Salmonellosis Health Article

SALMONELLOSIS

Salmonellosis is a common enteric disease caused by rod-shaped, gram-negative bacteria. The name is derived from the American veterinary surgeon, Daniel A. Salmon, who described Salmonella choleraesuis as the cause of hog cholera in 1885. Since then over 2,200 Salmonella serotypes have been described; each is distinguished by its unique combination of cell wall, flagella, and capsular antigens. Many serotypes are further subdivided, usually for epidemiological studies, by their sensitivity to standard sets of bacteriophages (phage typing), and DNA fingerprinting methods. Salmonella are found in the intestinal tract of animals and birds, including domestic species (e.g., cattle, poultry), wild animals, and pets. Most human infections are caused by a few serotypes, commonly S. typhimurium and S. enteritidis. In most countries that keep national statistics, the majority of human cases are due to only five to ten common serotypes.

Salmonellosis is characterized by diarrhea, headache, abdominal pain, fever, and vomiting, beginning 6 to 72 hours (usually 6 to 36 hours) after infection. Healthy people normally recover within a week. Some individuals, however, are more susceptible to serious illness (see Table 1), and there is increasing evidence of longer-term sequelae occurring in a small proportion of cases.

Specific Salmonella serotypes are adapted to specific hosts, in which they usually cause septicaemia. For example, Salmonella typhi, is the cause of typhoid in man. Human infection is linked to a diverse variety of foods, possibly contaminated by animal or human feces during slaughter or during cultivation, harvesting, and preparation. Foods most commonly linked to illness include those of animal origin, such as meat products, unpasteurized milk, poultry, and eggs; foods contaminated during cultivation or preparation including vegetables, salads, fruit; and, less commonly, processed foods such as chocolate and snack products. Human infection has also been linked to exotic pets such as turtles, reptiles, and small mammals. People recovering from infection or with mild symptoms excrete salmonellae in their feces, and they may become a source of infection for others. Person-to-person spread is a particular risk where hygiene standards are difficult to maintain, as in institutions, day-care facilities, nursing homes, and households with ill individuals.

Most cases are apparently sporadic, though outbreaks occurring in the general population are not unusual and may be linked to a social event or institution such as a hospital or nursing home, or large-scale catering issues such as hotels, restaurants, and canteens. More rarely, large national and international outbreaks have been associated with manufactured or processed food products—in 1998 over 800 cases of S. enteritidis in Canada were associated with a pre-packed lunch product. Probably the largest recorded Salmonella outbreak affected an estimated 185,000 individuals who drank improperly pasteurized milk in the United States in 1985. It is recognized that even in countries which keep national statistics most cases are not reported. For example, only an estimated 1

Table 1

percent of cases in the United States are reported, and the estimated morbidity and economic burden is high. Current public health concern centers around the emergence of multiple antibiotic resistant salmonellae, which make serious illness, such as blood infection, difficult to treat.

PAUL N. SOCKETT

(SEE ALSO: Food-Borne Diseases)