Importance of Breeding Pigs in the Spread of Salmonella on Farms

Background: Pigs infected with Salmonella represent a high risk for contamination and cross-contamination of carcasses with Salmonella from feces, tonsils and mesenteric lymph nodes, which occurs on the slaughter line. The environment itself represents a potential source of infection, as it is Salmonella infection of other individuals in the herd (sows, boars, piglets, fattener pigs). One of the leading sources of infection is contaminated feed for pigs, or its ingredients. If an infection occurs at any stage and category of production, those individual animals can become a potential source of infection for all others on farm. Materials, Methods & Results: This study examines the importance of breeding categories of pigs in the spread of salmonellosis in piglets and fattener pigs on seven farrow-to-finish farms in Serbia. From each farm, 30 sows, 121 boars and 90 piglets were tested from the weaning to the time of dispatch to the slaughterhouse. Pigs were examined for the presence of Salmonella-specific antibodies in blood sera and the presence of Salmonella in feces. Pigs feed on the farms were also examined for the presence of Salmonella. The identification of serotypes was conducted according to the Kauffmann-White scheme. For antimicrobial sensitivity and resistance testing, isolates were tested by disc diffusion against a panel of 11 antimicrobials. The numbers of pigs with at least one positive faecal isolation and with at least one positive serological result were evaluated descriptively. The McNemar test was used to compare the number of shedding pigs and the number of seropositive pigs at different time points. The statistical significance of differences between means was at the level of P < 0.05. Overall, 60.0% of sows were seropositive, and this was significantly higher (P < 0.05) than the seropositivity of the boars. In addition, isolation of Salmonella from feces of sows was more common than from boars. Seroprevalence ranged from 0 to 90% in fattener pigs. Salmonella Typhimurium and Salmonella Derby were the most prevalent serovars, representing 69.77% and 19.77% of isolates serotyped, respectively. Antimicrobial sensitivity and resistance testing was carried out on 88 Salmonella isolates. Discussion: As it is well known that pigs are get easily infected orally with Salmonella and contaminated environments such as feed and litter direct contact between pigs are thought the main source of infection. When compared to higher number of seropositive animals, low number of Salmonella positive feed samples and feces positive animals show the lower correlation between the presence of Salmonella antibodies and Salmonella isolation in the current study. In addition, results obtained from this study shows there may be low correlation between serology and culture test results and low number of culture positivity can originate from latently infected animals or lower sensitivity of the culture methods. On farrow-to-finish farms in Serbia, because sows, boars and fatteners are kept together on the same farm, and for that reason sows and boars may play an important role in the transmission of Salmonella. Among the positive samples, Salmonella Typhimurium and Salmonella Derby represented 89.54% of all isolates. The percentage of Salmonella isolates sensitive to amoxicillin and ceftriaxone exceeded 75%, and sensitive to penicillin was seen in 58%. The antimicrobials showing resistance in this study are similar to those in reports from other authors, in particular the high rate of resistance to tetracyclines, streptomycin, neomycin and doxycycline. The results show that the farms with high prevalences of Salmonella in sows also had a higher prevalence of Salmonella in fattener pigs; this would lead to higher risk of contamination of carcasses. Sows may spend up to 10 years on farms and can be a continuous source of Salmonella infection.