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ARS Home » Southeast Area » Athens, Georgia » U.S. National Poultry Research Center » Endemic Poultry Viral Diseases Research » Research » Research Project #436348

Research Project: Effect of Vaccination Programs on the Enhancement of Spontaneous Avian Leukosis-like Tumors in Broiler Chickens

Location: Endemic Poultry Viral Diseases Research

Project Number: 6040-32000-083-002-T
Project Type: Trust Fund Cooperative Agreement

Start Date: May 1, 2019
End Date: Apr 30, 2022

There has been sporadic reports of tumors found in commercial broiler breeder hens in recent years. 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 commercial 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.

For this project we will use our unique experimental chicken line, 0.TVB*S1, commonly known as Rapid Feathering Susceptible (RFS) line, which lacks all endogenous avian leukosis virus (ALV) and are fully susceptible to all subgroups of ALV including ALV-E. The first goal is to isolate virus from the current cases and compare with previous isolates. This includes comparison of virus sequences as well as biological comparisons of virulence using challenge experiments with birds that lack endogenous virus. The second goal of this study is to evaluate the effect of Marek’s disease virus (MDV) serotype 2 vaccination on enhancement of spontaneous tumors, with comparisons of two different serotype 2 vaccines. The third major goal is to evaluate the effect of intensive early vaccination programs for infectious bursal disease virus (IBDV) and Reovirus (Reo).