Persistence of S. typhimurium in a large dairy herd.

Salmonella typhimurium 49a infection in a large dairy herd persisted for 3.5 years. Illness initially occurred in cows and calves but latterly although there were fewer clinical cases milk filters were culturally positive on 26 out of 73 samplings. Three associated human disease incidents occurred. Individual milk samples identified one cow as an excreter and the organism was recovered from the mammary gland of this animal at slaughter. Correlation between calving pattern, the times of calving and the occurrence of positive milk filters suggest that the cow may have been excreting the organism intermittently from the udder for 2.5 years.