How the new viruses are being created

July 13, 2012

Researchers from the University of Melbourne have shown that two different veterinary vaccines – used simultaneously to control the same condition in chickens – have combined to produce new infectious viruses, prompting action from Australia’s veterinary medicines regulator.

The vaccines were used to control a respiratory disease, infectious laryngotracheitis (ILT), which can have up to a 20 per cent mortality rate in chicken flocks.

The Melbourne research has found that when two different vaccine strains were used in the same populations, they combined into two new virus strains, resulting in disease outbreaks.

Neither the original ILT virus or the new strains can be transmitted to humans or other animals, and do not pose a food safety risk.

The research has been published today in the international journal Science.

Researcher Dr Joanne Devlin said the combining of live vaccine virus strains outside of the laboratory was previously thought to be highly unlikely, but this study showed that it was possible and had led to disease outbreaks in poultry flocks.

“We alerted the Australian Pesticide and Veterinary Medicines Authority (APVMA) to our findings and they are now working closely with our research team, vaccine registrants and the poultry industry to determine both short and long term regulatory actions,” she said.

“Short-term measures include risk assessment of all live virus vaccines currently registered by the APVMA in regard to the risk of recombination and could include changes to product labels, which may result in restrictions on the use of two vaccines of different origins in the one animal population.”

The ILT vaccines are “live attenuated vaccines: which means that the virus has some disease-causing factors removed but the immune system still recognises the virus to defend against a real infection.

“Live vaccines are used throughout the world to control ILT in poultry. For over 40 years the vaccines used in Australia were derived from an Australian virus strain. But following a vaccine shortage another vaccine originating from Europe was registered in 2006 and rapidly became widely used,” Dr Devlin said.

“Shortly after the introduction of the European strain of vaccine, two new strains of ILT virus were found to be responsible for most of the outbreaks of disease in NSW and Victoria. So we sought to examine the origin of these two new strains.”

The team sequenced all of the genes of the two vaccines used in Australia, and the two new outbreak strains of the virus. After analysis, they found that the new disease-causing strains were combinations of the Australian and European origin vaccine strains, and the new strains could cause more severe disease.

Fellow researcher Prof Glenn Browning said recombination was a natural process that can occur when two viruses infect the same cell at the same time.

“While recombination has been recognised as a potential risk associated with live virus vaccines for many years, the likelihood of it happening in viruses like this in the field has been thought to be so low that it was considered to be very unlikely to lead to significant problems,” he said.

“Our studies have shown that the risk of recombination between different vaccine strains in the field is significant as two different recombinant viruses arose within a year. We also demonstrated that the consequences of such recombination can be very severe, as the new viruses have been responsible for the deaths of thousands of Australian poultry.”