Through administering two different vaccines to combat the acute respiratory disease, laryngotracheitis (ILT), which affects chickens internationally, researchers revealed that new strains of infectious viruses had been created. The discovery is said to have prompted a quick response from Australia’s veterinary medicines regulator.
ILT is responsible for up to 20% of deaths in some flocks and has a significant impact on the welfare and economic stability of the poultry industry, experts say. But through vaccinating with two different live ILT inoculations, a process known as recombination happens, which means two new strains of virus develop, resulting in further disease outbreak.
The research was led by Dr Joanne Devlin, Professor Glenn Browning and Dr Sang-Won Lee as well as researchers at the Asia-Pacific Centre for Animal Health at the University of Melbourne and the NICTA Victoria Research Laboratory.
Previous to research, experts believed the recombination of live vaccine viruses and virus strains outside a laboratory was extremely unlikely. However, Devlin said the study showed that it was “possible and has led to disease outbreaks in poultry flocks”.
Devlin explained: “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.” Experts said it was soon after the European vaccine was introduced that two new types of ILT virus were found. The new strains were thought to be the cause of the majority of the outbreaks in New South Wales and Victoria, which resulted in the research.
Research has also shown that the new strain of virus is a combination of the original and European vaccine strains and Lee believed the new strains caused “more severe disease” and could replicate to a higher level than the parent vaccine strains that gave rise to them.
According to Browning, recombination is 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,” said Browning.
The Australian Pesticide and Veterinary Medicines Authority (APVMA) were alerted to the findings and are now working with the research team to reduce the possible impact of the new strains and prevent similar future occurrences.
The vaccine registrants and poultry industry are now developing short- and long-term regulatory actions including risk assessment of all “live virus vaccines”.