WHO Update on fatal H5N1 case in Mexico

Last week, we discussed the first H5N1 human case in Mexico.  (See Mexico Reports First Human H5N1 Case). This case was in a three year old girl, hospitalized in serious condition.  The situation evolved quickly and unfortunately the child died from respiratory complications in the hospital.  (See Fatal H5N1 Cases in Children and Risks to the Food Chain).  

On April 17, 2025, the WHO published a Disease Outbreak News bulletin on this fatal H5N1 case.  

From this bulletin we learn several new things about the human H5N1 case from Mexico.

The H5N1 virus was genotype D1.1:

From this report we learn that this H5N1 virus was genotype D1.1.  This genotype has been linked to more severe H5N1 human cases in the U.S. and Canada.  Genotype D1.1 has been circulating through wild birds and is different from the main genotype infecting dairy cattle.

The source of the child’s H5N1 infection remains unknown:

We still do not know where this child was exposed to H5N1 or how she was infected.  We previously noted that the girl lived in Durango state, a heavily agricultural region, but the agricultural authorities reported no known H5N1 outbreaks in poultry.

As previously discussed, part of the health authorities investigation was sampling local and wild birds in the area where the girl lived.  

From the WHO report we learn that 75 H5N1 outbreaks occurred in poultry farms between January 2022 and August 2024, but none in the same state as this case.  

There are, however, several mentions of H5N1 in captive or wild birds in this report.  H5N1 was detected in a vulture at a zoo in Durango state in January 2025.  Additionally, a canada goose exhibiting “neurological and hemorrhagic” symptoms reportedly died at a dam in the area.  The report notes that 25 sick birds were reported and H5N1 infection confirmed by laboratory testing.  The report also notes H5N1 was detected in a bird at a local park.  

The report makes no definitive statements as to whether this child may have been exposed to sick captive or wild birds, such as these referenced above.  But these cases highlight the fact that H5N1 is present in the surrounding environments.  It certainly seems possible that the child was exposed to a sick bird or contaminated feces, or some sort of contaminated environment, given the lack of known connection to sick poultry.  Why else would the authorities mention all these dead birds in the area where she lived?

H5N1 case investigation shows no further human cases:

According to the WHO, contact tracing identified 91 contacts, 21 of which were household contacts to the patient.  60 contacts were healthcare workers, and 10 contacts were “individuals from a child care center.”  Samples were collected from 49 of those contacts, all of which tested negative.  

The report does not specifically state why only 49 out of 91 contacts were tested.  It may be that some declined testing, or that only people with flu-like symptoms were tested.  The important thing is that no further H5N1 cases have been detected.  

The implications of this H5N1 case for human health:

The WHO maintains that the risk to humans remains low.  The investigation into this case has identified no additional H5N1 cases.  

Under its risk assessment, the WHO notes: “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.”  

This has been a typical message in these types of risk reports, although it doesn’t make me feel any better.  Especially since in the Mexican case, the girl was not exposed to sick poultry.  From this report the WHO’s risk profile does not appear altered, and yet they mention specific cases of H5N1 in dead captive or wild birds.  Does that mean there is an increased level of risk, like for zoo visitors or tourists at nature reserves?  

Another notable aspect of this report is the discussion of how providers should treat “confirmed or suspected human infection with a novel influenza A virus of pandemic potential.”  WHO recommends providers get details about animal exposure, travel history, identifying close contacts, “even prior to laboratory confirmation.”

That stood out to me for a related reason.  In this case, prompt diagnosis didn’t happen.  The girl first experienced fever, malaise, and vomiting on March 7.  She was hospitalized on March 13 “due to respiratory failure” and began antiviral treatment.  She was transferred to a tertiary care unit on March 16.  

It was not until March 18 that samples were collected for influenza testing.  Because the sample came back as influenza of an unknown subtype, it was sent to a laboratory.  The non-subtypable result was confirmed, and the sample was again forwarded to another laboratory “where it was molecularly identified as influenza A(H5).”  The sample was then sent to another laboratory where, on April 1, H5N1 was confirmed by PCR.  The girl died on April 8.  

That seems like a lot of hoops to jump through to get a positive H5N1 result, especially for a hospitalized patient.  We know that prompt identification of these cases is key to treatment because antivirals should be given as soon as possible.  

Moreover, it’s a crucial element of public health and pandemic preparedness to be promptly aware of novel flu cases in order to get a jump on managing a potential outbreak.  The WHO speaks to this issue in its report: 

Fortunately in this case, no further cases have been identified.  

Hopefully we get some more information as to how the child was exposed to the virus.  Otherwise this will join the ever growing list of cases with unknown H5N1 origin.  WHO and other health authorities remind us that such are to be expected, but it still makes me uneasy.  

It just didn’t used to be that easy to catch H5N1.

Until next time.