Protective efficacy of high-passage avian pneumovirus (APV/MN/turkey/1-a/97) in turkeys

A U.S. isolate of avian pneumovirus (APV), APV/MN/turkey/1-a/97, was attenu ated by serial cell culture passages in chicken embryo fibroblasts (seven p assages) and Vero cells (34 passages). This virus was designated as APV pas sage 41 (P41) and was evaluated for use as a live vaccine in commercial tur key flocks. The vaccine was inoculated by nasal and ocular routes in 2-to-4 -wk-old turkeys in 10 turkey flocks, each with 20,000-50,000 birds. Only 2 birds per 1000 birds were inoculated in each flock with. the expectation th at bird-to-bird passage would help spread the infection from P41-exposed bi rds to their respective flock mates. The virus did spread from vaccinated b irds to the entire flock within 10 days as detected by reverse transcriptio n-polymerase chain reaction. Mild respiratory illness was observed in a few birds 12 days postvaccination in 2 of 10 flocks. Within 3 wk postvaccinati on, all flocks became seropositive for APV antibodies as measured by enzyme -linked immunosorbent assay. In an additional flock, the virus was administ ered to all turkeys simultaneously in drinking water and seroconversion occ urred within 2 wk. All 11 flocks remained seropositive until 10 wk postvacc ination. When compared with unvaccinated flocks on the same farm from the p revious year, the medication cost, total condemnation, and mortality rates attributed to APV were lower in P41-vaccinated flocks. When birds from vacc inated flocks were challenged with virulent APV under experimental conditio ns, no clinical signs were observed at 2, 6, and 10 wk postvaccination, whe reas in the control unvaccinated birds, respiratory illness and virus shedd ing occurred after challenge. These results indicate that P41 administered by the nasal and ocular routes, and by drinking water, causes seroconversio n and induces protection from virulent APV challenge for at least 10 wk.