Late Marek's Disease in Adult Chickens Inoculated with Virulent Marek's Disease Virus

Abstract

Recently, excessive losses from Marek's disease (MD) have been noted in adult laying flocks over the age of 40 weeks. We defined these late outbreaks in adult chickens as "late MD", and experimentally reproduced that using adult SPF P2 line (50-week-old) or commercial line (74-week-old) chickens inoculated with a virulent strain of Marek's disease virus (MDV). Commercial line chickens were given MDV vaccines (HVT and CVI 988) at hatch. The occurrence of MD was evaluated periodically based on such neurologic signs as paralysis, torticollis, ataxia, and/or nervous tics, as well as histopathological examinations. In P2 line chickens, neurologic signs and MD lymphoma were observed from day 21 onward, and they tended to increase in a time-dependent manner. Meanwhile, in commercial line chickens, only one chicken exhibited MD lymphoma on day 70 post inoculation, but its pathogenesis was questionable. No regression of MD lymphoma was noted in either case. The lesions in the visceral organs, thymus, peripheral nerves, and feather pulps of P2 line chickens were characterized by proliferation of variably sized lymphoid cells. In the feather follicle epithelium, numerous inclusion bodies were noted on day 21 post-inoculation, which subsequently tended to decrease. The morphological findings obtained resembled late MD. In conclusion, our results demonstrate that adult SPF P2 line chickens are susceptible to virulent MDV, and would be useful for investigation of late MD.