- Also called:
- avian influenza
- Related Topics:
- Bird
- Poultry
- influenza type A virus
- H5N1 virus
- fowl plague
bird flu
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What is bird flu?
Bird flu is a respiratory illness caused by type A influenza viruses that infect birds. While most cases of avian influenza in humans come from contact with infected poultry, the first known human cases were reported in 1997, when the H5N1 subtype infected people in Hong Kong.
The symptoms of bird flu in humans are similar to those of human influenza, including fever, sore throat, cough, headache, and muscle aches. A vaccine to protect humans against one subtype of the H5N1 virus was approved by the FDA in 2007.
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What causes bird flu in birds?
Bird flu in birds is caused by type A influenza viruses, with subtypes distinguished by variations in hemagglutinin (H) and neuraminidase (N) proteins on the virus's surface. Wild waterfowl are primary hosts, carrying the viruses in their intestines and spreading them through feces. This contaminates the environment and infects domestic birds through saliva, nasal secretions, and feces. The disease spreads between farms via contaminated materials or wild animals and across regions through migratory birds or international poultry trade. Some strains can be transmitted to other vertebrates including humans especially after undergoing mutation.
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How can bird flu spread from birds to humans?
Bird flu spreads from birds to humans through close contact with sick birds. Poultry farmers and slaughterhouse workers face the greatest risk of infection via virus-contaminated surfaces and intermediate hosts like pigs. Sick birds transmit the virus through saliva, nasal secretions, and feces. Within a region, the virus spreads from farm to farm through airborne, feces-contaminated dust and soil, contaminated clothing, feed, and equipment, or by wild animals carrying the virus on their bodies. Though person-to-person transmission is rare, it can occur. Symptoms in humans resemble those of human influenza, including fever, sore throat, and cough.
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What are common symptoms of bird flu in people?
Symptoms of bird flu in humans are similar to those of the human variety of influenza. They include fever, sore throat, cough, headache, and muscle aches, and they appear after an incubation period of several days. Severe infection can result in conjunctivitis or life-threatening complications like bacterial or viral pneumonia and acute respiratory illness.
The people at greatest risk of contracting bird flu are those in close contact with sick birds, such as poultry farmers and slaughterhouse workers.
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How is bird flu prevented and controlled in poultry farms?
To prevent and control bird flu in poultry farms, strict sanitary precautions, intelligent use of antibiotics and vaccines, and widespread use of cages have been effective. Rapid culling of infected farm populations and decontaminating farms and equipment remain the most effective methods for controlling outbreaks in poultry. Early detection using methods like polymerase chain reaction (PCR) is also important in preventing and controlling outbreaks. These actions reduce the chances of human exposure to the virus.
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Why is it important to monitor and report bird flu outbreaks?
Monitoring and reporting bird flu outbreaks is crucial for preventing and controlling epidemics. Early detection, often through methods like polymerase chain reaction (PCR), helps identify the virus in blood or tissue samples. Because the disease can spread rapidly between farms and regions via migratory birds or international trade, rapid culling of infected populations remains the most effective measure. Monitoring and reporting outbreaks is also essential to reduce human exposure to the virus, as humans in close contact with infected birds are at the greatest risk of becoming infected.
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News •
bird flu, a viral respiratory disease mainly of poultry and certain other bird species, including migratory waterbirds, some imported pet birds, and ostriches, that can be transmitted directly to humans. The first known cases in humans were reported in 1997, when an outbreak of avian influenza A virus subtype H5N1 in poultry in Hong Kong led to severe illness in 18 people, one-third of whom died.
Between 2003 and late 2005, outbreaks of subtype H5N1, the most deadly variety of bird flu, occurred among poultry in Cambodia, China, Indonesia, Japan, Kazakhstan, Laos, Malaysia, Romania, Russia, South Korea, Thailand, Turkey, and Vietnam. Hundreds of millions of birds in those countries died from the disease or were killed in attempts to control the epidemics. Similar culling events have taken place since then, including culls in countries in Africa, Asia, and the Middle East.
Bird flu in humans
According to the World Health Organization, about 900 people were infected with H5N1 between 2003 and 2024; more than 50 percent of those individuals died. The majority of human H5N1 infections and deaths occurred in Egypt, Indonesia, and Vietnam. The first human death from H5N1 in the United States was reported in early January 2025.
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Small outbreaks of bird flu caused by other subtypes of the virus have also occurred. A less severe form of disease associated with H7N7, for example, was reported in the Netherlands in 2003, where it caused one human death but led to the culling of thousands of chickens; since then the virus has been detected in the country on several occasions. In 2013 a strain of H7N9 capable of causing severe pneumonia and death emerged in China, with the first confirmed cases detected in February that year and dozens more reported in the following months. It was the first H7N9 outbreak reported in humans.
Symptoms of bird flu in humans resemble those of the human variety of influenza and include fever, sore throat, cough, headache, and muscle aches, which appear following an incubation period of several days. Severe infection can result in conjunctivitis or such life-threatening complications as bacterial or viral pneumonia and acute respiratory illness.
Subtypes of bird flu virus
Bird flu in avian species occurs in two forms, one mild and the other highly virulent and contagious; the latter form is called fowl plague. Mutation of the virus causing the mild form is thought to have given rise to the virus causing the severe form. The infectious agents of bird flu are any of several subtypes of type A influenza virus, which is classified as an orthomyxovirus. Other subtypes of this virus are responsible for most cases of human influenza and for the great influenza pandemics of the past (see influenza pandemic of 1918–19 and influenza pandemic (H1N1) of 2009). Genetic analysis suggests that the influenza A subtypes that afflict mainly nonavian animals, including humans, pigs, whales, and horses, derive at least partially from bird flu subtypes.
All the subtypes are distinguished on the basis of variations in two proteins found on the surface of the viral particle—hemagglutinin (H) and neuraminidase (N). H5N1, the cause of the 1997 bird flu outbreak in Hong Kong, was first identified in terns in South Africa in 1961. It has been responsible for the vast majority of laboratory-confirmed bird flu infections in humans and for the most devastating outbreaks in poultry. Eight other H5 subtypes have been discovered as well as nine H7 subtypes and nine H9 subtypes. Several H10 and H6 viruses have also been found in birds and in humans. A subtype known as H7N9 was first isolated from birds and humans in 2013, having caused severe illness in most infected persons.
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Avian influenza viruses are further distinguished as highly pathogenic avian influenza (HPAI) or low pathogenicity avian influenza (LPAI). With few exceptions (e.g., H5N1, H7N9), most H5, H7, and H9 subtypes are LPAI viruses. LPAI viruses can still cause mild to moderate disease, however. For example, infection with H7N2, H7N3, H7N7, or H9N2 can produce mild to moderate symptoms of illness in birds and humans.
To better understand the pandemic potential of H5N1, scientists have genetically altered H5N1 to make it transmissible between ferrets, which respond to influenza in much the same way that humans do. The possibility for its transmission to humans, however, raised concern about its potential use as a biological weapon.
Transmission
Waterfowl such as wild ducks are thought to be primary hosts for all bird flu subtypes. Though normally resistant to the viruses, the birds carry them in their intestines and distribute them through feces into the environment, where they infect susceptible domestic birds. Sick birds pass the viruses to healthy birds through saliva, nasal secretions, and feces. Within a single region, bird flu is transmitted readily from farm to farm by airborne feces-contaminated dust and soil, by contaminated clothing, feed, and equipment, or by wild animals carrying the virus on their bodies. The disease is spread from region to region by migratory birds and through international trade in live poultry. Humans who are in close contact with sick birds—for example, poultry farmers and slaughterhouse workers—are at the greatest risk of becoming infected. Virus-contaminated surfaces and intermediate hosts such as pigs can also be sources of infection for humans.
Although isolated instances of person-to-person transmission appear to have occurred since 1997, sustained transmission has not been observed. However, through a rapid evolutionary process called antigenic shift, two viral subtypes—e.g., one a bird flu virus such as H5N1 and the other a human influenza virus—can combine parts of their genetic makeup to produce a previously unknown viral subtype. If the new subtype causes severe disease in humans, spreads easily between people, and has a combination of surface proteins to which few people have immunity, the stage will be set for a new influenza pandemic to occur.