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#18,594
#18,594
Last June Mexico's MOH issued a lengthy Statement on a Fatal H5N2 Infection In Mexico City, This was the first confirmed human infection with this subtype, although seroconversion has been observed in poultry workers (see Taiwan: Three Poultry Workers Show H5N2 Antibodies).
The patient, we learned, was male, with a history of `chronic kidney disease, type 2 diabetes, and long-standing systemic arterial hypertension'. The timeline provided stated:
- The patient died in the hospital on April 24th, but H5N2 wasn't identified until two weeks later (May 8th)
- Notification of WHO/PAHO occurred on May 23rd, two weeks after the H5N2 test results were obtained
- Samples were collected from contacts at the hospital on May 27 & 28, 3 weeks after H5N2 had been identified, and 5 weeks after the index patient died. While some were reportedly mildly symptomatic post exposure - all tested negative - although serological tests are still pending
This timeline illustrates (again) how difficult picking up novel flu infections can be - even in a big city hospital, and during a time of increased awareness (see CDC HAN: Accelerated Subtyping of Influenza A in Hospitalized Patients).
As to the delays following the initial identification of the virus, no details were provided.
A week later, in WHO Update On H5N2 Case In Mexico City, we learned that the H5N2 virus had been identified as a 99% match to an LPAI H5N2 virus reported in birds earlier last year in neighboring Texcoco State.
While reassuringly no signs of additional cases were found, the $64 question remained:
How did a chronically ill bedridden individual in Mexico City get exposed to an LPAI H5N2 virus?
Yesterday the Journal Viruses published a review of this case, which - while informative - is unable to explain how this patients was exposed to this virus. I've posted some excerpts below, but follow the link to read the (brief) report in its entirety. I'll have a bit more after the break.
Mexico’s Laboratory-Confirmed Human Case of Infection with the Influenza A(H5N2) Virus
Joel Armando Vázquez-Pérez 1, Claudia Wong-Arámbula 2, Mario Solís-Hernández 3, Eduardo Becerril-Vargas 1, Gisela Barrera-Badillo 2, Víctor Hugo Ahumada-Topete 1, Santiago Avila-Rios 1,
Rogelio Pérez-Padilla 1, Fidencio Mejía-Nepomuceno 1, Enrique Mendoza-Ramírez 1, Marisol Karina Rocha-Martinez 4, Carlos Javier Alcazar-Ramiro 3, Alfredo Cruz 1, Joaquin Zúñiga 1,5, Karolina Bozena Piekarska 1, Dayanira Sarith Arellano-Suarez 2, María Natividad Cruz-Ortiz 2, Tatiana Ernestina Núñez-García 2, Eréndira Molina-Gómez 2, Laura Adriana Flores-Cisneros 6,add Show full author list
Viruses 2025, 17(2), 205; https://doi.org/10.3390/v17020205
Published: 31 January 2025
Abstract
In April 2024, the Instituto Nacional de Enfermedades Respiratorias of Mexico City identified a case of unsubtypeable Influenza A in a 58-year-old immunocompromised patient with renal failure due to diabetic nephropathy and bacterial peritonitis. Through sequencing the M, NS, NA, NP, and HA complete segments, we identified an H5N2 influenza virus with identity of 99% with avian influenza A(H5N2) from Texcoco, Mexico, in 2024. This case is the first reported with direct evidence of human infection caused by the H5N2 influenza virus; the relationship of the virus with the severity of his condition remains unknown.
1. Introduction
Human infections with avian influenza viruses (AIVs) have been reported since the 1990s [1]. The main subtypes causing occasional respiratory diseases in humans are H5N1, H7N9, and H9N2 [2]. Additionally, H5N1 or H7N3 viruses were recently discovered in patients from North America who had mild symptoms such as conjunctivitis but no respiratory system involvement [3,4]. Furthermore, the avian H5N2 subtype is primarily seen in migratory birds and commercial and backyard poultry [5]. The first avian influenza H5N2 strain was initially described as infecting chickens in Pennsylvania in April 1983 [6]. To date, viral clades of H5N2 have been described in several countries, including Mexico, infecting chickens and other bird species [5].
In 1994, Mexican health authorities reported cases of low-pathogenicity H5N2 infections in chickens. However, at the end of 1994, highly pathogenic H5N2 strains were detected on several poultry farms, causing serious economic consequences amongst poultry farmers [6].
In March 2024, a high-pathogenicity H5N2 outbreak was identified on a poultry farm in Michoacán State. During the same month, two low-pathogenicity H5N2 outbreaks were reported in backyard poultry in the State of Mexico, in the municipalities of Texcoco and Temascalapa [7].
Some serological reports suggest potential subclinical H5N2 infections that were not detected [8,9]. However, to our knowledge, no human H5N2 infection has previously been documented. We present a case of respiratory sickness caused by the avian influenza H5N2 subtype in a patient who was admitted to the National Institute for Respiratory Diseases (INER) in Mexico City in April 2024.
(SNIP)
The epidemiological investigation included communication with household contacts and an interview with his partner, who denied the presence of respiratory symptoms, living with animals, contact with birds, or any poultry-related activities. Seventeen suspected cases were identified, including contacts at the hospital and family members. Other surveillance activities included intentionally searching for suspected cases of zoonotic influenza across the two blocks around the home of the confirmed case. No identified contact was confirmed to have influenza H5N2.(SNIP)
Discussion
The detection and molecular characterization of influenza virus H5N2 in a respiratory sample confirmed the first report of human infection due of this subtype in Mexico.
Molecular evidence suggests that the human isolate of this study (INER_INF645_24) and the avian isolates from 2022, 2023, and 2024 possibly derive from a common avian H5N2 ancestor from 2019 from Central Mexico (Influenza A virus (A/chicken/Queretaro/CPA-04673-1/2019(H5N2)).
The observation of the highest homology (99%) of the study virus being with an avian H5N2 isolate from Texcoco, State of Mexico (2024), suggests a direct relationship between these isolates. Although direct contact between the patient in this study and poultry or other domestic animals could not be confirmed, it is plausible that this avian virus causing high disease burden in chickens in this geographical area in 2024 could be the source of the human case described here, as human-to-human transmission seems unplausible.
This is the first report of a human case of influenza H5N2 infection in Mexico. Further studies are required to determine the predicted pathogenicity of the virus and to predict its capability of human-to-human transmission and potential threat to human health. Unfortunately, several comorbidities in the case described here led to a fatal outcome, but the pathogenicity of the isolate needs to be further studied.
Since no cases of H5N2 influenza in humans have been reported so far, we are unaware of the clinical outcomes that this influenza virus subtype may have in humans. At admission, the patient was severely ill, with renal failure and bacteremic infection. It is uncertain what contribution the influenza virus H5N2 made to the final clinical status of the patient, and it is also unknown how the patient acquired the influenza virus, which is very similar to bird viruses identified in the Valley of Mexico in 2024.
Based on this report, the epidemiological investigation doesn't appear to have included serological testing of close contacts.
The challenges of identifying and tracking novel influenza infections are topics we've looked at repeatedly. Last summer the ECDC published Enhanced Influenza Surveillance to Detect Avian Influenza Virus Infections in the EU/EEA During the Inter-Seasonal Period which cautioned:
Sentinel surveillance systems are important for the monitoring of respiratory viruses in the EU/EEA, but these systems are not designed and are not sufficiently sensitive to identify a newly emerging virus such as avian influenza in the general population early enough for the purpose of implementing control measures in a timely way.Similarly, in 2023's analysis from the UKHSA (see TTD (Time to Detect): Revisited), they estimated there could be dozens or even hundreds of undetected human H5N1 infections before public health surveillance would likely detect them, potentially over a period of weeks or months.
While most of these spillovers continue to be dead-end infections, each is another opportunity for the virus to better adapt to humans. Should one stumble upon the right set of mutations to make it easily transmissible, that status quo could change overnight.
As the following quote by a former HHS Secretary reminds us:
“Everything you say in advance of a pandemic seems alarmist. Anything you’ve done after it starts is inadequate."
- Michael Leavitt, Secretary of HHS
