Reports of Avian Influenza (AI) "Bird Flu" Outbreaks Increased Globally from 2013–2022

CDC Update

September 7, 2023—A new CDC study shows the number of reported avian influenza (AI) "bird flu" outbreaks in animals and infections in people are increasing worldwide. During 2013–2022, bird flu outbreaks in animals and infections in people not only increased in numbers but also were detected over a growing geographic area as well as among a growing number of different animal categories. These findings are particularly relevant given ongoing H5 bird flu outbreaks globally and in the United States among commercial and backyard poultry and wild birds and sporadically in some mammals, with one reported human case in the United States and others reported internationally.

Study authors looked at bird flu outbreaks in animals globally reported to the World Organization for Animal Health (WOAH) and other animal health authorities and bird flu infections in people reported to the World Health Organization during 2013–2022. They compared these findings with reports submitted during 2005–2012.

Study findings include:

• The number of reported bird flu outbreaks in animals increased. Animal categories with bird flu outbreaks included wild birds, captive birds, birds in live bird markets, backyard and commercial poultry and non-human mammals, such as mink.
• From January 2013 to June 2022, there were reports of 34 bird flu virus subtypes during more than 21,000 bird flu outbreaks in animals and 2,000 human infections with bird flu viruses globally.
  • On average 42 member states reported AI outbreaks in animals each year.
  • On average, four member-states reported human AI virus infections each year.
• In total, 16 new bird flu subtypes were detected; 10 of these were new highly pathogenic avian influenza (HPAI) virus subtypes identified in animals, including birds (almost double the number of subtypes identified during the previous eight years).
  • Six low pathogenic avian influenza (LPAI) virus subtypes identified in animals, including birds, were reported for the first time during 2013–2022.
• Four HPAI H5 virus subtypes accounted for almost 90% of reported animal outbreaks: H5N1 (47%), H5N8 (32%), H5N2 (6%), H5N6 (4%).
  • H5N8 was the virus subtype that accounted for the most global animal outbreaks each year since 2017, except during 2019 and 2022.
• During 2013–2022, 26 member-states reported their first animal AI outbreak.
• Eight member-states did not report a human case prior to 2013.
• Bird flu viruses also moved into more animal categories.
  • From 2016 to 2022, on average, 14% of outbreaks reported each month occurred in a new animal category, with wild birds representing the highest percentage (37%).

Increases in geographic and animal category spread could be attributed to increased poultry production and commercial trade, increased exposure to wild birds through repeated annual migrations of infected birds, and changes in migration patterns because of climate change or land conversion to agricultural production. These increases could also reflect better AI virus surveillance and reporting.

Spread of bird flu viruses among birds, animals and people can have important public health implications. As animal outbreaks increase, the risk of spread to people can also increase, along with opportunities for viral reassortment that can enhance transmissibility of avian influenza viruses. The most effective AI surveillance would include comprehensive surveillance, inclusive of all virus subtypes, as well as timely reporting in domestic and wild birds, humans and, ideally, non-human mammals especially pigs, as pigs can play a crucial role in novel influenza virus reassortment. Efficient and sustained transmission among people could result in a flu pandemic.

While the recent detections of H5N1 bird flu in U.S. poultry and wild birds are thought to pose a low risk to the health of the general public at this time, human infections are possible. As of August 9, 2023, more than 58.7 million poultry and more than 7,100 wild birds have been affected in the United States. While only one human case of H5N1 has been found in a person in the United States, CDC continues to help monitor for additional infections among people with contact with infected birds. To date, more than 6,500 people in the United States who have had exposure to birds/poultry infected with H5N1 have been or are being monitored for symptoms.

The trends highlighted in this report underscore that the reasons for concerns around bird flu are growing rather than diminishing and the importance of continued monitoring and timely reporting of bird flu in animals and humans in the United States and internationally.