H5N1 Bird Flu: What You Need to Know

[Originally published: May 16, 2024; Updated: Nov. 11, 2024.]

Note: Information in this article was accurate at the time of original publication. We encourage you to visit the websites of the Centers for Disease Control and Prevention (CDC), World Health Organization (WHO), and your state and local government for the latest information on H5N1.

Avian influenza A (H5N1), or bird flu, has killed millions of wild birds, and caused sporadic outbreaks among poultry and an ongoing multistate outbreak among cows in the United States. While there has been no known person-to-person transmission of the virus, there have been 46 reported human cases in the U.S. in 2024.

At the end of October, a pig on a backyard farm in Oregon was found to have the virus, raising a new concern, since pigs can be infected with both bird and human flu at the same time and serve as mixing pots, potentially creating a new flu virus that can spread to humans (more on that below).

So far, most of the human cases of H5N1 involved people working on farms where they were exposed to sick cows or poultry in Colorado, Michigan, Missouri, Texas, and, most recently, California and Washington. But one person, who was infected in September in Missouri, developed the illness despite reporting no contact with sick animals, which raised questions about how they might have been infected.

The people who have been infected with bird flu have reported mild illnesses. A few experienced eye infections, while others reported flu-like symptoms, including chills, coughing, fever, sore throat, and runny nose, according to the Centers for Disease Control and Prevention (CDC).

Meanwhile, the good news is that H5N1 is not considered a major public health threat in the U.S. at this time. The CDC has categorized the risk to the general public as “low,” and notes that people with exposure to infected animals are at higher risk of infection.

“In the short term, there is very little threat,” says Yale Medicine infectious diseases specialist Scott Roberts, MD. “In the long term, in the coming years or decades, however, I’m much more concerned.” He gives two reasons: One is that there has been a mortality (or death) rate of about 50% in the almost 900 people around the world who have been infected with bird flu between 2003 and 2024.

However, 50% may be an overestimate, Dr. Roberts says, adding that there may be cases where people have no symptoms, are only mildly symptomatic, or haven’t sought care for their symptoms. He also noted that if the virus did spread in humans, the percentage might be significantly lower if preventive approaches, including a vaccine, and treatments were made widely available.

Another reason for concern is that bird flu is now being detected in new animal species. In addition to outbreaks in cows and pigs—both relatively new developments for the bird flu virus— sporadic infections in some wild mammals were first detected in Canada and the U.S. in 2022, and are ongoing. “It’s natural to wonder if it’s only a matter of time before this virus is able to spread among humans,” Dr. Roberts says.

Below, Dr. Roberts answers eight common questions people have about bird flu.

"Avian influenza A (H5N1)," "bird flu," and "H5N1 bird flu" all refer to an illness caused by influenza type A viruses, which primarily affect birds. H5N1 bird flu was first identified in geese in China in 1996 and in people in Hong Kong the following year. Almost 25 years later, in 2020, a new variant of H5N1, referred to as Highly Pathogenic Avian Influenza (HPAI), was detected in wild birds in Europe; it was first seen in birds in the U.S. and Canada in late 2021, and has since been detected in a variety of wild bird species in all 50 states.

(The term “highly pathogenic” relates to how the virus impacts birds, not necessarily humans. There is also a low pathogenic avian influenza [LPAI] that usually causes mild illness in birds and poultry; however, some LPAI strains can mutate into strains that can cause severe illness and even death in poultry.)

In February 2022, the virus began causing sporadic outbreaks of HPAI H5N1 in backyard and commercial poultry flocks in the U.S., causing serious illness and death in infected chickens. The number of outbreaks has increased and spread over time—in November 2024, the CDC reported poultry outbreaks in 48 states.

In addition, there have been sporadic infections in mammals (including bears, bobcats, minks, mountain lions, raccoons, skunks, and others), according to the CDC.

The first reported case of bird flu in humans in the U.S. was reported in April 2022 in Colorado; a person reported mild symptoms after being exposed to poultry. (However, this case may have been a contamination of the nasal passages with the virus as opposed to an actual infection, according to the CDC.) More than two years later, in 2024, there have been 46 more reported cases involving exposures to poultry and cows.

At the same time, there is an ongoing multi-state outbreak in dairy cattle. The CDC reported the first of four cow-to-human transmission in April 2024 in a dairy worker in Texas; that case was described in a letter to the editor published in The New England Journal of Medicine. In the months that followed, more cases have been reported in other Western and Midwestern states.

Officials are still investigating the one case in Missouri in September in which the potential exposure remains unknown. The case was identified through the state’s flu surveillance system. The patient, who was hospitalized, had underlying medical conditions, and was treated with flu and antiviral medications before being discharged.

It’s possible that there are more human cases of bird flu than are being reported. This past summer, the CDC took blood samples from more than 115 farm workers in Colorado and Michigan, and found evidence of recent bird flu infection in eight of them, half of whom said they did not notice symptoms.

Both pigs and dairy cows are “new and unexpected" hosts of the H5N1 virus, and the spread of the virus among cows is a sign that it could mutate in them, making it easier for it to spread to other animals or potentially humans, according to the CDC.

Experts have been concerned about the possibility of the virus infecting pigs at some point, because pigs have been infected in the past with both human and animal flu viruses, adds Dr. Roberts. “Then, we have this sort of mixing pot, where you can get a genetic reassortment of different flu strains,” he says. In 2009, the virus H1N1, also known as “swine flu,” was the result of a reassortment of multiple virus strains in pigs; it spread rapidly in people and led to a worldwide pandemic that lasted more than a year.

While the risk of a bird flu infection is very low for most people in the U.S., “people who work with birds or poultry, own them, or have recreational exposure to them are at higher risk than other people,” Dr. Roberts says. “I would say that anyone who works on a farm in a state that has reported animal infections, where the farmer has identified illness in their animals, should monitor for flu-like symptoms and contact public health officials if any symptoms develop.”

The CDC, and state and local health departments, are monitoring people for 10 days after an exposure to infected birds, poultry, dairy cows, or other animals.

At this point, there is no publicly available H5N1 test, and commercial tests cannot distinguish human influenza A viruses (such as H1N1) from bird flu (H5N1). The CDC says tracking the specimens that test positive for influenza A can be helpful in monitoring for unusual increases in flu activity that may be an early sign of the spread of new influenza A viruses, including H5N1. Dedicated bird flu testing can only be performed at public health laboratories.

Any test for bird flu would need to be sent out to public health departments, says Dr. Roberts. “If you are concerned, alert your doctor, and they will need to talk to the state public health department,” he says. “However, at this point, I don't think they would authorize a test unless somebody was a farm worker or was working in an area or on a site where there was known H5N1 infection, since the risk for the general public is so low.”

Bird flu in the milk supply has been a major concern among consumers, but the Food & Drug Administration (FDA) has been sampling retail milk and other dairy products, such as cottage cheese and sour cream, as well as powdered milk formula and powdered milk products sold as toddler formula; it reports that although dead byproducts of the virus have been present, it found no live infectious virus, thanks to pasteurization, a food production process of heating liquids for short periods of time to kill harmful bacteria.

However, the FDA advises people not to drink, manufacture, or sell raw cow milk and products, such as cheeses, ice cream, and yogurt, made with raw milk. (Raw, unpasteurized milk and milk products are sold legally in most states.) While neither of the two human infections was linked to raw milk, more than half of a resident population of domestic cats on a farm in Texas died after they were fed raw, unpasteurized colostrum (milk produced during the first few days after birth) from H5N1-infected cows, according to a CDC report.

There also is no evidence that anyone in the U.S. has gotten bird flu after eating properly handled and cooked poultry products, but the CDC notes that uncooked poultry and poultry products (such as blood) may have caused a small number of H5N1 infections in people in Southeast Asia.

Poultry and other animal foods should be safe to eat as long as they are prepared properly, according to the CDC. This includes cooking poultry and eggs to an internal temperature of 165°F, which can kill viruses, including H5N1. (Use a food thermometer to be sure.) The FDA provides more information on raw milk on its website.

They range from no symptoms to mild flu-like illness to severe illness that requires hospitalization. The dairy worker infected in the U.S. in April reported mild illness, with the main symptom being conjunctivitis (irritation or inflammation of the conjunctiva, a clear mucous membrane that protects the eye). Other common symptoms are consistent with influenza, according to the CDC:

• Cough
• Fatigue
• Fever
• Headaches
• Muscle or body aches
• Runny nose
• Shortness of breath

There are currently no vaccines for bird flu, but the CDC reports that there has been preliminary work toward a targeted vaccine, in case one is needed. (The seasonal flu shot cannot prevent bird flu infection.)

If anyone were to become infected with bird flu—or is suspected of having an infection—the CDC recommends treatment as soon as possible with antiviral drugs.

There are several FDA-approved antiviral drugs for seasonal flu that can be used for bird flu, such as oseltamivir phosphate (available generically or under the brand name Tamiflu®) and baloxavir marboxil (Xofluza®). Both are oral prescription medications that should be taken as soon as possible, within 48 hours of experiencing symptoms.

Antiviral drugs can also be given soon after a person has had unprotected exposure to an infected animal or sick or dead poultry, meaning they did not wear gloves, face masks, eye protection, or other protective gear, according to the CDC.

Since bird flu isn’t spreading among humans, most people don’t need to worry about precautions, Dr. Roberts says. But people who are exposed to cows or poultry, especially in states where bird flu has been identified, should consider them, he says. So should hunters who handle wild birds and people who raise chickens at home.

One problem is that experts are still learning how bird flu is transmitted. “We understand much less about H5N1 transmission than we do about COVID,” Dr. Roberts says. But most people infected with bird flu get it from unprotected contact with infected birds, according to the CDC. The agency adds that infection also can happen when people inhale the virus in droplets or dust from the infected animals, or touch a contaminated surface.

The CDC has a list of recommended precautions, including:

• Avoid sick or dead birds. Check with your state or local government about their policy for reporting wild birds and other animals that look sick or are dead. Avoid wild birds even if they don’t look sick, since they can have an infection without symptoms.
• Don’t touch surfaces or materials that could be contaminated. That includes animal litter or feces, mucus, raw milk, or saliva.
• Avoid poultry farms and bird markets. If you must visit those places, wear a well-fitting face mask and don’t touch the animals.
• Use the following protective measures if you are unable to avoid contact with wild birds or sick or dead animals:
  • Wear gloves and, after removing them, wash your hands properly.
  • Wear an N95 respirator or a well-fitting mask.
  • Use eye protection, such as goggles.

It’s important to throw away your gloves and face mask after any exposure.

Check the CDC website for further advice for specific situations and groups, including hunters and people who raise chickens in their backyards. It also offers special advice for people who work with animals on farms.

For anyone who wants to keep up with the news about bird flu, the CDC is providing regular updates based on its flu surveillance systems.

Information provided in Yale Medicine articles is for general informational purposes only. No content in the articles should ever be used as a substitute for medical advice from your doctor or other qualified clinician. Always seek the individual advice of your health care provider with any questions you have regarding a medical condition.