When Vaccines Turn Vicious

July 12, 2012

12Jul (THE SCIENTIST) – The very vaccines used to prevent a respiratory disease in chickens caused several recent outbreaks of the same disease at farms across Australia, according to a report published today (July 12) in Science. Different weakened versions of a live herpes virus used in the vaccines exchanged portions of their genomes, resulting in virulent, disease-causing strains. This suggests that such in-the-field genetic recombination is more common than  previously thought, and has implications for both animal and human health.

“I think that we completely underestimate the role of recombination in [vaccine] viruses,” said veterinary virologist Etienne Thiryof the University of Liège in Belgium, who was not involved in the work. “All RNA and DNA viruses do recombine to different extents,” he said. In fact, recombination, also known as reassortment, was part of the method used by one of the two groups that recently succeeded in making the H5N1 bird flu virus transmissible between ferrets in a laboratory setting.

But while researchers were well aware that viruses could recombine, “we didn’t really think that recombination could be a problem in the field,” said Joanne Devlin, a veterinary scientist at the University of Melbourne who led the study. “We should probably reassess that risk.”

Prior to 2006, two closely related vaccines had been used to prevent infectious laryngotracheitis virus (ILTV) from spreading among chickens in Australia. This herpes virus causes mild or severe respiratory disease that can lead to reduced egg production in poultry, and sometimes death, resulting in substantial economic losses for farmers. In 2006, a third European ILTV vaccine, less similar to the other two, was introduced and, within a few years, outbreaks of the disease started to occur, in some instances killing nearly 18 percent of the birds.

WEAKENED VIRUSES USED IN VACCINES CAN SWAP GENES TO PRODUCE DISEASE-CAUSING STRAINS

Devlin and her team isolated viruses from infected chickens and found that they were similar to the European vaccine strain. They presumed that the weakened virus had somehow mutated into virulent forms. However, whole genome sequencing of the three vaccines and the disease-associated strains revealed that the European vaccine virus had recombined with one or both of the Australian vaccine viruses, generating new, more dangerous strains.

Although recombination of herpes virus strains can occur readily in the laboratory, for it to occur in the field, Devlin said, two separate strains would have to infect one cell in the same animal at the same time. “It was thought that just wouldn’t occur or would be so unlikely as to be not worth worrying about,” she said.

For more information visit  The Scientist.com

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