One of the world's largest outbreaks of bird flu, which led to the slaughter of millions of chickens to limit its spread, appears to be spilling over into mammals.
The U.S. Department of Agriculture said the avian influenza A virus has been detected in mammals including skunks, bears, a raccoon and a red fox.
While most cases were detected in Oregon, positive tests in mammals were confirmed in the following states, according to the USDA's Animal and Plant Health Inspection Services:
Meanwhile in the United Kingdom, the Animal and Plant Health Agency found positive cases in otters and foxes. The agency also said a cat test positive in France and the highly contagious pathogen caused a large outbreak in a Spanish mink farm.
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How is the bird flu being transmitted?
Most of these infections are likely independent cases where a mammal eats an infected bird, said Jürgen Richt, professor and director of the Center on Emerging and Zoonotic Infectious Diseases at Kansas State University.
It may be happening more often during this outbreak as viral transmission increases among wild birds, like ducks, geese and swans.
The outbreak at the Spanish mink farm could be a special case where the virus transmitted from mammal to mammal, Richt said.
What does this mean for humans?
While more research is needed, it doesn't bode well for humans.
"If this virus has mammalian adaption and can transmit between mammals, humans are immunologically naive … and humans are mammals," Richt said.
The Centers for Disease Control and Prevention said the "bird flu situation remains primarily an animal health issue." Only four human infections have ever been reported in the U.S., the CDC said.
Bird flu is not a foodborne illness, so poultry and eggs found in grocery stores are safe to eat, Gregory Martin, a poultry educator at Penn State Extension, told USA TODAY last month.
Symptoms of bird flu in humans
The bird flu virus can cause mild to severe symptoms, including:
Runny or stuffy nose
Muscle or body aches
Shortness of breath
Less common symptoms include diarrhea, nausea, vomiting or seizures.
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Contributing: Amanda Pérez Pintado, USA TODAY. Follow Adrianna Rodriguez on Twitter: @AdriannaUSAT.
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This article originally appeared on USA TODAY: Bird flu H5N1 virus found in mammals. What does this mean for humans?