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.