Two different vaccines have for the first time been discovered to have spontaneously combined to create new, infectious viruses.
The vaccines were used to control infectious laryngotracheitis (ILT), an acute respiratory disease occurring in chickens worldwide. But when two different ILT vaccine strains were used in the same populations, they combined into two new strains, resulting in disease outbreaks.
While neither the ILT virus or the new strains can be transmitted to humans or other animals, the news has prompted the Australian authorities to investigate.
The combining of live vaccine virus strains outside of the laboratory was previously thought to be highly unlikely.
"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," says Dr Joanne Devlin of the University of Melbourne.
"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."
The team sequenced the genome of the two vaccines used in Australia, and the two new outbreak strains of the virus to establish that the new disease-causing strains were combinations of the Australian and European origin vaccine strains.
Comparisons of the vaccine strains and the new recombinant strains showed that both the recombinant strains cause more severe disease, or replicate to a higher level than the parent vaccine strains.
"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," says Professor Glenn Browning.
"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."