Mexico Reports First Human H5N1 Case

On Friday, April 4, 2025, Mexico reported the country’s first human case of H5N1 bird flu.

The details about the first human H5N1 case in Mexico:

The patient is a three year old girl. She is currently hospitalized.  She was treated with oseltamivir and remains in serious condition.

The Mexican Government issued a press release on April 4, providing the limited details we have on this new case.

Laboratory testing confirmed H5N1 infection on April 1.  It is not clear when the patient was first admitted to the hospital nor when she first suffered symptoms.  So we don’t know how long after infection that treatment started.  

Notably, the source of H5N1 exposure is unknown. The girl lives in the Durango State region of Mexico, which is heavily agricultural.  There may be a farming or animal connection to this case.  However, the state agricultural authority reported no H5N1 outbreaks in commercial poultry in the country. 

Previous Human H5N2 Case in Mexico:

Last year, Mexico reported its first H5N2 human case.  In June 2024, WHO reported that a 58 year old man in Mexico City died of H5N2.  This patient was actually hospitalized in late April 2024. It wasn’t until late May that Mexico reported it to the WHO. 

The source of that man’s infection was unknown. There were H5N2 outbreaks in poultry in March, but that patient had no known contact with poultry or other animals.

The situation is similar to this new case, the first H5N1 human case. As before, the three year old girl had no known contact with poultry.

H5N1 cases in North America with unknown exposure source:

We have already seen recent H5N1 human cases with unknown exposure sources. The human H5N1 case in Missouri did not have a clear exposure source. None was ever determined.  The H5N1 case in a teenager in Canada also had no known exposure source.  The investigation was ultimately closed without a known exposure. This new case in Mexico adds to the growing list of human cases with no clear exposure route to bird flu.

The scientific community states that such unknown exposure source cases are bound to crop up because there is so much H5N1 virus circulating.  Indeed, Michael Osterholm, one of the world’s most predominant bird flu scientists, said in a recent podcast that he fully expected there to be many more H5N1 cases of unknown origin.

But that doesn’t make me feel any better.

In Mexico, efforts are underway to test wild and local birds near the patient’s home.  Local health services are following up with known contacts to look for “cases of suspected viral illness.”  

The implications of Mexico’s first H5N1 human case and the risk for human health:

According to the press release, the risk to the general population is low, “so eating well-cooked chicken or eggs does not pose a risk to human health.”  This may be routine advice or may reflect another potential infection pathway under investigation.

Sequencing of the virus is not yet available, so we don’t know what genotype the virus belongs to or where it fits in the larger H5N1 family tree.  That information may shed light on how this girl became infected.

In the meantime, as always, the public must remain vigilant.  The general risk of H5N1 to humans remains low,  but there is just so much H5N1 out there now in the environment.  We will likely continue to see human H5N1 cases with no clear route of exposure. While expected, it is still unsettling.

Until next time.