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At age 30, Dutch biologist Freek Vonk has built up a respectable career as a snake scientist. But in his home country,...
Since arriving on the island of Guam in the 1940s, the brown tree snake ( Boiga irregularis ) has extirpated native...
An animal rights group known as the Nonhuman Rights Project filed lawsuits in three New York courts this week in an...
Researchers have been hot on the trail of the elusive Denisovans, a type of ancient human known only by their DNA and...
Thousands of scientists in the Russian Academy of Sciences (RAS) are about to lose their jobs as a result of the...
Dyslexia, a learning disability that hinders reading, hasn't been associated with deficits in vision, hearing, or...
Exotic, elusive, and dangerous, snakes have fascinated humankind for millennia. They can be hard to find, yet their...
Researchers have sequenced and analyzed the first two snake genomes, which represent two evolutionary extremes. The...
- 5 December 2013 11:26 am , Vol. 342 , #6163
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Chicken Vaccines Combine to Produce Deadly Virus
12 July 2012 2:00 pm
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.