EFFECT OF VACCINATION PROGRAMS ON THE ENHANCEMENT OF SPONTANEOUS AVIAN LEUKOSIS-LIKE TUMORS IN BROILER CHICKENS

In recent years, Aviagen has detected sporadic reports of tumors found in Elite, Grandparent and Parent hens. Most were incidental findings during post-mortem examination of hens between 35-45 weeks of age with no significant impact on reproduction performance and overall flock mortality. Lesions were consistent with myelocytomas, with lymphomas frequently found in liver, spleen, kidney and occasionally other organs. Tumors resembled those induced by avian leukosis virus (ALV), but in all cases there was no evidence of infection by any known exogenous ALV, as determined by virus isolation and Polymerase chain reaction (PCR). Only endogenous ALV (ALV-E) was detected. Most if not all chickens harbor ALV-E genes, particularly in those carrying the slow feather gene termed ev21. Under the previous agreement, two individual ALV-E field strains were isolated from breeder farms, designated AF227 and AF229. Vaccination with serotype 2 Marek¿s disease virus (MDV), in addition to AF227 or AF229 inoculation, significantly enhanced spontaneous LL-like tumor incidence in one of the Avian Disease & Oncology Laboratory (ADOL) bird lines that lack endogenous virus. Incidence increased from 17% when administered AF227 alone, to 33-42% when inoculated with AF227 and SB-1 vaccine. Currently, spontaneous tumors are being detected again from Aviagen broiler breeders of various ages, starting at 35 weeks. Tumors are lymphoma with intrafollicular transformation in the bursa and very homogenous cell population. One recent sample was a histiocytic sarcoma with abundant myelocytic infiltration. As before, tumors have been testing negative for MDV, Reticuloendotheliosis virus (REV) and exogenous ALVs. In the bursa, many follicles are transformed and the incidence appears high among culled birds, suggesting this could be a more virulent endogenous virus. The current vaccine plan includes use of SB-1. Current objectives include: 1. Isolate endogenous ALV (ALV-E) from affected chickens. 2. Sequence and compare to previous ALV-E isolate AF227. 3. Evaluate the effect of MDV serotype 2 vaccination on enhancement of spontaneous tumors. 4. Compare effect of different MDV serotype 2 vaccines (SB-1 vs. 301B/1). 5. Evaluate effect of early intensive vaccination for infectious bursal disease virus (IBDV) and Reovirus (REO) on enhancement of spontaneous tumors.