Emerging Pathogens

SPOTLIGHT ON Burkholderia pseudomallei and Burkholderia mallei

B. pseudomallei (disease: melioidosis) and B. mallei (disease: glanders) are genetically related Select Agent pathogens that inhabit very different ecological niches.  B. pseudomallei is a saprophytic Gram-negative bacteria that is readily cultured from moist soils in the tropics, particularly from the endemic regions of Southeast Asia and northern Australia.  Monsoons in endemic areas are typically associated with outbreaks of melioidosis, where the rising water tables and high winds are thought to aerosolize the bacteria leading to respiratory acquisition of disease.  Persistence of B. pseudomallei in standing water, such as rice paddies, can lead to opportunistic infections through cuts or breaks in the skin.  In contrast, B. mallei has been identified as a clone of B. pseudomallei that has lost the ability to persist in the environment.   B. mallei is maintained within soliped hosts, including horses, donkeys, and mules with either chronic or acute disease presentation.  While B. mallei has been eradicated from equine populations in many developed countries, disease is still endemic in parts of South America, the Middle East, Asia, and Africa.  Humans are opportunistically infected by B. mallei following interactions with infected horses which release bacteria in nasal discharges (glanders) or spread the bacteria found in skin lesions (farcy).

The progress of disease and clinical risk factors are very similar for B. pseudomallei and B. mallei, although more reports of the clinical description of disease exist for melioidosis than for glanders.  Risk factors for disease include diabetes, renal issues, severe alcoholism, and thalassemia, although under certain circumstances healthy adults may become infected, possibly due to factors of dose, strain virulence, and route of infection.  The primary routes of infection are percutaneous inoculation and inhalation, although incidents of infection by ingestion, sexual contact, and mother-to-fetus transmission have been reported.  Regardless of the route of infection, susceptible hosts develop a septicemic disease affecting all major organs.  Computerized tomography has revealed that pathologies are most commonly associated with the lung (50% of cases) with frequent involvement of the spleen and liver.

B. pseudomallei and B. mallei are emerging pathogens in the USA, with disease most commonly associated with travel to endemic areas.  Laboratory acquisition of disease is possible, but the highest risk is associated with clinical laboratories not prepared to handle the organisms.  Safe handling of B. pseudomallei and B. mallei requires high containment BSL-3 laboratories.  These pathogens have also been classified as Select Agents in several countries including the USA, as BSL-3 Burkholderia have the potential for malicious use in biowarfare.  As such, research to identify novel therapeutics and diagnostics is a high priority, particularly for Burkholderia respiratory disease models.

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