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Nine days after its initial announcement, the WHO has published an updated DON on the H5N2 case from Mexico City. Much of the details remain unchanged, although the H5N2 virus has been identified as a 99% match to an LPAI H5N2 virus reported in birds earlier this year in neighboring Texcoco State.
This is the first known human infection with H5N2, and while reassuringly no signs of additional cases have been found (serological tests are still pending), the $64 question remains:
How did a chronically ill bedridden individual in Mexico City get exposed to an LPAI H5N2 virus?
The WHO's risk assessment for the general population (Low) remains unchanged. Excerpts from the updated report follow.
Disease Outbreak News
Avian Influenza A(H5N2) - Mexico
14 June 2024
Situation at a glance
Following further investigation and information sharing by national authorities in Mexico, this is an update to the Disease Outbreak News (DON) on human infection caused by avian influenza A(H5N2) published on 5 June 2024. As was previously reported, on 23 May 2024, the Mexico International Health Regulations (IHR) National Focal Point (NFP) reported to PAHO/WHO a confirmed case of human infection with avian influenza A(H5N2) virus in a patient with underlying co-morbidities who subsequently died.This is the first laboratory-confirmed human case of infection with an influenza A(H5N2) virus reported globally and the first avian A(H5) virus infection in a person reported in Mexico. Although the source of exposure to the virus in this case is currently unknown, A(H5N2) viruses have been reported in poultry in Mexico.In their update, Mexican authorities informed WHO that a national group of experts determined on 6 June that the patient died due to complications related to his co-morbidities and that genetic analysis performed by the national authorities identified that this virus has a 99% similarity with the strain obtained during 2024 in birds in Texcoco State of Mexico. Based on available information, WHO’s risk assessment is unchanged: the current risk to the general population posed by this virus is low. The risk assessment will be reviewed should further epidemiological or virological information become available.
Description of the situation
As summarized in the previous DON (link), on 23 May 2024, the Mexico IHR NFP reported to PAHO/WHO a confirmed case of infection with avian influenza A(H5N2) virus detected in a 59-year-old resident of the State of Mexico who was hospitalized on 24 April 2024 in Mexico City. The case had no known history of exposure to poultry or other animals. He had multiple underlying medical conditions. His relatives reported that he had been bedridden for three weeks, for other reasons, prior to the onset of acute symptoms.On 22 May, sequencing of the sample by the national authorities confirmed the influenza subtype was A(H5N2). The genetic analysis by the national authorities identified that this virus has a 99% similarity with the low pathogenicity (in birds) avian influenza A/chicken/Texcoco, México/CPA-01654/2024 (H5N2) strain, obtained during 2024 from birds in Texcoco State of Mexico. The detailed H5N2 virus genetic sequence from the patient specimen has been uploaded to GISAID.
A national multidisciplinary group of experts was formed to investigate the cause of death. It included infectious disease specialists, pneumonologists, microbiologists and intensive care professionals. Upon review of the patient’s clinical history and records, the national multidisciplinary team concluded on 6 June that, although the patient had a laboratory-confirmed infection with avian influenza A(H5N2) virus, he died due to complications of his co-morbidities.
No further cases were reported during the epidemiological investigation. The 17 contacts identified and monitored at the hospital where the man died and 12 additional contacts near his residence, were tested and the results were negative for influenza viruses. Samples from these persons were taken a month after the acute disease onset in the patient with confirmed influenza A(H5N2) infection. The results of the serological samples are pending.
In March 2024, an outbreak of low pathogenicity avian influenza (LPAI) A(H5N2) was identified in poultry in Texcoco, State of Mexico, and a second outbreak of LPAI A(H5N2) in April in the municipality of Temascalapa in the same state (1).
Additionally, in March 2024, a high pathogenicity avian influenza A(H5N2) outbreak was detected in a backyard poultry farm in the state of Michoacán. A study describing the continuous circulation of low pathogenicity avian influenza H5N2 viruses in Mexico and spread to several other countries was published in 2022 (2), indicating that both LPAI and HPAI A(H5) subtypes were reported in avian species in Mexico recently and in past years.
(SNIP)
WHO risk assessment
This new information does not change WHO’s risk assessment. This is the first laboratory-confirmed human case of infection with an influenza A(H5N2) virus reported globally, and the first A(H5) virus infection in a person reported in Mexico. The case had multiple underlying conditions, and although the source of exposure has not been definitively determined, genetic analysis by authorities in Mexico identified that the virus from the patient has a 99% similarity with the strain obtained during 2024 in birds in Texcoco State of Mexico.
Whenever avian influenza viruses are circulating in poultry, there is a risk for infection and small clusters of human cases due to exposure to infected poultry or contaminated environments. Therefore, sporadic human cases are not unexpected. Human cases of infection with other A(H5) subtypes including A(H5N1), A(H5N6) and A(H5N8) viruses have been reported previously. Epidemiological and virological evidence available so far suggests that A(H5) viruses from previous events have not acquired the ability to sustain transmission between humans, thus the risk of sustained human-to-human spread remains assessed as low. According to the information available thus far, no further human cases of infection with A(H5N2) associated with this case have been detected.
There are no specific vaccines for preventing influenza A(H5) virus infection in humans. Candidate vaccines to prevent A(H5) infection in humans have been developed for pandemic preparedness purposes. Close analysis of the epidemiological situation, further characterization of the most recent viruses (in human and birds) and serological investigations are critical to assess associated risks and to adjust risk management measures in a timely manner.
Based on the available information, WHO assesses the current risk to the general population posed by this virus to be low. If needed, the risk assessment will be reviewed should further epidemiological or virological information, including information on A(H5N2) viruses detected in local animal populations, become available.
