Some scientists are very concerned about the possibility that bird flu could jump from birds to cows and into humans.
That same scenario has already played out twice in the United States since the spring of 2024, when the country’s first cows became sick with highly pathogenic avian influenza (HPAI).
Researchers from Iowa State University have now conducted a study of two dairy cows in Texas that were infected with the H5N1 strain to better understand how these transmissions occurred.
In the respiratory tissue and mammary glands of both infected cows, the team found receptors used by a wide range of flu strains, including those from birds, pigs and even humans.
If all those receptors are together, it is possible that a virus that infects a cell could learn its ‘secrets’ and mutate to attach to other receptors on the cell, such as those commonly found in humans.
Although bird flu is deadly among birds, it appears to cause a sharp, short-lived, non-fatal drop in milk production when it infects cows. Dairy cows in the U.S. are the first confirmed cases worldwide, and experts still don’t know how the virus, which evolved to infect birds, spreads from cow to cow.
The new study points to the milking process as a mechanism of spread. It identifies H5N1-friendly receptors in the mammary glands of cows, helping to explain the unusual impact of the virus on the animals’ milk.
Dairy workers say that when a cow is infected, the milk looks thick and discolored. If the milk is raw and unpasteurized, scientists suspect it could be a source of transmission to other mammals, including ourselves.
According to officials at the U.S. Centers for Disease Control and Prevention (CDC), high levels of the H5N1 virus have been detected in contaminated cow’s milk. However, they note that “sporadic human infections without ongoing spread would not change the CDC’s risk assessment for the U.S. public, which CDC considers low.”
Fortunately, the two dairy farmers in the US who contracted the H5N1 virus from dairy cows showed only mild symptoms and recovered without infecting others. But when scientists analyzed the pathogen that infected them, it showed disturbing signs of adaptation to mammalian bodies.
Whether these adaptations occurred in the dairy cows or the dairy farmers is unknown, but scientists at Iowa State would like to learn more.
Influenza A viruses, such as H5N1, are known to infect cells through receptors made of sialic acids. Those strains of the virus that have evolved to infect birds prefer certain sialic acid receptors compared to those that have evolved to infect mammals. Even down to the species level, influenza A viruses are quite specific about where they attach to cells.
Researchers from Iowa State University and the University of Georgia are concerned about the expression and distribution of sialic acids they found in the respiratory tract and mammary glands of dairy cows infected with H5N1.
Both locations appear to be susceptible to avian influenza and there are signs of possible virus replication.
Like the coronavirus, the influenza virus is “inherently error-prone,” the authors write. “Infection of a new host species provides the virus with additional opportunities to replicate and subsequently mutate, making it more adaptable to new hosts.”
“The spread of [influenza A virus] “Transmission of the bacteria to tissues outside the respiratory tract is rare,” the research team writes.
They believe it is possible that the slightly acidic environment in the cow’s mammary gland, combined with the presence of sialic acid receptors, may make cows more susceptible to H5N1 infection.
These hypotheses require further investigation, but the findings highlight the “urgent need” to understand how the bird flu virus infects so many mammalian species.
It is also an important reminder for consumers to always drink pasteurized milk.
The research was published in Emerging infectious diseases.