Prevalence, clustering, and risk factors for pathogens of public health significance in the Ontario swine industry

This thesis investigates prevalence, spatial clustering, and risk factors for swine influenza virus, Toxoplasma gondii, and Salmonella enterica in Ontario swine populations. Herds included in these studies were a part of four-year project initiated in 2001: the "Ontario Sentinel Swine Project". During that period, a subset of 112 farms representative of the Ontario swine industry was visited yearly for blood and fecal sample collection. The apparent pig-level H1N1 seroprevalence in 2001 was 61.1% and 24.3% in sows and finisher pigs, respectively. The apparent pig-level seroprevalence for H3N2 A/Swine/Colorado/1/77 and A/Swine/Texas/4199-2/98 in 2003 in sows was 0.6% and 0.7%, respectively. Apparent seroprevalence for Toxoplasma gondii in finisher pigs was 1.4%, 0.06%, and 0.25% in 2001, 2003, and 2004, respectively. Prevalence of Salmonella enterica shedding in 2004 was 11% at the pig-, and 46% at the herd level. The most frequently isolated serovar was S. Typhimurium, followed by S. Infantis. Sow herds positive for H1N1 subtype, A/Swine/Colorado/1/77 (H3N2) and A/Swine/Texas/4199-2/98 (H3N2) strains spatially clustered. The location of the spatial cluster of sow H1N1-positive sow herds was in an area of high pig-farm density. For H1N1-positivity in finisher pig herds, the risk factors were large herd size, and farm-density, and the protective factor was a separate site finishing pig facility. Herds of finishing pigs were more likely to be positive if the sow herd was positive, but 45% of finisher herds were negative despite a positive sow herd. The H1N1-positivity in sows was positively associated with parity, but there was also a significant between-farm random variation. Presence of cats in the barn increased the odds of T. gondii-seropositivity. A completely closed barn was identified as a protective management procedure for Salmonella enterica-positivity at different levels, while pelleted feed, disinfection and washing with cold water, and larger number of pigs per barn were identified as risk factors. Pig weight tended to be associated univariably with pigs shedding Salmonella. Recommendations regarding establishment of monitoring systems, sampling, study design of analytical observational studies, and statistical analysis as related to these three pathogens are given.