Diagnosis
A definitive diagnosis is made by culturing the organism from any clinical sample, because the organism is never part of the normal human flora.
A definite history of contact with soil may not be elicited as melioidosis can be dormant for many years before manifesting. 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 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 misidentify 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. There is almost complete cross-reactivity with B. thailandensis. There exists a commercial ELISA kit for melioidosis which appears to perform well. but no ELISA test has yet been clinically validated as a diagnostic tool.
It is not possible to make the diagnosis on imaging studies alone (X-rays and scans), but imaging is routinely performed to assess the full extent of disease. Imaging of the abdomen using CT scans or ultrasound is recommended routinely, as abscesses may not be clinically apparent and may 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 presence of honeycomb abscesses in the liver are considered characteristic, but are not diagnostic.
The differential diagnosis is extensive; melioidosis may mimic many other infections, including tuberculosis.
Read more about this topic: Melioidosis