Submitted to: Avian Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/20/2006
Publication Date: 9/1/2006
Citation: Fadly, A.M., Silva, R.F., Hunt, H.D., Pandiri, A., Davis, C. 2006. Isolation and characterization of an adventitious avian leukosis virus isolated from commercial Marek's disease vaccines. Avian Diseases. 50:380-385. Interpretive Summary: Avian leukosis virus (ALV) is the most common naturally occurring avian retrovirus that can cause cancer-like diseases and other production problems in chickens. Because ALV can be transmitted vertically from breeder hens to offspring, embryos from such infected breeder hens and ingredients prepared from such embryos for production of poultry vaccines may harbor ALV. Thus, chicken-embryos from ALV-infected hens could serve as a source of ALV contamination of live virus vaccines of poultry including Marek's disease (MD), an economically important cancer-like disease of chickens induced by a virus named MD virus (MDV). The objective of this work was to test suspected commercial MD vaccines produced by two manufacturers for contamination with ALV, and to characterize the extraneous ALV isolated from vaccines. Our data indicate that commercial MD vaccines produced by two manufacturers tested positive for contamination with both endogenous and exogenous ALV. Characterization of the extraneous ALV proved that the exogenous ALV belong to subgroup A and that its disease inducing potential is less than that of reference strain of ALV-A termed Rous-Associated virus-1 (RAV-1). This important information is essential to vaccine manufacturers, breeder companies who use MD vaccines, regulatory government agencies, namely USDA-APHIS-CVB. The information has resulted directly in revising the protocol recommended by USDA-APHIS-CVB for testing live virus vaccines of poultry for contamination with ALV.
Technical Abstract: Commercial Marek's disease (MD) vaccines produced by two manufacturers were tested for possible contamination with avian leukosis virus (ALV). Samples of MD vaccines manufactured by two companies (A & B) were received from a breeder company; samples were also received directly from vaccine company B. Using virus isolation tests, samples initially tested positive subgroup E (endogenous) ALV. However, upon re-passage, the vaccines also tested positive for exogenous ALV. The isolated exogenous ALV proved to be a subgroup A, as determined by flowcytometry using polyclonal chicken antibodies specific for various subgroups of ALV, and by DNA sequencing of the envelope glygoprotein (gp85). The exogenous ALV isolated from MD vaccines was inoculated in chickens from ADOL lines 15I5 X 71 and 0 to determine its pathogenicity and compare it with that of Rous-associated virus-1 (RAV-1), the prototype strain of ALV-A. Each chicken from each line was inoculated with approximately 10,000 infectious units of RAV-1 or B-39 virus at 7th day of embryonation. At hatch, and at 4, 8, and 16 weeks of age, chickens were tested for viremia and cloacal shedding; chickens were also observed for ALV-induced tumors within 16 weeks of age. Viremia and cloacal shedding results suggest that chickens from both lines were susceptible to infection with either virus. Within 16 weeks of age, the incidence of ALV tumors induced by strain B-39 in line 0 and 15I5 X 71 chickens was 0% and 12%, respectively; compared with 62% and 67% in chickens inoculated with RAV-1. The data indicate that commercial MD vaccines produced by two manufacturers were contaminated with endogenous subgroup E and an exogenous subgroup A ALV. Further, data from biological characterization suggest that the ALV-A isolated from commercial MD vaccines is of low oncogenicity, compared with that of RAV-1.