Increased resistance of vaccinated turkeys to experimental infection with an H7N3 low-pathogenicity avian influenza virus

A trial was performed to establish whether turkeys vaccinated against avian influenza with a vaccine containing a strain with a heterologous neuraminidase to the challenge virus required a higher infectious dose to develop infection than naive birds. Birds were vaccinated with a commercially available, inactivated oil emulsion product containing the strain A/ty/Italy/99/(H7N1) and challenged with different dilutions of a LPAI isolate A/ty/Italy/8000/02(H7N3) obtained during the 2002 to 2003 Italian epidemic. Groups of 10 vaccinated and 10 unvaccinated birds. were infected experimentally with a virus suspension containing 10(2,) 10(4) and 10(6) median embryo infective dose (EID50)/0.1 ml. Infected birds were observed daily with tracheal and cloacal swabs collected at regular intervals for antigen detection, virus isolation and real-time reverse transcription-polymerase chain reaction. Pre-infection and post-infection serology was also performed. The results of the experiment indicate that infection is achieved in naive birds with 10(4) EID50, while vaccinated birds are resistant at this challenge dose. Vaccinated and unvaccinated birds were susceptible to infection with 10(6) EID50, although the duration and/or the number of birds shedding was reduced in the vaccinated group. The data presented indicate that heterologous vaccination in the framework of a 'Differentiating Infected from Vaccinated Animals' strategy can be a valid tool to support eradication measures in areas with high densities of susceptible animals.