Bird Flu Shows Increased Ability to Infect Mammals
H5N1, commonly known as bird flu, is becoming a growing concern in the U.S. While the virus typically only affects poultry and wild birds, it is increasingly able to infect mammals. Cases have been reported in cattle, and a small number of people have contracted mild illnesses and recovered.
Recent research suggests that the current strain of H5N1 may be more easily transmissible to mammals like cows. Led by Yoshihiro Kawaoka, researchers at the University of Wisconsin—Madison and the University of Tokyo, have identified mutations in the current H5N1 strain that enhance its ability to infect mammalian cells. While this adaptation is more pronounced than previous avian H5N1 strains, Kawaoka emphasizes that it remains limited.
The research team tested the virus obtained from infected cows in both mice and ferrets, the latter serving as a model for human influenza virus behavior. Only one in four uninfected ferrets housed with infected ones tested positive for H5N1. However, the virus’s spread is highly dependent on its concentration, and H5N1 in dairy cattle currently does not appear to concentrate in respiratory tissues. Instead, it is primarily found in the mammary glands, thus contaminating milk.
This poses a concern for dairy workers and consumers of raw, unpasteurized milk. Kawaoka warns, “We should be concerned because there is a huge amount of virus present in milk. 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 quantified the viral load in infected cow milk, published in the journal “Nature.” They discovered a high concentration of infectious particles in a single milliliter of raw cow’s milk. Pasteurization effectively inactivates the virus, but Kawaoka notes that its effectiveness depends on the duration of heat treatment and the initial viral concentration.
While the virus does not appear to spread easily among ferrets, its transmission among cows is not entirely clear. Kawaoka suggests that direct transmission between cows is unlikely. Instead, the high concentration of H5N1 in mammary glands and milk suggests that contamination through improperly cleaned milking machines between milkings is a more likely transmission route.
While the findings are reassuring in the sense that H5N1 does not readily transmit from animal to animal, the emergence of a new strain with increased mammalian infectivity is concerning. Health officials are closely monitoring samples from infected cattle and dairy workers who tested positive for H5N1 to track potential mutations and prevent widespread human transmission. If such an event occurs, a stockpile of three vaccines exists, capable of protecting against severe infections.