AsianScientist (Jul. 16, 2012) – Researchers in Australia have shown that two different viral vaccines – used simultaneously in chickens – have combined to produce new infectious viruses.
Infectious laryngotracheitis (ILT) is an acute respiratory disease in poultry that is responsible for 20 percent of mortality in some flocks. Prevention of ILT is through the use of live-attenuated ILT vaccines – vaccines with some disease-causing factors of the virus removed but which are still capable of stimulating the production of immune cells to defend against a real infection.
University of Melbourne researchers have found that when two different ILT vaccine strains were used in the same populations, they combined into two new strains (a process known as recombination), resulting in disease outbreaks. Neither the ILT virus nor the new strains can be transmitted to humans or other animals, and do not pose a food safety risk.
The study, published on July 13 in the journal Science, was led by researchers at the Asia-Pacific Center for Animal Health at the University of Melbourne and the Victoria Research Laboratory.
Senior author 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 is significant as two different recombinant viruses arose within a year.
“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.”
According to Devlin, live vaccines are commonly used throughout the world to control ILT in poultry. Over the last four decades, the vaccines used in Australia were derived from an Australian virus strain, but a vaccine shortage gave rise to the use of another vaccine originating from Europe which was registered in 2006.
“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 New South Wales and Victoria. So we sought to examine the origin of these two new strains,” she explained.
The team sequenced the genome of the two vaccines used in Australia, and the two new outbreak strains of the virus. Following bioinformatic analysis on the resulting DNA sequence, the team found that the new disease-causing strains were combinations of the Australian and European origin vaccine strains.
Professor Glenn Browning, a co-author on the paper, said recombination was a natural process that can occur when two viruses infect the same cell at the same time.
“The study suggests that regulation of live attenuated vaccines for all species needs to take into account the real potential for vaccine viruses to combine. Measures such as those now being taken for the ILT vaccines will need to be implemented,” he cautioned.
The article can be found at: Lee SW et al. (2012) Attenuated Vaccines Can Recombine to Form Virulent Field Viruses.
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Source: University of Melbourne.
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