Avian influenza, known more casually as bird flu and also called H5N1 influenza virus, is an infectious disease caused by a family of viruses that normally infect birds. Beginning in 1997, a subtype of avian influenza began infecting and causing a small number of deaths in humans worldwide.
The Avian influenza H5N1 influenza virus was first isolated in terns (migratory shore birds) in South Africa in 1961. The virus is highly contagious and sometimes fatal in birds; it did not appear to cause disease in humans until 1997. In that year in Hong Kong, H5N1 bird flu caused 18 confirmed cases of severe respiratory disease in humans, of which six were fatal. All but one of the individuals diagnosed with bird flu in Hong Kong had close contact with infected poultry. The exception was a case in which the disease was transmitted from child to parent but spread no further. In 2013, an Asian lineage of avian flu (H7N9) was reported in China. As of fall 2017, six total epidemics of the H7N9 flu had occurred in China. In all, a total of 1564 confirmed human infections with H7N9 had occurred by late 2017. In February 2018, the first human case of a new type of avian flu (H7N4) was reported, also in China. The avian flu has been detected several times in the United States in commercial operations, including a turkey farm in Missouri and a chicken farm in Texas. There were no reports of human infection with these findings.
According to the World Health Organization (WHO), between 2003 and 2017, there were 860 laboratory-confirmed cases of avian influenza in humans worldwide resulting in 454 deaths. The disease can affect people of all ages, genders, and ethnicities. Almost all individuals who have developed avian flu were in close contact with infected wild birds or poultry; only a few contracted the disease from extended contact with a sick family member. As of late 2017, nearly all confirmed cases of avian flu in humans had occurred in Asia or Africa, although one case and death was reported in Canada in 2013.
There are three types of influenza viruses: types A, B, and C. Only influenza A viruses causes serious, widespread illness in humans. Several different strains or subtypes of influenza A virus can cause infection in humans. Other subtypes of influenza A cause disease in birds, pigs, horses, ferrets, whales, and seals. In most cases, the subtypes that cause disease in one species do not spread the disease to other species. If they spread to humans, they are considered zoonotic diseases.
Influenza viruses are protein sacks with a core of eight loosely connected genes. Two proteins spike from the surface of the virus. The most plentiful protein is called hemagglutinin (H). The other surface protein is neuraminidase (N). The job of these proteins is to hook the influenza virus on to the surface of a cell in the animal it has infected and then help get the virus's genes inside that cell. Once inside, the virus gene takes over the host cell and reprograms it to make thousands of copies of the virus.
There are many subtypes of influenza A viruses. These are identified by a series of letters and numbers that refer to two surface proteins, H and N. There are 16 known types of H proteins and 9 known types of N proteins of influenza. The structure of the genes in the influenza virus allows these viruses to mutate (change) rapidly. Many combinations of the two proteins can result from these mutations, each representing a new subtype of influenza A.
A total of 16 different avian flu H subtypes and 9N subtypes can infect birds. Normally the subtypes of influenza that infect birds do not harm humans, although some can infect pigs. However, some avian influenza viruses have mutated in such a way that they can rarely infect humans. Many of the flu viruses affecting birds are of low pathogenicity and therefore difficult to detect among large and wild bird populations.
Avian flu seldom infects humans, but people can get the virus from bird saliva, mucus, or feces. Individuals then inhale the virus or get it into their eyes, nose, or mouth.
People at greatest risk of contracting avian flu are those who live or work closely with poultry. Potentially vulnerable people include those working on poultry on farms or in poultry processing plants or in live bird markets. Individuals living with a family member who has avian flu are also at risk of developing the disease.
Avian flu does not spread easily to humans, and many people who are exposed to infected birds do not get sick. Travelers to Asia and Africa who expect to handle birds or visit poultry farms should check the CDC Travelers' Health website for information on the country to which they are traveling.
An influenza virus that birds carry in their intestines causes avian flu. The virus spreads as infected birds excrete the virus through their saliva, nasal secretions, and feces. Birds vulnerable to the flu become infected when they come into contact with the excretions or surfaces contaminated by the virus.
Birds that survive the H5N1 infection can excrete the virus for at least ten days. This allows the H5N1 strain to spread through bird-to-bird contact from wild birds to domestic birds on farms and in live bird markets. The virus can also spread on surfaces, including manure, bird feed, equipment, vehicles, egg flats and crates, and the clothing and shoes of people who are exposed to the virus. Influenza is a respiratory disease. People must inhale the virus or carry it on their hands to their nose, eyes, or mouth to become sick. People cannot get the disease by eating properly cooked poultry.
In general, people who contract bird flu have symptoms similar to seasonal human influenza, including fever, cough, sore throat, fatigue, and aching muscles. Other symptoms are eye infections (conjunctivitis), pneumonia, difficulty breathing, and viral pneumonia. Influenza weakens the respiratory system and leaves the lungs vulnerable to infection. Many people with flu die from pneumonia caused by a secondary bacterial infection. Bacteria can grow in the lungs because the body's defenses have been weakened by the flu virus.
The symptoms of avian flu and seasonal influenza are similar. A physician may suspect avian influenza if an individual showing flu-like symptoms has been in close contact with poultry in an infected area. Nevertheless, laboratory testing is needed to confirm a diagnosis of avian influenza. Symptoms normally develop within seven days of exposure.
In April 2009, the U.S. Food and Drug Administration (FDA) approved a rapid detection test for the H5N1 strain of bird flu. All it requires is a swab from the nose or throat of an ill individual. This test produces results in about 40 minutes. Previous bird flu tests took at minimum four hours to complete. People suspected of having avian flu should have the test as soon as possible after signs and symptoms of the illness begins.
Some antiviral drugs appear to be partially effective against avian flu viruses if they are administered promptly, usually within 48 hours after the start of symptoms. These drugs do not cure flu, but they lessen its symptoms and duration. In the United States, three drugs have been approved by the U.S. Food and Drug Administration (FDA) for the treatment of influenza A viruses in otherwise healthy adults. These are in a class of drugs called neuraminidase inhibitors. Those used to treat avian flu are oseltamivir phosphate (Tamiflu), zanamivir (Relenza), and peramivir (Rapivab).
Antibiotics might be needed to treat secondary bacterial infections. Acetaminophen can be used to control fever and reduce aches.
In March 2005, people in South Korea began eating more kimchi to ward off avian flu infection, according to reports from the British Broadcasting Company and other news organizations. The public turned to the spicy vegetable dish after scientists at Seoul National University announced that kimchi aided in the recovery of 11 out of 13 infected chickens. The scientists fed the birds an extract of kimchi, a dish made by fermenting cabbage with red peppers, radishes, and large amounts of garlic and ginger. A week later, all but two birds showed signs of recovery. The researchers acknowledged that their study was unscientific. Research continues on whether bacteria in kimchi can help people who have avian flu.
Much of the treatment for influenza is supportive and consists of bed rest, drinking plenty of fluids to stay hydrated, and using a humidifier to relieve nasal congestion and ease breathing.
Although avian influenza in people is rare, scientists at WHO, the Centers for Disease Control and Prevention (CDC), and public health agencies of many other countries are concerned about the deadly consequences that could occur if H5N1 and other virus subtypes further mutate so they can spread easily from one human to another. A strain of bird flu spread by human-to-human contact could cause an influenza pandemic and sicken millions. WHO and CDC experts believe the question is not if another influenza epidemic will occur, but when it will happen and how severe it will be. For this reason, prevention strategies and pandemic planning are extremely important.
About 60% of individuals who develop avian influenza die. The rest typically recover completely.
As of 2018, bird flu was primarily a risk for people in infected areas who work with poultry. People working with birds in locations such as commercial poultry facilities, veterinary offices, and live bird markets should wear protective clothing. That equipment includes boots, coveralls, face masks, gloves, and headgear, according to the United States Department of Agriculture (USDA). Still, all people should avoid sources of exposure when possible.
Furthermore, poultry producers should implement security measures to prevent the outbreak of a highly pathogenic virus. Those actions include keeping flocks away from wild or migratory birds and providing clothing and disinfectant facilities for employees. Plastic crates are recommended for use at live bird markets because they are easier to clean than wood crates. Cleaning and disinfecting areas are also important for preventing an outbreak. Infected birds must be quarantined or destroyed.
In July 2007, FDA approved a vaccine for use against H5N1 bird flu. In 2013, the FDA approved a second vaccine for use in the United States in preventing H5N1 influenza. The 2013 vaccine is not commercially available but can be administered by public health officials as needed.
See also Flu pandemic ; H1N1 influenza A (2009) ; Zoonoses .
Ryan, Jeffrey. Biosecurity and Bioterrorism: Containing and Preventing Biological Threats. 2nd ed. Cambridge, MA: Elsevier, 2016.
Taylor, Kyle M., and Bruce O'Connor. Avian Influenza: Molecular Evolution, Outbreaks and Prevention/Control. Hauppage, NY: Nova Science, 2016.
Ramsey, A. M., et al. “Lessons Learned from Research and Surveillance Directed at Highly Pathogenic Influenza A Viruses in Wild Birds Inhabiting North America.” Virology 13, no. 518 (February 13, 2018): 55–63.
Schneider, Elena K., Jian Li, and Tony Velkov. “A Portrait of the Sialyl Glycan Receptor Specificity of the H10 Influenza Virus Hemagglutinin, a Picture of an Avian Virus on the Verge of Becoming a Pandemic?” Vaccines (Basel) 5, no. 4 (December 2017): pii–E51.
Centers for Disease Control and Prevention (CDC). “Information on Avian Influenza.” http://www.cdc.gov/flu/avianflu (accessed March 12, 2018).
MedlinePlus. “Bird Flu.” http://www.nlm.nih.gov/medlineplus/birdflu.html (accessed March 12, 2018).
World Health Organization (WHO). “Avian and Other Zoonotic Influenza.” http://www.who.int/influenza/human_animal_interface/en/ (accessed March 12, 2018).
World Health Organization (WHO). “Cumulative Number of Confirmed Human Cases of Avian Influenza A (H5N1) Reported to WHO.” http://www.who.int/influenza/human_animal_interface/H5N1_cumulative_table_archives/en/ (accessed March 12, 2018).
Centers for Disease Control and Prevention (CDC), 1600 Clifton Rd., Atlanta, GA, 30333, (404) 639-3534, (800) 232-4636, http://www.cdc.gov .
Tish Davidson, AM
Revised by Teresa Odle, BA, ELS