There are three types of influenza viruses: A, B and C. Human influenza A and B viruses cause seasonal epidemics, generally between October and May, of disease in the United States.
Wild aquatic birds are the natural hosts for all known influenza type A viruses - particularly certain wild ducks, geese, swans, gulls, shorebirds and terns. Influenza type A viruses can infect people, birds, pigs, horses, dogs, marine mammals, and other animals. Influenza type A viruses are divided into subtypes on the basis of two proteins on the surface of the virus: hemagglutinin (HA) and neuraminidase (NA). For example, an “H7N2 virus” designates an influenza A virus subtype that has an HA 7 protein and an NA 2 protein. Similarly an “H5N1” virus has an HA 5 protein and an NA 1 protein. There are 17 known HA subtypes and 10 known NA subtypes. Many different combinations of HA and NA proteins are possible. All known subtypes of influenza A viruses can infect birds, except subtype H17N10 which has only been found in bats. Only two influenza A virus subtypes (i.e., H1N1, and H3N2) are currently in general circulation among people. Some subtypes are found in other infected animal species. For example, H7N7 and H3N8 virus infections can cause illness in horses, and H3N8 virus infection can also cause illness in dogs.
Avian influenza A viruses are classified into two categories (low pathogenic and highly pathogenic) that refer to their ability to cause severe disease, based upon molecular characteristics of the virus and mortality in birds under experimental conditions. Infection of poultry with low pathogenic avian influenza A (LPAI) viruses may cause no disease or mild illness (such as ruffled feathers and a drop in egg production) and may not be detected. Infection of poultry with highly pathogenic avian influenza A (HPAI) viruses can cause severe disease with high mortality. Both HPAI and LPAI viruses can spread rapidly through poultry flocks. HPAI virus infection can cause disease that affects multiple internal organs with mortality up to 90-100% in chickens, often within 48 hours. However, ducks can be infected without any signs of illness. There are genetic and antigenic differences between the influenza A virus subtypes that typically infect only birds and those that can infect birds and people.
Three prominent subtypes of avian influenza A viruses that are known to infect both birds and people are:
Influenza A H5 Nine potential subtypes of H5 viruses are known (H5N1, H5N2, H5N3, H5N4, H5N5, H5N6, H5N7, H5N8, and H5N9). Most H5 viruses identified worldwide in wild birds and poultry are LPAI viruses. Sporadic H5 virus infection of humans, such as with highly pathogenic avian influenza A (H5N1) viruses currently circulating among poultry in Asia and the Middle East have been reported in 15 countries, often resulting in severe pneumonia with approximately 60% mortality worldwide.
Influenza A H7 Nine potential subtypes of H7 viruses are known (H7N1, H7N2, H7N3, H7N4, H7N5, H7N6, H7N7, H7N8, and H7N9). Most H7 viruses identified worldwide in wild birds and poultry are LPAI viruses. H7 virus infection in humans is uncommon, but has been documented in persons who have direct contact with infected birds, especially during outbreaks of H7 virus among poultry. Illness in humans may include conjunctivitis and/or upper respiratory tract symptoms.
In humans, LPAI (H7N2, H7N3, H7N7) virus infections have caused mild to moderate illness. HPAI (H7N3, H7N7) virus infections have caused mild to severe and fatal illness. On April 1, 2013, the first known human cases of infection with avian influenza H7N9 viruses were reported. These were associated with severe respiratory illness and death.
Influenza A H9 Nine potential subtypes of H9 are known (H9N1, H9N2, H9N3, H9N4, H9N5, H9N6, H9N7, H9N8, and H9N9); all H9 viruses identified worldwide in wild birds and poultry are LPAI viruses. H9N2 virus has been detected in bird populations in Asia, Europe, the Middle East and Africa. Rare, sporadic H9N2 virus infections of humans have been reported to cause generally mild upper respiratory tract illness.