Assessment of immunity to Pasteurella haemolytica in sheep by in vitro methods

The sera of conventionally-reared sheep which had been experimentally infected with Pasteurella haemolytica serotype A2 and Mycoplasma ovipneumoniae were compared with those of animals from the same source which had been vaccinated 3 times with an alhydrogel-adsorbed mixture of cell-wall extract and heat-killed cells of P. haemolytica A2 emulsified in Freund's incomplete adjuvant. Both groups of animals had high titres of anti-P. haemolytica A2 IgG by ELISA, high opsonic indices in a phagocytic assay involving ovine lung macrophages, and reacted with a large number of cell-wall antigens in an immunoblot test. Both types of sera also protected mice against challenge with P. haemolytica A2 in mucin. In contrast, sera from animals injected twice with a vaccine similar to the first but lacking oil adjuvant yielded much lower values in the ELISA, opsonic index and immunoblotting assays, and failed to protect mice against challenge. The correlations between the 3 in vitro tests and the passive protection test in mice suggest that evaluation of the efficacy of P. haemolytica vaccines may be possible by in vitro methods alone.