Avian influenza A (H5N1) virus infection

First bird flu death reported in the US outbreak; case of human infection detected in the UK

​Avian influenza A(H5N1) virus is currently causing a multistate outbreak in poultry and dairy cows in the US. While outbreaks have previously been reported in poultry, this is the first time the virus has been detected in cows. Cases of human infection linked to the outbreak in dairy cows have been reported, and these cases represent the first instances of likely mammal-to-human transmission globally.

As of 31 January 2025, 67 confirmed cases of human infection associated with exposure to infected poultry or dairy cows have been reported in the US since 2024 across 10 states - California, Colorado, Iowa, Louisiana, Michigan, Missouri, Oregon, Texas, Washington, and Wisconsin.[22]Centers for Disease Control and Prevention. H5 bird flu: current situation. Feb 2025 [internet publication]. https://www.cdc.gov/bird-flu/situation-summary/index.html ​ 

• Of the 67 cases, 40 occurred following exposure to infected dairy cows, 23 occurred following exposure to infected poultry on poultry farms and during culling operations, 1 occurred following exposure to other animals, and 3 had an unknown exposure source. One death has been reported.

• The first case of infection in a child in the US was reported in California in November 2024. The child experienced mild symptoms and received antiviral therapy. The case was identified via influenza surveillance systems. The exposure source is currently unknown.[38]Centers for Disease Control and Prevention. CDC confirms H5N1 bird flu infection in a child in California. Nov 2024 [internet publication].​ https://www.cdc.gov/media/releases/2024/p1122-h5n1-bird-flu.html

• The first case of severe infection in the US was reported in Louisiana in December 2024. The patient had exposure to sick and dead birds in backyard flocks. The virus was identified as belonging to clade 2.3.4.4b (D1.1 genotype), the virus currently detected in poultry and wild birds across the US.[39]Centers for Disease Control and Prevention. CDC confirms first severe case of H5N1 bird flu in the United States. Dec 2024 [internet publication]. https://www.cdc.gov/media/releases/2024/m1218-h5n1-flu.html ​ The patient died in early January 2025, and was the first person in the US to die as a result of H5N1 virus infection. No human-to-human transmission related to this case has been identified.[40]Centers for Disease Control and Prevention. First H5 bird flu death reported in United States. Jan 2025 [internet publication]. https://www.cdc.gov/media/releases/2025/m0106-h5-birdflu-death.html ​​

The Centers for Disease Control and Prevention (CDC) currently considers the public health risk to be low. Updated situation summaries are available from the CDC and are being updated regularly:

• CDC: H5 bird flu - current situation Opens in new window

​In light of the outbreak, the CDC has published interim guidelines for prevention, monitoring, and public health investigations.[41]Centers for Disease Control and Prevention. Highly pathogenic avian influenza A(H5N1) virus: interim recommendations for prevention, monitoring, and public health investigations. Dec 2024 [internet publication]. https://www.cdc.gov/bird-flu/prevention/hpai-interim-recommendations.html

Members of the public are advised to:

• Avoid exposure to sick or dead animals. If this is not possible, avoid unprotected (not using respiratory and eye protection) exposures to sick or dead animals, including wild birds, poultry, other domesticated birds, and other wild or domesticated animals (including cows).

• Avoid unprotected exposures to animal feces, litter, or materials contaminated by birds or other animals with suspected or confirmed H5N1 virus infection.

• Cook poultry, eggs, and beef to safe internal temperatures and only consume pasteurized milk and milk products.

Clinicians are advised to consider the possibility of HPAI A(H5N1) virus infection in anyone showing signs or symptoms of acute respiratory illness or conjunctivitis who has a relevant exposure history.

• People exposed to infected birds or other animals are advised to monitor themselves for new respiratory illness symptoms and/or conjunctivitis, for 10 days after their last exposure. Antiviral postexposure prophylaxis may be considered in certain people to prevent infection.

• Symptomatic people are advised to seek prompt medical care for diagnosis and treatment, and isolate themselves away from others. Oseltamivir is the recommended first-line antiviral treatment.

A confirmed case of H5N1 virus infection has also been reported in the UK in the West Midlands region.[42]UK Health Security Agency. Human case of avian flu detected in England. Jan 2025 [internet publication]. https://www.gov.uk/government/news/human-case-of-avian-flu-detected-in-england ​

• The person acquired the infection on a farm after close prolonged contact with a large number of infected birds. The case was detected during routine surveillance, and no onwards transmission from the case has been reported. The birds were infected with the D1.2 genotype, a different strain to the ones currently circulating among birds and mammals in the US. Only a small number of cases have occurred in the UK prior to this case. The risk to the wider public is considered to be very low.

Sporadic human cases of HPAI A(H5N1) infection have been reported globally since 1997, with a case fatality rate of approximately 50% in reported cases. Most human infections have occurred after unprotected exposure to sick or dead infected poultry. There has been no evidence of sustained human-to-human transmission in any country, and limited nonsustained human-to-human transmission has not been reported since 2007.

See Epidemiology

Original source of update