Penn Vet | Fact Sheet Detail



Avian Influenza

By: Wildlife Futures Team Date: Nov 15, 2020

Other names: Bird Flu, Fowl Plague, Fowl Pest, Avian Flu, Type A Influenza

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In nature Avian influenza (AI), most commonly found in waterfowl and some shorebird species, is caused by various strains of the influenza A virus. These strains are subdivided into two categories based on the severity of disease they cause in poultry. Low Pathogenic Avian Influenzas (LPAI) cause only mild clinical disease, while Highly Pathogenic Avian Influenzas (HPAI) cause more severe clinical disease and spread more rapidly. LPAI strains are capable of mutating into HPAI strains. AI viruses are very contagious and apparently asymptomatic wild birds can infect domestic poultry as well as other bird and mammal species.


AI is a global disease with the potential to threaten wildlife, agriculture and human health. AI viruses are named based on the outer proteins H (Haemogglutinin) and N (Neuramidnidase) that can combine to create different strains such as H5N1 and H9N2. New AI virus strains evolve frequently through recombination of different subtypes of H and N across different species.

HPAI strains are of great concern to domestic poultry producers. In December of 2014, AI (H5N2) was reported in a flock of backyard turkeys in British Columbia, Canada and wild waterfowl in Washington State, USA. Over 40 million poultry were euthanized to control this outbreak. Since this outbreak, cases of HPAI (H5N2, H5N8, and a new H5N1) have been reported in commercial chickens and turkeys and backyard ducks, chickens, and turkeys in Arkansas, California, Idaho, Indiana, Iowa, Kansas, Minnesota, Missouri, Montana, Nebraska, North Dakota, Oregon, South Dakota, Washington, and Wisconsin. For more information regarding these outbreaks, see the APHIS website.

Species Affected

Wild birds are natural reservoirs for all known strains of the avian influenza virus. Aquatic birds such as waterfowl and shorebirds are most often associated with the virus, but many other wild bird species can be infected. The virus can be transmitted to domestic birds including chickens, turkeys, pheasants, quail, ducks, and geese.

AI is primarily adapted to birds, so it is not easily transmitted to mammals. However, pigs, horses, dogs, cats, marine mammals, and humans can rarely become infected. The virus can evolve, and some strains can become better adapted to infecting humans or other mammalian hosts.

Neurologic Wood Duck-Wildlife Futures 
Neurologic wood duck with HPAI Photo courtesy of CDC.


AI viruses are distributed worldwide and have been isolated on all continents except Antarctica. Waterfowl and shorebirds can carry the disease long distances during their natural migrations. The virus can also spread from country to country by importation of infected birds and bird products. Avian influenza can spread from farm to farm via movement of infected birds or people carrying the virus on their clothing, and especially boots, contaminated equipment, and feed.


The AI virus reproduces primarily in the intestinal tract of infected birds and is shed in high concentrations in the feces. The virus is also shed in saliva and nasal secretions and can become airborne. Susceptible individuals become infected when they have directed contact with virus shedding birds or more commonly, indirectly through a contaminated environment.

AI virus can survive for long periods of time in the environment, especially in water at colder temperatures. It can also survive on straw and animal bedding, equipment, and on clothes and shoes, meaning humans can spread the disease if proper hygiene is not maintained. AI is sensitive to most detergents and disinfectants as well as to heat and drying.

Humans and other mammals can also become infected following direct contact with infected wild or domestic birds or contaminated materials. Human to human transmission of AI is unlikely.

Clinical Signs

Clinical signs for AI viruses are markedly different from LPAI and HPAI and depend on which species of bird is affected.

Wild birds usually show no clinical signs when infected with LPAI.  Domestic poultry infected with LPAI may show no clinical signs or mild illness such as sneezing, coughing, ocular and nasal discharge, sinusitis, ruffled feathers, poor fertility and drop in egg production. Morbidity and mortality are low with LPAI unless accompanied by other infections or aggravated by environmental stressors.

In contrast, HPAI strains can cause severe, systemic disease. Different species of wild birds vary in susceptibility to infection and may show a variety of clinical signs depending on which organ systems are affected. In domesticated birds HPAI infections are fatal with mortality rates as high as 100% in a few days. The following clinical signs may be observed; a swollen head, blue discoloration and swelling of the neck and throat, blood-tinged oral and nasal discharge, respiratory distress (gaping beak, coughing, sneezing, gurgling, rattling), greenish diarrhea and bruising of the skin (due to haemorrhages). Birds that survive infection can develop neurologic signs such as torticollis (twisted neck), opisthotonos (backward-arching head), incoordination, paralysis and drooping wings.


Clinical signs and post-mortem examination may lead to suspicion of AI, but a definitive diagnosis is reached by virus isolation. AI virus can be isolated from throat and cloacal swabs and HPAI viruses can be isolated from internal organs at the time of necropsy. Genetic characterization of the strain is an important part of the diagnosis.


There is no treatment for AI in wild birds, but depopulation of domesticated species can control the spread of the virus. Studies have shown that AI vaccines can prevent clinical disease and death, reduce shedding of the virus and eliminate ongoing AI epidemics when used in conjunction with strict biosecurity, rapid diagnostics and surveillance and elimination of infected poultry.


Avian influenza viruses are continuously changing, posing a constant threat to animal and public health. The Centers for Disease Control and Prevention (CDC), the United States Department of Agriculture (USDA), the World Health Organization (WHO), the World Organization for Animal Health (OIE), and the Food and Agriculture Organization (FAO) conduct routine surveillance to monitor influenza viruses. Migratory wild bird populations are monitored by testing live birds, hunter killed birds, and the environment for the presence of AI virus. Vaccination of wild birds is not considered an effective preventative measure.

To prevent the spread of AI to domesticated flocks, strict biosecurity measures must be implemented such as wearing dedicated clothing and shoes that are disinfected between sties, spraying down car tires and disinfecting equipment between sites.

People who handle wild birds may also be at risk of being exposed to the virus. Hunters should practice routine hand washing, wear gloves when handling healthy wild birds and make sure to cook meat thoroughly to at least 165F. If AI is suspected additional personal protective equipment such as face shields, respirators and Tyvek suits may be necessary.

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