Avian encephalomyelitis (AE) is caused by a picornavirus that occurs worldwide and infects the central nervous system of chickens, turkeys, quails and pheasants. While in young chicks, AE is clinically apparent as paralysis, ataxia and muscular dystrophy, in layers and breeders the infection is usually subclinical and results in egg production decline. Vertical transmission, as well as contact-infection are important routes of AE virus dissemination. Control of the ailment is achieved by vaccination of the breeder flocks during rearing with a live vaccine.