Disease Outbreak News: Avian Influenza A(H5N1) - Mexico, 17 April 2025

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17 Apr 2025

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17 Apr 2025

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Situation at a glance

On 2 April 2025, the International Health Regulations (IHR) National Focal Point (NFP) for Mexico notified the World Health Organization (WHO) of the country’s first laboratory-confirmed human infection with an avian influenza A(H5N1) virus in the state of Durango. In response, local and national health authorities have implemented a range of measures to monitor, prevent, and control the situation. There have been reports of A(H5N1) outbreaks in birds in Durango, although the exact source of infection in this case remains under investigation. To date, no further cases of human infection with influenza A(H5N1) linked to this case have been identified. In accordance with the IHR (2005), any human infection caused by a new influenza A virus subtype is considered a potentially significant public health event and is subject to mandatory notification to WHO. Based on the information currently available on this and previous cases, WHO assesses the risk to the general population posed by A(H5N1) viruses as low. For individuals with occupational exposure to these viruses, the risk of infection is considered low to moderate. The situation may change as more information is gathered and so WHO continues to closely monitor these viruses and the global epidemiological situation.

Description of the situation

On 2 April 2025, the Mexico IHR National Focal Point (NFP) notified the World Health Organization of a laboratory-confirmed human infection with an avian influenza A(H5N1) virus in the state of Durango.

This case represents the second reported human infection with avian influenza A(H5) in Mexico, and the first confirmed case of infection with an influenza A(H5N1) virus in the country.

The case is a child under the age of 10 years from the state of Durango who tested positive for influenza A(H5N1) at the Instituto de Diagnóstico y Referencia Epidemiológicos (InDRE, by its Spanish acronym). The case did not have any underlying medical conditions, had not received seasonal influenza vaccination, and had no history of travel.

Symptoms began on 7 March 2025 with fever, malaise, and vomiting. On 13 March, the case was admitted to hospital due to respiratory failure and antiviral treatment was initiated the next day. The case was transferred to a tertiary care hospital on 16 March and died on 8 April due to respiratory complications.

On 18 March, a nasopharyngeal swab was collected, and the sample was processed by real-time polymerase chain reaction (RT-PCR). The result was influenza A, non-subtypable. The sample was sent to the Centro de Investigación Biomédica del Noroeste (CIBIN, by its Spanish acronym), IMSS Monterrey, where the result was confirmed as influenza A, non-subtypeable, along with simultaneous detection of parainfluenza 3 virus. On 31 March, the sample was forwarded to the Laboratorio Central de Epidemiología (LCE, by its Spanish acronym) “La Raza,” where it was molecularly identified as influenza A(H5). On 1 April, the sample was received by InDRE, where the positive result for influenza A(H5N1) was confirmed by RT-PCR. The sample was further characterized as avian influenza A(H5N1) clade 2.3.4.4b genotype D1.1.

The source of infection remains under investigation. During contact tracing, 91 individuals were identified, including 21 household contacts, 60 healthcare workers, and 10 individuals from a childcare center. Pharyngeal and nasopharyngeal swab samples collected from 49 contacts tested negative for influenza A(H5N1). To date, no further cases of human infection with influenza A(H5N1) linked to this case have been identified.

According to information from the National Service for Agrifood Health, Safety and Quality (SENASICA per its acronym in Spanish), between January 2022 and August 2024, 75 outbreaks of A(H5N1) in poultry were reported across various regions of Mexico including: Aguascalientes (5), Baja California (4), Chiapas (1), Chihuahua (3), Guanajuato (2), Jalisco (17), México City (7), Michoacán (1), Nuevo León (1), Oaxaca (2), Puebla (2), Sonora (8), Tamaulipas (1), Veracruz (1), Yucatán (20). At the end of January 2025, SENASICA confirmed a new case of high pathogenicity avian influenza (HPAI) A(H5N1) in a sick vulture at the Sahuatoba Zoo in Durango, the state where the case lived. Following this, the death of a Canada goose with neurological and hemorrhagic symptoms was reported at the Peña del Aguila dam in Durango. A total of 25 sick birds were reported, and the presence of HPAI A(H5) was confirmed at a laboratory in Gómez Palacio, in Durango. A positive case of avian influenza A(H5) was also recorded in a bird at Las Auras Park.

Epidemiology

Animal influenza viruses typically circulate within animal populations but some have the potential to infect humans. Human infections are predominantly acquired through direct contact with infected animals or exposure to contaminated environments. Based on the original host species, influenza A viruses can be categorized into subtypes such as avian influenza, swine influenza, and other animal-origin influenza subtypes.

Human infection with avian influenza viruses may result in a spectrum of illness, ranging from mild upper respiratory tract symptoms to severe, life-threatening conditions. Clinical manifestations include conjunctivitis, respiratory, gastrointestinal symptoms, encephalitis, and encephalopathy. In some cases, asymptomatic infections with the A(H5N1) virus have been detected in individuals with known exposure to infected animals and environments.

A definitive diagnosis of human avian influenza infection requires laboratory confirmation. WHO regularly updates its technical guidance on the detection of zoonotic influenza, utilizing molecular diagnostic methods such as RT-PCR. Clinical evidence indicates that certain antiviral agents, particularly neuraminidase inhibitors (e.g., oseltamivir, zanamivir), have been shown to shorten the duration of viral replication and improve patient outcomes in some cases.

From 2003 to 10 April 2025, 972 cases of human infections with avian influenza A(H5N1), including 470 deaths (CFR 48.4%), have been reported to WHO from 24 countries. Almost all of these cases have been linked to close contact with infected live or dead birds, or contaminated environments.

Public health response

In response to this detection, additional investigation and surveillance activities are underway, coordinated by local and national health authorities, with involvement from both the animal and environmental sectors. These activities include:

Conducting a comprehensive epidemiological investigation of the case and contacts, including the collection of respiratory samples from symptomatic household contacts and healthcare workers, alongside ongoing follow-up and monitoring of close contacts.
Strengthening surveillance of respiratory viruses (including influenza-like illness [ILI] and severe acute respiratory infection [SARI]), with a particular focus on influenza viruses, in order to identify and analyze any atypical respiratory patterns or trends in the region.
Activating the One Health approach, involving SENASICA and national environmental agencies (SEMARNAT and CONANP), to ensure inter-institutional coordination, assess potential animal health risks, evaluate possible exposures within the community and animal populations, and notify the National Epidemiological Surveillance Committee (CONAVE).
Undertaking regular reporting of HPAI incidence in poultry and wild birds to the World Organisation for Animal Health (WOAH). The latest update on 5 March 2025 reported active surveillance and timely outbreak notifications, with a recent detection in the State of Durango. Consequently, local poultry farmers have been urged to reinforce biosecurity measures on their farms and backyard operations, and to immediately report any observed anomalies in animal health to the relevant authorities. Ongoing epidemiological surveillance is being maintained in poultry production units, backyards, federally inspected slaughterhouses, municipal slaughterhouses, and through national-level monitoring of wild bird populations.

WHO risk assessment

This case represents the second documented case of human infection with avian influenza A(H5) in Mexico and the first confirmed case of infection with an A(H5N1) influenza virus. Ongoing investigations are focused on identifying the source of infection and monitoring contacts. To date, no additional human cases of A(H5N1) virus infection have been identified in relation to this case, nor have any been detected through routine influenza surveillance.

Outbreaks of HPAI A(H5) viruses, including A(H5N1), in poultry have been reported across multiple states in Mexico since January 2022, including in the state of Durango.

When avian influenza viruses circulate in poultry populations, there is an inherent risk of human infection through exposure to infected birds or contaminated environments. As such, sporadic human cases are expected.

Although limited human-to-human transmission of A(H5) viruses was observed in isolated events between 1997 and 2007, sustained human-to-human transmission of A(H5) viruses has not been documented. Available epidemiological and virological data suggest that A(H5) viruses from previous outbreaks have not acquired the capacity for sustained human-to-human transmission.

Based on current information, the WHO assesses the overall public health risk associated with A(H5) viruses as low. However, for individuals with occupational exposure, the risk of infection is considered low to moderate.

The risk assessment will be updated as new epidemiological or virological information emerges in relation to this event.

WHO advice

This event does not change the current WHO recommendations on public health measures and surveillance of influenza.

Given the dynamic and evolving nature of influenza viruses, WHO underscores the critical importance of robust global surveillance systems to detect and monitor virological, epidemiological, and clinical changes associated with emerging or circulating influenza strains with implications for human or animal health. Timely sharing of virus isolates remains essential to inform comprehensive risk assessments.

In instances where humans are exposed to outbreaks of influenza A viruses in domestic poultry, wild birds, or other animal species—or when a human case is confirmed or suspected—enhanced surveillance of potentially exposed populations should be promptly initiated.

Surveillance strategies should account for healthcare-seeking behaviors within the affected population and may include a combination of active and passive methods, such as intensified case finding through ILI/SARI sentinel systems, active hospital-based screening, and targeted surveillance among high-risk occupational groups. Additional data sources, including traditional healers, private healthcare providers, and non-governmental diagnostic laboratories, should also be integrated where applicable to enhance the comprehensiveness of surveillance.

Considering the widespread detection of avian influenza viruses among poultry, wild birds, and select mammalian species, the general public is advised to avoid direct contact with sick or deceased animals. Any suspected dead birds or mammals, as well as any requests for their removal, should be reported to the appropriate local veterinary or wildlife authorities to ensure safe handling and testing.

All poultry products, including eggs and meat, should be thoroughly cooked and handled with proper food safety precautions. Consumption of raw milk is discouraged due to potential health risks. WHO advises consuming pasteurized milk and if pasteurized milk is not available, heating raw milk until it boils to make it safer for consumption.

In the event of a confirmed or suspected human infection with a novel influenza A virus of pandemic potential, including avian-origin strains, a comprehensive epidemiological investigation should be initiated. This should include detailed assessment of animal exposure history, travel history, and identification of close contacts, even prior to laboratory confirmation. The epidemiological investigation should also include early identification of unusual events that could signal person-to-person transmission of the novel virus. Clinical specimens from confirmed or suspected cases should be tested and referred to a WHO Collaborating Centre for further virological characterization. Additional samples should be collected from animals, the environment or any foods suspected to be sources of infection.

Individuals working in poultry production and processing environments should take additional health precautions as they are at higher risk of exposure to avian influenza and other zoonotic diseases due to their close contact with birds and potentially contaminated environments.

Farm workers who have direct or close contact with animals or materials infected or contaminated with avian influenza A(H5) virus should wear appropriate personal protective equipment (PPE) to minimize their risk of exposure.

Currently, there are several vaccines licensed for preventing influenza A(H5) virus infection in humans, although their availability is limited. Candidate vaccine viruses for pandemic preparedness have been selected to protect against A(H5) disease in humans based on circulating strains. Existing seasonal influenza vaccines are unlikely to provide protection against avian influenza A(H5) viruses, based on currently available data. However, it is important that individuals who may have frequent exposure to infected or potentially infected birds or other animals get a seasonal influenza vaccine, as it would contribute to decreasing the risk of co-infection and possible genomic recombination of avian and human viruses, which could result in new strains with pandemic potential. Close monitoring of the epidemiological, clinical and virological situation, further characterization of recent human, poultry, and other animal influenza viruses, and serological investigations remain essential for assessing risk and adjusting risk management measures as needed.

WHO advises travelers to regions experiencing animal influenza outbreaks to avoid contact with live animal markets, farms, slaughter sites, or any environments with potential contamination from animal excreta. Adherence to hand hygiene and safe food handling practices is strongly recommended. Should infected individuals travel internationally, detection may occur during travel or upon arrival through routine health screening. However, further community-level spread is considered unlikely, as this virus has not yet acquired the ability to transmit easily among humans.

All human infections caused by a novel influenza A virus subtype are notifiable under the International Health Regulations (IHR 2005) and State Parties to the IHR are required to immediately notify WHO within 24 hours of any laboratory-confirmed case of a recent human infection caused by an influenza A virus due to the potential to cause a pandemic. Evidence of illness is not required for this report. WHO has updated the influenza A(H5) confirmed case definition on the WHO website.

WHO does not recommend special traveler screening at points of entry or other restrictions due to the current situation of influenza viruses at the human-animal interface.

Further information

Citable reference: World Health Organization (17 April 2025). Disease Outbreak News; Avian Influenza A (H5N1) – Mexico. Available at: https://www.who.int/emergencies/disease-outbreak-news/item/2025-DON564

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