Field Avian Metapneumovirus evolution avoiding vaccine induced immunity

Infection of turkeys and chickens with Avian Metapneumovirus (AMPV) leads to disease and serious economic losses in unprotected birds. In the late 1980s, live AMPV vaccines became available in Europe and these largely brought the disease under control. However disease has still been encountered and much of this remains unexplained. There is evidence that in some instances disease might be due to the involvement of another subtype or in some cases the disease may be caused by the vaccine itself. However many disease outbreaks remain unexplained especially in situations where they are observed after apparently successful vaccination with virus of the same subtype. Longitudinal studies were undertaken in Italian turkey farms and demonstrated that subtype B AMPV was frequently detected some period after subtype B vaccination. Sequencing showed that these later viruses were not derived from the previously applied vaccine, and have genome differences in key antigenic regions. An experimental infection of birds was carried out to discover whether the consistent mutations observed in the more recent Italian isolates might allow the viruses to avoid an immune response induced by the common subtype B vaccine, which has virus sequence close to AMPV strains isolated in Italy in the late 80s. The results of the study demonstrated that the more recent field viruses were able to overcome vaccine induced immunity and are consistent with the hypothesis that the field virus has changed in key antigenic regions in order to thrive within a groups of well vaccinated birds.