Glanders and melioidosis
Description and importance of the disease: Glanders is a contagious and fatal disease of horses, donkeys, and mules, caused by infection with the bacterium Burkholderia mallei. The pathogen causes nodules and ulcerations in the upper respiratory tract and lungs. A skin form also occurs, known as ‘farcy’. Melioidosis is an infectious disease caused by Burkholderia pseudomallei in humans and animals and sometimes resembles glanders in horses. This chapter focuses on the disease in horses. Burkholderia mallei has evolved from B. pseudomallei by reduction of genetic information and is phylogenetically considered as a clone, i.e. a pathovar of B. pseudomallei. Control of glanders and melioidosis requires testing of suspect clinical cases, screening of apparently normal equids, and elimination of reactors. Stable hygiene and manure management are imperative. As B. mallei and B. pseudomallei can be transmitted to humans, all infected or contaminated (or potentially infected or contaminated) material must be handled in a laboratory with appropriate biosafety and biosecurity controls following a biorisk analysis. Identification of the agent: Smears from fresh material containing B. mallei bacteria may reveal Gram-negative nonsporulating, nonencapsulated rods. Burkholderia mallei grows aerobically and prefers media that contain glycerol. Standard media for isolation of B. pseudomallei can be used and selective enrichment techniques have been developed. The presence of a capsule-like cover has been demonstrated by electron microscopy in both agents. Unlike the Pseudomonas species and the closely related bacterium B. pseudomallei, B. mallei is nonmotile. For identification, biochemical phenotyping can be used. Commercially available biochemical identification kits lack diagnostic sensitivity. MALDI-TOF spectra have been made available for both agents in the past years. Whole genome sequences have been published. Specific monoclonal antibodies and polymerase chain reaction (PCR) as well as real-time PCR assays are available. Serological tests: Complement fixation is an accurate and reliable serological method for diagnostic use in glanders in equids. A rose bengal plate agglutination test for glanders has been developed. An immunoblot test based on a crude formalin preparation of B. mallei antigens from isolates of different geographical regions is also a sensitive and specific assay. These tests may also be positive in horses with melioidosis. Enzyme-linked immunosorbent assays show promise for specific diagnosis in equids once their validation is complete. Mallein test: The mallein test is a hypersensitivity skin test against B. mallei. The test is not generally recommended because of animal welfare concerns, however it can be useful in remote endemic areas where sample transport or proper cooling of samples is not possible. Mallein, a water soluble protein fraction of the organism, is injected intradermo-palpebrally. In infected animals, the eyelid swells markedly within 1–2 days. This test may also be positive in horses with melioidosis.