Melioidosis in aspects of epidemiology, clinic, and laboratory diagnostics

Abstract

Melioidosis is a life-threatening infection caused by Burkholderia pseudomallei, an environmental Gram-negative bacterium, inhabitant of moist soils in the tropics and subtropics. There is no licensed vaccine against melioidosis. The main routes of B. pseudomallei infection are percutaneous inoculation, inhalation, or ingestion. Individual cases of vertical, sexual, zoonotic, and nosocomial transmission of melioidosis are described. Risk factors for infection are contact with soil or water (especially during the rainy season). The age over 45, type 2 diabetes, alcoholism, liver disease, chronic lung disease, chronic renal disease, and thalassemia, as well as long-term use of steroids and immunosuppressive therapy, are the main susceptibility factors for melioidosis. Among the affected adult residents of endemic regions, 80% had one or more predisposing factors, among children — about 20%. No significant influence of concomitant diseases on the development of melioidosis in travelers was found. Less than 50% of patients had predisposing factors. The incubation period of melioidosis ranges within 1—21 days; on average, 9 days, in case of sizeable infectious dose, it can be less than one day. There is no post-infectious immunity, and reinfection can occur with a different B. pseudomallei strain after successful treatment. B. pseudomallei is a facultative intracellular pathogen that can invade and multiply inside a wide range of cells, including phagocytic. The acute form of melioidosis is characterized by pneumonia, multiple abscesses, bacteremia, and systemic sepsis. Chronic, subacute, and latent forms are also possible. Antimicrobial therapy is divided into the initial intensive phase and the subsequent eradication phase. B. pseudomallei is resistant to penicillins, first- and second-generation cephalosporins, aminoglycosides, macrolides, chloramphenicol, fluoroquinolones, tetracyclines, trimethoprim, and in some cases to co-trimoxazole, and rarely to ceftazidime. Early diagnosis and appropriate management are crucial in reducing severe complications leading to high mortality, and in preventing disease recurrences. However, there is no pathognomonic melioidosis-specific feature, and the disease is not well known to physicians and microbiologists. The results of serological tests for detection of specific antibodies are ambiguous. The bacterial load of the tested sample limits the detection of antigens. Among the accelerated methods for identifying the causative agent of melioidosis, PCR has the highest sensitivity and specificity. Automated identification using microbiological analyzers generally shows good results, but about 15% of isolates are misidentified. Time-of-flight mass spectrometry with matrix-assisted laser desorption ionization is potentially useful for rapid identification of B. pseudomallei. However, existing databases require optimization by adding the reference spectra for B. pseudomallei.