Bird Flu (Avian Influenza H5N1): Symptoms, Spread, Diagnosis, Treatment, and Prevention

Article Summary

Bird flu, also called avian influenza, is caused by influenza A viruses that mainly infect birds. Some strains, especially H5N1, can occasionally infect people and other mammals. Human infection usually follows close contact with infected animals or contaminated environments. Symptoms can range from eye redness and mild flu-like illness to severe pneumonia. Diagnosis requires specialized laboratory testing. Antiviral medicines such as oseltamivir are used when human infection is suspected or confirmed. Most people are at low risk, but poultry workers, farmers, veterinarians, and others with frequent animal exposure face greater risk. Avoiding unprotected contact with sick animals is the main preventive measure.

Cite as: Bird Flu (Avian Influenza H5N1): Symptoms, Spread, Diagnosis, Treatment, and Prevention. Brisbane (AU): Exon Publications; 2024 Jul 07 [updated 2026 Sep 11].

Introduction

Bird flu is an infection caused by influenza A viruses that naturally circulate among birds. Most of these viruses do not infect people. However, certain strains can cross from animals to humans and cause illness. H5N1 is currently the avian influenza virus of greatest international concern because it has spread widely among wild birds and poultry and has also infected several mammal species. Human infections remain uncommon and are usually linked to close exposure to infected animals (1-3).

What Is Bird Flu?

Bird flu, or avian influenza, refers to infection caused by influenza A viruses adapted mainly to birds. Wild aquatic birds are natural hosts for many of these viruses and may carry them without appearing seriously ill.

Influenza A viruses are named partly according to two proteins on their surface: hemagglutinin, represented by the letter H, and neuraminidase, represented by the letter N. This produces names such as H5N1, H7N9, and H9N2.

Many avian influenza viruses remain largely restricted to birds. Others occasionally infect mammals or humans. H5N1 has received particular attention because modern H5N1 viruses have spread across large geographic areas and into an increasing range of animal species (1, 3).

What Is H5N1 and What Does Highly Pathogenic Mean?

H5N1 is a subtype of influenza A virus. Several different genetic groups of H5N1 have circulated over time. The currently widespread H5N1 viruses include viruses belonging to clade 2.3.4.4b, which have caused major outbreaks among wild birds and poultry and have also infected mammals (2, 3).

The term highly pathogenic avian influenza (HPAI) describes how severely a virus can affect poultry, especially chickens. It does not mean that every infected person will develop severe disease.

Human H5N1 infections have ranged from no symptoms or mild illness to severe pneumonia and death. Recent infections linked to occupational exposure in the United States have often been mild, with eye inflammation being particularly common, while human infections reported in other settings have sometimes been severe (1, 2).

How Does Bird Flu Spread to People?

Most human infections occur after direct or close contact with infected birds, mammals, or environments contaminated by their body fluids or waste.

Virus may be present in respiratory secretions, saliva, feces, and other material from infected birds. H5N1 virus has also been found in high amounts in milk from infected dairy cattle during outbreaks (2, 3).

Infection can occur when virus-containing material reaches the eyes, nose, mouth, or respiratory tract. This can happen while handling infected animals, slaughtering or culling poultry, cleaning contaminated areas, or working closely with infected livestock without adequate protection.

Recent investigations of infected dairy and poultry workers have strengthened evidence that occupational exposure is an important route of human infection (2).

Can Bird Flu Spread From Person to Person?

Avian influenza viruses do not currently spread efficiently between people. Limited person-to-person transmission has occasionally been reported with some avian influenza viruses in the past, usually after very close contact, but sustained human-to-human transmission of current H5N1 viruses has not been identified (1, 6).

This distinction is important. A virus that occasionally passes from an animal to a person is very different from one that spreads easily through communities.

Public health agencies closely investigate every human infection because influenza viruses can change over time. If an avian influenza virus developed the ability to spread efficiently and continuously between people, the public-health risk would increase substantially.

Who Is Most at Risk of Bird Flu?

People who regularly work with infected or potentially infected animals have the greatest risk.

Higher-risk groups can include poultry workers, dairy farm workers, veterinarians, people involved in animal culling, laboratory workers handling avian influenza viruses, wildlife workers, and people who regularly handle sick or dead wild birds (2, 5).

People keeping backyard poultry may also face increased risk during outbreaks if they handle infected birds without appropriate protection.

For people without these exposures, the risk is much lower. The joint Food and Agriculture Organization of the United Nations, World Health Organization, and World Organisation for Animal Health assessment published in 2026 continued to rate the global public-health risk from circulating H5 viruses as low. Risk for people with frequent occupational exposure was assessed as low to moderate, depending on local conditions and protective measures (6).

What Are the Symptoms of Bird Flu in Humans?

Bird flu can produce a wide range of symptoms. Some infected people have had no symptoms, while others have developed mild illness or severe respiratory disease (1, 2).

Possible symptoms include fever or feeling feverish, cough, sore throat, runny nose, headache, tiredness, and muscle aches.

Eye inflammation, called conjunctivitis, can cause redness, irritation, watering, or discomfort. Conjunctivitis was particularly common among several recent groups of people infected after exposure to dairy cattle or poultry (2).

More serious infection can cause shortness of breath, pneumonia, and severe breathing problems. Some people may develop complications affecting several organs.

Fever is not present in every case. A person's exposure history is therefore important when deciding whether bird flu should be considered.

How Is Bird Flu Diagnosed?

Bird flu cannot be reliably diagnosed from symptoms alone because many of its symptoms resemble seasonal influenza and other respiratory infections.

Diagnosis requires laboratory testing for influenza A virus, followed by specialized testing to determine whether an avian subtype such as H5 is present.

Samples may be collected from the nose or throat. When conjunctivitis is present, an eye swab may also be useful. People with severe respiratory illness may require samples from deeper in the respiratory tract (1, 4).

Because human infection with a novel avian influenza virus has public-health importance, testing is usually coordinated with public-health laboratories and authorities.

Routine home influenza tests cannot determine whether a person has H5N1.

How Is Bird Flu Treated?

Antiviral treatment should begin promptly when avian influenza is strongly suspected or confirmed. Treatment should not necessarily be delayed while waiting for final laboratory confirmation when a person has compatible symptoms and a significant exposure history (4).

Oseltamivir is the antiviral medicine most commonly recommended. It blocks an enzyme that influenza viruses use to release newly formed virus particles from infected cells.

Evidence from human infections and animal studies indicates that antiviral treatment is most useful when started early. Treatment may still be appropriate later in people who are seriously ill or hospitalized (4).

Other influenza antivirals may have activity against some avian influenza viruses, but treatment recommendations depend on the virus, illness severity, drug availability, and evidence of antiviral resistance.

People with severe disease may also need hospital care, oxygen, breathing support, treatment of complications, and other supportive care.

Can Antiviral Medicine Be Given After Exposure?

Antiviral medicine may sometimes be given to people who have had a high-risk exposure but have not yet developed symptoms. This is called post-exposure prophylaxis.

Whether prophylaxis is recommended depends on the type and intensity of exposure, whether protective equipment was used, the outbreak circumstances, and guidance from public-health authorities (4).

People who have been exposed to infected animals may also be monitored for symptoms for a period after the exposure. New respiratory symptoms or conjunctivitis during that period should be reported promptly so that testing and treatment can be considered.

How Can Bird Flu Be Prevented?

The most effective way for the general public to reduce risk is to avoid unprotected contact with sick or dead birds and other animals that may be infected.

People should not touch sick or dead wild birds with bare hands. Local animal-health or wildlife authorities should be contacted when unusual numbers of dead or sick birds are found.

People who work with infected poultry, livestock, or contaminated materials may need gloves, protective clothing, appropriate respiratory protection, and eye protection. Proper handwashing and safe removal of protective equipment are also important (5).

Equipment, clothing, vehicles, and work areas can carry contaminated material, so animal-control programs also use cleaning, disinfection, and other biosecurity measures.

Seasonal influenza vaccination does not specifically protect against H5N1. However, keeping seasonal influenza vaccination up to date can reduce the chance of becoming infected with seasonal influenza at the same time as an avian influenza virus.

Can Bird Flu Be Caught From Poultry, Eggs, or Milk?

Properly handled and thoroughly cooked poultry and eggs are not considered an important source of human H5N1 infection. Normal food-safety practices remain important.

Raw animal products require greater caution. H5N1 virus has been found in unpasteurized milk from infected dairy cattle. Pasteurization uses controlled heating to destroy harmful microorganisms and greatly reduces infectious influenza virus in milk (3).

Unpasteurized, or raw, milk should therefore be avoided, particularly where H5N1 is circulating in cattle.

People preparing poultry should follow normal food-safety practices, including preventing raw poultry from contaminating other foods, washing hands after handling it, and cooking poultry and eggs thoroughly.

Is There a Bird Flu Vaccine for People?

Vaccines against selected H5 influenza viruses have been developed, and some countries maintain H5 vaccine supplies as part of pandemic preparedness. Researchers continue to update vaccine candidates as H5 viruses change (3).

These vaccines are different from the seasonal influenza vaccine. Routine H5N1 vaccination of the general population is not currently a standard public-health program in most countries because human infection remains uncommon and sustained person-to-person transmission has not developed.

If the virus changes significantly or the public-health risk increases, vaccination strategies could also change.

Could Bird Flu Cause a Human Pandemic?

Bird flu viruses have pandemic potential, but pandemic potential does not mean that a pandemic is occurring or inevitable.

For H5N1 to cause a human influenza pandemic, the virus would need to spread efficiently and sustainably between people. Current H5N1 viruses have not demonstrated this ability (1, 6).

Scientists nevertheless monitor H5N1 closely because influenza viruses continually change. They can accumulate genetic changes, and influenza viruses can sometimes exchange gene segments when more than one influenza virus infects the same host.

The widespread circulation of H5N1 among birds and its ability to infect mammals create more opportunities for the virus to change. Surveillance of animal outbreaks, human infections, viral genetics, and antiviral susceptibility is therefore an important part of pandemic preparedness (3, 4).

What Is the Current Public-Health Risk?

Bird flu remains primarily an animal disease, even though sporadic human infections continue to occur.

As of the 2026 global assessment, international health agencies considered the overall public-health risk from circulating H5 viruses to be low. People who have frequent or occupational exposure to infected animals face a higher risk than the general population (6).

This assessment is not permanent. Influenza viruses evolve, animal outbreaks change, and new human infections are investigated continually. Public-health recommendations may therefore be updated when new evidence appears.

The current concern is not that H5N1 is spreading widely between people. The concern is that extensive circulation among birds and mammals creates continuing opportunities for human exposure and viral evolution.

Conclusion

Bird flu is caused by influenza A viruses that mainly infect birds, with H5N1 currently receiving the greatest public-health attention. Human infections remain uncommon and usually follow close contact with infected birds, livestock, or contaminated environments. Illness can range from conjunctivitis or mild flu-like symptoms to severe pneumonia. Diagnosis requires specialized laboratory testing, and antiviral treatment should begin promptly when infection is strongly suspected. Most people remain at low risk, while people with frequent animal exposure need greater protection. Avoiding contact with sick animals, using appropriate protective equipment, following food-safety measures, and maintaining strong animal and human surveillance are central to prevention.

References

1. Patauner F, Durante-Mangoni E, Bertolino L. Avian flu: a swooping threat or just a volatile menace? What clinicians need to know. Eur J Intern Med. 2025;136:27-30. https://doi.org/10.1016/j.ejim.2025.04.032


2. Garg S, Reinhart K, Couture A, et al. Highly pathogenic avian influenza A(H5N1) virus infections in humans. N Engl J Med. 2025;392(9):843-854. https://doi.org/10.1056/NEJMoa2414610


3. Neumann G, Kawaoka Y. Highly pathogenic H5N1 avian influenza virus outbreak in cattle: the knowns and unknowns. Nat Rev Microbiol. 2024;22(9):525-526. https://doi.org/10.1038/s41579-024-01087-1


4. Uyeki TM, Belser JA. Antivirals for novel influenza A virus infections. J Infect Dis. 2025;232(Suppl 3):S191-S209. https://doi.org/10.1093/infdis/jiaf296


5. Catalan Saenz HS, Cruz-Ausejo L. Preventive, safety and control measures against avian influenza A(H5N1) in occupationally exposed groups: a scoping review. One Health. 2024;19:100766. https://doi.org/10.1016/j.onehlt.2024.100766


6. Food and Agriculture Organization of the United Nations, World Health Organization, World Organisation for Animal Health. Updated joint FAO/WHO/WOAH public health assessment of recent high pathogenicity avian influenza A(H5) virus events in animals and people. Geneva: World Health Organization; 2026 May 18. https://www.who.int/publications/m/item/updated-joint-fao-who-woah-public-health-assessment-of-recent-high-pathogenicity-avian-influenza-a%28h5%29-virus-events-in-animals-and-people

This article is part of the 'Public Education Series' initiative by Exon Publications.

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