Vaccines combine to produce deadly virus

Vaccines combine to produce deadly virus





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.





Source:-
http://www.stuff.co.nz/science/7272611/Vaccines-Combine-to-Produce-Deadly-Virus