Seven of Cambodia's eleven H5N1 cases this year were reported in June alone

Cambodia has reported 11 laboratory-confirmed human infections with avian influenza A(H5N1) between 1 January and 1 July 2025, and seven of them were notified in June — a monthly increase the World Health Organization describes as unusual [s1]. Six of the 11 people died, a case fatality ratio of 54% [s1].

WHO published the details on 5 July in a Disease Outbreak News item [s1]. The numbers are small in absolute terms, and the agency's overall risk assessment has not changed. But a country that recorded six human cases in all of 2023 and ten in all of 2024 has now recorded 11 in six months, most of them in a single month [s1].

What was reported

The 11 cases came from seven provinces: Siem Reap (4), Takeo (2), Kampong Cham (1), Kampong Speu (1), Kratie (1), Prey Veng (1) and Svay Rieng (1) [s1]. Males accounted for 63% of cases [s1]. Three were under five years old, two were aged 5 to 18, and six were aged 18 to 65 [s1].

Where virus sequences are available, the cases are clade 2.3.2.1e, formerly classified as 2.3.2.1c [s1]. This is the lineage that has circulated in Cambodian poultry for years, and it is genetically distinct from the 2.3.4.4b viruses that have driven the panzootic in wild birds, poultry and dairy cattle elsewhere.

All 11 cases reported direct exposure — handling or culling — to sick poultry, often birds kept in backyards [s1]. That is the consistent finding across Cambodia's cluster, and it is the basis on which WHO judged sustained human-to-human transmission unlikely in this event, while noting that human-to-human transmission cannot be formally ruled out [s1].

The longer record

Avian influenza A(H5N1) was first detected in Cambodia in December 2003, initially in wild birds [s1]. From then to 1 July 2025 the country has reported 83 human infections with 49 deaths, a case fatality ratio of 59% [s1].

The record has a nine-year hole in it. Between 2014 and 2022, Cambodia reported no human A(H5N1) infections at all; the virus continued circulating in birds throughout [s1]. Human cases re-emerged in February 2023 [s1]. Since that re-emergence Cambodia has reported 27 laboratory-confirmed human infections including 12 deaths, a case fatality ratio of 44%, across eight provinces: Prey Veng (6), Siem Reap (5), Svay Rieng (4), Takeo (4), Kampot (3), Kratie (3), Kampong Cham (1) and Kampong Speu (1) [s1].

Globally, from 2003 to 1 July 2025, WHO has been notified of 986 human A(H5N1) infections including 473 deaths — a case fatality ratio of 48% — from 25 countries [s1]. Almost all have been linked to close contact with infected live or dead birds or mammals, or with contaminated environments [s1].

Those global case fatality ratios deserve a caveat WHO does not spell out. Severe cases are far more likely to be detected and tested than mild ones, so a ratio built from notified cases is an upper bound on the true infection fatality risk, not an estimate of it. WHO itself notes that asymptomatic infections with A(H5N1) have been reported in people with known exposure [s1].

The response

Cambodia's national and sub-national rapid response teams deployed to investigate, in coordination with local authorities, the Ministry of Environment and the Ministry of Agriculture, Forestry and Fisheries [s1]. The work has focused on identifying the source of transmission in humans and animals, enhanced surveillance, detection of suspected cases and prevention of community transmission; close contacts were monitored, and health education campaigns ran in affected villages [s1].

On the animal side, the General Directorate of Animal Health and Production and sub-national teams investigated poultry outbreaks, with interventions centred on disinfection and limiting animal movement between villages, alongside community awareness work and poultry disease surveillance [s1].

The risk assessment, and what it rests on

WHO assesses the risk to the general population as low [s1]. For people occupationally exposed — farm workers, for instance — the risk is judged low to moderate depending on the control measures in place [s1]. Additional cases in people exposed to sick or dead poultry are to be expected [s1].

That assessment is conditional in a way worth stating plainly. WHO notes that close analysis of the epidemiological situation, further characterisation of the most recent A(H5N1) viruses in human and animal populations, and serological investigations are all critical to updating the risk assessment [s1]. In other words, the current judgement is based on what sequencing and case investigation have shown so far, and the agency says explicitly that it will be reviewed if new information arrives.

Two other points from the WHO item bear on how any individual case is handled. Current seasonal influenza vaccines are unlikely to protect against A(H5N1) infection, though vaccines against influenza A(H5) in humans have been developed and licensed in some countries [s1]. And neuraminidase inhibitors such as oseltamivir and zanamivir have been shown to shorten viral replication and improve outcomes in some cases, with WHO noting these should be given within 48 hours of symptom onset [s1].

On that last point there is independent laboratory evidence. A CDC analysis published earlier this year tested clade 2.3.2.1c viruses from human infections in Cambodia during 2023-2024 alongside clade 2.3.4.4b viruses from human infections in the Americas [s2]. In neuraminidase inhibition assays, all the viruses were susceptible to oseltamivir, zanamivir, peramivir, laninamivir and the investigational AV5080, and all were susceptible to the polymerase inhibitors baloxavir and tivoxavir and to pimodivir [s2]. Two viruses isolated from Cambodia were exceptions to susceptibility to M2 ion-channel blockers in cell-culture assays, and oseltamivir was roughly four-fold less potent against clade 2.3.4.4b neuraminidase than against clade 2.3.2.1c [s2]. The authors' own conclusion is that resistant viruses can emerge spontaneously or through reassortment, so continued monitoring is essential [s2] — susceptibility measured in 2023-2024 isolates is not a guarantee about the viruses circulating now.

What to watch

The concentration of seven cases in June is the signal, and the question it raises is whether it reflects a change in the virus, a change in poultry exposure, or a change in how hard Cambodia is looking. Enhanced surveillance finds more of what it is looking for, and Cambodia has had enhanced surveillance running since the 2023 re-emergence. Distinguishing a real increase in incidence from an increase in ascertainment requires the serological investigations WHO says are needed [s1].

The second thing to watch is exposure. Every one of the 11 cases involved handling or culling sick backyard poultry [s1]. As long as that remains true of each new case, the epidemiology is consistent with repeated animal-to-human spillover rather than anything else. A case without a poultry exposure would be a different kind of event.

Sources

• [s1] Avian Influenza A(H5N1) — Cambodia, Disease Outbreak News 575 — World Health Organization, 5 July 2025. https://www.who.int/emergencies/disease-outbreak-news/item/2025-DON575
• [s2] Antiviral Susceptibility of Influenza A(H5N1) Clade 2.3.2.1c and 2.3.4.4b Viruses from Humans, 2023-2024 — Emerging Infectious Diseases, Crossref record created 13 March 2025 (April 2025 issue). https://doi.org/10.3201/eid3104.241820