Efficacy of avirulent hemorrhagic enteritis virus propagated in turkey leukocyte cultures for vaccination against hemorrhagic enteritis in turkeys.

Avirulent hemorrhagic enteritis virus (HEV-A) propagated in turkey leukocyte cell culture was tested as a vaccine to prevent hemorrhagic enteritis (HE) in turkeys in experimental and field trials. Immunization of turkeys with live HEV-A resulted in protection against a challenge with virulent HEV (HEV-V) as measured by the serological response and the absence of clinical disease and HEV antigen in spleens. In field trials, 19 out of 20 flocks seroconverted within 21 days after vaccination with live HEV-A distributed in the drinking water. The average immune response of the turkeys in these flocks was 96%. Most importantly, neither clinical HE nor other adverse effects caused by HEV-A vaccination were observed in any of the vaccinated flocks. Because maternal antibodies can interfere with the immune response to the vaccine, the optimum time for vaccination was determined. Using an established half-life value of 4.25 days for turkey antibody, and knowing the enzyme-linked immunosorbent assay titer of the maternal antibodies and age of the turkey, the time of vaccination could be calculated taking into account that maternal antibody titers should be lower than 40 to vaccinate the turkeys successfully and induce protection. In vivo tests with HEV-A preparations confirmed the replication of the virus in turkey leukocyte cultures and the potential to pass it in culture. Furthermore, in vivo analysis showed that most infectious virus was associated with preparations of the nonadherent cell population, confirming the results obtained by in vitro analysis. The potency of HEV-A preparations was dependent on the production method and varied from an average of 570 to 8135 doses per ml.