Prevalence of Fecal Salmonella Shedding by Cull Dairy Cattle Marketed in Washington State

On nine occasions over a 1-year period, cull dairy cattle (n = 1,289) at four saleyards and one abattoir in Washington State were surveyed for salmonellae shedding by bacterial culture of duplicate rectal swabs, 251 single fecal samples and duplicate rectal swabs, and 225 mesenteric lymph node and duplicate rectal swabs. Using parallel selective enrichment and brilliant green media, salmonellae were isolated from six cattle, from rectal swabs only, and consisted of five isolates of Salmonella typhimurium and one of Salmonella dublin . In the two rectal swab- positive cattle for which mesenteric nodes were also sampled, 1-g samples of the nodes were negative. The rate of fecal shedding of cull dairy cattle marketed in Washington State as detected by this methodology is estimated to be 4.6 per 1,000 head (95% confidence interval of 1.9 to 10.6) and is expected to be no higher than 9.2 per 1,000 head if larger fecal samples were used. Based on antibiograms and plasmid profiles, none of the six isolates matched any of the 280 previously characterized isolates of the same serotypes obtained from human salmonellosis cases 2 years previously by the State health department. Four of the five S. typhimurium isolates matched three of 215 S. typhimurium isolates obtained from bovine submissions to the State's animal disease diagnostic laboratory and by a field animal disease investigation unit. The S. dublin isolate matched 17 of the 165 S. dublin isolates in those submissions. In this State, swab sampling of cull dairy cows at the point of first market concentration does not appear to be an efficient method of detecting salmonellae- infected dairy herds.