Immunogenic Potency of an Oil-Emulsified Escherichia coli Bacterin

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Abstract
Immunogenicity of an oil-emulsified E. coli (O1:K1) bacterin with an aqueous phase-to-oil phase ratio of 1:4 was evaluated in chickens. Chickens were vaccinated s.c. with 0.5 ml of the bacterin at 4 and 6 wk of age. At 8 wk, the vaccinated chickens and unvaccinated controls were challenged via air sacs with 104 colony-forming units (CFU) of homologous E. coli. Vaccinated chickens were protected against active respiratory infection in that they gained body weights comparable to those in unvaccinated, unchallenged chickens; suffered no morbidity or mortality; had gross lesions so mild that the scored values were comparable statistically to the 0 lesion scores of the negative controls; and 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 .ltoreq. 0.05) different from the vaccinated and negative control chickens. 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 2nd experiment, although the challenge dose was 1.06 .times. 106 CFU of E. coli. The immunity 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 .ltoreq. 0.05).