#18,913
Three days ago in the latest PAHO H5N1 Epidemiological Report, there was a brief mention of an H5 case in Mexico. While I've been hoping for a more detailed report, so far, details remain scant.
What we do know is the patient - a woman in her 20's - developed symptoms on September 14th, and after two weeks of increasing respiratory distress was finally hospitalized and a bronchoalveolar lavage sample was obtained, which tested positive for an unsubtypable influenza A on Sept 29th.
The sample sent out for further testing, and Avian influenza A(H5) was confirmed on September 30th. The patient was then put on oseltamivir. Determination of the NA type remains pending.
The first occurred in April of 2024 (see Mexico announced the death of a 58-year-old man) when a patient - who also suffered from serious comorbidities - tested positive for H5N2, becoming the first laboratory-confirmed human case of influenza A(H5N2) infection reported globally.This is the third H5 case reported from Mexico since 2024, and the source of all three remain undetermined.
A year later (April 2025), a 3-year old girl from Durango State died following an H5N1 infection (see WHO DON Update On Mexico's Fatal H5N1 Infection).
While this latest case went > 2 weeks with symptoms before being diagnosed, the good news is once hospitalized, the infection was quickly identified and treatment was begun.
First the ECDC summary, after which I'll have a postscript.
Human infection with avian influenza A(H5) virus - Mexico - 2025
Overview: On 15 October 2025, Pan American Health Organization, PAHO/WHO reported a new human case with avian influenza A(H5) virus infection in Mexico City, Mexico. A young woman in her 20s with no recent travel history and no recent influenza vaccination developed respiratory symptoms (rhinorrhea and cough) on 14 September 2025.
The symptoms progressed to fever and odynophagia followed by hemoptysis and chest pain between 21 and 28 September when she was hospitalised at the National Institute of Respiratory Diseases (INER as per acronym in Spanish). On 29 September bronchoalveolar lavage sample was collected and tested positive for unsubtypable influenza A. Avian influenza A(H5) was confirmed on 30 September by real-time RT-PCR (designation of the influenza virus neuraminidase is pending), same day the patient received treatment with Oseltamvir. The patient was discharged on 11 October 2025.
During the epidemiological investigation, 41 contacts were identified. All samples taken from the contacts tested negative for avian influenza; all contacts received prophylactic treatment with Oseltamivir.
Several animals (two pigeons, a pet dog in the domicile of the case, and poultry birds) were identified in the courtyard of the residential place of the individual. Bird droppings were identified in several areas of the house, including a poorly sealed water cistern that supplies water to all apartments in the building. Samples taken from some animals tested positive for avian influenza A(H5) at the Official Laboratory of the National Service for Agrifood Health, Safety, and Quality (SENASICA, per its acronym in Spanish). Results of the testing of the environmental samples are pending.
This is the third human case with avian influenza A(H5) infection reported in Mexico since 2024. The previous case (with avian influenza A(H5N1)) of infection was reported in April 2025. Since 2003 and as of 13 October 2025, 991 human cases of avian influenza A(H5N1) virus infection, including 476 deaths (CFR 48%), were reported from 25 countries worldwide.
Acknowledgements: we gratefully acknowledge all data contributors, i.e. the authors and their originating laboratories responsible for obtaining the specimens, and the submitting laboratories for generating the genetic sequence and metadata and sharing via the GISAID Initiative.
ECDC assessment:
Sporadic human cases of different avian influenza A(H5Nx) subtypes have previously been reported globally.
Despite widespread transmission of avian influenza viruses in animals, transmission to humans with avian influenza remains infrequent and no sustained transmission between humans has been observed.
Overall, the risk related to zoonotic influenza for the general population in EU/EEA is considered low.
Direct contact with birds and other infected animals, their secretions or a contaminated environment is the most likely source of infection, and the use of personal protective measures for people exposed to dead animals or their secretions will minimise the associated risk. The recent severe cases in Asia and the Americas in children and people exposed to infected, sick or dead backyard poultry underlines the risk of unprotected contact with infected birds in backyard farm settings.
Actions:
ECDC monitors avian influenza strains through its influenza surveillance programme and epidemic intelligence activities in collaboration with the European Food Safety Authority (EFSA) and the EU Reference Laboratory for Avian Influenza in order to identify significant changes in the virological characteristics and epidemiology of the virus. Together with EFSA and the EU Reference Laboratory for Avian Influenza, ECDC produces a quarterly report on the avian influenza situation. The most recent report was published in September 2025.
Once again, we don't know if this is H5N1, H5N2, or some other subtype.
Making the negative results from close contacts - who are often tested weeks after exposure - less probative than they might appear.
While most known human H5 infections have been epidemiologically linked to a specific agricultural exposure (cows, chickens, wild birds, etc.), over the past 18 months we've seen a handful (U.S. x 4, Mexico x 3, Canada x 1) where the source of exposure remains unexplained.
Given the limits of surveillance and testing (see above), it would not be terribly surprising if there are other cases in the community that have not been officially confirmed.
Particularly since some percentage of infections are asymptomatic or very mild (see MMWR: Serologic Evidence of Recent Infection with HPAI A(H5) Virus Among Dairy Workers).
Had this patient's condition not worsened enough over 2 weeks for her to seek hospital care - and the proper tests were performed - it would likely never have come to light.
While there is still no evidence of efficient human-to-human transmission, that is something that can likely only be detected after it's begun in earnest (see UK Novel Flu Surveillance: Quantifying TTD).
