Abstract

Transient paralysis (TP) is a non-neoplastic neurological disorder in poultry caused by Marek’s Disease Virus (MDV). It is characterized by temporary ataxia and paresis, primarily due to inflammatory damage in the central nervous system (CNS) rather than neoplastic transformation. The present study reported an outbreak of MDV-induced TP in a commercial 80,000 Lohmann LSL-Lite layer flock aged 95 days in Alborz Province, Iran, where affected hens exhibited sudden-onset paralysis and ataxia, with those that survived recovering spontaneously within three to five days without intervention. Although the flock had been vaccinated against MDV at hatch, neurological signs appeared at 95 days of age. Cerebellar samples were collected from ten clinically affected hens, including five collected postmortem from deceased chickens and five from live chickens that were humanely euthanized. Real-time polymerase chain reaction (PCR) confirmed infection with MDV field strains through strong amplification of pp38-Vir (1) and pp38-Vir (3) probes. In contrast, the CVI988 vaccine strain was not detected in samples collected at 95 days post-vaccination, which raised concerns regarding the quality of the vaccine and the proper administration of vaccination protocols. Histopathological examination of cerebellar tissues from three hens revealed vasogenic edema, perivascular cuffing, vasculitis, and widespread inflammatory cell infiltration. No evidence of tumor formation, mitotic activity, or neoplastic lymphoid proliferation was observed. These findings helped to distinguish TP from classical neoplastic Marek’s Disease. The present study highlighted the need for early and accurate differentiation of TP from other neurological disorders, such as ionophore toxicity, botulism, and vitamin deficiencies, given its transient condition. The occurrence of TP in a vaccinated flock emphasizes the complexity of MDV pathogenesis, possible shortcomings in vaccine efficacy or its application practices, and the need for improved immunization strategies and biosecurity measures. Early molecular diagnostics and histopathological evaluation are essential for managing MDV-induced TP and mitigating its impact on poultry farms.