There are two groups of avian influenza viruses (AIVs), categorized by disease severity (pathogenicity) in poultry: low pathogenic avian influenza (LPAI) viruses and highly pathogenic avian influenza (HPAI) viruses. Human avian influenza (AI) has been associated with both HPAI and LPAI viruses (e.g., various H5, H7 and H9 strains).1
Although AI outbreaks affect the United States, countries more often linked to AIV outbreaks are located in Asia and the Middle East, such as Bangladesh, China, Egypt, India, Indonesia, and Vietnam.1
Surveillance indicates that AIVs are routinely present in wild bird populations in the U.S., and since 1997, there have been sporadic outbreaks of LPAI in several commercial poultry flocks in the west and Midwest.2 Concern increased with HPAI virus detection in U.S. commercial poultry flocks in 2004, the first outbreak of HPAI (H5N2) in the U.S. in 20 years, and in 2015, with reports of HPAI virus outbreaks in 21 U.S. states.
Low Pathogenic Avian Influenza (LPAI) Viruses
LPAI viruses pose a minimal threat to human health.
Wild, aquatic birds are the natural reservoirs (i.e., hosts) for LPAIs worldwide.3,4 Such birds include seagulls, terns and shorebirds, as well as waterfowl, such as ducks, swans, and geese. These wild, aquatic birds often have lung and intestinal infections without being sick. LPAI viruses generally cause minor illness in domestic poultry, such as chickens, ducks, and turkeys, although severe illnesses can lead to death. LPAI viruses also have caused illness in other mammals, including cats, dogs, and ferrets.5
Highly Pathogenic Avian Influenza (HPAI) Viruses
Some HPAI viruses have caused serious illness and death among people. These HPAI viruses include strains of H7N2, H7N7, and, more recently, H7N9.
Avian influenza is generally more of a threat to animal health than to the health of people. HPAI viruses are very contagious and extremely deadly in poultry. Evidence that HPAI viruses are spreading has led the World Organization for Animal Health (officially, Organisation Mondiale de la Santé Animale, or OIE) to classify all H5 or H7 strains as notifiable--meaning that member countries, including the United States, must report H5 or H7 AIV cases to OIE.6
Unlike with LPAI viruses, wild birds generally are not believed to be reservoirs for HPAI viruses.4,5 Rather, some evidence suggests that HPAI viruses evolve from LPAI viruses, which can change genetically to become more pathogenic. That means they may be able to cause disease more readily, or cause more severe disease.
Avian Influenza in People
Although it is rare for people to get AIV infections from any source, AIV transmission primarily results from direct human contact with infected birds or animals (including intermediate hosts, such as pigs) or from contaminated environments, such as poultry farms and markets.7
Human-to-human transmission has not occurred in the U.S.8 However, case reports from abroad suggest that sporadic human-to-human transmission of certain AIVs is possible.
Over time, influenza viruses can change genetically. The greatest public health threat regarding avian influenza outbreaks in poultry exists in the potential for AIVs to change, possibly increasing the ability of the viruses to cause severe disease and/or more widespread human-to-human transmission. This possibility, plus the increasing resistance of some AIVs to currently available antiviral treatment and lack of complete vaccination effectiveness, raises the risk to human health.10
1 Center for Infectious Disease Research and Policy (CIDRAP), Avian Influenza (Bird Flu): Implications for Human Disease.
2 Bui, C. M., Gardner, L., & MacIntyre, C. R. (2016). Highly pathogenic avian influenza virus, Midwestern United States. Emerging Infectious Diseases, 22(1), 138.
3 Centers for Disease Control and Prevention (CDC), Avian Influenza in Birds.
4 Clark, L. and Hall, J. Avian Influenza in Wild Birds: Status as Reservoirs, and Risks to Humans and Agriculture, Ornithological Monographs, 60, 3-29 (2006).
5 Worker Education and Training Program, Animal and Plant Inspection Service (APHIS), U.S. Department of Agriculture (USDA), and National Institute of Environmental Health Sciences (NIEHS), National Institutes of Health (NIH), U.S. Department of Health and Human Services (HHS), “Protecting Yourself from Avian Influenza.”
6 Avian Influenza: Infection with Avian Influenza Viruses, World Organization for Animal Health (OIE) Terrestrial Manual, 2015.
7 Centers for Disease Control and Prevention (CDC), Transmission of Avian Influenza A Viruses Between Animals and People.
8 Centers for Disease Control and Prevention (CDC), H5 Viruses in the United States.
9 Centers for Disease Control and Prevention (CDC), Avian Influenza A Virus Infections in Humans.
10 World Organization for Animal Health (OIE), Avian Influenza, General Disease Information Sheets.