A longitudinal study of the Salmonella status on Ontario swine farms within the time period 2001-2006

In order to describe the farm-level Salmonella status, 113 Ontario swine farms were tested for Salmonella one to five times within the time period 2001-2006. During 422 visits, 6844 fecal samples were collected and cultured for Salmonella. Salmonella was recovered from 437 (6.38%) of the fecal samples, and 69 (61%) of the farms had at least one positive sample over the entire period of the study. Salmonella was not recovered on 11 farms of the 54 farms visited five times, nor from 7 of the 17 farms visited four times. On seven farms Salmonella was not recovered over the first four visits but were cultured on the fifth visit. The isolates belonged to 30 different serovars, and serogroup B and C1 were the most common serogroups. Salmonella Typhimurium (including var. Copenhagen) was the most common serovar recovered from 35.5% of the farms with DT104 as the most frequent phage type. Only 24% of the total random variance in prevalence of Salmonella was due to repeated measurement, while the variation in prevalence of Salmonella Typhimurium (including var. Copenhagen) and DT104 due to repeated measurement was 37% and 52% of total random variance, respectively. Although the observed trends may be partly attributed to the different culturing methods, different types of samples, and sampling strategies used in each year, it may also denote the dynamics of Salmonella as a bacterial population on swine farms. These findings indicate that monitoring over time may be useful to detect changes in Salmonella on swine farms.