H5N1 avian influenza, more commonly known as bird flu, has infected more than 100 million birds in the U.S. and almost 500 dairy cattle herds across 15 states. The virus has popped up in mammals including elephant seals, goats, foxes, and house cats.
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Despite its prolific spread among animals, federal health authorities maintain that the risk to the U.S. public remains low. There have only been 46 confirmed human cases in the U.S. during the current outbreak. All but one of those people had a known exposure to affected poultry or cattle, according to the U.S. Centers for Disease Control and Prevention (CDC), and all of their illnesses were mild. The CDC says there is no evidence that the virus is spreading from person to person at this time.
Recent news, however, has some people feeling uneasy. On Nov. 12, Canadian health authorities announced that an otherwise healthy teenager there who caught bird flu from an unknown source is in critical condition and struggling to breathe, underscoring the illness’ potential severity—and its sometimes-mysterious spread.
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In October, the U.S. Department of Agriculture also raised alarm bells when it announced that a pig in Oregon had tested positive. That’s a worrying development, because pigs can be infected with swine, human, and bird flu viruses, making them prime “mixing vessels,” says Meghan Davis, an associate professor of environmental health and engineering at the Johns Hopkins Bloomberg School of Public Health. If a pig were to be infected with multiple types of influenza at the same time, the viruses could potentially combine to form a new strain capable of spreading widely among humans, Davis explains. That’s what happened during the 2009 “swine flu” pandemic.
“There are a lot of things I worry about,” Davis says. “This is one of them.”
Another cause for concern: flu season is underway in the U.S., and as seasonal influenza virus goes around, “humans, ourselves, could be a mixing vessel,” Davis says. If a person were simultaneously infected with bird flu and seasonal flu, the two viruses could theoretically combine to create a more transmissible strain.
The good news is that, as of now, there’s no evidence that the avian virus has undergone significant-enough changes to easily infect and spread among humans, says Troy Sutton, an assistant professor of veterinary and biomedical sciences at Penn State University.
Why have some people gotten sick at all, if the virus isn’t good at infecting humans? At a high-enough dose—if a farm worker is in close contact with sick animals, for example—the virus can sometimes get into human cells, even though it’s not built to do exactly that, Sutton says. But, crucially for public health, the virus doesn’t seem to have evolved in a way that makes it easy for those who get sick to pass on the illness to others.
That seems to be, in part, because the virus isn’t good at growing in the human nose, Sutton says. The seasonal flu is highly contagious because it takes root in the upper respiratory tract. When a sick person coughs, sneezes, or even talks, they may expel infectious respiratory droplets. The bird flu virus isn’t as prevalent in the upper airways, which seems to make it less transmissible among humans, Sutton says.
That’s not to say respiratory spread is impossible, though. Two recent studies in ferrets—one by researchers at the CDC, and one led by a researcher from the University of Wisconsin-Madison—raised that possibility. The researchers isolated the bird flu strain that sickened the first person infected in the current outbreak and tested how infectious it was among ferrets. Although it wasn’t as contagious as the seasonal flu, the bird flu virus was capable of spreading among ferrets by droplets, the researchers found.
But there are important caveats, Sutton says. Ferrets—while commonly used in influenza research—are not a perfect parallel for humans. And the studied strain is similar but not identical to the one spreading widely among cows right now. Overall, the CDC concluded, “the virus still is not capable of spreading efficiently among people via respiratory droplets compared to seasonal influenza viruses.”
Still, health authorities are getting ready in case that situation changes. In October, the U.S. Administration for Strategic Preparedness and Response (ASPR) gave $72 million to pharmaceutical companies that make H5 influenza vaccines, directing them to use the funds to get shots prepped “should they be needed now or in the future,” according to an agency statement. ASPR representatives said they took that step out of an “abundance of caution.” Federal health officials have not recommended that anyone get vaccinated against H5N1, and shots are not publicly available.
Getting a regular seasonal flu shot does not protect against bird flu, but it’s a good idea to get one anyway—especially for people who work or regularly come into contact with animals. The CDC also recommends that farm workers wear personal protective equipment, such as masks and goggles, and take flu antivirals as soon as possible in case of exposure.
But for the general public, health authorities say, there’s not much to do at this point. Just refrain from drinking raw milk (since the virus can persist without pasteurization), avoid touching dead or sick animals, and get your seasonal flu shot.
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