Two different vaccines used simultaneously to control an acute respiratory disease in chicken have combined to produce new infectious viruses in Australia, says new research from the University of Melbourne. The viruses cannot be transmitted to humans and are not a food safety hazard.
The acute respiratory disease - infectious laryngotracheitis, or ILT, has fatality rates of up to 20% in some flocks and is said to be a major economic burden to the livestock industry. For the last 40 years, the disease had been controlled by using a vaccine prepared from the Australian strain of the virus.
Due to vaccine shortage, a European vaccine was registered for use in Australia in 2006. Like the Australian vaccine, the European vaccine was of the 'live attenuated type'. It contained harmless or less virulent versions of the virus which could activate an immune response.
Soon after the introduction of the European vaccine, two new strains of ILT virus were found to be associated with the disease outbreak in the states of New South Wales and Victoria. To find out the origin of these new strains, scientists at the Asia-Pacific Center for Animal Health at the University of Melbourne, and NICTA's (previously known as National ICT Australia Ltd), analyzed the DNA of the vaccines and the new strains.
Genome mapping of the vaccines and the two new virus strains clearly showed that the new strains were combinations of the Australian and European vaccine strains. The research found out that genetic combination of the two strains of the vaccine occurred (a process called recombination), when they were used simultaneously on a population, leading to the outbreak of the disease. While the potential risk of such a recombination had always existed, the possibility of its occurrence in the field was thought to be very low till now.
"Comparisons of the vaccine strains and the new recombinant strains have shown that both the recombinant strains cause more severe disease, or replicate to a higher level than the parent vaccine strains that gave rise to them," said Sang-Won Lee, one of the lead authors of the study.
The findings of the study have been brought to the notice of Australian Pesticide and Veterinary Medicines Authority, or APVMA, which is now working closely with the research team, the poultry industry and vaccine registrants to determine both short and long term regulatory actions.
The researchers say that short-term measures could include assessment of the recombination risk of all live virus vaccines. It may lead to stoppages on the use of two vaccines of different origin in one animal population. It could also include changes to product labels.
The results were published in the international journal science on July 13.
by RTT Staff Writer
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