Around 1995, a form of highly pathogenic avian influenza known as H5N1 emerged in waterfowl in China. About a decade later, it began to infect more wild birds, and spread around the world. While it wreaked havoc in a variety of wild bird populations, it also started to affect farms where poultry was raised. Millions of birds died or had to be culled. For a time, other flu viruses became far more dominant. But by 2021, a new genetic form or clade of H5N1 was detected in wild birds around the world. Poultry farms were again affected. By 2022, over 50 million birds had died or been culled because of the virus. The number has only risen since then. While there are some vaccines available, there are many challenges to applying them effectively. Though there were some efforts to use such vaccines, H5N1 continued to spread, and started to show up in other animals.
Scientists have documented H5N1 infections in a huge number of wild animals. Some experts are growing increasingly concerned that so many infected animals are giving the virus untold chances to mutate into new forms, some of which may threaten people. It's long been assumed that this virus did not pose a major threat to the public, because people have to be infected after contact with an infected bird. Even when they are infected, there has not been any evidence that one person can spread H5N1 to another.
But some researchers are still sounding the alarm, and have suggested that we both do more to stop this virus from continuing to spread, and also that we start to develop vaccines for this strain of the flu. Right now, there are promising candidates for human H5N1 vaccines.
There is also new evidence that the concern is warranted. For the first time, researchers have determined that avian influenza is now moving, in some cases, from one infected mammal to another. The report, which was published in Nature, has shown that H5N1 infected cows, then spread to other cows, and then cats, and finally, a raccoon.
"This is one of the first times that we are seeing evidence of efficient and sustained mammalian-to-mammalian transmission of highly pathogenic avian influenza H5N1," said study co-author Diego Diel, associate professor of virology at Cornell University, among other appointments.
In this work, the investigators assessed samples of the virus that had been isolated from infected animals. This effort did not reveal any genetic mutations that are thought to promote the transmission of the virus in people. The researchers did find that some viral samples could move from one mammal to another, which means it may adapt even more in mammals.
Dairy cows that are infected with H5N1 probably first got it from wild birds. There are high viral levels in milk from infected cows, though it is thought that when applied correctly, pasteurization kills the virus in milk.
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This study also used whole genome sequencing to demonstrate that the virus moved from cow to cow after infected cows were moved from Texas to a farm in Ohio. After that, it moved to cats. The cats were likely drinking raw milk from the infected cows. A raccoon and wild birds also tested positive for the virus on the Ohio farm.
There have been eleven confirmed cases of H5N1 in people in the United States, and each individual only experienced mild symptoms. These cases were in agricultural workers who either helped to cull herds or worked with dairy cows. Four recent cases in Colorado are thought to be linked to the same county.
While people can be infected, the efficiency of viral replication is still low in humans. But the virus will keep changing.
"The concern is that potential mutations could arise that could lead adaptation to mammals, spillover into humans and potential efficient transmission in humans in the future," Diel said.
Right now, we must keep monitoring animals, and potentially infected humans, noted Diel. There are testing programs now available at no cost from the U.S. Department of Agriculture. We must make an effort to contain the virus and stop further spread, added Diel.
Sources: Cornell University, Nature