The virus is also in milk
Studies have shown lower numbers of viruses in cows' nasal mucus and blood, but very high numbers in milk.
According to scientists, the H5N1 avian influenza virus is transmitted from cows to cats and raccoons. Their research provides further evidence for the transmission of viruses from one mammalian species to another.
Now scientists fear that further mutations could make the virus more dangerous to humans. The research by a team led by Diego Diel from Cornell University in Ithaca was published in the journal Nature.
From mammal to mammal for the first time
This is one of the first evidence of effective and sustained transmission of highly pathogenic H5N1 avian influenza from mammal to mammal, Diel said. Viral genome sequences obtained from dairy cows, birds, domestic cats and raccoons indicate the virus can be transmitted between animal species, the team writes.
However, the risk of human-to-human transmission remains low. Sequencing of the entire virus genome did not reveal any mutations that would lead to an increased possibility of transmitting the H5N1 virus to humans. However, the data clearly showed mammal-to-mammal transmission of the virus, which is concerning because the virus can adapt.
Transport of the virus by a cow in the USA
Beginning in 2005, a highly contagious variant of avian influenza spread from Southeast Asia across the globe, initially in wild birds and farmed poultry, and later in mammals. Diel and colleagues now examined viruses and animals on nine U.S. farms in four states that reported dairy cow illnesses in 2024. H5N1-infected cows ate less, had nasal discharge and feces, and produced less milk.
Five of the cows were in Texas, two in neighboring New Mexico, one in nearby Kansas and one in distant Ohio. On March 8, 2024, 42 apparently healthy dairy cows were transported from Texas to a farm in Ohio. Five days later, symptoms of the disease appeared in cattle on the farm of origin, and twelve days later also in cows on a farm in Ohio. These results indicate transmission of highly pathogenic H5N1 avian influenza between subclinically infected cows, said Diel's team, which also analyzed the viruses.
Virus reproduction in the mammary glands
Studies have shown lower numbers of viruses in cows' nasal mucus and blood, but very high numbers in milk. Scientists have discovered that H5N1 viruses of the B3.13 genotype can multiply in some mammary gland cells. This is further evidence that, in addition to respiratory infections, infection can also occur through milk and udders, for example through insufficiently cleaned milking machines.
Cats absorb viruses through milk
In several farms examined, the presence of dead cats fed with raw milk was found. A dead raccoon found on the farm could also have drank such milk.
Genetic analyzes reveal the development of viruses
Based on these and other findings, especially genetic studies, scientists outline the following sequence of events: The B3.13 genotype developed in an unknown host animal. It was first detected in a goose in Wyoming, a peregrine falcon in California and a skunk in New Mexico in early 2024. Another unknown host animal, possibly a wild bird, then passed the virus to cows, which then found other hosts – in addition to cats and raccoons, pigeons and other birds.
In the US, eleven human cases of avian influenza since April 2022 have so far been mild with mild symptoms. Infected people worked on poultry and cattle farms and probably became infected directly from the animals. Scientists now fear that the virus may continue to adapt, potentially increasing its infectivity and transmissibility to other species, including humans.
