Chicken vaccines produce new viruses

Chickens are usually vaccinated when they are 7 days old. Image: Lepas/Shutterstock

Two herpesvirus vaccines have recombined to produce new virus strains.

The herpesvirus infectious laryngotracheitis virus causes mild to severe respiratory disease in poultry worldwide. Live attenuated viruses are used to vaccinate against this disease and, while herpes virus recombination has been seen before in the laboratory, this is the first time that recombination has occurred in the field.

The new viruses emerged in NSW and Victoria in 2008/2009 shortly after a new European-origin vaccine was brought into Australia during a shortage of the original Australian vaccine, according to lead author Dr Joanne Devlin from the University of Melbourne. The use of the different attenuated vaccines in the same poultry populations resulted in two recombinant viruses.

“To try to understand the origin of the new field strains, what we did is sequence the entire genome of the new field strains and we also sequenced the genome of the vaccine strains at the time,” Devlin says. The new viruses were found to contain similar elements to the two vaccines.

When tested under field conditions, they were also found to be more virulent than the vaccine strains that gave rise to them. They had high mortality rates, with an average of seven per cent, sometimes as many as 17 per cent, of the birds dying according to Devlin.

“These findings highlight the risks of using multiple different attenuated herpesvirus vaccines, or vectors, in the same populations,” the authors wrote in the paper published in Science.

But according to Professor Ian Gust, also of the University of Melbourne, there is little cause for humans to worry. Chickens and humans are vaccinated using different methods and with different strains of the herpes virus, minimising the likelihood of recombination.

In the case of humans, herpesvirus is delivered along with measles, mumps and rubella, which cannot combine to produce a new disease. “They are as distinct as a giraffe, a zebra, an elephant and an orangutan,” Gust says. “The viruses only breed with themselves, we don’t have interspecies breeding.”

Recombination concerns have also been raised with influenza vaccinations, especially with the emergence of the virulent H5N1 virus. But influenza vaccinations are performed well before the flu season and not given to anyone who is already sick in order to prevent recombination between different strains.

Gust also says that is unlikely that H5N1 will recombine with the easily transmitted human strains. “It’s not to say that it can’t happen on theoretical grounds, but you’d have to say the chances are probably the same as getting kicked to death by a duck.”

If you’d like to read more about vaccines and how they are changing, check out our vaccines feature in Issue 16.

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