Avian Encephalomyelitis in Layer Pullets Associated with Vaccination.

Avian encephalomyelitis (AE) was diagnosed in three flocks of leghorn layer pullets following AE vaccination. Ages of the birds were 11, 12, and 14 wk. The submissions came from three different companies located in two geographic areas of the Central Valley of California. The clinical signs included birds down on their legs, unilateral recumbency or sitting on their hocks, lethargy, reluctance to move, dehydration, unevenness in size, low weight, tremors of the head in a few birds, and mildly to moderately elevated mortality. The flocks had been vaccinated against fowl pox and AE with a combined product in the wing-web 2 wk prior to the onset of AE clinical signs. Histopathologic examination revealed lesions consistent with AE, including lymphocytic perivascular infiltration and neuronal central chromatolysis in the brain and spinal cord, as well as gliosis in the cerebellar molecular layer. The AE virus was detected by reverse-transcriptase PCR in the brain homogenate from three cases and peripheral nerves in one case. Additionally, the AE virus was isolated in specific-pathogen-free (SPF) embryonated eggs from brain tissue pool samples. Other avian viral infections capable of causing encephalitis, including avian paramyxoviruses, avian influenza virus (AIV), West Nile virus (WNV), eastern equine encephalitis virus (EEEV), and western equine encephalitis virus (WEEV), were ruled out by attempting virus isolation and molecular procedures.