Three swine production units in Illinois were examined with respect to the role of purchased gilts in introducing new genetic variants of Salmonella into a herd, and the risk of incoming gilts acquiring Salmonella infection from resident pigs. On each farm a cohort of at least 100 gilts was followed from day of arrival until the time of introduction into the breeding section of the herd. Fecal and floor samples were obtained from the gilts and from resident pigs in adjacent pens or rooms after each movement of the cohort gilts to a new location, and cultured for Salmonella. Polymerase chain reaction (PCR) techniques were used to detect and genetically classify Salmonella isolates to identify transmission of the pathogen.
Salmonella was detected at varying times and locations in the production systems examined, although only a small percentage of pigs (usually less than 10%) were shedding Salmonella at any time. The close genetic similarity of isolates obtained from samples collected within a room or building on a given farm visit indicates that there was limited mixing of Salmonella from different sources on the farm, that is, there is usually a low level of transmission among pigs within different locations on the farm. Close genetic relatedness of some isolates from gilt samples with some isolates from resident pig samples suggests that a small but detectable amount of transmission of Salmonella was occurring between gilts and resident pigs that resided in adjacent locations.
Whereas most Salmonella infection is subclinical and therefore rarely a problem in reducing productivity, the proliferation of virulent strains of Salmonella, some of which may have antibiotic resistance, is a serious concern for swine and human health. Separation of pigs from different sources through all stages of production, even in the breeding herd, as well as biosecurity measures restricting movement of personnel across sections of the herd with pigs from different sources, coupled with exclusive use of artificial insemination, is recommended as the optimal strategy to minimize transmission of different genetic variants of Salmonella across different sections of the herd.
