Molecular characterization of the <i>meq</i> oncogene of Marek’s disease virus in vaccinated Brazilian poultry farms reveals selective pressure on prevalent strains

Marek’s disease virus (MDV) has become an increasingly virulent pathogen in the poultry industry despite vaccination efforts to control it. Brazil has experienced a significant rise of Marek’s disease (MD) outbreaks in recent years. Our study aimed to analyze the complete meq gene sequences to understand the molecular epidemiological basis of MD outbreaks in Brazilian vaccinated layer farms. We detected a high incidence rate of visceral MD (67.74%) and multiple circulating MDV strains. The most prevalent and geographically widespread genotype presented several clinical and molecular characteristics of a highly virulent strain and evolving under positive selective pressure. Phylogenetic and phylogeographic analysis revealed a closer relationship with strains from the USA and Japan. This study sheds light on the circulation of MDV strains capable of infecting vaccinated birds. We emphasize the urgency of adopting preventive measures to manage MDV outbreaks threatening the poultry farming industry.尽管家禽养殖业已开展疫苗接种防控工作，但马立克氏病病毒（Marek’s disease virus, MDV）已逐渐演变为对该行业威胁日益加重的病原。近年来，巴西的马立克氏病（Marek’s disease, MD）暴发案例显著增加。本研究旨在通过分析完整meq基因序列，解析巴西免疫蛋鸡场马立克氏病暴发的分子流行病学机制。本研究检测到内脏型马立克氏病发病率达67.74%，且存在多种流行的马立克氏病病毒毒株。其中流行率最高、地理分布最广的基因型毒株，兼具强毒株的多项临床与分子特征，并处于正选择压力下持续演化。系统发育及系统地理学分析显示，该毒株与美国、日本分离的毒株亲缘关系更为密切。本研究阐明了能够突破疫苗免疫保护的马立克氏病病毒毒株的流行现状。研究强调，亟需采取防控措施以应对威胁家禽养殖业的马立克氏病病毒暴发风险。