1st-known human case of H5N2 bird flu remains under investigation

A person in Mexico recently died after catching the first known human case of H5N2 bird flu.

The case marks the first time someone in Mexico has been sickened by any type of influenza A(H5) virus, a broad group of related bird flu viruses, the World Health Organization (WHO) reported June 5. 

Initially, authorities reported that the infection was a "confirmed fatal case" of H5N2. However, in an update issued June 14, they said that, although the man had tested positive for the flu virus around his time of death, he likely actually died of complications related to his underlying medical conditions.  

Specifics about those conditions haven't been released, but the investigators said that the patient's medical records pointed to this conclusion. Notably, the man had reportedly been "bedridden" for other reasons three weeks prior to developing any bird flu symptoms.  

Related: 'Increased evidence that we should be alert': H5N1 bird flu is adapting to mammals in 'new ways'

The case is still notable, though, because it demonstrates that H5N2 can spread to humans and cause disease. 

The broad category of virus that H5N2 belongs to also includes the one currently circulating among U.S. dairy cows, called H5N1. H5N1 recently sickened three people who had close contact with cattle. When it's caused sporadic infections in people, H5N1 has led to severe pneumonia and death in at least 50% of cases. In general, bird flu-related deaths have been reported in people with and without underlying medical conditions, the WHO says. 

The spread of bird flu to people is concerning, in part, because these viruses can sometimes be deadly. Another reason for concern is that the more times bird flu jumps into people, the more chances it has to pick up mutations and gain the ability to spread easily between humans. As of yet, no A(H5) viruses can sustainably spread from person to person, the WHO reported — but that could change in the future. 

For now, though, "based on available information, WHO assesses the current risk to the general population posed by this virus [H5N2] as low," the agency reported June 14. 

The recent case in Mexico involved a 59-year-old who developed fever, shortness of breath, diarrhea, nausea and general malaise in mid-April. The person sought medical treatment at a Mexico City hospital on April 24 but died the same day.

A sample of the patient's respiratory fluids, taken April 24, revealed he'd had H5N2. The virus is similar to H5N1, in that both viruses carry the same type of protein — H5 — on their surfaces, but a second protein called neuraminidase (N) is a slightly different shape on each virus. 

A close genetic analysis of the man's sample revealed that the virus was 99% similar to a strain of H5N2 detected this year in birds in Texcoco, Mexico. The strain is known to have low pathogenicity in birds, meaning it causes few to no symptoms in poultry. Highly pathogenic avian influenza viruses, in contrast, cause severe disease and death in poultry. 

Both highly and low pathogenic H5N2 viruses have caused outbreaks among birds in several Mexican states this year. 

According to the WHO, the man had had no recent contact with poultry or other animals; typically, people catch bird flu through contact with sick or dead animals, or from contact with materials those animals contaminated with the virus. As of June 14, authorities are still investigating the potential source of the patient's infection.

So far, none of the person's close contacts have tested positive for bird flu, and there's no evidence the virus has spread between people. "This case and this update do not change the current WHO recommendations on public health measures and surveillance of influenza," the agency reiterated in its June 14 statement.

Since people generally catch A(H5) viruses from animals or from handling those animals' bodily fluids, you can reduce the risk of bird flu by avoiding sick and dead animals and regularly washing your hands with soap and water. In the U.S., dairy workers have been advised to wear personal protective equipment around potentially sick animals, as an additional safety measure, and authorities have also emphasized that drinking raw milk could pose a risk to consumers. 

Again, the risk of transmission to the general public is low and overall these bird flu infections are rare. Should you catch an A(H5) virus, existing drugs for seasonal flu can help treat the infection.

Editor's note: This story was updated on June 14, 2024, following an update from the WHO that clarified the man likely died of comorbidities. The original story was published on June 6. 

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