Protective Efficacy of High-Passage Avian Pneumovirus (APV/MN/turkey/1-a/97) in Turkeys

Abstract

A U.S. isolate of avian pneumovirus (APV), APV/MN/turkey/1-a/97, was attenuated by serial cell culture passages in chicken embryo fibroblasts (seven passages) and Vero cells (34 passages). This virus was designated as APV passage 41 (P41) and was evaluated for use as a live vaccine in commercial turkey 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 that bird-to-bird passage would help spread the infection from P41-exposed birds to their respective flock mates. The virus did spread from vaccinated birds to the entire flock within 10 days as detected by reverse transcription-polymerase chain reaction. Mild respiratory illness was observed in a few birds 12 days postvaccination in 2 of 10 flocks. Within 3 wk postvaccination, all flocks became seropositive for APV antibodies as measured by enzyme-linked immunosorbent assay. In an additional flock, the virus was administered to all turkeys simultaneously in drinking water and seroconversion occurred within 2 wk. All 11 flocks remained seropositive until 10 wk postvaccination. When compared with unvaccinated flocks on the same farm from the previous year, the medication cost, total condemnation, and mortality rates attributed to APV were lower in P41-vaccinated flocks. When birds from vaccinated flocks were challenged with virulent APV under experimental conditions, no clinical signs were observed at 2, 6, and 10 wk postvaccination, whereas in the control unvaccinated birds, respiratory illness and virus shedding occurred after challenge. These results indicate that P41 administered by the nasal and ocular routes, and by drinking water, causes seroconversion and induces protection from virulent APV challenge for at least 10 wk.

Experimental and field evaluation of a live vaccine against avian pneumovirus

“…Attenuation of virulent APV by repeated passages in cell cultures has been reported previously (Cook et al, 1989a,b;Williams et al, 1991a;Gulati et al, 2001). Live attenuated viruses are good vaccine candidates as they induce strong immune responses and protection (Murphy et al, 1988;Cook et al, 1996;Collins et al, 1999).…”

“…We reported earlier on the development of an attenuated APV (named P41) by passaging a field isolate in vitro for 41 passages. Under field conditions, P41 was found to produce mild clinical signs in some flocks (Gulati et al, 2001). In the present study, we have further passaged P41 for an additional 22 passages in Vero cells with the idea that further passages would result in a safer and protective vaccine.…”

“…Earlier reports from our laboratory showed the efficacy of P41 in inducing protection in turkeys under field conditions using the approach of controlled exposure (Gulati et al, 2001). The idea was to expose a small number of turkeys in a flock and co-mingle them with other birds in the entire flock so that the vaccine virus would gradually spread within the whole flock by contact.…”

“…The idea was to expose a small number of turkeys in a flock and co-mingle them with other birds in the entire flock so that the vaccine virus would gradually spread within the whole flock by contact. This method was found to be useful in protecting turkeys from APV outbreaks, thus causing reduction in economic losses (Gulati et al, 2001).…”

“…We have also attenuated a US isolate of APV (APV/MN/turkey/ 1-a/97) by continuously passaging it in Vero cells. Although serially passaged virus (designated P41) produced immunity against APV under experimental and field conditions (Gulati et al, 2001), it was found to be partially attenuated because it caused mild clinical signs in vaccinated birds. The present study was undertaken to further attenuate APV and to study its potential as a live attenuated vaccine in turkeys under experimental and field conditions.…”