Researchers identify “unpredictable mutations” in vaccine against acute, highly contagious respiratory poultry disease infectious bronchitis virus.

Scientists at The Pirbright Institute have shown infectious bronchitis virus (IBV) vaccines produced in chicken eggs have unpredictable mutations that may be prone to reversion in the field.

The live attenuated vaccine virus is generated by infecting eggs and allowing the virus to grow, removing it and infecting another batch of eggs, and so on, sequentially until the virus is sufficiently weakened – this is known as serial passage.

Mutations

However, little is known about how this method of attenuation affects the virus genetically and, therefore, the mechanisms by which it is being weakened.

The team compared IBV genomes that had been passaged through eggs more than 100 times to find out which genes are involved in weakening the virus.

They discovered mutations occurred throughout the IBV genomes, but nine mutations were found more than once across the viruses.

Risk

Erica Bickerton, head of the coronaviruses group at Pirbright, said: “This shows using egg passages as a means of creating attenuated IBV vaccines produces unpredictable changes in the virus.

“The presence of so few consistent mutations that may weaken the virus indicates the risk that a vaccine strain generated this way could revert back to a more virulent form in the field.”

Targeted

The team’s research, published in the Journal of Virology, reinforces the need for more targeted genetic modification of infectious bronchitis vaccine viruses to ensure the changes are both reliable and achieve the best protection.

Dr Bickerton added: “We will use the mutation hotspots that we have identified in this study to inform the development of live attenuated vaccines that are genetically modified in the lab.”

Effective control

IBV is a highly infectious avian pathogen that affects the respiratory tract, gut, kidney and reproductive systems.

About 44 billion chickens are produced worldwide every year, and as a heavily relied on food source, it is important diseases such as infectious bronchitis are controlled effectively.

It has been estimated every 10% reduction in IBV would be worth about £654 million to the global poultry industry.