Is there a new strain of bird flu to be worried about?
On Wednesday, the World Health Organization 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.
The case adds to growing anxiety about the risk of bird flu spreading among people, especially because the man had no history of exposure to poultry or other animals, according to WHO.
The strain is different from the outbreak of bird flu virus, called H5N1, that is circulating among herds of dairy cows in the United States and has caused mild infection among three farm workers.
What is H5N2?
H5N2 is just one of several kinds of avian influenza viruses. Does it actually pose a significant health risk to humans?
An exposure to H5 virus in Mexico isn’t surprising, said Dr. Troy Sutton, an assistant professor of veterinary and biomedical sciences at Penn State. H5 viruses have been circulating among poultry and wild birds in Mexico since the mid-1990s. However, unlike other avian influenza strains that have caused outbreaks in humans — such H1 and H3 viruses — H5 viruses rarely infect humans.
The viruses are classified based on two types of protein on their surfaces: hemagglutinin, or H, which plays a crucial role in allowing the virus to infect cells, and neuraminidase, or N, which helps the virus spread. Many different combinations of H and N proteins are possible.
H5N2 belongs to a family of bird flu viruses called H5, which primarily infects wild birds. There are a total of nine known subtypes of H5 viruses, according to the Centers for Disease Control and Prevention.
H5N1, which was detected in dairy cows in the U.S. in March, also belongs to this family. It is commonly associated with highly contagious strains of H5 viruses called the “Goose Guangdong lineage” that have caused numerous outbreaks in poultry over the last 20 years and sporadic infections in humans, said Sutton.
H5N1 has infected people in 23 countries since 1997, according to the CDC, resulting in severe pneumonia and death in about 50% of cases.
“They are a separate lineage with a separate history and a separate sort of story around the disease they cause,” Sutton said of the H5N2 and H5N1 viruses.
Should people be concerned?
The patient in Mexico had been bedridden for several weeks prior to developing symptoms.
According to WHO, on April 17, the man developed fever, nausea, diarrhea, shortness of breath and general malaise. A week later, on April 24, he was hospitalized and died that day.
Sutton said that it’s important to note that the man had multiple underlying medication conditions, which likely exacerbated his infection.
“The person may have already been quite sick,” Sutton said. “That changes the calculation a little bit more than, say, a healthy farm worker getting infected.”
The WHO said no other cases were reported during its investigation. Of the 17 contacts identified and monitored at the hospital where the patient died, one reported a runny nose.
However, experts still don’t know how the man became infected with the virus, as he wasn’t exposed to poultry or to other animals. If he was infected by another human, that suggests that there could be additional unidentified cases.
“It is concerning that a new virus subtype has infected a human,” Sutton said.
Dr. Michael Osterholm, an infectious disease expert at the University of Minnesota, said human-to-human transmission is unlikely. “They likely picked it up from the same place.”
Data shows that the H5N2 virus that infected the man is a low pathogenic virus, meaning it is unlikely to cause severe illness, said 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,” Osterholm said. “And the readily transmitted low path oftentimes can infect any number of animal species with little or no symptoms.”
What scientists want to know
Dr. Paul Offit, an infectious disease expert and the director of the Vaccine Education Center at Children’s Hospital of Philadelphia, said the main question among experts is whether the H5N2 has mutated in some way that has made it easier to spread to humans.
H5 viruses, in general, he said, struggle to infect people because the cell receptors they target in birds are much different than ours.
That’s often why, he said, people become infected with H5 viruses through direct contact with birds and poultry, not other humans.
H5N1, which has evolved to cause infections in humans infrequently, has never caused widespread human-to-human transmission, he added.
“The fact that it is H5N2, as compared to H5N1, I don’t think is meaningful in terms of representing something that is more likely to be associated with a pandemic,” Offit said. “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.”
Sutton said that scientists still need to conduct further genetic sequencing of the H5N2 virus that infected the Mexican man before they’re able to determine whether it’s a risk to humans.
“Until we have that information, it’s really hard to draw a lot of conclusions.”
Osterholm said the H5N1 is really the one we need to be “laser focused” on.
The H5N1 virus has taken off in dairy cows in the U.S., infecting at least 84 herds across nine states, according to the U.S. Department of Agriculture, raising the possibility that it could acquire mutations that allow it to spill over in humans.
