Marek's disease virus (MDV) is a lymphoproliferative neuropathic disease affecting domestic fowl. Despite intensive vaccination, Egyptian poultry farms continue to experience recurrent infections of MDV. Our study aimed to investigate MDV in three Egyptian layer chicken farms that showed MDV-like symptoms, such as depression, ataxia, and emaciation. In this study, a total of nine field tissue samples (three from each farm) were collected from 7-10-month-old chickens vaccinated with CVI988 and HVT commercial MDV vaccines. The processed samples were inoculated on the chorioallantoic membrane (CAM) of SPF-ECEs for three serial passages, resulting in embryos with stunted growth, body curling, enlarged liver, and thickened CAM. Subsequently, the harvested isolates were tested for MDV using the agar gel precipitation test (AGPT), which confirmed that all these isolates tested negative. Furthermore, the isolates underwent conventional PCR for MDV using specific primers based on the ICP4 gene, confirming that all of them were negative. Therefore, we used specific primers for other tumor viruses to determine which viruses caused tumors on these farms. Avian leukosis virus subgroup J (ALV-J) and Reticuloendotheliosis virus (REV) were tested based on the gp85 and LTR genes, respectively. To our surprise, out of the three examined isolates, one tested positive for ALV-J, another was co-infected with ALV-J and REV, while the third was negative for both. Overall, this study underscored that the absence of MDV infection in vaccinated chickens demonstrates the efficacy of commercial MDV vaccines; however, tumors observed in the chicken farms studied were caused by ALV-J and REV infections. |
