Coagulase-positive staphylococcal mastitis in a herd with low somatic cell counts.

An increase in clinical mastitis infections was observed in a high-producing 77-cow Holstein herd. Low bulk tank somatic cell counts and individual cow Dairy Herd Improvement Association somatic cell counts observed before, during, and after the epizootic were suggestive of herd environmental mastitis. However, bacteriologic culture survey of the total herd indicated that, in addition to infections possibly attributable to environmental pathogens, 22% (17/77) of the cows were infected with coagulase-positive Staphylococcus spp. Conceivably, investigative efforts and management changes, without bacteriologic culturing, might have resulted in reduction of the clinical infection rate in this herd. However, the continued high prevalence of a contagious pathogen with potential future implications would have gone unnoticed. Somatic cell count in milk from individual cows generally is a useful tool for monitoring the probability of intramammary infection, but must be complemented with bacteriologic culture of milk to determine whether contagious or environmental pathogens are responsible.