Pathogenicity and transmissibility of Marek's disease virus isolated from chickens in Thailand

Marek's disease (MD) is a highly contagious lymphoproliferative disease of chickens caused by serotype 1 Marek's disease virus (MDV-1). Frequent outbreaks of MD in vaccinated chicken flocks have been reported worldwide, including in Thailand. Recently, we reported the circulation of several genotypes/clusters of MDV-1 with molecular characteristics of highly virulent strains in chicken populations in Thailand. However, no information is available regarding the pathogenic characteristics of MDV-1 field strains circulating in Thailand. In this study, we investigated the pathogenicity and transmissibility of a predominant cluster 1 Thai MDV-1 field strain in chickens. Our results demonstrated that the Thai MDV-1 field strain was pathogenic and oncogenic, as reflected by moderate morbidity and mortality, and a relatively high tumor incidence rate. We also observed a significant reduction in body weight gain and lymphoid organ atrophy in chickens inoculated with this strain. Correspondingly, the virus replicated efficiently in chickens, as evidenced by high viral loads with prolonged duration across a range of sample types. Additionally, the Thai MDV-1 field strain is highly transmissible, efficiently spreading to contact chickens via the airborne route, even in the absence of close contact. Overall, this study enhances our understanding of the pathogenicity, transmissibility, and replication dynamics of the Thai MDV-1 field strain in chickens, providing valuable insights for the development of effective control and prevention strategies for MD.