Use of multiple‐locus variable‐number tandem‐repeats analysis (MLVA) typing to characterize Salmonella Typhimurium DT41 broiler breeder infections

Aims: To characterize isolates of Salmonella Typhimurium DT41 obtained from infected flocks of broiler breeders by multiple‐locus variable‐number tandem‐repeats analysis (MLVA) and compare results with a diverse strain collection from Germany and United Kingdom and isolates from Danish patients. Methods and Results: A total of 102 isolates of Salm. Typhimurium phage type DT41 were MLVA typed. MLVA typing showed 4, 12, 25, 9 and 8 different alleles at the five MLVA loci 9, 5, 6, 10 and 3, respectively. A dendrogram based on MLVA types was constructed, and one large group, nine minor groups and 29 more unrelated MLVA types were obtained. The major group included 20 of the 30 human isolates. Isolates obtained from broiler breeders demonstrated major diversity, indicating the existence of several independent introductions of DT41 at farm level. When comparison was made to isolates included from Germany and England, DT41 seems to be ubiquitous in the wild fauna which might represent a risk factor for poultry. Conclusions: Transmission from Danish broilers to humans was not demonstrated, neither was the transmission from rearing farms to broiler breeder farms. Sources of infection at broiler breeder farm level remained unidentified. Significance and Impact of the Study: Major diversity was demonstrated for DT41 MLVA types. A persisting problem with infection of broiler breeder flocks with DT41 was not reflected in broiler flocks originating from these flocks.