Molecular characterization of Marek’s disease viruses circulating in small chicken flocks from England and Wales

Marek’s disease (MD) is common in chickens from small flocks, most of which are not vaccinated against this disease. However, little is known about the virulence of strains of Marek’s disease virus (MDV) that are currently circulating in small flocks in England and Wales, and whether they pose any risk to commercial poultry. Between 2017 and 2025, 31 samples with histopathological lesions consistent with MD, submitted to the Animal and Plant Health Agency, were confirmed by real-time PCR at the Pirbright Institute’s MDV Reference Laboratory. The age, gender, breed of chicken, flock size, MD vaccination status, clinical signs, and pathology due to MD were recorded. Sequencing of the MDV meq gene, which is closely associated with virulence, showed a range of Meq protein “isoforms” (standard, long, very long, and a previously unreported isoform designated very very long) with, respectively, 4, 7, 10, and 12 tetraproline repeat motifs. Phylogenetic analysis showed that British small flocks are a reservoir of MDV strains of diverse origins. While the majority of Meq sequences lie within well-established European MDV clades (including an ancient clade of low-virulence strains, and the very virulent plus (vv+) MDV Polen clade), 10 strains cluster phylogenetically with MDV from a diverse range of geographic locations, including Asia and South America.RESEARCH HIGHLIGHTS Molecular characterization of MDV strains from British small chicken flocks showed great diversity.Sequencing showed a variety of Meq protein isoforms: standard, long, very long, and very very long.Phylogenetic analysis identified MDV strains similar to those from diverse geographical regions, varying in predicted virulence.