Outbreak of mastitis caused by one strain of Staphylococcus aureus in a closed dairy herd.

The Washington State University dairy experienced an outbreak of intramammary infections (IMI) caused by Staphylococcus aureus during autumn 1993 through summer 1995. The outbreak was believed to be a result of transmission of 1 strain of S aureus in a herd that historically had excellent control of contagious mastitis. Control practices included strict hygiene at time of milking and preferential culling of cows infected with S aureus. Mastitis caused by Streptococcus agalactiae was not found in this herd. Despite excellent control practices, the strain of S aureus caused a new infection rate of approximately 3% of the herd per month. Moreover, a second strain of S aureus, isolated from a cow with mastitis, was introduced into the herd experimentally, and it failed to transmit disease. The outbreak of S aureus mastitis in this herd was eventually controlled by maintaining a program of strict milking time hygiene, by intensifying the program of preferentially culling infected cows, and by segregating cows with S aureus IMI in a separate pen and milking these infected cows last.