The goal of this project was to understand why Arkansas (Ark) infectious bronchitis vaccines are not effective when delivered by a hatchery spray cabinet. Considering that Massachusetts (Mass) type infectious bronchitis vaccines are effective when given by a hatchery spray cabinet and that Ark vaccines are effective when given by eye drop, the objectives as listed in the original proposal were:
- Determine the dose of Ark vaccine virus (vaccine titer) from different manufacturers delivered to chicks using a hatchery spray cabinet.
- Determine the minimum infective/immunizing dose of Ark IBV vaccines from different manufacturers using a hatchery spray cabinet.
Spraying either a 7ml or 21ml vaccine volume per 100 chicks, we collected samples and ran quantitative RT-PCR (qRT-PCR) and virus titrations in embryonated eggs to evaluate how much vaccine was getting to the chicks. We observed a 10 to 100-fold drop in titer from the spray nozzle to the level of the chicks in the chick box for three different Ark vaccines and one Mass type vaccine.
A similar decrease in titer for the Ark and Mass vaccines indicates that the spraying process did not affect the Ark vaccines any more than the Mass vaccine. The uniformity of the spray pattern and coverage of the chick box were examined, and we found that 40psi was the ideal pressure and larger nozzle sizes (used with the 21ml spray volume) gave more uniform spray patterns, less vaccine loss by aerosolization and more uniform coverage of the chicks.
To determine if sheering forces associated with spray vaccination damaged the Ark vaccine, we observed the structure of the virus particles and counted the average number of spikes (e.g.; infectious bronchitis virus has spikes on its surface used for attaching to chicken cells) associated with each virus particle prior to and after spray using an electron microscope and compared that data to a Mass vaccine control. No statistical differences in the number of spikes on the virus particles were observed, indicating that the spraying process apparently did not damage the virus. In addition, we verified that the viruses were still infectious by titration of each vaccine before and after spray.
Finally, we examined the amount of Ark vaccine necessary to infect 1-day old chicks using a hatchery spray cabinet, and we found that approximately 25 percent of the chicks were infected when a 10X dose of vaccine was given, whereas nearly 95 percent of the chicks were infected when a 100X dose of vaccine was given. Examining the Ark vaccine reisolated from the chicks, we found that the vaccine virus infecting the chicks was a minor virus subpopulation within the original vaccine. Apparently a 100X dose is needed to provide enough of that minor subpopulation to infect the chicks.
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