Between 1 January and 1 July 2025, the World Health Organization (WHO) was notified by Cambodia’s International Health Regulations (IHR) National Focal Point (NFP) of 11 laboratory-confirmed cases of human infection with avian influenza A(H5N1) virus. **Seven of the 11 cases were reported in June, an unusual monthly increase.**

Avian influenza A(H5N1) was first detected in Cambodia, in December 2003, initially affecting wild birds. Since then, 83 cases of human infection with influenza A(H5N1), including 49 deaths (case fatality ratio [CFR] of 59%), have been reported in the country. While the virus continued to circulate in avian species, no human cases were reported between 2014 and 2022, after which, the virus re-emerged in humans in February 2023. Since the re-emergence of human A(H5N1) infections in Cambodia in 2023, a total of 27 cases have been reported (six in 2023, 10 in 2024, and 11 to date in 2025), of which 12 were fatal (CFR 44%).

Seventeen of the cases occurred in children under 18 years old. Avian influenza A(H5N1) is circulating in wild birds, poultry and some mammals around the world, and occasional human infections following exposure to infected animals or contaminated environments are expected to occur. In cases detected in Cambodia, exposure to sick poultry, often poultry kept in backyards, has been reported. 

According to the IHR, a human infection caused by a novel influenza A virus subtype is an event that has the potential for high public health impact and must be notified to the WHO. Based on currently available information, WHO assesses the current risk to the general population posed by this virus as low. For those occupationally exposed to the virus, such as farm workers, the risk is low to moderate, depending on the measures in place. WHO routinely reassesses this risk to factor in new information.

**Description of the Situation**

Between 1 January and 1 July 2025, the National IHR Focal Point (NFP) of the Kingdom of Cambodia notified WHO of 11 laboratory-confirmed case of human infection with avian influenza A(H5N1) virus (clade 2.3.2.1e- formerly classified as 2.3.2.1c; from cases where virus sequences are available to date) including six deaths [CFR: 54%]. These cases are reported from the provinces of Siem Reap (4), Takeo (2), Kampong Cham (1), Kampong Speu (1), Kratie (1), Prey Veng (1), Svay Rieng (1). Of the total cases reported in 2025, seven cases were reported in June 2025.

Males account for 63% of the cases. Of the 11 cases, three cases were reported in less than five-year-olds, two cases were between the age of 5 and 18 years and six cases were reported in the age group 18-65 years. All cases had exposure – handling or culling - of sick poultry, often kept in backyards.

Avian influenza A(H5N1) was detected for the first time in Cambodia in December 2003, initially affecting wild birds. Between 2014 and 2022, there were no reports of human infection with A(H5N1) viruses. However, the re-emergence of human infections with A(H5N1) viruses in Cambodia was reported in February 2023. Since this re-emergence, Cambodia has reported 27 cases of laboratory confirmed human infection with avian influenza A(H5N1) including 12 fatalities (CFR 44%).  The cases have been reported from eight provinces: Kampong Cham (1), Kampong Speu (1), Kampot (3), Kratie (3), Prey Veng (6), Svay Rieng (4), Siem Reap (5), Takeo (4). 

**Epidemiology**

Animal influenza viruses typically circulate within animal populations, but some have the potential to infect humans. Human infections are predominantly acquired through direct contact with infected animals or exposure to contaminated environments. Based on the original host species, influenza A viruses can be categorized such as avian influenza, swine influenza, and other animal-origin influenza subtypes.

Human infection with avian influenza viruses may result in a spectrum of illness, ranging from mild upper respiratory tract symptoms to severe, life-threatening conditions. Clinical manifestations include conjunctivitis, respiratory, gastrointestinal symptoms, encephalitis (brain swelling), and encephalopathy (brain damage). In some cases, asymptomatic infections with the A(H5N1) virus have been reported in individuals with known exposure to infected animals and environments.

A definitive diagnosis of human avian influenza infection requires laboratory confirmation. WHO regularly updates its technical guidance on the detection of zoonotic influenza, utilizing molecular diagnostic methods such as RT-PCR. Clinical evidence indicates that certain antiviral agents, particularly neuraminidase inhibitors (e.g., oseltamivir, zanamivir), have been shown to shorten the duration of viral replication and improve patient outcomes in some cases. This antiviral agent should be administered within 48 hours of symptom onset.

**From 2003 to 1 July 2025, 986 cases of human infections with avian influenza A(H5N1), including 473 deaths (CFR 48%), have been reported to WHO from 25 countries. Almost all of these cases have been linked to close contact with infected live or dead birds, or contaminated environments. From 2003 to the present, 83 cases of human infection with influenza A(H5N1), including 49 deaths (case fatality ratio [CFR] of 59%), have been reported in Cambodia.**

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**Risk Assessment**

From 2003 to 1 July 2025, a total of 986 human cases of infection of influenza A(H5N1) have been reported globally to WHO from 25 countries, including this case.  **Almost all of these have been linked to close contact with A(H5N1) infected live or dead birds or mammals, or contaminated environments.** Human infection can cause severe disease with a high mortality rate: of the 986 infections reported globally, there have been 473 deaths (CFR 48%). 

In this event, cases have been reported from seven provinces in 2025. All cases have reported direct exposure to sick/dead poultry. While human-to-human transmission cannot be ruled out, the more likely source of exposure of these cases is infected poultry of contaminated environment.  

**Based on information available at this time, the overall public health risk from currently known influenza viruses circulating at the human-animal interface has not changed and remains low. For those occupationally exposed to the virus, such as farm workers, the risk is low to moderate, depending on the measures in place.** Additional cases in persons with exposure to sick/dead poultry is to be expected.

 **The occurrence of sustained human-to-human transmission in this event based on currently available information is currently considered unlikely.** This can, however, change and the risk assessment will be reviewed as needed if additional information becomes available. 

Close analysis of the epidemiological situation, further characterization of the most recent influenza A(H5N1) viruses in both human and animal populations, and serological investigations are critical to update associated risk assessments for public health and promptly adjust risk management measures. 

Current seasonal influenza vaccines are unlikely to protect humans against infections with influenza A(H5N1) viruses. Vaccines against influenza A(H5) infection in humans have been developed and licensed in some countries. WHO continues to update the list of zoonotic influenza candidate vaccine viruses (CVVs), which are selected twice a year at the WHO consultation on influenza virus vaccine composition, and on an ad hoc basis as needed. The list of such CVVs is available on the WHO website, see reference below. In addition, the genetic and antigenic characterization of contemporary animal and zoonotic influenza viruses are published here. 

This risk assessment will be reviewed as needed if additional information becomes available.



WHO: Avian Influenza A(H5N1) - Cambodia Situation Report, July 5, 2025