Genetic surveillance of endemic bovine Salmonella Infantis infection

BACKGROUND: Salmonella serovar Infantis is endemic in Finnish food-producing animals since the 1970s. The purpose of this study was to describe the molecular epidemiology of the infection in cattle during 1985–2005, to follow the persistence of the feed-related outbreak strain from 1995 in the cattle population, and to analyse the stability of XbaI-banding patterns in individual herds during long-lasting infections. METHODS: Salmonella Infantis isolates from 478 cattle herds (n = 588), covering 73% of the subclinically or clinically infected herds, were typed by pulsed-field gel electrophoresis (PFGE) using XbaI. DNA fragments larger than 125 kb were counted in PFGE types because of high plasmid background. Ribotyping and IS200-typing with BanI-digested DNA were done on 57 selected isolates. RESULTS: The isolates associated with the infection consisted of 51 PFGE types with genetic similarity (F value) between 0.58 and 0.95. From 1985 to 2003, the major type appeared on 68% of the farms. The three most common types, with F values of 0.90 to 0.95, accounted for 80% of the isolates. Only 17% of the isolates had F values below 0.80, and 1% below 0.70. Ribotyping and IS200-typing classified 89% of the analysed isolates into the major ribotype and IS200 type combination, and the rest fell into closely related types. Analysis of successive isolates from 142 herds revealed changes in XbaI-banding patterns in 21% of the herds with two analysed isolates and in 38% of the herds from which three or more isolates were analysed. The feed-related S. Infantis genotype from the 1995 outbreak had disappeared by 1999, at the time when the incidence of bovine salmonella, and S. Infantis in particular, strongly decreased. CONCLUSION: The study showed how genetic surveillance, as part of salmonella control, provides tools to follow the persistence of particular infections, and to assess the efficacy of control measures. Testing of several isolates from a herd in outbreak investigations is advisable, because minor changes in PFGE banding patterns frequently occur during long-lasting infections.