Preliminary antigenic characterization of an avian pneumovirus isolated from commercial turkeys in Colorado, USA

An avian pneumovirus (APV) isolated from turkeys showing respiratory disease in Colorado, USA, shared some characteristics with earlier subgroup A and B APV strains. This virus, designated the Colorado isolate (Colorado), when used after either seven passages in chick embryo fibroblasts (CEF), or seven passages in CEF followed by six turkey passages, induced clinical signs in turkeys that were similar to those caused by earlier APV strains. Although it induced an antibody response in specific pathogen free chickens, clinical signs were not seen. Unlike subgroups A or B, Colorado did not cause ciliostasis in tracheal organ cultures, but produced a cytopathic effect in chick embryo fibroblasts typical of that seen with other APV strains. Monospecific antisera to A or B strains did not neutralize Colorado and vice versa; nor did monoclonal antibodies, which neutralize subgroup A or B strains, neutralize Colorado. However, it was partially neutralized by a subgroup A hyperimmune serum. A homologous enzyme-linked immunosorbent assay (ELISA) antigen was essential for the detection of Colorado antibodies, since ELISAs in which subgroup A or B strains were used detected antibody to Colorado very poorly. Subgroup A and B vaccines protected turkeys against challenge with Colorado. However, while Colorado protected turkeys, and to some extent chickens, against subgroup A strains, protection against a subgroup B challenge was less good in both species. These results indicate that Colorado should be classified as an APV, but the antigenic differences suggest that it does not belong to subgroups A or B, and represents a separate subgroup (subgroup C) or possibly a separate serotype.