The World Health Organization (WHO) recently reported the first confirmed human case of the H5N2 type of bird flu in a 59-year-old man in Mexico who died in April. This case has added to the growing anxiety about the risk of bird flu spreading among people, particularly because the man had no history of exposure to poultry or other animals, according to the WHO.
H5N2 is one of several kinds of avian influenza viruses. Dr. Troy Sutton, an assistant professor of veterinary and biomedical sciences at Penn State, explained that H5 viruses have been circulating among poultry and wild birds in Mexico since the mid-1990s. Unlike other avian influenza strains that have caused outbreaks in humans, such as H1 and H3 viruses, H5 viruses rarely infect humans. However, the presence of a human case is always a cause for concern due to the potential for viruses to mutate.
The H5N2 virus belongs to a family of bird flu viruses classified by two types of proteins on their surfaces: hemagglutinin (H), which allows the virus to infect cells, and neuraminidase (N), which helps the virus spread. Many different combinations of H and N proteins are possible. There are nine known subtypes of H5 viruses, according to the Centers for Disease Control and Prevention (CDC).
The H5N1 strain, which was detected in dairy cows in the U.S. in March, also belongs to this family. H5N1 is associated with highly contagious strains called the “Goose Guangdong lineage,” which have caused numerous outbreaks in poultry over the last 20 years and sporadic infections in humans, Sutton noted. H5N1 has infected people in 23 countries since 1997, resulting in severe pneumonia and death in about 50% of cases.
The patient in Mexico who contracted H5N2 had been bedridden for several weeks before developing symptoms. On April 17, he developed fever, nausea, diarrhea, shortness of breath, and general malaise. He was hospitalized a week later on April 24 and died the same day. Sutton emphasized that the man had multiple underlying medical conditions, which likely exacerbated his infection. “The person may have already been quite sick,” Sutton said, noting that this changes the risk calculation compared to a healthy individual getting infected.
The WHO reported no other cases during its investigation. Among the 17 contacts identified and monitored at the hospital where the patient died, only one reported a runny nose. The source of the man’s infection remains unknown, as he had no known exposure to poultry or other animals. This raises concerns about the potential for unidentified cases or human-to-human transmission. However, Dr. Michael Osterholm, an infectious disease expert at the University of Minnesota, believes human-to-human transmission is unlikely and suggests the man likely picked up the virus from an environmental source.
Data indicates that the H5N2 virus that infected the man is a low pathogenic virus, meaning it is unlikely to cause severe illness, according to Osterholm. “There’s high path and low path, and the high path has certain genetic changes. It makes it much more likely to cause serious illness,” he explained. Low pathogenic viruses can infect various animal species with little or no symptoms, making them less of a concern for severe outbreaks.
Dr. Paul Offit, an infectious disease expert and director of the Vaccine Education Center at Children’s Hospital of Philadelphia, highlighted that the main question among experts is whether the H5N2 virus has mutated to make it easier to spread to humans. H5 viruses typically struggle to infect people because the cell receptors they target in birds are different from those in humans. This is why human infections with H5 viruses usually occur through direct contact with birds and poultry rather than through human-to-human transmission. Offit emphasized that the difference between H5N2 and H5N1 is not necessarily significant in terms of pandemic potential. “If the virus can’t reproduce itself well in the upper respiratory tract, it’s not going to be able to spread easily from human to human,” he said.
Further genetic sequencing of the H5N2 virus that infected the Mexican man is necessary to determine its risk to humans, Sutton added. Until this information is available, drawing definitive conclusions is challenging. Osterholm reiterated that H5N1 remains the primary concern due to its presence in dairy cows in the U.S., infecting at least 84 herds across nine states. The possibility that H5N1 could acquire mutations allowing it to spill over to humans is particularly troubling, given its high pathogenic potential and severe outcomes in infected individuals.
The discovery of the H5N2 infection in a human highlights the importance of ongoing surveillance and research into avian influenza viruses. Scientists are keen to understand whether the H5N2 virus has undergone any mutations that could facilitate its transmission to humans. The ability of avian influenza viruses to mutate and potentially become more transmissible among humans is a significant concern for public health.
While H5N2 is currently considered low pathogenic, the unpredictability of viral evolution necessitates caution and preparedness. The case in Mexico underscores the need for robust monitoring systems and rapid response strategies to detect and contain any emerging threats. Public health authorities must remain vigilant, especially in regions where avian influenza viruses are known to circulate among bird populations.
Preventive measures, including biosecurity practices on poultry farms and the monitoring of wild bird populations, are crucial in mitigating the risk of avian influenza outbreaks. Educating the public about the risks of direct contact with birds and promoting safe handling practices can also help reduce the likelihood of human infections.
As researchers continue to investigate the H5N2 virus, it is vital to support ongoing scientific research and funding for studies focused on avian influenza and other zoonotic diseases. Understanding the mechanisms of viral transmission, pathogenicity, and host adaptation can inform the development of effective vaccines and treatments.
