Sci-tech information: The avian flu virus strain seen adapting to mammals, humans
A genetic analysis of the avian flu virus responsible for at least nine human deaths in China portrays a virus evolving to adapt to human cells, raising concern about its potential to spark a new global flu pandemic.
The collaborative study, conducted by a group led by Masato Tashiro of the Influenza Virus Research Center, National Institute of Infectious Diseases, and Yoshihiro Kawaoka of the University of Wisconsin-Madison and the University of Tokyo, appears in the current edition (April 11, 2013) of the journal Eurosurveillance. The group examined the genetic sequences of H7N9 isolates from four of the pathogen's human victims as well as samples derived from birds and the environs of a Shanghai market.
"The human isolates, but not the avian and environmental ones, have a protein mutation that allows for efficient growth in human cells and that also allows them to grow at a temperature that corresponds to the upper respiratory tract of humans, which is lower than you find in birds," says Kawaoka, a leading expert on avian influenza.
The findings, drawn from genetic sequences deposited by Chinese researchers into an international database, provide some of the first molecular clues about a worrisome new strain of bird flu, the first human cases of which were reported on March 31 by the Chinese Center for Disease Control and Prevention. So far, the new virus has sickened at least 33 people, killing nine. Although it is too early to predict its potential to cause a pandemic, signs that the virus is adapting to mammalian and, in particular, human hosts are unmistakable, says Kawaoka.
Access to the genetic information in the viruses, he adds, is necessary for understanding how the virus is evolving and for developing a candidate vaccine to prevent infection.
Influenza virus depends on its ability to attach to and commandeer the living cells of its host to replicate and spread efficiently. Avian influenza rarely infects humans, but can sometimes adapt to people, posing a significant risk to human health.
"These viruses possess several characteristic features of mammalian influenza viruses, which likely contribute to their ability to infect humans and raise concerns regarding their pandemic potential," Kawaoka and his colleagues conclude in the Eurosurveillance report.
Kawaoka, a faculty member in the UW-Madison School of Veterinary Medicine who also holds a faculty appointment at the University of Tokyo, explains that the majority of the viruses in the studyâ€"from both humans and birdsâ€"display mutations in the surface protein hemagglutinin, which the pathogen uses to bind to host cells. Those mutations, according to Kawaoka, allowed them to easily infect human cells.
In addition, the isolates from patients contained another mutation that allows the virus to efficiently replicate inside human cells. The same mutation, Kawaoka notes, lets the avian virus thrive in the cooler temperatures of the human upper respiratory system. It is in the cells of the nose and throat that flu typically gains a hold in a mammalian or human host.
Kawaoka and his colleagues also assessed the response of the new strain to drugs used to treat influenza, discovering that one class of commonly used antiviral drugs, ion channel inhibitors which effectively bottle up the virus in the cell, would not be effective; the new strain could be treated with another clinically relevant antiviral drug, oseltamivir.
The new type of bird flu in China, called H7N9, continues to infect and kill people, with the World Health Organization reporting new cases daily. News of the virus is unfolding quickly as public health officials and scientists struggle to learn about it. Among the latest discoveries:
First case with no symptoms
Though most confirmed cases have meant pneumonia and difficulty breathing, Chinese officials have identified a 4-year-old Beijing boy as a carrier of the virus: DNA tests revealed that he is infected but has no symptoms, according to a news report from the Xinhua News Agency.
The boy represents only the second confirmed case of the disease in the city. His lack of symptoms raises the possibility that more people may have contracted the virus without anyone being aware.
First pair of cases within a family
Thus far, only one family member of a flu victim, the husband of a 64-year-old woman who died of H7N9, has been found to also have the virus, according to another Xinhua report. Chinese officials said no evidence exists that he caught it from his wife or that the virus is capable of sustaining person-to-person transmission.
Birds as a possible reservoir
In a risk assessment published April 13, the World Health Organization confirmed that the H7N9 virus has turned up in poultry and pigeons in some poultry markets. But the virus in the birds doesn’t carry all the mutations that help it adapt to humans, so officials don’t yet know if people caught the virus from the birds or from some other animal.
Double jump from animals to humans
A study published in Eurosurveillance April 11 suggests that the first man found to be infected with H7N9 (an 87-year-old from Shanghai) carried a virus similar to one that infected a pigeon. Two other people who died of the flu share a virus similar to one in a chicken from a Shanghai market. Those findings agree with a New England Journal of Medicine study that suggests the virus may have moved from animals into humans twice
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