Animal sources of salmonellosis in humans.

N ontyphoidal salmonellosis is one of the leading causes of acute bacterial gastroenteritis in the United States, responsible for an estimated 1.4 million cases of illness annually. 1 Many animals, both domestic and wild, are colonized by Salmonella spp, usually harboring the bacteria in their gastrointestinal tracts with no apparent signs of illness. Hence, salmonellae are often present in feces excreted by healthy animals and frequently contaminate raw foods of animal origin through fecal contact during production and slaughter. Although the genus Salmonella consists of more than 2,400 serovars, most human cases of salmonel-losis in the United States are caused by 4 serovars. For example, in 1995 the US Centers for Disease Control and Prevention (CDC) reported that approximately 60% of human cases were caused by Salmonella enteri-2 These same 4 serovars represented 46.4% of the isolates from nonhuman sources that year. Two major changes occurred in the United States during the past 2 decades in the epidemiologic characteristics of nontyphoidal salmonellosis. These were the evolution of 2 pandemic serovars, S ser Enteritidis and S ser Typhimurium DT104, that have caused marked increases in the percentage of foodborne human Salmonella infections. Salmonella ser Enteritidis infections are largely associated with fresh shell eggs and egg products, in which the bacteria contaminate the interior contents of the egg through transovarian transmission. Salmonella ser Enteritidis infects the ova or oviduct of the hen' s reproductive tract, which leads to contamination of the vitelline membrane, albumen, and possibly the yolk. Internal contamination of the egg' s content renders egg-sanitizing practices, which focus on reducing pathogen contamination on the eggshell surface, ineffective. Salmonella ser Typhimurium definitive phage type DT104 emerged in the early 1990s as the dominant type of Salmonella spp. Most isolates possess chromo-somally encoded resistance to 5 antimicrobials, specifically ampicillin, chloramphenicol, streptomycin, sul-fonamides, and tetracycline (R-type ACSSuT). There is evidence that some penta-resistant DT104 strains are also acquiring resistance to quinolones and trimethoprim. 3 Evidence in Europe suggests that the emergence of DT104 in cattle was the precursor to its spread to other animals used for food production. 4 Although DT104 is presently the dominant penta-resistant clone of S ser Typhimurium, many other phage types (DT29, DT204, DT193, and DT204C) of this serovar have also been associated with multidrug resistance. 5 Understanding the factors that influence the emergence of these dominant serovars of Salmonella spp and the factors contributing to the spread and …