How Does Bird Flu Spread to Humans?

A global avian flu outbreak is infecting record numbers of birds and has spread to some mammals, raising questions in the scientific community about whether the variant circulating right now might jump to humans.

The outbreak is the largest on record in the United States, already infecting more than 58 million chickens, according to the Centers for Disease Control and Prevention (CDC). Every state has found cases in wild birds, and all but three had outbreaks in poultry.

Bird flu is also spreading to mammals, according to the U.S. Department of Agriculture (USDA). A variety of animals have been infected, including skunks, bobcats, foxes, raccoons, mountain lions, bears, possums, dolphins, seals, and coyotes.

Right now, the risk to humans is still very low, Tedros Adhanom Ghebreyesus, PhD, director-general of the World Health Organization (WHO), said in a February 8 statement. Since bird flu emerged in the late 1990s, transmission to humans has been very rare, “but we cannot assume that will remain the case,” he added.

Here, Richard Webby, PhD, an infectious disease researcher at St. Jude Graduate School of Biomedical Sciences in Memphis, Tennessee, and director of the WHO Collaborating Center for Studies on the Ecology of Influenza in Animals and Birds, answers the most pressing questions on bird flu.

Q: What is bird flu, and how is it different from other kinds of flu?

A: Bird flu is a terrible term. Essentially it encompasses a wide variety of different influenza viruses. In the current situation the best way to help people think about this is to think about COVID-19 with many variants. Influenza also has a range of different variants. The main types that cause seasonal flu in people in the United States are limited to four variants. For avian flu, there are 16 variants in birds. What we have right now [in the current outbreak] is one of those avian flu variants, the H5 virus.

Q: Why are so many birds and mammals getting bird flu?

A: The problem of H5 is that it has become very adept at infecting migrating birds like ducks. When birds move they bring the virus with them and they spread it to other bird species. Birds of prey like eagles or raptors are probably getting it from eating sick ducks. We are seeing mammals like foxes or skunks getting it, and they are scavenging and probably getting it from feeding on carcasses of infected birds.

Q: Can people get bird flu?

A: This virus does have the potential to transform from a bird virus to a human virus, but it seems right now to be very difficult for this particular H5 virus to jump to humans; the virus maintains all the characteristics of a virus that will stay within birds. But the risk is not zero, and people have been infected.

Q: What happens to people who get bird flu? Can they get very sick or die?

A: We have seen H5 for 25 years and in all this time we have seen hundreds of people get infected. We haven’t ever seen it move from human to human.

The human fatality rate is running at about 50 percent, but we have to take this number with a little bit of a grain of salt because we are only detecting people who get really sick and see a doctor. The virus is probably infecting more people who don’t get sick, who get infected and are asymptomatic.

What we do know about this virus is that it has a bigger potential risk for severe disease than seasonal flu because the infection can go deeper down into the lungs.

Q: Are there certain people who are more likely to get bird flu?

A: Anything you do that increases the risk of coming into contact with an infected bird could increase your risk. Over the past 25 years, most cases in humans have been in countries like China and Egypt, where many people keep birds and there are large poultry markets. Coming in contact with live or sick birds is the highest risk.

Q: Can you talk about bird flu symptoms, diagnosis, and treatment in humans?

A: Symptoms of bird flu can be similar to seasonal flu symptoms and you can get tested. There’s a generic test for flu, and if somebody has bird flu this would be positive. Then if it’s positive, there is a separate test for H5 that can detect bird flu. If people have flu-like symptoms and have been in contact with poultry or bird populations, that elevates the possibility that it might be bird flu.

Two drugs that work for seasonal flu can also work for bird flu — baloxavir (Xofluza) and oseltamivir (Tamiflu). The key is getting people treated early. The sooner people get treated, the better these drugs will work.

Q: What can people do to prevent catching bird flu? Is there a vaccine?

A: There is a licensed H5 vaccine in the United States, but it’s not for this particular version of H5 in circulation right now. It would probably provide some protection against what we’re seeing right now, though. The United States has some stockpiles of some H5 vaccines, and manufacturers know how to make it. But people can’t get a vaccine right now. It’s all being held within national stockpiles.

Q: What about my cat and my dog — can they get bird flu?

A: If they’re running around in the neighborhood and see a dead bird they chew on, there is a risk. Big cats in the zoo do catch it; it’s not like household cats and dogs are immune. We haven’t seen any evidence of infected cats or dogs passing it to their owners.

Q: Big picture: How worried should people be about bird flu right now? Is this the next pandemic?

A: It’s really difficult to put this into context. Right now, this is a virus that for a human is very, very difficult to catch.

But if there’s a flu virus I wouldn’t want to catch — it’s this virus. We’ve been following it for 25 years and it hasn’t become widespread in humans. But we haven’t seen it in South America before [it has been identified in animals there recently] and we’ve seen it only transiently in the United States in the past.

The risk may be very low for humans right now, but the relative risk is as high now as it has ever been. We know influenza viruses can make this jump from an animal virus to humans. Other types of bird flu — H1, H2, and H3 — have successfully made the leap.

With the history of knowing that flu viruses can do this, there is absolutely the worry about this becoming a pandemic. There are 16 types of flu in birds and only three have successfully made it to humans. Some people read this as the other 13 types probably don’t have the capacity to move to humans. I think they still have the potential to do this, but they don’t do it easily and it’s a much lower risk. It’s just not zero.

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