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Avian encephalomyelitis following oral vaccination.

Outbreaks of clinical avian encephalomyelitis (AE) which developed after vaccination of 14-week-old birds by the oral route are reported. Mortality during weeks 2 to 5 following vaccination reached 2%. Experimental studies showed that, in contrast to popular opinion, AE vaccine virus 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 before AE vaccination. It is postulated that non-CAV-induced immunosuppression allowed vaccine virus to produce the severe lesions and deaths occurring during the disease outbreaks, and that in one case, Marek's disease virus was the immunosuppressive agent.