Avian Influenza

Highly pathogenic avian influenza (HPAI) viruses cause severe disease and high mortality in infected poultry. While most strains of A(H5) and A(H7) are low pathogenic (LPAI) viruses, there are several strains of A(H5) and A(H7) that are considered highly pathogenic (HPAI).

These HPAI A(H5) or A(H7) virus infections can cause disease that affects multiple internal organs with mortality up to 90% to 100% in chickens, often within 48 hours. However, ducks can be infected without any signs of illness.

HPAI A(H5) and A(H7) virus infections in poultry also can spill back into wild birds, resulting in further geographic spread of the virus as those birds migrate. 

While some wild bird species can be infected with some HPAI A(H5) or A(H7) virus subtypes without appearing sick, other HPAI A(H5) and A(H7) virus subtypes can cause severe disease and mortality in some infected wild birds as well as in infected poultry.

Low pathogenic avian influenza viruses cause either no signs of disease or mild disease in chickens/poultry (such as ruffled feathers and a drop in egg production). Most avian influenza A viruses are low pathogenic and cause few signs of disease in infected wild birds. In poultry, some low-pathogenic viruses can mutate into highly pathogenic avian influenza viruses.

HPAI and LPAI designations do not refer to or correlate with the severity of illness in cases of human infection with these viruses; both LPAI and HPAI A viruses have caused mild to severe illness in infected humans. 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.