The genus Salmonella comprises commensal and pathogenic bacteria found in humans, mammals, reptiles, birds, and insects worldwide. These gram-negative, largely motile bacilli are highly adaptable facultative anaerobes 2-3 µm long that reside mainly in the intestines of their hosts. Salmonellae are classified in two species, S. enterica and S. bongori; the former is divided into six subspecies and more than 2,500 serotypes (or serovars) according to their somatic, surface, and flagellar antigens and their habitats (Box 5.1) (Center for Infectious Disease Research and Policy 2006; Pegues et al. 2005).
Classification of Salmonella
subspecies enterica (I)
subspecies salmae (II)
subspecies arizonae (IIIa)
subspecies diarizonae (IIIb)
subspecies houtenae (IV)
subspecies indica (VI)
SOURCE: Pegues et al. 2005.
Salmonella enterica serotypes Typhi and Paratyphi cause life-threatening typhoid fever and paratyphoid fever (typhoidal salmonellosis), respectively. Those diseases’ severity, short incubation period, and other salient characteristics would lead to rapid detection, diagnosis, and treatment in deployed US military personnel (CDC 2005b; Olsen et al. 2003). In contrast, uncomplicated infection with nontyphoidal salmonellae causes an array of generally milder illnesses that appear similar to other diarrheal diseases and usually resolve without medical attention. Therefore, the committee devotes attention exclusively to infection with nontyphoidal salmonellae in this chapter.
Nontyphoidal salmonellae are most commonly transmitted by the ingestion of contaminated food, especially food of animal origin. Food derived from infected animals that is uncooked, inadequately cooked, unpasteurized, or inadequately pasteurized may transmit the bacteria to humans. Alternatively, such products may cross-contaminate other food that then becomes a vehicle for transmission. Outbreaks of salmonellosis also have arisen from the consumption of fresh produce contaminated with human or animal feces containing salmonellae (Pegues et al. 2005).
Drinking contaminated water infrequently leads to transmission of nontyphoidal salmonellae to humans (Pegues et al. 2005). Exposure to salmonella-infected pets, especially reptiles, can lead to transmission to humans. Rarely, transmission occurs through the transfusion of tainted blood products (Wilson 1991).