Factors affecting the development of respiratory disease complex in chickens.

Factors playing a part in the development of respiratory disease complex in chickens were investigated in a series of experiments. The experimental infection was produced by exposing chickens to Mycoplasma gallisepticum and the B1 vaccine strain of Newcastle disease virus and later exposing them to aerosols containing the O1:K1 serotype of Escherichia coli. Chickens became susceptible (pericarditis or death) to E. coli 8 days after mixed respiratory disease challenge. One day after respiratory disease challenge, lesions consisted of edema and infiltration with lymphoid cells and heterophils. At the time of susceptibility to E. coli, the lesions were strongly lymphoid with many dense follicular areas and very few heterophils. The incidence of pericarditis and death was similar when the concentration of bacteria in the aerosol inoculum ranged between 10(9)/ml and 10(5)/ml. At the time of maximum susceptibility to aerosol challenge, chickens were less susceptible to intravenously administered E. coli than were the uninfected controls. Resistance of chickens that had been selectively bred for a high (HA) or low (LA) antibody response to sheep erythrocytes was compared. HA chickens were more resistant to respiratory agents and less resistant to E. coli than LA line chickens. When the lines were exposed to respiratory disease followed by exposure to aerosols containing E. coli, the HA line had the lowest incidence of pericarditis and death.