The effect of inoculation of coagulase-negative staphylococci into the ovine mammary gland.

In each of two experiments, the effect of inoculation of coagulase-negative staphylococci (C-NS) into the ovine mammary gland was studied. In the first experiment, ewes were inoculated with one of six different C-NS to study differences in the pathogenicity of the isolates. In the second, ewes were inoculated with Staphylococcus simulans to study the sequential histopathological features of the infection. The existence of subclinical mastitis was established by consistent isolation of bacteria, the demonstration of high somatic cell counts in milk and an inflammatory reaction in biopsy samples of mammary tissue. A variation in the pathogenicity of the different isolates was evident: one Staph. chromogenes isolate caused clinical mastitis, four Staph. simulans isolates caused subclinical mastitis and one Staph. xylosus isolate caused a transient increase of somatic cell counts. Neutrophilic infiltration was the predominant histopathological finding. Lysis of neutrophils, destruction of alveoli and, in late stages of the infection, infiltration of lymphocytes and fibrous tissue proliferation were also noted.