Effect of immunisation on the early influx of neutrophils during staphylococcal mastitis in ewes.

The neutrophil influx into mammary secretions was studied in unimmunised, and in systemically and locally immunised, lactating and non-lactating ewes experimentally infected with Staphylococcus aureus. Systemic immunisation was effected by subcutaneous injection of live bacteria or by intramuscular injection of killed bacteria in Freund's incomplete adjuvant. The initial inflammatory response to infection of the mammary glands of systemically immunised lactating ewes was at first comparable with that for unimmunised, lactating ewes. However by eight hours after challenge the leucocyte concentrations were lower in systemically immunised ewes. In non-lactating ewes immunised with live vaccine, higher leucocyte concentrations and higher proportions of neutrophils were recorded four hours after infection (hpi) than for unimmunised ewes or ewes immunised with killed vaccine. Prior local immunisation, by unilateral infusion of killed bacteria into mammary glands, enhanced the initial neutrophil influx in comparison to infection of unimmunised contralateral glands of non-lactating ewes by four hpi. For these animals the proportion of neutrophils in washings from immunised glands were significantly greater than for unimmunised glands four and six hpi. In two of three locally immunised lactating ewes there was a larger neutrophil influx into secretions of locally immunised glands than unimmunised glands six hpi. Neutrophils comprised more than 93 per cent of leucocytes from all infected glands by eight hpi. The results suggest that differences in the rate of influx of neutrophils into infected mammary glands of immunised and unimmunised ewes could be attributed to immunological enhancement of neutrophil recruitment or to limitation of toxic damage to tissues with consequently diminished neutrophil invasion.