Can H3N8 Bird Flu Spread Between Humans?

The death of a Chinese woman, who was the first person to die from a type of bird flu rarely seen in humans, has raised questions about whether there is any danger of the virus being passed between humans.

News of the 56-year-old woman's death in China caused some alarm on social media as the world is just recovering from the devastating COVID pandemic that scientists believe may have been caused by a virus jumping from animals to humans.

The woman from the southern province of Guangdong contracted the A(H3N8) bird flu virus in February, before being hospitalized on March 3 with severe pneumonia, a statement from the World Health Organization (WHO) released on April 11 said. She subsequently died on March 16, 2023.

H3N8 is a type of flu virus that usually infects birds, but has previously infected other mammals, including dogs, horses and seals: one paper in the journal mBio from 2012 describes how 162 harbor seals died from an outbreak of H3N8 in 2011 in New England.

However, until the three recent cases of H3N8, no cases had been reported in humans.

"This virus like other influenza viruses is undergoing constant mutations and genetic reassortments, which have led to the emergence of new strains of H3N8," Shayan Sharif, a professor of immunology at the University of Guelph in Canada, told Newsweek. "The novel strain that has infected humans in China is different from other H3N8 strains in that it appears to be a reassortant with some of its genes closely related to another virus subtype, H9N2. Perhaps, these genes have enabled the virus to gain enhanced pathogenicity [ability to cause severe disease] in humans."

Luckily for us, this strain of H3N8 is currently thought to be unable to spread between humans, or even between mammals of other species.

"There is no evidence of mammal-to-mammal transmission of this virus," Sharif said. "Therefore, at the moment, the risk of transmission from animals to humans and sustained human-to-human transmission is very low. It appears that human infections are happening sporadically and they are occurring in individuals with a history of exposure to poultry."

The WHO also gave reassurances that, based on available information, "it appears that this virus does not have the ability to spread easily from person to person, and therefore the risk of it spreading among humans ... is considered to be low."

The Guangdong woman is thought to have caught the virus as a result of exposure to poultry, possibly at a market, while the previous two cases, both in children and both also in China, were also associated with close contact with poultry. Their close contacts are being tracked and monitored for symptoms, just in case.

"All of the known H3N8 cases in people were linked to exposure from birds, not horses, dogs, or people, with no evidence of person-person transfer afterwards," Treana Mayer, a postdoctoral fellow in microbiology at Colorado State University, told Newsweek. "This is good news, and let's hope it stays that way."

"The exact details as to why this subtype is infecting people for the first time are still being worked out," Mayer said. "Many other groups within Influenza Type A have caused a few human infections on rare occasion."

Jumping between species requires the virus to be able to bind to proteins in the cells of multiple species, which can arise from mutations in the virus' genetic code.

"Whether a bird flu virus can spread from one species to another depends on the tools that virus is equipped with," Mayer said. "The HA protein (hemagglutinin) acts as the key to unlock an animal cell to enter and cause infection. Some of these keys can unlock more types of cell doors than others. If the code for this key changes a lot, it might gain the ability to enter cells in a new species, like going from bird to mammal."

"Over long periods of time, a flu virus can adapt and become endemic in a mammal, meaning it is regularly circulating in that species. This includes our own seasonal flu in humans, which is less deadly but more contagious. For some H3N8 viruses, they are already considered endemic in horses (equine influenza) and dogs (canine influenza), spreading from horse-horse and dog-dog in stables and kennels," Mayer said.

The H5N1 virus is another type of bird flu that has been spreading across wild birds and poultry in the U.S. and worldwide. This also infects humans, but in very low numbers: according to the Centers for Disease Control and Prevention (CDC), fewer than 10 human cases of H5N1 have been reported globally since December 2021.

"It is mostly the uncertainty of what comes next that is unsettling, not that a newer virus is always going to be more dangerous than a previously known one," Mayer said. "Both H3N8 and H5N1 type bird flus have only caused isolated cases in humans, so it is not considered an outbreak in people just yet. Continued monitoring of new cases with more research as to how these viruses are evolving in people will be critical for preventing a bigger threat to public health in the future."

Therefore, while H3N8 may not be able to spread between humans, it is still indicative of how easily diseases can jump from animals to humans, and as a result, how vulnerable we are to another pandemic.

"We are living in times when humankind is squeezing the wild while expanding their footprint—as a result viruses and flu is what we are getting in return," Parwinder Kaur, an associate professor of biotechnology at the University of Western Australia, told Newsweek. "We are living in close proximity to these wild species more than ever before so zoonosis [an infectious disease transmitted from animals to humans or vice versa] crossing species borders is something we surely need to worry [about]."

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