| Summary: | ABSTRACT: The reticuloendotheliosis virus (REV) and the Marek's disease virus (MDV) cause reticuloendotheliosis (RE) and Marek's disease (MD) in poultry, respectively. According to epidemiological results obtained in our laboratory from 2010 to 2017, the positive rates of REV and MDV co-infection remained at low levels. In the present study, during the period of October 2018 to July 2020, 4 clinical cases with high morbidity (5%-20%) and mortality (2%-10%), caused by the co-infection of REV and vv+ MDV-like strains, were diagnosed and analyzed by histopathological observation, cell cultures and detection with ELISA and IFA, and the PCR and by sequencing of the isolates’ genes. Sequencing and the sequence analysis on the complete genomes of the REV strains and the meq genes of the MDV strains were performed. The results, based on the complete genome, LTR, gag, pol, and env genes’ nucleotide sequences of the REV strains, showed that the REV isolates and 68.0 % (17/25) of the reference strains were in a same branch, and all had a high sequence similarity (>99.0%). The similarities between the four isolates and a vv+MDV strain GX18NNM4 were very high, up to 99.3-99.8%. Also, the amino acid residuals at locations 71, 77, 80, 115, 139, 176, and 217 were all the same as A, E, Y, A, A, R, and A, respectively, in the meq gene of the four MDV isolates. In addition, the substitutes at P176R and P217A interrupted the stretches of the proline-rich repeat PPPP, indicating that these strains belonged to the vv+ MDV-like category. Our findings indicated that the more recent and frequent reemergence of REV and the subsequent co-infection with vv+ MDV-like strain has become one of the causes of the clinical outbreaks of tumors and is undoubtedly a threat to the poultry industry in southern China.
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