Pathology of Marek’s Disease in Layer Hens at Selected Commercial Poultry Farms in Ogun State, South West Nigeria

Marek's disease (MD), a lymphoproliferative disease of chickens caused by Marek's disease virus (MDV), continues to be a major cause of mortality and economic losses in commercial layer operations across Nigeria's South West zone. This study examined the gross and histopathological features of Marek's disease in layer hens from five selected commercial farms in Ogun State, from March to October 2023. Thirty-eight layer hens showing progressive leg paresis, weight loss, and mortality were submitted for necropsy. Tissues from peripheral nerves, visceral organs including spleen, liver, ovary, proventriculus, as well as the brain were processed for histopathological evaluation. Gross findings included asymmetric enlargement of brachial and sciatic nerve trunks with loss of cross-striations, presence of tumour nodules in the ovary, liver, spleen, and proventriculus, and grey discolouration of the iris. Microscopically, affected nerves showed infiltration by a pleomorphic population of lymphocytes, plasma cells, and large lymphoblastoid cells with mitotic figures, consistent with type B neuropathology. Visceral organ sections revealed infiltrating pleomorphic lymphoid cells with features consistent with lymphoma. These findings confirm MD as a significant cause of mortality in vaccinated flocks and suggest field virus variants may be overcoming vaccine-induced immunity. Improved cold chain for vaccine storage and hermetic poultry house biosecurity are recommended. Keywords: Marek's disease, lymphoma, layer hens, neuropathology, Ogun