Dairy herd problem with mastitis caused by a rapidly growing Mycobacterium species

Intramammary infections by a nonspeciated Mycobacterium of Runyon group IV increased in the dairy herd at the Beltsville, Md, Agricultural Research Center during 4 years, and were cultured from 9.7% of 300 quarter samples from clinical mastitis cases in 1982. The infection was mildly acute, without systemic involvement, and refractory to therapy but self-terminating, usually at the end of lactation. Invasion of the mammary gland commonly followed stress, either calving or previous clinical mastitis, and was most frequent in August and September. Transmission of this mycobacterial intramammary infection from cow to cow was far more likely in free-stall barns than in tie stalls, and seemed not to be associated with machine milking.