USDA: Fact Sheet - Avian Influenza Updated
There are two types of avian influenza (AI) that are identified as H5N1. A difference exists in the virus classification; one is low pathogenic (LPAI) and the other is highly pathogenic (HPAI). Pathogenicity refers to the ability of the virus to produce disease.
HPAI H5N1, often referred to as the “Asian” H5N1, is the type causing worldwide concern. LPAI H5N1, often referred to as the “North American” H5N1, is of less concern. Following is an explanation of the differences between them.
LPAI H5N1 (”North American” H5N1)
LPAI, or “low path” AI, commonly occurs in wild birds. In most cases, it causes minor sickness or no noticeable signs of disease. It is rarely fatal in birds. LPAI strains are not a human health concern. This includes LPAI H5N1.
Evidence of LPAI H5N1 has been found in wild birds in the United States in recent years and is not closely related to the more severe HPAI H5N1 circulating overseas. Examples of historical reports of LPAI H5N1 received by USDA include:
1981 and 1985 - the University of Minnesota conducted a sampling procedure in which sentinel ducks were monitored in cages placed in the wild for a short period of time and LPAI H5N1 was
1983 - LPAI H5N1 was detected in ring-billed gulls in Pennsylvania.
1986 - LPAI H5N1 was detected in a wild mallard duck in Ohio as part of routine sampling, not as a result of noticeable illness in the birds.