Pathology and Distribution of Velogenic Viscerotropic Newcastle Disease Virus in the Reproductive System of Vaccinated and Unvaccinated Laying Hens (Gallus gallus domesticus) by Immunohistochemical Labelling

This study investigated the pathological changes in the reproductive system of laying hens that lead to the poor egg production and quality in Newcastle disease (ND) and the distribution of the virus in the system. Two hundred and forty Isa-Brown pullets were divided randomly into vaccinated and unvaccinated groups (n = 120 each). The vaccinated group was given Hitchner B1 vaccine at 1 day of age, La Sota vaccine at 4 weeks of age and Komarov vaccine at 9 and 16 weeks of age. At the peak of egg production, the laying hens (32 weeks old) were assigned randomly into four groups (n = 60): VC, vaccinated with ND vaccines and inoculated intramuscularly with velogenic viscerotropic ND virus (vvNDV); VU, vaccinated unchallenged; UC, unvaccinated challenged; and UU, unvaccinated unchallenged. UC hens showed depression, diarrhoea and later torticollis. Mortality in UC hens was 90%. VC hens showed mild anorexia. The body weights of the UC hens were significantly (P <0.05) lower than those of UU hens. VC and UC hens showed a significant (P <0.05) drop in egg production. Only UC hens produced abnormal eggs and initially had swollen, oedematous, hyperaemic oviducts followed by atrophy and shortening of the reproductive tract with atresia of the ovarian follicles. The histopathological changes were of necrosis of the epithelium and secretory glands. VC hens showed mild inflammatory changes in the oviduct. Immunohistochemical labelling showed extensive presence of the virus in the ovary, infundibulum, magnum, isthmus, uterus and vagina of UC hens and in the ovary of VC hens. These changes will be the cause of serious egg production problems, especially in vaccinated layers in countries where vvNDV is enzootic.