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 also been detected in mammals, including elephant seals, goats, foxes, and house cats.
Despite its widespread presence among animals, federal health authorities maintain that the risk to the U.S. public remains low. Only 46 confirmed human cases have been reported in the U.S. during the current outbreak. All but one of these individuals had a known exposure to affected poultry or cattle, according to the Centers for Disease Control and Prevention (CDC), and all their illnesses were mild. The CDC also states that there is no evidence that the virus is spreading from person to person at this time.
However, recent news has caused some concern. On Nov. 12, Canadian health authorities reported that an otherwise healthy teenager there who caught bird flu from an unknown source is in critical condition and struggling to breathe. This highlights the potential severity of the illness and its sometimes-mysterious spread.
In October, the U.S. Department of Agriculture raised alarm bells when it announced that a pig in Oregon had tested positive for bird flu. This is a worrying development because pigs can be infected with swine, human, and bird flu viruses, making them potential “mixing vessels,” according to 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 simultaneously, the viruses could potentially combine to form a new strain capable of spreading widely among humans, Davis explains. This was observed 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 is that flu season is underway in the U.S. As seasonal influenza virus circulates, “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 spread between 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 person 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 another led by a researcher from the University of Wisconsin-Madison—raised that possibility. The researchers isolated the bird flu strain that sickened the teenager 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 maintains, “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) awarded contracts to 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 bird flu vaccines, and shots are not publicly available.
Getting a regular seasonal flu shot does not protect against bird flu, but it’s a good idea, 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 has been detected in cows), avoid touching dead or sick animals, and get your seasonal flu shot.