Three poultry vaccines meant to prevent respiratory disease have swapped genes, creating two new lethal strains of the virus that have killed tens of thousands of birds across two Australian states since they were discovered in 2008, according to scientists.

The strains appeared a year after Australia began using a European vaccine along with two similar Australian vaccines to fight acute respiratory disease in poultry. The virus usually kills roughly 5 percent of the birds it infects; this new type kills up to 17 percent. "What could have happened was one chicken was vaccinated with one vaccine and later was exposed to the other vaccine somehow, from nearby chickens," said Joanne Devlin, lead author of the paper published on the scientists' findings.

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Australia's agricultural authorities have been informed of the scientists' results and are working on how to prevent crossovers from happening in the future. "Use of only one vaccine in a population of birds will prevent different viruses from combining," said Devlin. "Authorities are reviewing labels on vaccine to change the way vaccines are used and prevent different vaccines being used in one population."