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, 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.
Epidemiology
Animal influenza viruses normally circulate in animals but can also infect humans. Infections in humans have primarily been acquired through direct contact with infected animals or contaminated environments. Depending on the original host, influenza A viruses can be classified as avian influenza, swine influenza, or other types of animal influenza viruses.
Avian influenza virus infections in humans may cause mild to severe upper respiratory tract infections and influenza-associated deaths have been reported in persons with or without comorbidities. Conjunctivitis, gastrointestinal symptoms, encephalitis and encephalopathy have also been reported.
Laboratory tests are required to diagnose human infection with influenza. WHO periodically updates technical guidance protocols for the detection of zoonotic influenza using molecular methods, e.g. RT-PCR. Evidence suggests that some antiviral drugs, notably neuraminidase inhibitors (oseltamivir, zanamivir), can reduce the duration of viral replication and improve survival prospects for some cases.
Public health response
Local and national health authorities report implementing the following public health measures:
Conducting an ongoing investigation into the source of infection.
Formed a multi-disciplinary group experts to determine the cause of death. It comprised of professionals from Secretaría de Salud: Comisión Coordinadora de Institutos Nacionales de Salud y Hospitales de Alta Especialidad (CCINSHAE) of the Centro Nacional de Programas Preventivos y Control de Enfermedades (CENAPRECE); Dirección General de Información en Salud (DGIS), Dirección General de Epidemiología (DGE-InDRE) and Dirección General de Promoción de la Salud (DGPS); Secretarías de la Defensa Nacional (SEDENA); Secretaría de Marina Armada de México (SEMAR); and Instituto Mexicano del Seguro Social.
Issued official press releases including updates of national investigations and actions with a multisectoral approach, as well as general information and recommendations for the population.
The below actions were previously reported
Epidemiological investigation of case and contacts.
Monitoring of health care workers with a history of contact with the patient.
Monitoring and surveillance of influenza-like respiratory illness (ILI) and severe acute respiratory illness (SARI) in neighbouring municipalities (within the same health region), in order to analyze the behaviour and trends of respiratory syndromes and viruses in the region.
Analysis of the trends of pneumonia and bronchopneumonia, acute respiratory infections, and conjunctivitis by the health services of Mexico City and the State of Mexico.
Identification of transmission chains and risk factors in the municipality where the case resided, the State of Mexico and surrounding areas.
Training on the National Guide for preparedness, prevention and response to an outbreak or zoonotic influenza event at the animal-human interface.
Communicated with animal and environmental health authorities to strengthen surveillance activities in poultry and wild birds near the case’s residence and areas with a history of low pathogenic avian influenza A(H5N2) outbreaks.
PAHO/WHO implemented the following measures:
Strengthening routine and event surveillance on the human-animal interface with WHO Collaborating Centers and strategic partners.
Improvement of molecular diagnostic capacity for detection of zoonotic diseases through knowledge transfer, training and technical support with recent emphasis in avian influenza A(H5N1).
Strengthening national capacity for the prompt shipment of human and animal samples to WHO collaborating centers for additional characterization and/or vaccine composition analysis.
Regular risk assessment for transmissibility and severity for zoonotic viruses.
Update of guidelines on influenza surveillance and response at the human-animal interface.
Revision of experiences in response and lessons learned from countries that experienced zoonotic influenza outbreaks.
Technical strengthening of risk communication capacities for events at the human-animal interface.
Clinical management training on zoonotic influenza treatment, infection prevention and control (IPC), and reorganization of health services.
Animal carcass handling training, including IPC technical aspects.
PAHO published recommendations to strengthen intersectoral work in surveillance, early detection, and research at the human animal interface.
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.
WHO advice
This case and this update do not change the current WHO recommendations on public health measures and surveillance of influenza.
Due to the constantly evolving nature of influenza viruses, WHO continues to stress the importance of thorough and timely epidemiologic investigation and global surveillance to detect and monitor virological, epidemiological and clinical changes associated with emerging or circulating influenza viruses that may affect human and animal health and timely virus-sharing for risk assessment.
When there has been human exposure to a known outbreak of an influenza A virus in domestic poultry, wild birds or other animals or when there has been an identified human case of infection with such a virus, enhanced surveillance in potentially exposed human populations becomes necessary. Enhanced surveillance should consider the healthcare-seeking behaviour of the population. It could include a range of active and passive healthcare and/or community-based approaches, including enhanced surveillance in local ARI/ILI/ SARI systems, active screening in hospitals and of groups that may be at higher occupational risk of exposure, and inclusion of other sources such as traditional healers, private practitioners and private diagnostic laboratories.
In the case of a confirmed or suspected human infection caused by a novel influenza A virus with pandemic potential, including avian influenza virus, a thorough epidemiologic investigation (even while awaiting the confirmatory laboratory results) of a history of exposure to animals and/or travel should be undertaken along with contact tracing. The epidemiologic investigation should include early identification of unusual events that could signal person-to-person transmission of the novel virus. Clinical samples collected from suspected human cases should be tested and sent to a WHO Collaboration Centre for further characterization.
Travellers to countries with known outbreaks of animal influenza should avoid farms, contact with animals in live animal markets, entering areas where animals may be slaughtered, or contact with any surfaces that may have been contaminated with animal faeces. Travellers should also wash their hands often with soap and water. Travelers should follow good food safety and good food hygiene practices. Should infected individuals from affected areas travel internationally, their infection may be detected in another country during travel or after arrival. If this occurs, further community-level spread is considered unlikely as this virus has not acquired the ability to transmit easily among humans.
All human infections caused by a novel influenza A virus subtype are notifiable under the IHR, and State Parties to the regulations are required to immediately notify WHO of any laboratory-confirmed case of a recent human infection caused by an influenza A virus with the potential to cause a pandemic. Evidence of illness is not required for this report.
WHO does not advise special traveller screening at points of entry or restrictions regarding the current situation of influenza viruses at the human-animal interface.
