Bird Flu May Be Getting Better at Infecting Mammals

H5N1, known as bird flu, is becoming an increasing problem in the U.S. The virus normally only infects poultry and wild birds, but it's becoming more adept at infecting mammals; so far, it's infected cattle in 12 states. A small number of people are also falling ill—several dairy workers have been contracted mild illnesses and recovered.

The latest research published in Nature suggests that the current strain may be more adept at infecting mammals like cows. Researchers led by Yoshihiro Kawaoka, a professor at the University of Wisconsin—Madison and the University of Tokyo, report that the current version differs from previous H5N1 strains and has mutations that make it easier for the virus to infect mammalian cells. Kawaoka says that this ability “is more than what we saw in the past with avian H5N1 viruses, but it is still limited.”

The group tested virus obtained from infected cows in both mice and ferrets—the latter of which are a good model for how influenza viruses may act in people. Only one in four uninfected ferrets that were housed in the same cages as infected ones tested positive for H5N1. But how well a virus spreads depends a lot on how much virus is present, and at the moment H5N1 in dairy cattle does not appear to concentrate in the respiratory tissues—meaning it’s not saturating airborne particles the animals may exhale and inhale. Instead, it’s primarily found in the mammary glands, and therefore in the milk the cows produce.

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That’s a concern for dairy workers and anyone drinking raw, unpasteurized milk. “We should be concerned because there is a huge amount of virus present in milk,” says Kawaoka. “The biosecurity in farms is limited, so there is a lot of virus out there, and it’s very important to contain this virus as soon as possible.”

Kawaoka and his team recently quantified the amount of virus in the milk of an infected cow in a report published in the New England Journal of Medicine. They found a very high number of infectious particles in a single milliliter of raw cow’s milk. Pasteurization inactivated the virus, but Kawaoka says the effectiveness of the heat pasteurization depends on how long the milk is treated and how much virus is present.

If the virus doesn’t spread easily among ferrets, how is it transmitting so readily among cows? Kawaoka says it’s not likely that cows are actually passing the virus among themselves. Instead, because H5N1 is so highly concentrated in the mammary glands and milk, it’s likely that the virus is spreading via contaminated milking machines that aren’t cleaned thoroughly between each milking.

The results are reassuring in the sense that for now, H5N1 doesn’t seem to pass easily from animal to animal. But the fact that the virus is different from previous strains and has picked up the ability to more easily infect mammals is concerning, says Kawaoka. Health officials are closely monitoring samples from both infected cattle as well as the handful of dairy workers who have tested positive for H5N1 to ensure the virus isn’t mutating further to spread easily among people. If it does, they say they are ready with three vaccines, currently in the vaccine stockpile, that can protect against severe infections.