AVIAN ENCEPHALOMYELITIS FOLLOWING ORAL VACCINATION

Outbreaks of clinical avian encephalomyelitis (AE) which developed aft er vaccination of 14-week-old birds by the oral route are reported. Mo rtality during weeks 2 to 5 following vaccination reached 2%. Experime ntal studies showed that, in contrast to popular opinion, AE vaccine v irus given orally can spread to the central nervous system and produce encephalomyelitis (mild). The severity of the vaccine induced lesions was not affected by chicken anaemia virus (CAV) infection 2 weeks bef ore AE vaccination. It is postulated that non-CAV-induced immunosuppre ssion allowed vaccine virus to produce the severe lesions and deaths o ccurring during the disease outbreaks, and that in one case, Marek's d isease virus was the immunosuppressive agent.