Epidemiology of Salmonella enterica related to swine production system and food safety

Salmonella infection is the second leading causes of foodborne illness in the U.S. There are over 2,500 Salmonella serovars and all are considered to cause salmonellosis in humans. Salmonellae are widely distributed in swine production systems in most parts of the world. Pork and pork products; therefore, have been considered as a major reservoir for this pathogen, and have a potential to contaminate the food supply. Although two of these serovars, Salmonella Choleraesuis and Salmonella Typhimurium, are considered important pathogens causing clinical disease in swine, a large number of Salmonella serotypes have been implicated as causing foodborne illness transmitted into the human food chain due to swine carrier state of other serovars.;The main objective of the dissertation was an attempt to evaluate control strategies, understand the epidemiology of Salmonella on swine farm and associated environment, and to better understand an epidemiology of a common predominant Salmonella serovars and phenotypes associated with human illness in the United States (Chapter 3, 5). In addition, antimicrobial resistance and diversity of drug-resistant Salmonella isolated from swine production systems with the use of different levels of antimicrobial drugs administrations were evaluated (Chapter 2). Distribution and clonality of Salmonella from the environmental sources, including waste disposal from swine farm and watershed systems in the national forest, have also been determined to track the source of the Salmonella contamination of the environment (Chapter 4).;Salmonellae are commonly associated with swine production systems in the United States. In general, Salmonella were recovered in fecal samples from 711 (37.4 %) of a total of 1,899 samples in 34 swine herds in the first study. Antimicrobials resistance to at least one drug was found in 98.5% of all Salmonella isolates and nearly 85% shown multidrugs resistant with the majority (∼30%) of pentaresistant pattern (R-type: AmClSuStTe). Five (∼0.7%) Salmonella isolates were detected resistant to the third generation cephalosporin ceftriaxone, and there was no Salmonella isolates resistant to the fluoroquinolone ciprofloxacin. Effect on the recovery of Salmonella resistant to third generation cephalosporin was not found to be correlated to the use of third generation cephalosporin.;The second part of the study focused on characterization of one of the predominant serovars, Heidelberg, commonly detected in pigs and humans. Multidrug resistance S. Heidelberg is an emerging public health issue worldwide. All Salmonella Heidelberg (n=95) isolates from swine were resistant to one or more of the antimicrobials tested and the majority showed multidrug resistance (MDR) to streptomycin, tetracycline, and kanamycin (R type: StTeKm) while 80% of the S. Heidelberg isolates of human origin were pansusceptible. Among the multidrug resistant S. Heidelberg isolates, Class 1 integrons with variable number and sizes of the gene cassettes were detected both from human and swine isolates, which carried a gene encoding for aminoglycoside adenyltransferase ( aadA). Dendrogram analysis of pulsotypes indicated possible clonality of Salmonella Heidelberg between isolates of human and swine origins.;The next study looked at the comparative significance of various environmental sources and the phenotypic and gentotypic relationship of Salmonella isolates. The distribution of Salmonella in the waste disposal lagoon system from a swine farm and various other watershed systems were included. A higher prevalence of Salmonella was detected from residential/industrial area (58.8%) followed by a national forest (57.1%). Fifty percent of the water samples from crop agriculture and 41.7% of the water samples from swine production were Salmonella also positive. Serotyping results revealed the diversity of Salmonella serovars with the majority being Anatum, Gaminara and Inverness and serovars of high public health significance including Muenchen and Newport were also detected. Antimicrobial resistance patterns of Salmonella isolated from environmental sites demonstrated the majority being pan-susceptible. Results of this study suggested that while all the various areas in the study could be important contributors of Salmonella enterica contamination, antimicrobial resistance appears to be of concern in swine production systems.;The finding of the intervention study demonstrated that when fed in low doses, sodium chlorate reduced Salmonella prevalence in piglets. Early weaning age (10 days of pig age) was associated with reduced shedding of Salmonella (lower concentration and prevalence) in fecal samples collected at 10 days postweaning (DPW). The protective effects persisted through the end of the study at 14 DPW. Disinfectant treatment also reduced shedding in fecal samples at 14 DPW. Chlorate treatment, topical disinfection, and younger weaning age may be useful tools for reducing Salmonella shedding on farms that practice segregated weaning and where sow-to-piglet transfer of Salmonella is an important source of infection in nursery pigs. This finding provides a potential practical approach for reducing Salmonella on-farm.