Avian pneumovirus infection in broiler chicks inoculated with Escherichia coli at different time intervals

The effects of dual infection of 1-day-old broiler chicks with a chicken isolate of avian pneumovirus (APV) and a pool of pathogenic Escherichia coli strains were studied by supraconjunctival application of the bacteria simultaneously with the virus, or at 4, 7 or 11 days afterwards. When the agents were given together, the clinical disease was significantly more severe than that caused by the virus alone, but when the bacterium was given later the signs were less severe. None of the infections resulted in swollen head syndrome by 32 days. All mixed infections caused moderate to severe congestion in the turbinates, when birds were examined at 32 days of age, at which time no such lesions were present in birds having been infected with APV alone. E. coli was isolated from almost 100% of birds with mixed infections, while rates of those given only E. coli isolation varied between 56 and 67%. Furthermore, E. coli colony counts were consistently higher from mixed infection groups. Virus persistence in the choanal cleft was slightly prolonged in birds with the simultaneous mixed infection. Although the pool of E. coli included O2, O78 and O18 serotypes, only those of the O2 serotype and a small number of untypable strains were re-isolated from selected mixed and single E. coli-infected groups. Mixed APV and E. coli infection did not affect APV enzyme-linked immunosorbent assay antibody titres at 21 or 32 days. Thus, experimental infection of broiler chicks with APV and E. coli, simultaneously or at intervals afterwards, demonstrated a synergistic effect between the two agents, but none of the infection protocols caused swollen head syndrome.