COMPARATIVE LEUKOTOXICITIES OF PASTEURELLA-HAEMOLYTICA ISOLATES FROM DOMESTIC SHEEP AND FREE-RANGING BIGHORN SHEEP (OVIS-CANADENSIS)

Twenty-eight isolates of Pasteurella haemolytica from domestic sheep ( n = 14 isolates) and bighorn sheep (n = 14 isolates) were evaluated fo r leucotoxicity against peripheral blood neutrophils of bighorn sheep by adding bacterial culture supernatants to bighorn sheep neutrophils in vitro. Leukotoxic isolates of P. haemolytica, defined as causing >5 0% neutrophil death as measured by release of lactate dehydrogenase in to culture supernatants, were identified from eight of 14 domestic she ep isolates and from 0 of 14 bighorn sheep isolates. The in vitro assa y of isolates of P. haemolytica may provide a valid predictive measure of strain virulence of P. haemolytica, and of potential pneumonic epi sodes in bighorn sheep populations.