Immunity against Escherichia coli infection in chickens assessed by viable bacterial counts in internal organs

Septicemic strains of Escherichia coli cause systemic infection in chickens after the intra-airsac inoculation. We have investigated whether levels of immunity can be determined by the viable organism count in the internal organs of infected birds. The intra-airsac inoculation of O1:K1 strain caused acute systemic infection in 6 hr. The viable count was highest in the lung followed by the liver, spleen, and blood. The count was significantly (P < 0.05) lower in the liver or spleen of vaccinated birds at 6, 12, or 24 hr after inoculation than in controls. Vaccines containing various adjuvants were tested in this system, and three oil-based adjuvants demonstrated significant (P < 0.05) immunity, whereas an alum-precipitated vaccine or one without an adjuvant failed to do so compared with nonvaccinated controls. An oil-adjuvanted vaccine showed some deterioration in its immunogenicity after prolonged storage or heating at 100 C. The acute phase response induced by intravenous injection of killed O1: K1 cells or lipopolysaccharide purified from Salmonella typhimurium in aqueous suspension induced significant immunity against the E. coli infection. These results indicate that the method referred to as "in vivo viable count method" produces quantitative results in a reproducible manner and suggest that it may be used as an alternative method to mortality measurement.