Chicken vaccines recombine to produce virulent viruses

New Australian research showing that poultry vaccines have recombined to produce more virulent viruses has prompted the Australian Pesticides and Veterinary Medicines Authority (APVMA) to examine regulatory controls over the approval and use of veterinary vaccines.

The research study, by Dr Joanne Devlin, Professor Glenn Browning and Dr Sang-Won Lee and colleagues at the Asia-Pacific Centre for Animal Health at the University of Melbourne and NICTA, was published today in the international journal Science. The researchers found that two different types of vaccine viruses used to control a chicken respiratory disease - infectious laryngotracheitis or ILT - have recombined (crossed) to form two new virulent forms of the ILT virus.

“Vaccines protect animals from disease. Some vaccines use a milder (attenuated) version of a live virus to build resistance to a full virus. Live attenuated viruses have been used safely in vaccines worldwide for many years,” Dr Allen Bryce, APVMA Veterinary Medicines Program Manager, said.

Australian strains of ILT vaccine were developed from the 1960s. A European strain was registered in 2006 in response to demand. Shortly afterwards, the two new and previously unknown ILT strains were identified as responsible for outbreaks of disease in New South Wales and Victorian flocks.

“We have informed our state and Australian Government regulatory partners and are now working through the implications of these findings with the researchers, vaccine registrants and the poultry industry,” Dr Bryce said.

“While there are clearly a number of regulatory issues raised by this research, these can be readily accommodated within our current regulatory approach.

“There are a number of short-term measures we are considering, including changes to product labels, which may result in restrictions on the use of two vaccines of different origins in the one animal population.

“We will also assess all live virus vaccines currently registered by the APVMA in regard to the risk of recombination.

“Longer-term measures may include changes to the regulatory process for new vaccine applications to address the potential for recombination in our scientific risk assessments.”

The Melbourne University team used genetic sequencing to demonstrate that the two new strains arose from genetic recombination between the Australian and European strains. Both new strains are more virulent than their parent strains.

Recombination can occur naturally when two viruses infect the same cell at the same time. While the process has been recognised as a potential risk associated with live virus vaccines, the likelihood of it happening in the field was thought to be insignificant.

Devlin, Browning, Lee and colleagues showed that recombination could occur in the field if poultry flocks were simultaneously exposed to two different vaccine virus strains.

The current vaccines are considered effective in providing protection against both the older as well as the new recombinant strains of the infectious laryngotracheitis viruses. Producers should administer only one ILT vaccine at a time and ensure that they carefully follow label instructions. Producers can also minimise the risk of introducing disease by following good biosecurity practices, such as:

• Keeping equipment and yards clean.
• Preventing contamination of food and water.
• Restricting contact between flocks and wild birds.