Live Avian Vaccines Recombine to Cause Disease They Were Intended to Prevent / Wired and ScienceNOW

From Wired and ScienceNOW comes this very worrisome piece about how two different, attenuated live virus vaccines for infectious laryngotracheitis in chickens recombined to form a virus twice as deadly as the disease the vaccines were intended to prevent.  (The death rate in chickens increased from 8% to 17%.)  Chickens in multiple parts of Australia were affected, beginning in 2008.

The new strains (termed class 8 and 9) have since become dominant, edging out the wild-type viruses.  There is no comment as to whether the strains have spread beyond the island nation of Australia.

Articles discussing problems with vaccine safety or efficacy always end with a statement that the information provided in no way challenges the great benefits of vaccinations, in general--and this article is no exception.  However, an article from the Australian Broadcasting Company makes clear the implications for humans as well as livestock are real:

Live vaccines, where a weaker version of the virus is introduced to allow the immune system to build up its own defences, are quite commonly used for animals and humans, and include polio, measles, mumps, rubella, chicken pox and rabies.

The Australian Pesticides and Veterinary Medicines Authority is now consulting with industry to control the use of these vaccines.

'Wider implications'

Co-researcher Professor Glenn Browning says other creatures could also be under threat of vaccines combining.

"We suspect that this sort of event could potentially happen in other animal species as well and with other viruses in addition to infectious laryngotracheitis virus," he says.

"So we believe that what we've seen here has potentially wider implications than just this particular disease in poultry..."

Farmed chickens are dying from a recombined vaccine. Image: NRCS/USDA

By Kai Kupferschmidt, ScienceNOW

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,” says 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,” says 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,” says 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 is not clear what mutations keep the vaccine strains from causing disease in the first place, they were probably lost when the viruses recombined, says Browning, whose team reports its findings online today 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 agrees. But the SA2 strain might have spread into an unvaccinated population that was later vaccinated with the Serva strain, he suggests.

The data for the recombination is “convincing,” says 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,” adds Thomas Mettenleiter, head of the Federal Research Institute for Animal Health also on Riems. Only well-characterized live vaccines, rendered harmless by mutations in the same or overlapping regions, should be used in order to minimize the risk of recombination to a new virulent strain, he argues.

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,” says Browning. And Farrell stresses vaccines have 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.