Two virus strains used to vaccinate chickens against a deadly disease appear to be mutating into a single new virus that is sickening and killing birds on Australian farms, Science reports.

Chickens worldwide are susceptible to a respiratory disease called infectious laryngotracheitis, which kills one in five infected birds. To combat the illness, farmers vaccinate chickens with strains of the virus. In Australia, there are two vaccines that are widely used, one of which was introduced from Europe in 2006.

Two years after the new vaccine came into play, new strains of the deadly disease began to surface in Australian chickens. Scientists studying the situation say the new vaccine seems to be combining with the old one and mutating into a disease-causing form.

“This shows that recombination of such strains can happen and people need to think about it,” Glenn Browning, an Australian veterinary microbiologist, told Science in an interview.

Scientists studying the scenario say vaccines are an important disease management control that cause much less harm than good. But they said it’s important to be aware of the possibility of recombination and mutation.

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