Flu season is not just for humans: every year, multiple strains of avian influenza (aka bird flu) virus wreak havoc on birds and poultry (not to mention poultry farmers) across the globe. 

Last year, highly pathogenic strains forced epidemiologists to euthanize hundreds of thousands of ducks, chickens, and turkey, from Norway to Egypt to Japan, in an effort to prevent the viruses from spreading. Earlier this week, bird flu outbreaks in Bulgaria led to a cull of almost 8,000 ducks.

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What the rapidly mutating pathogen will do this year is still anyone's guess, but epidemiologists are on high alert. "Bird flu is our number one, two, and three priority," says Michael Osterholm, Ph.D., director of the Center for Infectious Disease Research and Policy at the University of Minnesota. "For both birds and humans." 

Scientists have good reason to worry. The greatest flu pandemic in human history—one that killed 50 million to 100 million people between 1918 and 1919—started out as bird flu. And most experts agree that the next pandemic will arrive in much the same way. 

So far, contemporary reports of human bird flu infection around the world have been rare, and epidemiologists say that most of the strains now circulating have yet to leap into human hosts at all. But Osterholm and others say that could change at any moment. 

Here's a quick primer on the current state of bird flu outbreaks and what they mean for your health, safety, and holiday turkey.  

What Is Bird Flu?

Avian influenza refers to a collection of flu viruses that occur naturally in wild aquatic birds like ducks and geese. These viruses are very contagious among birds and can infect domestic poultry (usually when wild ducks intermingle with farm birds) along with other animal species.

There are many different subtypes of bird flu, and they mutate and swap genes with one another frequently. Epidemiologists are constantly monitoring these viruses to see which strains might pose a threat to commercial poultry operations or to humans.

How Does It Differ From Seasonal Flu?

Unlike seasonal flu, bird flu does not readily infect humans or spread easily from one person to another. In past outbreaks, when bird flu has made the leap to humans, the symptoms have been similar to seasonal flu. But bird flu has been far more lethal—in nearly half of all reported infections the patient died, frequently from pneumonia triggered by the virus, or from acute respiratory distress. 

The flu vaccine doesn't protect against bird flu, but you should still get your flu shot. While seasonal flu is less deadly than bird flu (meaning that any given infection is less likely to result in a fatality), it infects so many more humans than its avian cousin that it actually kills many, many more people every year—between 3,000 and 50,000 people, according to the Centers for Disease Control and Prevention. 

Where Is Bird Flu Spreading Now?

Along with the outbreak among birds in Bulgaria, an outbreak that spread to humans in China earlier this year sickened 759 people—almost as many people as in the four previous outbreaks combined.

And various strains of bird flu have recently circulated among birds in countries across Africa, Asia, and Europe. All of them have been categorized as "highly pathogenic," meaning that if they were allowed to spread unchecked, they could potentially cause up to 90% mortality in affected bird flocks. 

According to the World Health Agency, some of these strains have infected humans in the past; for other strains, the risk of human infection is low but cannot be ruled out. 

In 2015 and 2016, bird flu outbreaks were reported in several U.S. states, including California, Utah, Maryland, Washington State, and Indiana. In response to those outbreaks, affected bird flocks were culled (i.e., tens of millions of birds were euthanized in an effort to stop the virus from spreading). And as a result, the virus strains in question were eradicated from the U.S.

Osterholm says that the states seem to be free of the most worrisome avian flu strains at the moment, but that that could change quickly. The USDA conducts routine monitoring and surveillance of bird populations to detect emerging infectious disease threats like avian influenza.  

What About Human Infections?

Some strains of bird flu (namely the H7N9 variety) appear to have become increasingly virulent to humans—the outbreak earlier this year in China is one example.

But the Centers for Disease Control says that overall, human cases are still rare and sporadic. 

Avian influenza is passed from bird to bird via direct contact and through contaminated surfaces (infected animals shed the virus in their saliva, nasal secretions, and feces). For any given strain of bird flu to pass from birds to humans or from humans to humans, the virus itself has to mutate or swap genes with a human flu virus. While such mutations have clearly occurred in the past, they are impossible to predict ahead of time. 

So far, almost all human bird flu cases have resulted from close contact between humans and infected birds. People who butcher or pluck birds or spend time in areas contaminated with their blood and feces are the most likely to become infected.

In very rare cases, the virus can also pass from humans to other humans, though so far, this is even less common than bird-to-human transmission. According to the CDC, it is thought to have happened only in a handful of cases, almost all of which resulted from very close contact between an infected person and a caregiver.

Are Your Poultry Purchases Safe?

Yes. So far, there haven't been any reported cases of bird flu transmission through food. And if you live in the U.S., your poultry is especially safe. The current outbreaks haven't spread here, and the country doesn't generally import poultry from affected regions.

The USDA says that when outbreaks do occur, infected bird populations are quickly culled (also called “stamping out” or “depopulating”), and nearby flocks are then subject to additional surveillance.

Having said all that, you should always make sure that your poultry is well-cooked. It should be heated until the interior is no longer pink, and eggs should be cooked until the yolk is firm. 

What About Overseas Travel?

At the moment, bird flu poses very little risk to overseas travelers. But it's always a good idea to take certain health precautions when you travel. Check with the CDC's travelers’ health website for the latest updates on disease risks and for recommendations specific to your destination.

If you’re visiting a region that has been affected by avian influenza, you should avoid contact with chickens, ducks, and geese, and with live food markets, poultry farms, and other places likely to be contaminated with poultry excrement.