Bursal Lesions in Chickens Inoculated with Marek's Disease Vaccines

The effects on the bursa of Fabricius (BF) of a herpesvirus, isolated from turkeys (HVT) and used as a vaccine in chickens for the prevention of Marek's disease (MD), were examined. In 25 HVT-vaccinated unchallenged chickens, histologic examination of the BF failed to reveal detectable lesions. Thirty-two of 33 nonHVT-vaccinated MDHV-challenged chickens had BF lesions, compared with 17 of 30 HVT-vaccinated MDHV-challenged chickens. The BF lesions observed were much less severe (0.5 on an arbitrary 0-to-3 scale) in the HVT-vaccinated MDHV-challenged group than in the unvaccinated MDHV-challenged group (2.2 on the same scale). The BF lesions observed were similar to those previously described in MDHV-infected chickens. Marked elevation of the serum gamma-globulin was observed at 7 weeks but not 4 or 8 weeks in HVT-vaccinated MDHV-challenged chickens.