Molecular characterization and phylogenetic analysis of a virulent Marek’s disease virus field strain in broiler chickens in Japan

Marek’s disease is a lymphoproliferative disease causing a serious threat in poultry production. Field strains of Marek’s disease virus (MDVs) are continuously re-emerging, causing great economical losses to the poultry industry worldwide in spite of the intensive vaccination and restrictive management policy used. Histopathological and molecular characterizations of MDVs are essential for monitoring the changes of viruses and evaluating the effectiveness of existing vaccines. During 2016, 190 visceral tumour tissues representing 30 vaccinated chicken flocks from the Gifu prefecture, Japan, were analysed. A pathological examination revealed the presence of lymphoproliferative lesions in the visceral organs. Polymerase chain reaction screening of tissue specimens using specific primers for avian leucosis virus, reticuloendotheliosis virus, and MDV was positive only for MDV. The polymerase chain reaction products of meq, pp38, virus-induced IL-8 homology, and glycoprotein MDV genes were sequenced and used for homology, phylogenetic, and similarity level analysis with the published reference of MDVs in the database. The results revealed high similarity between the field isolates, vv and vv+ strains of MDV from the USA and China. Several point mutations in the nucleotide sequence of the field isolates and their deduced amino acid sequences were detected in those genes. The present molecular analyses indicated that nucleotide and amino acid changes could be valuable criteria for differentiation and determination of the pathogenicity and oncogenicity of MDVs according to the Avian Disease and Oncology Laboratory pathotyping in vivo studies. Furthermore, the results suggest that development of a new vaccine must be considered to overcome this devastating avian oncogenic viral disease.

马立克氏病（Marek’s disease）是一种淋巴增生性疾病，对家禽养殖业构成严重威胁。尽管全球家禽业已实施强化免疫接种与限制性管理政策，但马立克氏病病毒（Marek’s disease virus, MDV）野外毒株仍持续重现，给全球家禽业造成了巨额经济损失。对MDV开展组织病理学与分子特征分析，是监测病毒变异、评估现有疫苗保护效力的关键手段。2016年，研究人员对采自日本岐阜县30个免疫鸡群的190份内脏肿瘤组织样本展开分析。病理学检查显示，受试内脏器官存在淋巴增生性病变。采用针对禽白血病病毒（avian leucosis virus）、网状内皮组织增殖症病毒（reticuloendotheliosis virus）及MDV的特异性引物对组织标本进行聚合酶链式反应（Polymerase chain reaction, PCR）筛查，结果仅MDV检测呈阳性。对meq基因、pp38基因、病毒诱导IL-8同源基因以及糖蛋白MDV基因的PCR扩增产物进行测序，并与数据库中已发表的MDV参考序列开展同源性、系统发育及相似性水平分析。结果显示，本次获得的野外分离株与美国、中国的超强（vv）及超超强（vv+）MDV毒株具有较高的序列相似性。在上述基因的野外分离株核苷酸序列及其推导的氨基酸序列中，共检测到多处点突变。本项分子分析结果表明，依据美国禽病与肿瘤学实验室（Avian Disease and Oncology Laboratory）体内致病型分型研究标准，核苷酸与氨基酸变异可作为区分MDV毒株、判定其致病性与致瘤性的有效指标。此外，本研究结果提示，需研发新型疫苗以防控这种极具破坏性的禽源性致瘤病毒病。