, and evidence of renal and hepatic impairment. Such patients often deteriorate rapidly, developing widespread metastatic abscesses, particularly in the lungs, liver, and spleen, and metabolic acidosis with Kussmaul’s breathing. Once septic shock has supervened, the untreated mortality approaches 95%, with many patients dying within 48 hours of hospital admission. Other poor prognostic features include absence of fever, leukopenia, azotemia, and abnormal liver function tests.22 A high level of bacteremia (>50 CFU/ mL) is associated with a fatal outcome.33 If the patient survives this acute phase, the manifestations of the multiple septic foci that result from bacteremic dissemination become prominent. Any site or tissue may be involved, but the most common foci are the lungs, liver, spleen, prostate, and skin and soft tissues. An abnormal chest radiograph is found in 60–80% of patients, the most common pattern being widespread shadowing34 (Fig. 33.1). Multiple liver and splenic abscesses are also common (Fig. 33.2). Cutaneous pustules or subcutaneous abscesses occur in 10–20% of cases.22 Secondary lesions
Figure 33.3 Child with suppurative parotitis. In this case the overlying skin is involved, and there is facial nerve palsy.
may occur in any other tissue or organ (e.g., kidneys, bones and joints, brain). “Neurological melioidosis,” characterized by peripheral motor weakness, brainstem encephalitis, aseptic meningitis, and respiratory failure, is now thought to reflect direct invasion of the central nervous system rather than being toxin-mediated.16,35
Localized Melioidosis Localized melioidosis in the lung can be confused with tuberculosis, with cavitating pneumonia accompanied by profound weight loss.36 Relative sparing of the apices and the infrequency of hilar adenopathy may help to distinguish the two.34 There is a predilection for the upper lobes, although any lung zone may be affected. Complications include pneumothorax, empyema, and purulent pericarditis, and ultimately progression to septicemia. Acute suppurative parotitis (Fig. 33.3) is a characteristic manifestation of melioidosis in Thai children, accounting for approximately
Pathology Although B. pseudomallei is a pyogenic organism, the pathological features of lesions of melioidosis may vary from an acute, necrotizing inflammation with abscess formation to a chronic granulomatous inflammation, depending on the duration of the infection, and sometimes a mixed picture is seen.38 It is difficult to make a specific histopathologic diagnosis, but features that may be helpful include the presence of intracellular “globi” of Gram-negative bacilli combined with giant cells against a background of acute necrotizing inflammation.39
PATHOGENESIS AND IMMUNITY Bacterial Virulence Factors The organism possesses several potential virulence determinants and strains of the organism undoubtedly vary in virulence.40 Lipopolysaccharide (LPS) is presumably important during septicemia, although the biologic effects of B. pseudomallei LPS differ from those of enterobacterial LPS.41 Polysaccharide capsule is required for virulence in experimental animal models.42,43 Capsule expression is induced in the presence of serum and appears to interfere with deposition of complement factor C3b on the bacterial cell surface.43 Other putative virulence determinants include a heat-labile, lethal exotoxin with a molecular weight of approximately 31 kDa,44 various other toxins and enzymes (e.g., hemolysin, lecithinase, lipase, and proteases),45 a siderophore (malleobactin),46 flagella,47 type IV pili,48 and a type III secretion system.49 Six type VI secretion systems have been noted in the B. pseudomallei genome,50 although the investigation of their role in disease pathogenesis has not yet been published. Quorum sensing, a cell-density dependent communication system involved in coordination of gene regulation via N-acyl-homoserine lactones, has been described for B. pseudomallei, disruption of which leads to reduced bacterial virulence in an animal model.51 B. pseudomallei is able to survive intracellularly,52 which probably contributes to the recalcitrant nature of melioidosis, its potential for long periods of latency, and its tendency to relapse.53 Spread between cells occurs via a process involving actin rearrangement into a comet tail appearance within membrane protrusions.54 A type III secretion system (TTSS3) that shares homology with the inv/ spa/prg TTSS of Salmonella enterica serovar Typhimurium and the ipa/mxi/ spa TTSS cluster of Shigella flexneri appears to play a central role in this process.49,55
Host Defense The innate and adaptive immune response to B. pseudomallei has been reviewed in detail elsewhere.56,57 Evidence from studies in experimental animals suggests that antibodies to exotoxin, LPS, flagellin, flagellin–LPS conjugates, and capsular polysaccharides may all have some protective effect. In human infections, the level of antibody to lipopolysaccharide but not capsular polysaccharide was significantly higher in patients who survived melioidosis than in those who died.58 Production of interferon-γ appears to be particularly important in protection in some mouse models.59 The fact that C57BL/6 mice are considerably more resistant to infection by B. pseudomallei than BALB/c mice also suggests a role for cellular immunity in host defense.60 On the other hand, an overaggressive host response may actually contribute to pathogenesis, and raised levels
DIAGNOSIS Melioidosis is difficult to diagnose on clinical grounds alone, so where possible laboratory confirmation by detection of B. pseudomallei should be sought. This requires relatively sophisticated facilities, which are not available in many endemic areas. Because of the potential for latency, the diagnosis should be considered in any patient who has ever visited an endemic area who presents with septicemia or abscesses, particularly if there is evidence of an underlying disease such as diabetes mellitus.
Chapter 33
of several proinflammatory cytokines have been found to be associated with a poor outcome in human melioidosis.61–63
Melioidosis
one-third of pediatric cases,37 but has rarely been reported elsewhere. This strong age–site association probably reflects ascending infection after oral contamination with muddy water. Most cases are unilateral and result in parotid abscesses that require surgical drainage, although they may rupture spontaneously into the auditory canal. Facial nerve palsy and septicemia are rare complications. Other sites include cutaneous and subcutaneous abscesses, lymphadenitis, osteomyelitis and septic arthritis, liver or splenic abscesses, cystitis, pyelonephritis, prostatic abscesses, epididymo-orchitis, keratitis, and brain abscesses.
Microscopy and Culture The organism should be sought in blood, pus, throat swab, sputum, urine, or any other specimen appropriate to the clinical presentation. Microscopy of a Gram-stained smear may reveal bipolar or unevenly staining Gram-negative rods, but this has a low specificity and sensitivity. Direct immunofluorescent microscopy may be helpful in endemic areas, but is not widely available.64 Isolation and identification of B. pseudomallei is diagnostic, since asymptomatic carriage has never been reported. The organism grows readily on most laboratory media, although it may take 48 hours or more to develop characteristic colonial morphology. The sensitivity of culture may be increased by the use of selective media.65 Culture of a throat swab alone using these selective techniques has an overall sensitivity of 36% for the diagnosis of melioidosis (79% in sputumpositive patients), which is particularly useful in children or others who cannot produce sputum.66 Identification of cultures may be conducted using conventional biochemical tests, commercial kits, or latex agglutination tests, but may be delayed or incorrect because many microbiologists are not familiar with the characteristics of the organism. Consequently, if melioidosis is suspected, the laboratory should always be warned to ensure that appropriate techniques and containment measures are employed.
Detection of Antigens and Nucleic Acids Several rapid diagnostic techniques for the detection of B. pseudomallei antigens have been developed, but most of these currently have suboptimal sensitivity when used directly on clinical samples and/or are not widely available.67,68 A conventional polymerase chain reaction (PCR) assay targeting the 16S rRNA gene for the detection of B. pseudomallei in clinical specimens was reported to have a high diagnostic sensitivity but lacked specificity.69 Several real-time PCR assays have been developed,70–74 and two of these have undergone clinical evaluation for the detection of B. pseudomallei in clinical specimens.73,74 One assay had a diagnostic sensitivity of 91%,73 but the second had a much lower sensitivity of 61%74; the reasons for this disparity are unclear. The diagnostic sensitivity of PCR is highest for pus samples but is low for blood.73,74 Loop-mediated isothermal amplification (LAMP) has been developed for the detection of B. pseudomallei, which on clinical evaluation was shown to have a diagnostic sensitivity of 44%.75 At the present time, molecular diagnostics are not sufficiently sensitive to replace culture for the diagnosis of melioidosis.
Detection of Antibodies The serodiagnostic test most widely used in endemic areas is an indirect hemagglutination (IHA) test, which detects antibodies to a mixture of crude heat-stable antigens.76 The test is poorly standardized, and there is considerable interlaboratory variation between the titers regarded as positive. The IHA and other currently available serologic tests are not useful in regions where melioidosis is endemic because the healthy indigenous population is often seropositive.31 Serologic testing may have a greater diagnostic utility in persons who do not normally reside in regions
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Section II PATHOGENS
TREATMENT AND PROGNOSIS
PART A: Bacterial and Mycobacterial Infections
endemic for melioidosis, including returning travelers and laboratory workers following accidental laboratory exposure to B. pseudomallei.77 Different IHA titers have been used to infer a positive result, but a single raised titer (>1 : 40) in someone from a non-endemic area, or a rising titer, is suggestive of exposure to B. pseudomallei. A negative serologic test does not rule out exposure or infection since some patients with cultureproven melioidosis do not have detectable antibodies.78
Supportive Treatment Patients with septicemic melioidosis usually require aggressive supportive treatment and should ideally be managed in an intensive care unit. Particular attention should be paid to correction of volume depletion and septic shock, respiratory and renal failure, and hyperglycemia or ketoacidosis. Abscesses should be drained whenever possible.
Specific Treatment B. pseudomallei is intrinsically resistant to many antibiotics, including aminoglycosides and early β-lactams. Until the mid-1980s, various empirical combination regimens were used to treat melioidosis, usually including a tetracycline, chloramphenicol, and co-trimoxazole. Several more recent studies, which have been well summarized elsewhere,10,30,79 have provided a sound evidence base for the treatment of severe meli oidosis. Treatment comprises two phases: an acute phase, the aim of which is to reduce mortality, and an eradication phase, the aim of which is to reduce the risk of relapse. The mortality of acute severe melioidosis has been substantially reduced by the use of ceftazidime, imipenem, or cefoperazone-sulbactam.80–85 A striking reduction in mortality from 95% to 10% in patients with melioidosis shock associated with the use of meropenem plus co-trimoxazole with adjunctive granulocyte colony-stimulating factor (G-CSF) was reported from the Royal Darwin Hospital, northern Australia in 2004.86 A subsequent randomized, placebo-controlled trial of G-CSF in ceftazidime-treated patients with severe sepsis caused by suspected melioidosis conducted in northeast Thailand did not show a mortality benefit associated with G-CSF therapy.87 Ceftazidime 50 mg/kg per dose (up to 2 g) every 6–8 hours, or meropenem 25 mg/kg per dose (up to 1 g) every 8 hours is currently the treatment of choice for melioidosis and should be given for a minimum of 10–14 days, and longer (4–8 weeks) for deep-seated infection according to the clinical response. Trial data do not support the addition of trimethoprim-sulfamethoxazole (TMP-SMX) during this initial phase, although some clinicians elect on grounds of drug penetration to add TMP-SMX 8/40 mg/kg (up to 320/1600 mg) every 12 hours for treatment of patients with neurologic, prostatic, bone, or joint melioidosis. Amoxicillin-clavulanate therapy is an alternative but is associated with a higher failure rate,88 and ceftriaxone and cefotaxime should not be used.89
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Following parenteral treatment, prolonged oral antibiotics are needed to prevent relapse, which occurs in up to 23% of patients,26,53 and is more common in patients with more severe disease. This can be reduced to less than 10% if antibiotics are given for a total of 20 weeks.90 The current treatment recommendation for oral eradication therapy is TMP-SMX plus doxycycline.91 TMP-SMX is given every 12 hours using a dose based on weight (2 × 160–800 mg (960 mg) tablets if more than 60 kg; 3 × 80– 400 (480 mg) tablets if 40–60 kg; and 1 × 160–800 mg (960 mg) or 2 × 80–400 (480 mg) tablets if adult less than 40 kg). Dosing of doxycycline is 2.5 mg/kg per dose up to 100 mg orally every 12 hours. A clinical trial being conducted in Thailand to determine the equivalence of TMP-SMX with or without doxycycline is nearing completion. Amoxicillin-clavulanate therapy is preferable for oral eradication treatment in children and in pregnant or lactating women, but is associated with a higher rate of relapse compared with TMP-SMX plus doxycycline.90,92 Consensus guidelines have been developed for dosing of amoxicillin-clavulanate in melioidosis, which in the oral eradication phase should be used at a dose of 20/5 mg/kg three times a day with a maximum dose of 1500/375 mg three times a day for patients >60 kg, and a lower maximum dose for patients <60 kg of 1000/250 mg three times per day.93 The fluoroquinolones, with or without azithromycin, and doxycycline alone are all associated with unacceptable relapse rates.92,94–96 In patients with mild localized disease, the preceding oral regimens may be used, although the optimal agents and duration of treatment remain to be defined.
Outcome and Follow-up Even with optimal treatment, the mortality from acute severe melioidosis is high (30–47%). In patients who survive, there is often chronic morbidity resulting from both the disease itself and the underlying conditions. Patients require long-term follow-up to detect relapse. Susceptibility tests should be carried out on isolates obtained during or after treatment, since resistance may emerge in 5–10% of cases.97
PREVENTION AND CONTROL B. pseudomallei is ubiquitous in the environment in endemic areas, so it is difficult for those whose occupations involve soil and water contact to avoid exposure. Common sense would suggest that those particularly at risk (e.g., diabetic patients) should avoid regular contact with contaminated environments or use hand and foot protection, although the effectiveness of this has not been evaluated. Elimination of the organism from soil using disinfectants was attempted in the French outbreak,12 but is probably futile. There is no B. pseudomallei vaccine licensed for human use, although experimental vaccines are under development and have been used in animals.98 The organism should be handled in containment level 3 facilities in the laboratory. Patients should ideally be nursed in standard isolation, although person-to-person spread is very rare.