#17,900
While reporting fewer cases than Indonesia, Egypt, and Vietnam (see WHO chart above) - between 2003 and 2014 Cambodia registered 56 human H5N1 cases, and 37 deaths - the bulk of which occurred in 2013 and 2014 (see Cambodia’s H5N1 Surge).
But after 2014, Cambodia went the next 9 years without reporting any cases. At least until early 2023, when the first (of now 8 cases) was reported.
That quiescent period - which by 2016 had spread globally - corresponded with the introduction and rapid spread of a new (clade 2.3.4.4) H5Nx virus in 2014. While far more easily carried and spread by birds than its predecessors, this new lineage has caused substantially fewer human infections.
Over the past 3 years we've seen clade 2.3.4.4b increase its ability to jump to mammals, raising alarms around the globe, but thus far its impact on humans has been limited (see CDC Chart below).
Although clade 2.3.4.4b may have conquered much of the rest of the world, in Cambodia the older clade 2.3.2.1c - which was associated with a number of serious infections a decade ago - still circulates. And over the past 12 months we've seen it infect 8 people, killing 4.
The first 6 cases (all in 2023) are summarized in the following CDC chart.
Today the WHO has published an update and risk assessment of the 7th & 8th cases, both of which were reported in late January (see here and here). As with the previous 6, these have been identified as due to clade 2.3.2.1c.
Avian Influenza A (H5N1) - Cambodia
8 February 2024
Situation at a Glance
Between 26 and 28 January 2024, the Cambodia National Focal Point (NFP) for the International Health Regulations (IHR) notified the World Health Organization (WHO) of two confirmed cases of human infection with avian influenza A(H5N1) virus. These cases were reported from Kampong Trabek district, Prey Veng province, and Puok district, Siem Reap province, Cambodia. These are the first two cases of human infection with influenza A(H5N1) reported in Cambodia in 2024. One case was detected through severe acute respiratory infection (SARI) sentinel surveillance and one by a physician at a non-SARI sentinel site. Both patients had contact with sick poultry. There is no evidence of an epidemiological link between the two cases.
In December 2003, Cambodia reported an outbreak of highly pathogenic avian influenza (HPAI) H5N1 for the first time, affecting wild birds. Following this, human cases due to poultry-to-human transmission were reported sporadically until 2014, after which there was a gap until the next cases reported in 2023: two cases each were reported in February, October, and November of 2023. H5N1 infection in humans can cause severe disease, has a high mortality rate, and is notifiable under IHR (2005).
Description of the Situation
Between 26 and 28 January 2024, the Cambodia IHR NFP notified WHO of two confirmed cases of human infection with avian influenza A (H5N1) virus. These cases were reported from Kampong Trabek district, Prey Veng province, and Puok district, Siem Reap province, Cambodia.
The first case, a 3-year-old, was reported on 26 January 2024, from Kampong Trabek district, Prey Veng province. The patient developed symptoms on 13 January 2024 and was admitted to hospital on 16 January 2024 with high fever, cough and runny nose. Samples were collected at the hospital and were transported to the National Institute of Public Health for testing. There, the samples tested positive for H5N1 through quantitative reverse transcription polymerase chain reaction (RT-qPCR) on 25 January 2024 and were confirmed by the Institut Pasteur du Cambodge (IPC) on 25 January 2024. The patient had a history of exposure to backyard chickens that were found dead around the residence. A total of 14 close contacts of the case were identified and samples were collected and tested, of which none were positive for influenza.
The second case, a 69-year-old, was reported on 28 January 2024, from Puok district, Siem Reap province, Cambodia. The patient had pre-existing hypertension and had onset of symptoms on 21 January 2024, including a fever exceeding 38°C, cough, and difficulty breathing. The patient was admitted to the hospital on 23 January 2024 and tested positive for H5N1 through RT-PCR at the National Institute for Public Health on 27 January 2024. The sample was confirmed positive by additional testing at IPC on 28 January 2024. Based on the initial investigation, the patient raised domestic poultry and fighting roosters. Three chickens tested were found to be positive for influenza A(H5N1). No sampling and testing of the environment was done. Four close contacts and 39 additional contacts were identified and tested, of which one was found positive for an unrelated influenza B/Victoria lineage.
The two confirmed cases of human infection with avian influenza A (H5N1) virus have since recovered. Genome sequencing and phylogenetic analysis revealed that the HA genes of A/H5 isolates in both confirmed cases belong to clade 2.3.2.1c.
In 2023, six human cases, including four deaths, were reported from Kampot Province (n=2), Prey Veng Province (n=3) and Svay Rieng (n=1). From 2003 through 28 January 2024, a total of 64 cases of human infection with influenza A(H5N1), including 41 deaths, have been reported from Cambodia.
WHO Risk Assessment
From 2003 to 28 January 2024, 884 human cases of influenza A (H5N1) infection, including 461 deaths, have been reported globally from 23 countries. Almost all cases of human infection with avian influenza A(H5N1) were sporadic infections and have been linked to close contact with infected live or dead birds, or influenza A(H5N1) contaminated environments. These animal influenza viruses do not easily infect humans, and human-to-human transmission appears to be unusual. However, severe disease with high mortality rates can occur as a result of human infection. Given that the virus continues to circulate in poultry, particularly in rural areas in Cambodia and other countries where the virus is endemic in poultry, the potential for further sporadic human cases can be expected.
Available epidemiological and virological evidence suggests that A(H5N1) viruses have not acquired the ability to sustain transmission among humans. Therefore, the likelihood of human-to-human spread is considered low. Based on available information, WHO assesses the risk to the general population posed by this virus to be low. 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 poultry populations, and serological investigations, are critical to assess associated risks to public health and promptly adjust risk management measures.
There are no specific vaccines for influenza A(H5N1) in humans. However, candidate vaccines to prevent influenza A(H5) infection in humans have been developed for pandemic preparedness in some countries. WHO continues to update the list of zoonotic influenza candidate vaccine viruses (CVV) twice a year at the WHO consultation on influenza virus vaccine composition. The list of such CVVs is available on WHO website. In addition, the genetic and antigenic characterizations of contemporary zoonotic influenza viruses are published on Global Influenza Programme (who.int).
As we've discussed often, the public health threats from HPAI H5 viruses are multi-faceted and continually evolving. In addition to the multiple clades of H5N1 circulating around the globe, China has been dealing with a decade-long battle against HPAI H5N6, which has infected (at least) 90 people, killing roughly half.
Raw data suggests that H5N1 clade 2.3.2.1c and HPAI H5N6 currently pose more of a threat to human health than does H5N1 clade 2.3.4.4b, but we missing a lot of critical information, and all of these viruses continue to evolve in unpredictable ways.
Whether any of these viruses has what it takes to spark a pandemic is the $64 question. So far, efficient transmission between humans has not been observed.
But we shouldn't expect nature to stop trying.
