In late March, a worker on a commercial dairy farm in Texas developed conjunctivitis. He eventually tested positive for the highly pathogenic H5N1 avian influenza A virus, also known as bird flu. Scientists were shocked to learn that cattle, not birds, were infected, and that large amounts of H5N1 were found in the infected cattle.
This year, four human cases have been confirmed after exposure to a dairy cow, and at least 143 dairy herds have been affected in a dozen states. Scientists have warned for years that bird flu could become another pandemic like COVID-19 (which isn’t exactly over). In other species, scientists have seen the H5N1 virus decimate populations of seals and cats. Now, a new study published in the journal Nature takes a closer look at the virus that infects cows, shedding light on the risk it poses to humans.
In the study, scientists found that after being isolated from an infected cow, the H5N1 virus spread to the mammary glands of mice and some ferrets.
“There’s good news and bad news,” Keith Poulson, study co-author and director of the Wisconsin Veterinary Diagnostic Laboratory, told Salon. The good news is that scientists have a model to infect lab animals and get them to shed the virus in their mammary glands. Specifically, in the study, scientists found that when mice and ferrets were infected with the virus, it spread to the brain, intestines, kidneys, heart, and lungs. It also spread to the mammary glands of mice and some ferrets. While transmission didn’t occur through direct contact between mice, female mice did pass it to their pups through their milk.
Typically, influenza viruses, like the ones that peak each winter but are much milder than bird flu, spread among mammals through droplets that are released into the air when an infected person coughs, sneezes or talks. This is called respiratory transmission. The study results suggest that the current strain is not very efficient at respiratory transmission. At first glance, this is good news. However, the latter part of the study left scientists concerned.
“The bad news is that this virus attaches to both sets of sialic acid receptors,” Poulson told Salon. These receptors are present in the upper respiratory tract of humans. In birds, H5N1 strains typically attack different sialic acid receptors. But it has now clearly adapted to mammalian receptors, which could give the virus greater ability to transmit between humans.
One of the main reasons why the H5N1 virus has not yet become a pandemic—and I emphasize the word “yet”—is that it is rather incapable of spreading from person to person. If it ever acquires this ability, which is quite possible, it could then easily spread within a population, creating a classic pandemic. In fact, this is exactly what SARS-CoV-2, a virus that first existed in bats, managed to do in 2019.
“So we really need to pay attention to this virus; we can’t let it continue to be endemic in our dairy herds,” Poulson said.
“We really need to pay attention to this virus.”
Indeed, in the final part of the study, scientists found that, unlike older strains of H5N1, this one has the ability to “double bind”, suggesting it might be possible for humans to become infected in their respiratory tracts – but scientists said they needed more information.
Poulson said that in light of the findings, he thinks it’s a good idea that last week, Human Services announced a contract with vaccine developer Moderna to make an mRNA vaccine that targets avian flu. “We have successfully learned the lessons of the COVID-19 pandemic and used them to better prepare for future public health crises,” HHS Secretary Xavier Becerra said in a statement. “As part of that, we continue to develop new vaccines and other tools to help fight influenza and strengthen our pandemic response capabilities.”
“We don’t want to find ourselves in a difficult situation like we did with COVID,” Poulson said. “It’s worth being prepared, so if that were to change, even though it’s very unlikely, if it were to change, we’d be ready.”
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Professor Ian Brown of the Pirbright Institute, who leads a research consortium investigating similar questions about receptor binding, said in a press release that “the predicted binding to cells provides new evidence for broader attachment, including to cells lining the human upper airway, but requires further study to understand the underlying factors.”
“Overall, the study results are not unexpected, but this report provides additional scientific insight into an evolving situation,” he said. “It underscores the need for rigorous monitoring and surveillance of affected or exposed populations, both animals and humans, to track future risks.”
Dr. Rajendram Rajnarayanan, of the New York Institute of Technology’s campus in Jonesboro, Arkansas, told Salon he was concerned by the latest study’s findings, but that the concern was not yet “alarming.”
“We expect certain things to happen if we don’t control the spread,” Rajnarayanan said. “It’s possible that if the virus is binding now, it’s trying to bind better. It’s always going to mutate and that’s a major concern.”
According to Rajnarayanan, genomic surveillance is essential to anticipate effective human-to-human respiratory transmission.
“In general, anything that infects the upper respiratory tract has a fairly high capacity for human-to-human transmission,” Rajnarayanan said. “So far, we haven’t seen that yet, but we have seen that this virus is able to bind to both receptors. That’s the information we have right now.”
Scientists should now turn their attention to how effective these connections are. “Once the virus is transmitted from human to human, it will spread in the same way as other potentially pandemic viruses,” he said.
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