Immunogenic potency of an oil-emulsified Escherichia coli bacterin.

Immunogenicity of an oil-emulsified Escherichia coli (O1:K1) bacterin with an aqueous-phase-to-oil-phase ratio of 1:4 was evaluated in chickens. Chickens were vaccinated subcutaneously with 0.5 ml of the bacterin at 4 and 6 weeks of age. At 8 weeks, the vaccinated chickens and unvaccinated controls were challenged via air sacs with 10(4) colony-forming units (CFU) of homologous E. coli. Vaccinated chickens were protected against active respiratory infection in that they (a) gained body weights comparable to those in unvaccinated, unchallenged chickens, (b) suffered no morbidity or mortality, (c) had gross lesions so mild that the scored values were comparable statistically to the 0 lesion scores of the negative controls, and (d) did not yield E. coli when their heart blood, pericardial sacs, livers, and air sacs were cultured. Unvaccinated challenged chickens had severe respiratory distress, suffered 36% mortality, and had average air sac, pericardial sac, and liver lesion scores significantly (P less than or equal to 0.05) different from both the vaccinated and negative control chickens. Also, the challenge strain of E. coli only was isolated from the affected tissues of 5 of 14 chickens. Protection against active respiratory infection was again demonstrated in a second experiment, though the challenge dose was 1.06 X 10(6) CFU of E. coli. The immunity, however, was partially overcome, as the vaccinated chickens gained less body weight and the scored values for lesions in the air sacs, pericardial sacs, and livers were significantly higher than those of the negative controls (P less than or equal to 0.05).