H5N1 Bird Flu Is Increasingly Spreading Between Mammals

Health authorities have long suspected that bird flu is passing reliably between mammals, raising the possibility that persistent outbreaks could eventually develop in humans.

"This is one of the first times that we are seeing evidence of efficient and sustained mammalian-to-mammalian transmission," says Cornell University virologist Diego Diel.

H5N1 bird flu and the threat of mammal-to-mammal spread

Since 2022, the highly virulent B3.13 variant of the H5N1 virus has resulted in the culling of 90 million domestic birds in the United States alone. The CDC currently assesses the H5N1 strain as posing a low risk to the wider public, and the 11 people infected in the United States so far have reported mild symptoms.

However, one confirmed death in Mexico involving a vulnerable person infected with the related H5N2 virus is a sobering indication of the danger should the pathogen evolve.

"The concern is that potential mutations could arise that could lead to adaptation to mammals, spillover into humans and potential efficient transmission in humans in the future," explains Diel.

Much of the concern surrounding H5N1 stems from its capacity to acquire genes from other influenza viruses, alongside its ability to move readily between mammalian species. It has now been identified in mammals from the Arctic to Antarctica, including polar bears, otters, squirrels, dolphins, elephant seals, Antarctic fur seals and goats.

"I don't think we have a good understanding of the molecular reason why these viruses, with this combination of gene segments, with these mutations, now have this inherent ability of infecting many more species than in the past," molecular virologist Daniel Perez told Will Sullivan at the Smithsonian Magazine earlier this year.

How H5N1 spread through US dairy cattle

The virus responsible for the United States dairy cow outbreak was first genetically sequenced from a Canada goose (Branta canadensis) in Wyoming on 25 January 2024. That goose was discovered on the Central North American migratory bird flyway. By 25 March 2024, dairy cattle on farms in several states along the same flyway had become infected with this version of the virus.

Genetic analyses verified that the virus spread from cow to cow after apparently healthy dairy cattle were moved across state lines from an affected farm to a site well away from the flyway. The findings also showed spread in multiple directions: cats and a raccoon were infected, presumably after drinking raw milk from infected cows.

Cornell University geneticist Leonardo Caserta and his colleagues likewise found that this strain favours mammary gland tissue. This results in milk that is heavily contaminated, discoloured and unusually textured.

Hygiene, reporting and control measures

Authorities around the world are advising people to practise good hygiene in areas inhabited by wild birds, including thorough hand sanitisation.

They also want people to report groups of sick or dead birds (US, 1 866 487 3297; UK, 03459 33 55 77; AU, 1800 675 888). In animals, signs of the illness include sleepiness, poor coordination, circling movements, a tilted head and/or an inability to stand or fly.

Australia and New Zealand have not yet reported any H5N1 cases. Nevertheless, both nations are preparing for what they consider its inevitable arrival and the severe effects it is expected to have on their distinctive wildlife. Australia is also undertaking mass poultry culls because of H7 bird flu outbreaks, which are causing widespread egg shortages.

The United States has begun vaccinating California condors to limit the consequences for this critically endangered species, although many other animals are likely to require support too.

At the same time, scientists are working intensively to produce a vaccine for people.

Following H5N1's spread among dairy cows, the USDA now requires dairy cattle to return a negative virus test before being moved into another state.

"These cases highlight the zoonotic potential of the virus underscoring the need for robust measures to prevent and control the infection and further spread of H5N1 in dairy cattle," the paper's authors write.

"This would reduce the risk of the virus adapting in this new mammalian host species, thereby decreasing the pandemic risk to humans."

This research has been accepted for publication in Nature.