Etiology

Salmonella as a generic category of bacteria is present worldwide. There are more than 2400 serovars currently known. Salmonella can cause disease in humans by contact with contaminated animal feces and/or animal products; there are at least 42 distinct serovars associated with broiler chickens, 19 with beef cattle, 29 with turkeys, and 23 with swine.

Common animal serovars include S. typhimurium, S. Kentucky, S. Heidelburg, S. Anatum, and S. Hader.

In a year-long study done, between 1998 and 1999 by the United States Food Safety Inspection Service, 14% of the broilers positive for salmonella were of the typhimurium serotype, 9.5% of beef cattle, 3.8% of turkeys, and 23% of swine.

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The strain of typhimurium known as DT104 is a particular focus of control efforts. CDR (Center for Disease Research, in the United Kingdom) reported that in 1998, 95% of isolates in human cases were DT104; in addition, these isolates also showed at least some drug resistance. In the last decade, the rise of mrDT104 (a multi-drug-resistant set of strains) has emerged as a worldwide issue; worse, resistance is to most of the drugs traditionally used to treat salmonellosis (aminoglycosides, tetracyclines, sulfonamides, and trimethoprin).
According to a report in the Journal of the American Veterinary Medical Association, in 1998 the incidence of salmonella infection in the United States was estimated to be 13.8 of 100,000 individuals.   In a study done in England and Wales, the incidence of infections with strain DT104 alone rose from 259 cases in 1990 to 4006 cases in 1996.

Salmonella DT104 was first identified in 1984 from a human in the UK. According to CDC reports, in 1986, it had not been cultured in the US, but by 1991, 13% of the Salmonella found was typhimurium species, and 1998, 16% of the identified S. typhimurium strains were mrDT104.

Worldwide, the strain remains nearly identical, promoting the idea that the bacterium is spread by a rapidly migrating species, such as humans or birds. However, in Brazil, S. agona remains the main isolate affecting humans, and in one study documenting the occurrence of different Salmonella strains, S. typhimurium was not among the eight serotypes identified.

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