Drs Mark Jackwood and Brian Jordan at the University of Georgia recently completed a research project, sponsored by the US Poultry and Egg Association, that looked at the possible reasons for frequent vaccine failures when broilers are vaccinated with Arkansas infectious bronchitis vaccine.

Arkansas infectious bronchitis vaccines (Ark) are not effective when delivered by a hatchery spray cabinet. The Massachusetts (Mass) type infectious bronchitis vaccines are effective when given by a hatchery spray cabinet while the Ark vaccines are effective when given by eye drop.

They found the vaccine contains a mixture of subpopulations of viruses, and one of the minor subpopulations is actually responsible for immunising the birds.

Spraying either a 7ml or 21ml vaccine volume per 100 chicks, Drs Jackwood and Jordan 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.

They observed a 10- to 100-fold drop in titre 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 titre 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 the researchers found that 40 pounds per square inch (psi) was the ideal pressure and larger nozzle sizes (used with the 21ml spray volume) gave more uniform spray patterns, less vaccine loss by aerosolisation and more uniform coverage of the chicks.

To determine if sheering forces associated with spray vaccination damaged the Ark vaccine, the researchers 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.

They found 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, they verified that the viruses were still infectious by titration of each vaccine before and after spray.

Finally, the Athens-based researchers examined the amount of Ark vaccine necessary to infect day-old chicks using a hatchery spray cabinet, and they found that approximately 25 per cent of the chicks were infected when a 10-times dose of vaccine was given, whereas nearly 95 per cent of the chicks were infected when a 100-times dose of vaccine was given.

Examining the Ark vaccine reisolated from the chicks, they found that the vaccine virus infecting the chicks was a minor virus subpopulation within the original vaccine. Apparently a 100-times dose is needed to provide enough of that minor subpopulation to infect the chicks.

Although the inefficient nature of delivering vaccines with a hatchery spray cabinet contributes to the problem, Jackwood and Jordan concluded that the failure of Ark IBV vaccine when delivered by a hatchery spray cabinet was not solely related to the mechanism of delivery but rather to problems inherent in the Ark vaccine.

Ark vaccines contain multiple virus sub-populations. One of the minor virus subpopulations in the vaccine is responsible for infecting the chicks and inducing immunity. A sufficient amount of this minor subpopulation in the Ark vaccine capable must be given for the vaccine to be effective.

February 2015