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

A significant incidence of bursal lymphomas with long latencies was noted i n transgenic breeders carrying a benign defective subgroup A avian leukosis provirus, ALVA6, in their germline and maintained free of exposure to avia n 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 incide nce. Although these bursal lymphomas had several characteristics similar to retroviral-induced bursal lymphomas, we found no evidence of retroviral in fluence based on many virological, immunological and molecular tests that w ere performed on plasma and tumour cells. These tumours were therefore clas sified as spontaneous bursal lymphomas, similar to those reported for some specific pathogen-free (SPF) chicken lines, Long-term in vivo experiments i n plastic isolators and carefully maintained pens with homozygous and hemiz ygous ALVA6 and ALVA6-free female chickens (line 0) that were either non-va ccinated, serotype 3 (herpesvirus of turkeys [HVT]; monovalent)-vaccinated, or HVT/SB-1 (bivalent) vaccinated, demonstrated that the incidence of spon taneous bursal lymphomas were significantly higher in those chickens that w ere vaccinated with the bivalent MD vaccine (P less than or equal to 0.05). In addition, this incidence did not depend on the ALVA6 proviral insert si nce there was no significant difference in spontaneous bursal lymphoma inci dence between bivalent vaccinated hemizygous ALVA6 and ALVA6-free line 0 fe male chickens. Thus, the increased incidence of spontaneous bursal lymphoma s is correlated solely with the presence of SB-1 and is not dependent on th e presence of ALVA6.