The World Health Organization (WHO), on June 5, confirmed the first human case of the H5N2 type of bird flu in a 59-year-old man from Mexico who died in April.
The case heightened concerns about the potential spread of bird flu among humans, especially since the man had no known contact with poultry or other animals.
“Although the source of exposure to the virus in this case is currently unknown, A(H5N2) viruses have been reported in poultry in Mexico,” stated the WHO, noting that it was unclear how the man contracted the virus.
H5 viruses
H5 viruses are distinguished by their surface proteins: hemagglutinin (H) and neuraminidase (N). There are nine known subtypes of H5 viruses, with H5N2 being part of this family. H5N1, another subtype, is known for its high pathogenicity and has caused significant outbreaks in poultry and sporadic human infections.
H5N2 is one of several avian influenza viruses. While exposure to H5 viruses in Mexico is not surprising, given their presence among poultry and wild birds since the mid-1990s, human infections are rare.
H5N2 virus case in Mexico
The 59-year-old man had been bedridden for several weeks before showing symptoms, which included fever, nausea, diarrhea, shortness of breath, and general malaise. He was hospitalized on April 24 and died the same day.
According to Dr. Troy Sutton, an assistant professor at Penn State, the man’s multiple underlying health conditions likely exacerbated his infection.
Despite extensive monitoring of 17 hospital contacts, only one person reported mild symptoms, and no other cases were detected.
Dr. Michael Osterholm from the University of Minnesota considerd human-to-human transmission unlikely, suggesting the man likely contracted the virus from the same environment where the virus was present.
Data indicated that the H5N2 virus in this case is a low pathogenic virus, which is less likely to cause severe illness. High pathogenic viruses, like certain strains of H5N1, undergo genetic changes that increase their severity.
Dr. Paul Offit from the Children’s Hospital of Philadelphia emphasized the importance of further genetic sequencing to determine if H5N2 has mutated to spread more easily among humans.
Historically, H5 viruses struggle to infect humans due to differences in cell receptors between birds and humans.
H5N1 outbreak in United States
The case in Mexico is not linked to the recent H5N1 outbreak in the United States, which infected three dairy farm workers.
The H5N1 virus also caused infections in mammals such as seals, raccoons, bears, and cattle due to contact with infected birds.
Andrew Pekosz, an influenza expert at Johns Hopkins University, highlighted the ongoing risk of H5 viruses adapting to infect humans more effectively, stressing the need for vigilant monitoring.
“So it continues to ring that warning bell that we should be very vigilant about monitoring for these infections, because every spillover is an opportunity for that virus to try to accumulate those mutations that make it better infect humans,” he said.