Prevalence and genomic characterization of Salmonella Weltevreden in commercial pig feed

Viet Nam is the world's fifth largest producer of pork meat. Salmonella is frequently found at farm level; however, risk factors for Salmonella infection in pigs have not been thoroughly investigated in the production system. In the current study, 123 commercial feed samples were obtained from 103 small, medium and large-scale pig farms in Viet Nam and investigated for the presence of Salmonella in 25 g of feed using the ISO 6579:2002/Cor 1:2004 method. Salmonella was detected in five samples (4.1%; 95% CI 1.75-9.16%). All five samples were found to contain S. Weltevreden as the only serovar. The isolates were subjected to phenotypic and whole genome sequencing analysis for further characterization. They all belonged to ST365 and were sensitive to the 14 antimicrobials tested. Four strains were found to belong to the continental lineage of S. Weltevreden, while one isolate was of the island type. This isolate, contrary to the remaining four isolates contained a prophage homolog to the Vibrio prophage X-29. The findings of only S. Weltevreden, which is often isolated from fish and aquatic samples, suggests that fishmeal used in the feed preparation was a likely source of contamination.