The causative organism, Burkholderia pseudomallei, was thought to be a member of the Pseudomonas genus and was previously known as Pseudomonas pseudomallei. This organism is phylogenetically related closely to Burkholderia mallei, the organism that causes glanders.
In northeast Thailand, 80% of children are positive for antibodies against B. pseudomallei by the age of 4; the figures are lower in other parts of the world. It affects humans as well as other animals such as goats, sheep, and horses. Cattle, water buffalo, and crocodiles are considered to be relatively resistant to melioidosis despite their constant exposure to mud. An outbreak at the Paris Zoo in the 1970s ("L’affaire du jardin des plantes") was thought to have originated from an imported panda.
The single most important risk factor for developing severe melioidosis is diabetes mellitus. Other risk factors include thalassaemia, kidney disease, and occupation (rice paddy farmers). The mode of infection is believed to be either through a break in the skin, or through the inhalation of aerosolized B. pseudomallei.
There is a clear association with increased rainfall: with the number (and severity) of cases increasing following increased precipitation.
A patient with active melioidosis usually presents with fever. Pain or other symptoms may be suggestive of a clinical focus, which is found in around 75% of patients. Such symptoms include cough or pleuritic chest pain suggestive of pneumonia, bone or joint pain suggestive of osteomyelitis or septic arthritis, or cellulitis. Intra-abdominal infection (including liver and/or splenic abscesses, or prostatic abscesses) do not usually present with focal pain, and imaging of these organs using ultrasound or CT should be performed routinely. It has been suggested that B. pseudomallei abscesses have a characteristic honeycomb architecture (hypoechoic, multi-septate, multiloculate).
There are regional variations in disease presentation: parotid abscesses characteristically occur in Thai children but this presentation has only been described once in Australia. Conversely, prostatic abscesses are found in up to 20% of Australian males but are rarely described elsewhere. An encephalomyelitis syndrome is recognised in northern Australia.
Patients with melioidosis usually have risk factors for disease, such as diabetes, thalassemia, hazardous alcohol use or renal disease, and frequently give a history of occupational or recreational exposure to mud or pooled surface water. However, otherwise healthy patients, including children, may also get melioidosis.
In up to 25% of patients, no focus of infection is found and the diagnosis is usually made on blood cultures or throat swab. Melioidosis is said to be able to affect any organ in the body except the heart valves (endocarditis). Although meningitis has been described secondary to ruptured brain abscesses, primary meningitis has not been described. Less common manifestation include intravascular infection, lymph node abscesses (1.2–2.2%), pyopericardium and myocarditis, mediastinal infection, and thyroid and scrotal abscesses and ocular infection.
A definite history of contact with soil may not be elicited as melioidosis can be dormant for many years before becoming acute. Attention should be paid to a history of travel to endemic areas in returned travellers. Some authors recommend considering possibility of melioidosis in every febrile patient with a history of traveling to and/or staying at endemic areas.
A complete screen (blood culture, sputum culture, urine culture, throat swab and culture of any aspirated pus) should be performed on all patients with suspected melioidosis (culture on blood agar as well as Ashdown's medium). A definitive diagnosis is made by growing B. pseudomallei from any site. A throat swab is not sensitive but is 100% specific if positive, and compares favourably with sputum culture. The sensitivity of urine culture is increased if a centrifuged specimen is cultured, and any bacterial growth should be reported (not just growth above 104 organisms/ml which is the usual cut off). Very occasionally, bone marrow culture may be positive in patients who have negative blood cultures for B. pseudomallei, but these are not usually recommended. A common error made by clinicians unfamiliar with the diagnosis of melioidosis is to only send a specimen from the affected site (which is the usual procedure for most other infections) instead of sending a full screen.
Ashdown's medium, a selective medium containing gentamicin, may be required for cultures taken from non-sterile sites. Burkholderia cepacia medium may be a useful alternative selective medium in non-endemic areas, where Ashdown's is not available. A new medium derived from Ashdown known as Francis medium may help differentiate B. pseudomallei from B. cepacia and may help in the early diagnosis of melioidosis, but has not yet been extensively clinically validated.
Many commercial kits for identifying bacteria may mis-identify B. pseudomallei (see Burkholderia pseudomallei for a more detailed discussion of these issues).
There is also a serological test for melioidosis (indirect haemagglutination), but this is not commercially available in most countries. A high background titre may reduce the positive predictive value of serological tests in endemic countries. A specific direct immunofluorescent test and latex agglutination, based on monoclonal antibodies, are used widely in Thailand but are not available elsewhere.
Imaging of the abdomen using CT scans or ultrasound is recommended routinely, as abscesses may not be clinically apparent and may be coexist with disease elsewhere. Australian authorities suggest imaging of the prostate specifically due to the high incidence of prostatic abscesses in northern Australian patients. A chest x-ray is also considered routine, with other investigations as clinically indicated.
The treatment of melioidosis is divided into two stages, an intravenous high intensity stage and an oral maintenance stage to prevent recurrence. Surgical drainage is usually indicated for prostatic abscesses and septic arthritis, may be indicated for parotid abscesses and not usually indicated for hepatosplenic abscesses.Intravenous intensive phase.
Relapse occurs in 10 to 20% of patients. While molecular studies have established that the majority of recurrences are due to the original infecting strain, a significant proportion of recurrences (perhaps up to a quarter) in endemic areas may be due to reinfection, particularly after 2 years. Risk factors include severity of disease (patients with positive blood cultures or multifocal disease have a higher risk of relapse), choice of antibiotic for eradication therapy (doxycycline monotherapy and fluoroquinolone therapy are not as effective), poor compliance with eradication therapy and duration of eradication therapy less than 8 weeks.
In endemic areas, people (rice-paddy farmers in particular) are warned to avoid contact with soil, mud and surface water where possible. Case clusters have been described following flooding and cyclones and probably relate to exposure. Other case clusters have related to contamination of drinking water supplies. Populations at risk include patients with diabetes mellitus, chronic renal failure, chronic lung disease or patients with an immune deficiency of any kind. The effectiveness of measures to reduce exposure to the causative organism have not been established. A vaccine is not yet available.
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