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Ohio State University Extension


Highly Pathogenic Avian Influenza

Veterinary Preventive Medicine
Scott P. Kenney, PhD, Assistant Professor, Ohio Agricultural Research and Development Center, Ohio State University Extension, Wooster

The flu is a contagious respiratory illness caused by influenza viruses. These viruses infect the nose, throat, and sometimes the lungs. Disease severity ranges from mild to severe illness and can cause death. The flu infects many different species including humans, pigs, horses, dogs, marine mammals, and birds, including wild birds and domestic poultry (Centers for Disease Control and Prevention 2017). There are four types of flu: Influenza A, B, C, and D (World Health Organization n.d.).

Flu A, B, and C can infect people, whereas D is currently thought to infect only cattle and pigs (Centers for Disease Control and Prevention 2021, and Snoeck C.J., et al. 2018). Influenza A infects the largest number of animal species and is typically responsible for pandemics.

Flu viruses are categorized into subtypes by two of their proteins: hemagglutinin (H) and neuraminidase (N). There are currently 18 H types and 11 N types of flu viruses.

The formula for naming a flu identifies the type of flu (A, B, C, or D) and the H and/or N subtype(s) of protein(s) present in the virus. For example, influenza A with a subtype 1 H and a subtype 2 N (Figure 1) is named A(H1N1), which was responsible for the 1918 Spanish flu and 2009 swine flu pandemics. Aquatic birds, such as ducks and geese, are the primary natural reservoir for most influenza A viruses. As hosts, they spread the influenza A virus worldwide during their natural migrations, which prevents eradication of the virus. Four of these bird migration pathways cross the United States, including major poultry producing areas (Figure 2).

A graphic displaying influenza types A, B, C, and D; listing avian, swine, and human subtypes of influenza A; and comparing highly pathogenic avian influenza versus low pathogenic avian influenza.

A 2012 USDA census map of broiler chicken production in the United States, overlaid with colors that display the four major waterfowl migration routes. Purple is titled Atlantic, chiefly covering Florida up through the east coast; yellow is titled Mississippi, running from Louisiana up through central northern states like Iowa, Wisconsin, Minnesota, and North Dakota; Orange is titled Central and runs mainly from Texas through Montana; and Green which is titled Pacific, covers California, Nevada, Oregon, and Washington.

Avian Influenza

All known subtypes of influenza A viruses can infect birds, except subtypes H17N10 and H18N11, which infect only bats (Centers for Disease and Control 2017). Influenza A viruses can be classified as “avian influenza” if they are first detected in birds. Humans can be infected with a number of avian influenza viruses including:

  • A(H5N1)
  • A(H7N9)
  • A(H9N2)

The majority of human cases of influenza A(H5N1) and A(H7N9) have been associated with direct or indirect contact with infected live or dead poultry. Controlling the disease in the animal source is critical to decreasing the risk to humans.

Highly Pathogenic Avian Influenza (HPAI)

Avian influenza A viruses are classified as low pathogenic avian influenza (LPAI) A viruses and highly pathogenic avian influenza (HPAI) based on the virus genes and ability to cause disease and death in chickens in a lab setting. LPAI viruses may cause no disease or mild illness (ruffled feathers and egg production drops), whereas HPAI causes severe disease and death. Both low and high pathogenic viruses can spread rapidly within a flock. HPAI H5 viruses that have infected birds in the United States have been caused by three subtypes: H5N1, H5N2, and H5N8. Most poultry outbreaks in the United States have been caused by HPAI (H5N2) viruses. To date, no human infections with HPAI H5 viruses have been identified in the United States, however 239 cases of human avian influenza infections with the A(H5N1) virus have occurred in Asia since 2003, resulting in 134 deaths (World Health Organization 2022).

Disease Severity of Bird Flu in Poultry

HPAI virus infection in poultry (H5 or H7 viruses) can cause disease that affects multiple internal organs resulting in a mortality rate of up to 100%, often within 48 hours. When H5 or H7 avian influenza outbreaks occur in poultry, infected flocks are typically depopulated or culled. The preferred method of stopping spread is to quarantine and screen flocks that are near or linked to the infected flock. If the virus is detected, these flocks are depopulated to stop further spread.

Disease Severity of Bird Flu in Humans

Infection of humans with avian flu strains results from very close contact with sick animals, is relatively infrequent, and can be deadly. Humans infected by A(H5) or A(H7N9) avian influenza viruses are typically sicker than humans infected with other types of flu. Initial symptoms are high fever (greater than or equal to 100.4 degrees Fahrenheit) and cough followed by difficulty breathing. Symptoms such as nasal discharge or congestion, sneezing, a sore throat, and a cough are less common. Complications of infection include severe pneumonia, respiratory failure, multi-organ dysfunction, septic shock, and secondary bacterial and fungal infections. The case fatality rate for A(H5) and A(H7N9) subtype virus infections among humans is much higher than that of seasonal influenza infections and can be over 50%. Human infections with avian influenza A(H7N7) and A(H9N2) viruses are typically mild or subclinical.

Notable HPAI Occurrences in North America

The 1983–84 HPAI H5N2 outbreak resulted in humanely euthanizing approximately 17 million chickens, turkeys, and guinea fowl in Pennsylvania and Virginia to contain and eradicate the disease. In 2015, HPAI A(H5N2) was associated with a large domestic poultry outbreak in North America that affected over 49 million domestic birds (International Steering Committee for Surveillance for Highly Pathogenic Avian Influenza in Wild Birds 2016). In late December 2021, H5N1 was detected in eastern Canada at an exhibition farm resulting in significant mortality prior to the remaining flock being euthanized (McCarthy 2021). On January 14, 2022, the United States Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) confirmed highly pathogenic Eurasian H5 avian influenza (HPAI) in a wild American wigeon in Colleton County, South Carolina. HPAI has since been detected in several other wild birds in the Carolinas, Virginia, Florida, and Maryland. Up to date detections can be found on the USDA APHIS website at

Action Items

  1. Poultry farmers can submit samples to the Ohio Department of Agriculture (ODA) Animal Disease Diagnostic Laboratory (ADDL) for HPAI testing. Directions for submitting samples are available at
  2. Avian influenza is a devastating pathogen to poultry flocks. It is essential to use all efforts to keep it out of your poultry houses.
  3. Create and review all line of separation (LOS) procedures (Figure 3) with anyone entering barns for any reason.

A graphic displaying how workers should change into biosecure clothing before entering a chicken house, how to enter chicken houses from a limited entry point, how to disinfect their boots upon chicken house entry and exit, and how to keep personal belongings out of the chicken house to prevent the introduction of disease.

  1. Reduce the number of people entering and exiting the barns and the number of trips those individuals make.
  2. Reduce the appeal of your farm for wild birds. Clean up spilled feed and remove open water sources such as filling puddles and covering culverts.
  3. Review hunting, fishing, trapping, and camping guidelines with all personnel. Influenza virus may be in and on the bird carcasses brought home. Dogs, their collars, your clothing, and your vehicles may all be contaminated.
  4. Anything that touches wild birds or their environments (mud, water sources, ground, etc.) should be considered highly dangerous to introducing influenza to poultry houses and should not have any contact with poultry flocks.
  5. Avoid contact with dead wild birds. Dead wild birds found on the farm should be avoided by farm personnel. Work out a pickup system that does not involve people who cross your line of separation.
  6. All garbage pickup trucks should remain outside of the farm perimeter buffer. If the dead birds are put in the garbage, don't allow that truck access to the farm.
  7. Off-site mortality disposal poses a constant risk. Pickup trucks that go from farm to farm put you into contact with dead birds from all farms on the pickup route.
  8. APHIS has materials about biosecurity, including videos, checklists, and a toolkit at More general influenza information is available at


Centers for Disease Control and Prevention. 2017. “Influenza Type A Viruses.” Influenza (Flu). Reviewed April 19, 2017.,HPAI%20viruses%20have%20been%20detected.

Centers for Disease Control and Prevention. 2021. “Types of Influenza Viruses.” Influenza (Flu). Reviewed November 2, 2021.

International Steering Committee for Surveillance for Highly Pathogenic Avian Influenza in Wild Birds. 2016. Highly Pathogenic Avian Influenza and North American Wild Birds: Frequently Asked Questions. Washington, DC: United States Department of Agriculture Animal and Plant Health Inspection Service. June 2016. PDF.

Iowa State University College of Veterinary Medicine. 2018. Understanding the Line of Separation. The Center for Food Security & Public Health. Powerpoint slideshow.

Iowa State University College of Veterinary Medicine. 2022. Poultry Biosecurity. The Center for Food Security & Public Health.

McCarthy, Ryan. 2021. “Canada Confirms HPAI at Exhibition Farm.” Meat + Poultry, December 28, 2021.

Snoeck C.J., Justine Oliva, Maude Pauly, Serge Losch, Felix Windschutz, Claude P. Miller, Judith M. Hubschen, et al. 2018. “Influenza D Virus Circulation in Cattle and Swine, Luxembourg, 2012–2016.” Emerg Infect Dis, Volume 24, Issue 7: 1388–1389.
DOI: 10.3201/eid2407.171937.

Webster, R. G., W. J. Bean, O. T. Gorman, T. M. Chambers, and Y. Kawaoka. 1992. “Evolution and Ecology of Influenza A Viruses.” Microbiological Reviews, Volume 56, Issue 1: 152–179. DOI: 10.1128/mr.56.1.152-179.1992.

World Health Organization n.d. “Influenza (Avian and Other Zoonotic).” Newsroom. Accessed February 11, 2022.

World Health Organization. 2022. Avian Influenza Weekly Update Number 827, Human Infection with Avian Influenza A(H5) Viruses, Manila, Philippines: World Health Organization, Western Pacific Region. February 4, 2022. PDF.

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Originally posted Feb 15, 2022.