Highlights in the History of Avian Influenza (Bird Flu)

Background and early history of bird flu

The first description of avian influenza (bird flu) dates to 1878 in northern Italy, when it was described as a contagious disease of poultry associated with high mortality, referred to as "fowl plague." At the turn of the 20^th century, it was determined that "fowl plague" was caused by a virus; however, it was not until 1955 that the virus was shown to be a type A influenza virus. In 1971, influenza viruses were first classified based on the antigenic properties of their structural and surface proteins: the nucleoprotein (NP) [type] and hemagglutinin (HA) and neuraminidase (NA) [subtype] proteins, as well as the species of origin. By 1980, researchers were using this system to classify influenza viruses regardless of the species of origin. This naming tradition is still used today. By 1981, the term "fowl plague" was substituted by the more appropriate term, avian influenza, at the First International Symposium on Avian Influenza.

First human infections with bird flu

The first known human infections with avian influenza A(H5N1) virus were reported in 1997 in Hong Kong. Animal to human transmission led to 18 human infections and six deaths. These were the first reported human infections with H5N1 bird flu that resulted in fatal outcomes. Human infections with avian influenza A viruses are uncommon but have occurred sporadically in many countries, usually after unprotected exposures (not using respiratory or eye protection) to infected poultry or virus-contaminated environments. In the United States, sporadic human infections with A(H5N1) viruses have occurred since 2022 mostly after direct or close exposure with infected poultry or dairy cows without using recommended personal protective equipment. More information is available at Current Situation: H5N1 Bird Flu in People.

Categories of avian influenza A viruses

Avian influenza A viruses are further classified into two categories: highly pathogenic avian influenza (HPAI) and low pathogenic avian influenza (LPAI) A viruses based on specific criteria (viral characteristics and mortality in experimentally infected chickens¹). Most bird flu viruses are low pathogenic and cause few or no signs of disease in infected wild birds. LPAI viruses also cause either no signs of disease or mild disease in chickens and other domestic poultry. In contrast, HPAI viruses can cause severe disease and high mortality in infected poultry yet cause little to no disease signs in certain wild aquatic bird species. While classification as an LPAI or HPAI virus refers to the severity of disease in infected poultry, both kinds of avian influenza A viruses have caused severe disease in infected humans.