Effects of infectious bronchitis virus vaccine on the oviduct of hens.

In Australia, currently, all pullets reared for egg production are vaccinated with live attenuated strains of infectious bronchitis virus. Various vaccines and protocols to control this viral disease have been developed, although the severity of the disease varies from place to place and flock to flock. In the present trial, the effects of vaccine strains A3 and Vic S on the oviduct of laying hens were assessed by histopathology, electron microscopy, serology and also by determining the presence and persistence of viral RNA in the oviduct by real time PCR following the experimental infection. Birds were either unvaccinated or vaccinated with both A3 and Vic S and then mock-infected, challenged with the same attenuated strains, either A3 or Vic S. Some respiratory signs were observed, but were mild and short-lived. There was no drop in egg production in any of the groups. However, there was visual loss of shell colour in the unvaccinated hens challenged with the Vic S strain. Mild histopathology was recorded only in terms of lymphocyte infiltration and occasional submucosal oedema in the infundibulum and very mild gland dilatation in the magnum. Microscopic lesions were not recorded in the isthmus, tubular shell gland or shell gland pouch. Cilia loss was not observed in any region of the oviduct using scanning electron microscopy. Both the A3 and Vic S vaccine strains were detected in the oviduct of vaccinated and unvaccinated hens, mainly on the 12th day p.i. These results indicate that the A3 and Vic S strains replicate at a low level in the oviduct without causing significant damage and hence are safe for the oviduct.