Histopathological and genetic characterisation of Marek's disease in Sebright chickens from Brazil

Abstract Marek's disease is a lymphoproliferative condition caused by Gallid alphaherpesvirus 2 (Marek's disease virus), affecting primarily birds of the order Galliformes. This study reports a case of Marek's disease in Sebright chickens in Rio de Janeiro, Brazil. Clinically, affected birds presented pelvic limb paralysis, anorexia and apathy. Postmortem examination revealed sciatic nerve enlargement and tumour lesions in visceral organs, including liver, spleen, kidneys and ovary. Histopathological evaluation demonstrated Type A and C lymphoid lesions in peripheral nerves and pleomorphic lymphocytic infiltration in various tissues. Polymerase chain reaction confirmed the presence of Marek's disease virus. Phylogenetic analyses of the Meq gene revealed close clustering of our strain (MSV UFF), other Brazilian strains and related international isolates, indicating potential regional circulation and evolutionary linkage. These findings underscore the persistence of virulent Marek's disease virus strains despite vaccination, and highlight the importance of integrating histopathology, molecular diagnostics and phylogenetic analysis for surveillance and control of Marek's disease.