Enhancement of spontaneous bursal lymphoma frequency by serotype 2 Marek's disease vaccine, SB-1, in transgenic and non-transgenic line 0 white leghorn chickens

A significant incidence of bursal lymphomas with long latencies was noted in transgenic breeders carrying a benign defective subgroup A avian leukosis provirus, ALVA6, in their germline and maintained free of exposure to avian retroviruses. Serotype 2 Marek's disease (MD) vaccine virus, strain SB-1, a component of the bivalent MD vaccine used to vaccinate the breeders, was suspected as a contributory factor in the increased bursal lymphoma incidence. Although these bursal lymphomas had several characteristics similar to retroviral-induced bursal lymphomas, we found no evidence of retroviral influence based on many virological, immunological and molecular tests that were performed on plasma and tumour cells. These tumours were therefore classified as spontaneous bursal lymphomas, similar to those reported for some specific pathogen-free (SPF) chicken lines. Long-term in vivo experiments in plastic isolators and carefully maintained pens with homozygous and hemizygous ALVA6 and ALVA6-free female chickens (line 0) that were either non-vaccinated, serotype 3 (herpesvirus of turkeys [HVT]; monovalent)-vaccinated, or HVT/SB-1 (bivalent) vaccinated, demonstrated that the incidence of spontaneous bursal lymphomas were significantly higher in those chickens that were vaccinated with the bivalent MD vaccine (P ≤ 0.05). In addition, this incidence did not depend on the ALVA6 proviral insert since there was no significant difference in spontaneous bursal lymphoma incidence between bivalent vaccinated hemizygous ALVA6 and ALVA6-free line 0 female chickens. Thus, the increased incidence of spontaneous bursal lymphomas is correlated solely with the presence of SB-1 and is not dependent on the presence of ALVA6.