Vaccines aren't supposed to cause disease. But that appears to be what's happening on Australian farms.



Scientists have found that two virus strains used to vaccinate chickens there may have recombined to form a virus that is sickening and killing the animals.



"This shows that recombination of such strains can happen and people need to think about it," said Glenn Browning, a veterinary microbiologist at the University of Melbourne, Parkville, in Australia and one of the co-authors on the paper.



Chickens worldwide are susceptible to a group of herpesviruses called ILTV, which target their upper respiratory tract. The resulting disease, known as infectious laryngotracheitis (ILTV), reduces egg production and can kill up to one-fifth of those infected.



"The birds effectively choke to death on blood and mucus," said Browning. The disease is not known to infect any other animals other than chicken and chicken-like birds.



To combat ILTV, farmers vaccinate their chickens with attenuated herpesviruses that can still infect and replicate but do not lead to disease.



Australia has used two vaccines, which are produced by Pfizer and called SA2 and A20. In 2006, however, the country purchased a new vaccine from European company Intervet called Serva.



Two years later, new strains of ILTV, called class 8 and 9, appeared. They are just as deadly as other strains. "But they seem to be dominating over the strains that were reported prior to 2007," said Browning.



Because the new strains appeared shortly after the European vaccine was introduced, scientists thought that the new vaccine strain might have reverted back to a disease-causing form.



But when the researchers sequenced the genomes of the two new strains and the three vaccine strains, they found that the new viruses were actually stitched together from the European and Australian vaccines. Although it was not clear what mutations kept the vaccine strains from causing disease in the first place, they were probably lost when the viruses recombined, said Browning, whose team reports its findings online in Science.



"This is quite possible but a bit surprising since it would imply that both vaccines have gone into the same animal, which would be required for recombination to occur," Paul Farrell, a virologist at Imperial College London, wrote in a statement released by the Science Media Centre.



Farmers do not deliberately vaccinate with both vaccines, Browning agreed. But the SA2 strain might have spread into an unvaccinated population that was later vaccinated with the Serva strain, he suggested.



The data for the recombination is "convincing," said Walter Fuchs, who heads the National Reference Laboratory for Infectious Laryngotracheitis of Poultry on the island of Riems in Germany.



The combination of vaccine strains to form a new virus is "a problem that needs to be taken seriously," added Thomas Mettenleiter, head of the Federal Research Institute for Animal Health also on Riems.



Only well-characterised live vaccines, rendered harmless by mutations in the same or overlapping regions, should be used in order to minimise the risk of recombination to a new virulent strain, he argued.



Live-attenuated vaccines are also used in humans, but a lot less than in poultry, and their sequence is usually known. "This is not a panic-button on vaccines," said Browning.



And Farrell stressed vaccines had been one of the great success stories of medicine.



"The type of important technicality raised in this article should not be allowed to detract from the enormous health benefit generally provided by vaccines," he wrote.

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