Submitted to: American Veterinary Medical Association Abstract
Publication Type: Abstract Only
Publication Acceptance Date: July 14, 2001
Publication Date: N/A
Technical Abstract: Serotype 1 Marek's disease (MD) viruses may be attenuated for oncogenicity by up to 100 serial passages (p100) in cell culture. This process occurs gradually. A fully attenuated virus no longer induces gross lymphomas in susceptible chickens while earlier passages retain various levels of oncogenicity. Studies were conducted to better understand the relationship between attenuation and induction of protective immunity by serially passaged serotype 1 viruses. 15x7 chickens with maternal antibodies to all 3 serotypes of MD virus were immunized at hatching with viruses passaged until fully attenuated or at selected passages prior to full attenuation. Vaccinated chickens were challenged at 5 days post vaccination with virulent MD virus. Lymphoma frequency was determined through 8 weeks post challenge. In tests with 4 different virus isolates, the partially attenuated viruses consistently induced higher levels of protection than fully attenuated viruses; the mean advantage was 41% compared to fully attenuated counterparts. In a second trial, a partially attenuated, mildly oncogenic (p80) preparation of strain 648A provided 83% protection compared to 49% by the fully attenuated (p100) preparation. Significant protection also was provided by overtly oncogenic preparations of 648A (p40-60). Partially attenuated, mildly oncogenic strains may induce strong protective immunity against virulent MD virus challenge. Even viruses with significant virulence are capable of inducing protective immune responses in some chickens. Partially attenuated viruses, although not currently permitted as commercial vaccines, may represent a unique strategy to induce strong protective immunity against MD.