Marek's disease in backyard chickens of Pakistan: A study of pathological lesions in correlation to viral load

Abstract

Marek's disease (MD) is an infectious, lymphoproliferative disorder caused by the alpha herpesvirus genus, Mardivirus, serotype 1 (Gallid Herpesvirus 2, or GaHV-2). There is no current information on Marek's disease virus (MDV) pathotypes and serotypes circulating in backyard poultry in Pakistan. This study aimed to characterize GaHV-2 in the backyard poultry of Rawalpindi division based on glycoprotein C and to investigate the correlation of a viral oncogene (meq) with the histological, lymphoproliferative infiltration in the visceral organs of naturally-infected birds. The study was performed from May 2019 to April 2020 in the Rawalpindi, Jhelum, Chakwal, and Attock districts of the Rawalpindi division. An overall 13.96% prevalence of MDV infection based on gross morphology and 18.43% using polymerase chain reaction (PCR) was recorded. The Pakistani GaHV-2 isolates could be divided into two clades, based on sequencing and phylogenetic analysis of the gC gene, closely related to the US virulent isolates and mildly virulent vaccine strain, CVI988, respectively. Of total PCR-positive cases, 25 (75.76%) samples revealed gross tumours. Median histopathological scores of lymphoproliferative infiltrations in the spleen and liver tissues were recorded as moderate (++), whereas in the heart and gonads, mild (+) changes were observed. The mean viral load quantified using real-time PCR of 14.72 copies of meq gene/β-actin revealed a strong (R² = 0.88) correlation with histopathology scores of spleen tissues. Differences in the meq gene/β-actin ratio were observed between non-tumorous and tumorous birds, which is indicative of pathological changes in individuals of the present study.