Cambodia’s Ministry of Health recently confirmed the country’s twelfth human case of H5N1 avian influenza so far this year. The patient, a five-year-old boy from Kampot province, is currently in intensive care with severe respiratory symptoms.
The announcement, on July 3, came just days after a 19-month-old child in neighbouring Takeo province died from the same virus.
To date, there is no evidence of human-to-human transmission. But the steady increase in cases has renewed attention to the risks posed by H5N1. This highly pathogenic bird flu virus spreads rapidly among poultry and occasionally jumps to humans, often with deadly consequences.
Since 2003, there have been at least 954 reported human infections globally, nearly half of them fatal, according to the World Health Organisation (WHO). Experts have long considered H5N1 a serious pandemic threat due to its high mortality rate and potential to evolve.
The recent Cambodian cases are linked to the 2.3.2.1e lineage of H5N1 (previously known as 2.3.2.1c), a strain that has circulated for decades in poultry across Cambodia, Laos and Vietnam.
From 2005 to 2014, Cambodia saw sporadic but severe human infections – then almost a decade passed without new cases.
That changed in 2023 when six human cases were reported. The numbers have since climbed: ten in 2024, and now 12 in the first half of 2025. Of these recent infections, at least 12 – about 43% – have been fatal. A troubling pattern is also emerging: seven of this year’s cases occurred in June alone, according to the WHO’s latest Disease Outbreak News update.
Animal pandemic
Globally, however, a different H5N1 lineage – 2.3.4.4b – has dominated in recent years. This strain sparked a devastating wave of avian outbreaks starting in 2021, sweeping across continents and decimating wild bird and poultry populations. It also spread to mammals, leading scientists to label it an “animal pandemic”.
Although it no longer causes mass die-offs, 2.3.4.4b remains widespread and dangerous, particularly because of its capacity to infect mammals. It has been linked to about 70 human cases in the US alone, with only one death recorded so far, and is under investigation for suspected mammal-to-mammal transmission in species, including US dairy cattle and seals.
Influenza viruses are notoriously prone to genetic reassortment – a process by which two or more strains infect the same host and exchange genetic material. These events can sometimes generate new, more transmissible or deadly variants. In April 2024, the UN Food and Agriculture Organisation reported the emergence of a reassortant virus in Vietnam. This new strain combines surface proteins from the long-standing 2.3.2.1e virus with internal genes from the globally dominant 2.3.4.4b.
Evidence suggests that this reassortant virus may be driving the rise in Cambodian human infections.
A 2024 study, which has not yet undergone peer review, found that the new virus carries genetic markers that could enhance its ability to infect humans – although it is not yet considered human-adapted. According to the study’s authors, this reassortant form has become the predominant strain found in poultry across the region in recent years.
So far, all confirmed human cases in Cambodia have been linked to direct contact with infected or dead poultry – often in small, rural backyards. This suggests that the country’s “one health” strategy, which aims to integrate human, animal and environmental health responses, is functioning as intended. Although some gaps clearly remain.
Food safety and food security remain serious concerns across much of Cambodia and Southeast Asia. Limited veterinary oversight, informal poultry markets, lack of compensation for poultry losses due to disease, and poor biosecurity may offer the virus opportunities to persist and evolve – and potentially reach more people.
Since the COVID pandemic, advances in disease surveillance and reporting have made it easier to detect and confirm human infections, Dr Vijaykrishna Dhanasekaran, head of the Pathogen Evolution Lab at Hong Kong University, told me over email. However, he notes that surveillance remains heavily concentrated in urban areas and the commercial poultry sector, while rural settings and interactions with wild birds are poorly monitored.
Expanding surveillance to these overlooked areas will be vital, he says, if the world hopes to better understand – and prepare for – the next potential influenza pandemic.
Divya Venkatesh is a BBSRC Discovery Fellow at the University of Oxford.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
