Prevalence, antimicrobial resistance and risk factors for Salmonella in 90 Alberta swine finishing farms

This thesis describes an investigation of the epidemiology of Salmonella infection in 90 Alberta swine finishing farms. The farms were visited three times, approximately one month apart. Thirty blood, 15 fecal and 5 farm-environment samples were collected from each farm. Farm management data were collected using a questionnaire. Conventional culture and VetScreen(TM) Salmonella covalent-mix ELISA were employed. The presumptive Salmonella-isolates were confirmed by serotyping/phagetyping, and tested for susceptibility to 18 antimicrobials.; The apparent Salmonella culture and serological prevalence were calculated at the sample/farm level. Among environmental samples, Salmonella were most frequently recovered from boots (38.6%) and the main drain (31.8%). At the farm-level Salmonella status varied frequently across three visits. The correlation between fecal and seroprevalence at the farm and farm visit level was 0.71 and 0.47, respectively. Serovars Typhimurium (78 isolates), Derby (71 isolates) and Infantis (47 isolates) were the commonest. Resistance to chloramphenicol (4.7%), ampicillin (7.8%), kanamycin (11.8%), sulfamethoxazole (21.1%), streptomycin (25.5%) and tetracycline (38.8%) was observed. None or less than 1% of resistance were observed to the remaining 12 antimicrobials. The most frequently resistant serovar was Derby, with 38% isolates resistant to ≥3 antimicrobials, including resistance to ≥5 antimicrobials among some isolates.; Associations among reported farm antimicrobial use, management practices and Salmonella status were assessed. Use of pelleted feed was significantly associated with an increased Salmonella shedding and sero-positivity. Housing in finishing barns from multi-site operations or individual grow-to-finish farms was associated with a higher risk for Salmonella shedding. Reported farm antimicrobial use was not significantly associated with Salmonella farm-shedding and sero-positivity, or had a modest protective effect.; The within-farm fecal and serological prevalence of ≤20% observed on 66.6% and 77.7% of the farms, respectively, and the absence of resistance to cephalosporins and fluoroquinolones are encouraging animal and public health findings. The results of this study show that longitudinal sampling/testing is required to properly evaluate the Salmonella farm status. The interpretation of existing tests for Salmonella in swine should reckon their imperfect sensitivity, their measurement targets, and the Salmonella serovar profile of the targeted population. Further research is warranted to better understand on-farm risk factors for Salmonella.