Marek's disease (MD) is a common lymphomatous and neuropathic disease of birds, especially chickens, and has caused significant losses to chicken production as a result of high mortality and morbidity. This study aims to determine the status of MD in backyard flocks in the Mekong Delta of Vietnam and to analyze clinical cases occurring during a year. The study was carried out from August 2018 to July 2019, during which time 16 suspected cases of chicken flocks with MD were observed, 40 chickens were subjected to anatomopathological examination, and PCR was performed for diagnosis of MD and determination of Marek's disease virus (MDV) serotypes. The results showed that all of the examined flocks were confirmed as experiencing MD. Nearly all cases were in an acute form with typical lesions of visceral lymphomas. Besides the presence of Marek's disease virus serotype 1 (MDV-1) from 100.0% of tested chicken flocks (16/16), nononcogenic turkey herpesvirus serotype 3 (MDV-3) was also found from 8 of 16 (50.0%) of examined chicken flocks. Morbidity and mortality at sampling time varied from 1% to 42.11% and 0.6% to 10%, respectively. Chicken flocks with MD vaccination had lower morbidity and mortality. These first findings confirm endemic MD in backyard chicken populations in Vietnam and confirm it continues to be a threat to chickens in backyard flocks and poultry production in general.

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