Submitted to: Avian Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 19, 2000
Publication Date: July 1, 2001
Interpretive Summary: The primary form of Newcastle disease (ND) in U.S. poultry is a respiratory disease that causes economic loss from reduced rate of gain, poor feed conversion, increased mortality, and carcass condemnation at processing. Infections with ND virus (NDV) strains of low virulence, similar to the vaccines used for ND control, are the cause of the disease. It was previously determined that some of the NDV field isolates had a more thermostable hemagglutinin, the virus protein responsible for cellular attachment to initiate infection, than the widely used vaccine strains. Seven NDV vaccines were subjected to a series of heat treatments to determine if viruses with a more thermostable hemagglutinin than the parent virus could be recovered from vaccine. After the second heat treatment all vaccine strains yielded progeny with a more stable hemagglutinin similar to the field isolates. Although viruses with thermostable hemagglutinins have been recovered from unvaccinated birds this study demonstrates that vaccines are also a previously unrecognized source of these thermostable virus populations. Therefore virus persisting from vaccination programs may be the source of ND in vaccinated flocks.
Technical Abstract: In a study of low virulence Newcastle disease virus (NDV) isolates from poultry, 38% of the isolates had a more thermostable hemagglutinin than the lentogenic reference strains B1 and La Sota or live vaccines derived from those strains. It was unknown whether those thermostable strains are truly indigenous or whether they could have originated from vaccines used in the flocks. Seven monovalent NDV vaccines of B1 or La Sota type and reference B1 and La Sota strains were heat treated at 56 C to select variants more thermostable than the parent virus. Four thermal treatment cycles were completed and virus propagated from the second and fourth heat treatment was assayed for changes in thermostability and antigenicity. The hemagglutinin thermostability of all vaccine and reference strain variants increased from the initial <10 min to >120 min after four treatments. Antigenic changes evaluated by hemagglutination-inhibition against NDV monoclonal antibodies identified changes in only the heat treated La Sota strains. The results demonstrate that the field isolates with a more thermostable hemagglutinin could have been derived by selection from the heterogenous NDV populations in vaccine strains and that minor antigenic changes may be a result of that selection.