Epidemiology

Highly pathogenic avian influenza (HPAI) A(H5N1) virus strains have infected poultry or wild birds in more than 108 countries since 2003.[35]

The first known outbreak of human cases occurred in Hong Kong in 1997. There were 18 cases and 6 deaths reported during May through December 1997.[36] Since January 2003, there have been sporadic human infections reported across 25 countries, with the largest single outbreaks occurring in Egypt in 2014-2015 (173 cases), and the US in 2024-2025 (70 cases). As of 28 July 2025, 1008 cases of human HPAI A (H5N1) infection have been reported to the World Health Organization (WHO) since 2003.[37] Approximately 49% of reported cases have been fatal.[38]

CDC: global human cases with influenza A(H5N1) 1997-2025 Opens in new window

US outbreak 2024-2025

  • ​​​​​​​Since March 2024, sporadic human infections have been reported in the US associated with exposure to poultry or dairy cattle as part of ongoing multistate outbreaks among poultry and dairy cattle. Seventy human cases and one death have been reported across 12 states as part of this outbreak. Of these cases, 41 occurred following exposure to infected dairy cattle, 24 occurred following exposure to infected poultry on poultry farms and during culling operations, 2 occurred following exposure to other animals, and 3 had an unknown exposure source. No cases have been reported in the US since February 2025.​​[22]​​​​

  • April 2024: one HPAI A(H5N1) human case was reported in Texas. The person developed conjunctivitis (eye redness) as their only symptom, and they had exposure to dairy cattle presumed to be infected with HPAI A(H5N1) virus.[39]​​[40]​​​

  • May 2024: a second human case was reported in a dairy worker in Michigan. Similar to the case in Texas, the person worked on a dairy farm where H5N1 virus had been identified in cows, and only reported eye symptoms.[41]​​

  • May 2024: a third human case was reported in a dairy worker in Michigan with exposure to infected cows. This was the first case in the US to report more typical upper respiratory tract symptoms including cough and eye discomfort with watery discharge.[42]

  • July 2024: a fourth human case was reported in a dairy worker in Colorado with exposure to infected cows. The person reported eye symptoms only.[43]

  • July 2024: a further nine confirmed cases of human infection associated with exposure to poultry at two facilities were reported in Colorado. All cases were in farm workers involved in the depopulation of poultry at a facility experiencing a H5N1 outbreak. The workers reported mild illness with conjunctivitis as the most common symptom.[44]​​

  • September 2024: a human case was reported in Missouri, and was the first case without a known occupational exposure to sick or infected animals. The case was identified through the state’s seasonal influenza surveillance system. No ongoing transmission was reported.[45]

  • ​​October 2024: three human cases were reported in people with occupational exposure to infected dairy cows in California. All cases experienced only mild symptoms (including conjunctivitis).[46][47]​​​ Several cases were reported in California since these initial three cases (38 in total), including the first case in a child in November 2024.​[48]

  • December 2024: the first case of severe infection was reported in Louisiana. The patient had exposure to sick and dead birds in backyard flocks. The virus was identified as belonging to the D1.1 genotype currently detected in poultry and wild birds in the US. The patient died in early January 2025, and was the first person in the US to die as a result of H5 virus infection. No human-to-human transmission related to this case was identified.[49][50]​​​

  • Cases have also been reported in Iowa, Oregon, Washington, Wisconsin, Nevada, and Ohio.[22]

  • Seroprevalence studies in bovine veterinarians suggest that there may be infected dairy cattle in states where infection in dairy cattle has not yet been identified.[51]

  • The risk to the general US population is considered to be low. The risk to populations in the US in contact with potentially infected animals or contaminated surfaces or fluids is considered to be moderate to high.[52]

  • Prior to this, the first human case of infection ever reported in the US was in 2022 in Colorado, and was associated with direct exposure to infected poultry during a culling process.

Global cases (outside of the US): 2024-2025

  • July 2025: eleven HPAI A(H5N1) cases were reported in Cambodia between 1 January and 1 July 2025, including 6 deaths. Seven of these cases were reported in June 2025.[53]

  • May 2025: one HPAI A(H5N1) case was reported in India. The virus was detected in a sample from a man in Karnataka state, and the patient subsequently died. There is limited information available on this case.[54]

  • May 2025: one HPAI A(H5N1) case was reported in China. The patient travelled from Vietnam and was detected at routine screening at port of entry in China. The likely source of exposure was domestic poultry at the patient’s home. The patient recovered.[55]

  • April 2025: one HPAI A(H5N1) case was reported in a 10-year-old child in Mexico. This is the first laboratory-confirmed human infection in the country. The patient died due to respiratory complications while in hospital. No further cases were identified, and the source of infection is under investigation.[56]

  • April 2025: one HPAI A(H5N1) case was reported in a child in Vietnam. The patient developed encephalitis, and reported close contact with dead poultry before becoming unwell.​[57]

  • ​April 2025: two HPAI A(H5N1) cases were reported in children in Bangladesh. Both patients recovered; however, there is limited information available on these cases.[55]

  • ​March 2025: one HPAI A(H5N1) case was reported in a child in India, who subsequently died. The child reported exposure to poultry. This was the second ever case reported in India.[57]

  • January 2025: one HPAI A(H5N1) case was reported in the West Midlands region in the UK. The case 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 was 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.[58]

  • November 2024: one HPAI A(H5N1) case was reported in a 13-year-old girl in British Columbia, Canada. The case was detected through enhanced hospital-based influenza surveillance, and the patient had no travel history. The source of infection is currently unknown. The patient had a history of mild asthma and elevated body mass index, and was hospitalised due to worsening respiratory symptoms and haemodynamic instability, which progressed to acute respiratory distress syndrome (ARDS).[59]

  • November 2024: one HPAI A(H5N1) case was reported in a 18-year-old boy in Vietnam. The patient resided in an area where a H5N1 outbreak in poultry and waterfowl had been reported, with the patient reporting exposure to sick and dead poultry. The patient was diagnosed with severe pneumonia, hospitalised, and treated with antiviral therapy, and has since recovered.[60]

  • September 2024: one HPAI A(H5N1) case was reported in a 15-year-old child in Cambodia. The patient was admitted to hospital after presenting with fever, cough, sore throat, and difficulty breathing. The patient was treated with oseltamivir, but died 3 days later. The patient was exposed to potentially infected chickens in the days prior to the onset of illness. This is one of ten cases reported in Cambodia in 2024.[61]

  • May 2024: one HPAI A(H5N1) case was reported in Victoria, Australia. The case occurred in a child who acquired the infection in India. The child was unwell in March 2024 and experienced severe infection, but has since made a full recovery. The source of exposure to the virus in this case is currently unknown. No further cases have been connected to this case. This is the first human case of infection reported in Australia.[62]​​

  • March 2024: one HPAI A(H5N1) case was reported in a 21-year-old man in Khanh Hoa Province, Vietnam. He developed fever and cough before being admitted to hospital with persistent abdominal pain and diarrhoea. He then developed severe pneumonia, sepsis, and acute respiratory distress syndrome, and died 12 days after initial symptom onset. The man went bird hunting in February 2024 and did not have contact with dead or sick poultry since then. No evidence of human-to-human transmission was identified.[63]

  • February 2024: two HPAI A(H5N1) cases were reported in epidemiologically-unrelated people in different provinces in Cambodia. A 3-year-old child exposed to dead backyard poultry was hospitalised with mild uncomplicated upper respiratory tract illness, and a 69-year-old patient who raised domestic poultry and fighting roosters was hospitalised with difficulty breathing. Both patients recovered, and no evidence of human-to-human transmission was identified.[64]

The WHO considers the global public health risk of influenza A(H5) viruses to be low. However, the risk to people who are occupationally or frequently exposed is low to moderate, depending on risk mitigation and hygiene measures in place, as well as the local avian influenza epidemiological situation.[21]

Current situation reports are available from the WHO, the US Centers for Disease Control and Prevention (CDC), and the UK Health Security Agency (UKHSA):

Prior to the current 2024-2025 outbreak in the US, most human HPAI A(H5N1) cases were reported among previously healthy children and young adults. The median age of patients was approximately 20 years, with an age range for all patients from under 1 year to 81 years.[65] The ratio of male-to-female cases was about equal; however, there was a higher case-fatality proportion in females, which may be due to many different epidemiological factors, such as delay in accessing healthcare, case age, and physician testing patterns.[26] From 2003-2010, patients under 20 years of age had a significantly lower risk of dying than those aged over 20 years (case-fatality proportions: 52% vs. 66%).[26] Mortality is associated with delayed recognition of disease and hospitalisation after symptom onset.[26] One study reported that the presence of rhinorrhoea appeared to indicate a better prognosis for children with HPAI A(H5N1).[32]

While rare, asymptomatic infection with HPAI A(H5N1) virus confirmed virologically and serologically has been reported, detections of A(H5N1) viral RNA in asymptomatic individuals exposed to infected poultry are more common.[64][66][67][68][69]​​​​​​​​ Most likely, this represents transient detection of viral RNA, and not evidence of HPAI A(H5N1) virus infection.​[20]​​[68]​​​​​

One systematic review and meta-analysis of human seroprevalence of H5N1 in China detected an overall seroprevalence of 2.45%. A higher seroprevalence of 7.32% was detected in central China.[70] One cohort study of human infections with HPAI A(H5N1) virus in households raising backyard poultry in Egypt found a very low seroprevalence of antibodies to H5N1 virus (0.4% at baseline and 0.2% at follow-up).[71]

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