Atypical Staphylococcal Mastitis in a Dairy Herd

A herd of cattle with a history of increased prevalence of clinical and nonclinical mastitis was investigated. Bacteriologic analysis of milk samples indicated approximately 50% of the herd was producing milk containing coagulase-positive staphylococci. Of these staphylococcal isolates, 55% had characteristics consistent with those of human strains of staphylococci, based on hemolysin production and phage patterns. Human beings in contact with the herd were nasal carriers of these staphylococci, which produced a granulartype coagulase reaction in bovine plasma, rather than the usually expected clot-type reaction. In the herd, the staphylococci caused mainly nonclinical mastitis, which was largely unresponsive to antibiotic therapy.