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Epidemiology and Pathology

The disease was originally described in 1912 by Whitmore and Krishnaswami, based on their findings on autopsies of male paupers and morphine addicts in Rangoon; they initially believed that the disease was the result of syringe inoculation. The causative organism, Burkholderia pseudomallei, is a small motile aerobic gram-negative bacillus believed to be a saprophyte in nature (Fig. 23.2). Since its identification by Whitmore in 1913, it has been variously named Pfeifferella whitmori, Pfeifferella pseudomallei, Malleomyces pseudomallei, Bacillus whitmori, Loefflerella whitmori, Flavobacterium pseudomallei, Actinobacillus pseudomallei and, until recently, Pseudomonas pseudomallei. Morphologically, it bears a close resemblance to Burkholderia mallei, the bacillus which causes glanders (pulmonary infection) or farcy (cutaneous-lymphatic involvement) in horses and asses (hence the name, melioidosis, which means "glanders-like"). It can be differentiated from the latter bacillus by culture, as well as by the fact that B. pseudomallei is more motile. A related species of the genus Burkholderia, B. cepacia, which is a major plant pathogen, shares with B. pseudomallei a predilection for the lung and for patients with altered host defenses. B. cepacia has been reported as a cause of progressive lung disease in some patients with cystic fibrosis.

Fig. 23.2 Burkholderia pseudomallei. Note the tiny pleomorphic, gram-negative bacteria with bipolar staining, resembling closed safety pins.

Burkholderia pseudomallei has been cultivated from the rice-growing areas of Southeast Asia. The principal source of infection was once thought to be food contaminated by the excreta of infected rodents, but large surveys of rats in Malaysia (undertaken to detect plague) have shown only a few rodents infected with melioidosis. The disease has been successfully transmitted from one guinea pig to another by the bite of the mosquito (Aedes aegypti) and the rat flea (Xenopsylla cheopis). The organism, B. pseudomallei, can cause epizootics in such animals as sheep, goats, cattle, cats, pigs, dogs, wallabies, monkeys, horses, and even dolphins.

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