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Source and unique characteristics of avian flu found: Why it infects humans so easily

Kathleen Blanchard's picture
Several gene donors to new avian flu, finds research

Researchers have found the source of deadly H7N9 Avian flu in China as the result of an analysis of virus samples collected from live poultry markets and the environment.

They have also analyzed the genome of the new Avian flu, finding some of the same characteristics of past flu pandemics. What makes humans susceptible to the newest bird flu mutation?

Professor Chen Hualan (National Avian Influenza Reference Laboratory, State Key Laboratory of Veterinary Biotechnology, Harbin Veterinary Research Institute, Chinese Academy of Agricultural Sciences) and colleagues say the virus is indeed circulating among birds in China, but it has some unique properties.

Hualan and his team analyzed 970 virus samples collected from drinking water, feces, contaminated soil, and live poultry from Shanghai live poultry markets, finding 20 that were positive for H7N9.

Among the samples that were positive, 10 came from chickens, 3 from pigeons, and 7 were from the environment. The analysis showed the same virus characteristics across 8 gene segments analyzed.

The researchers then sequenced the genome of three of the H7N9 viruses collected from a chicken, a pigeon, and a sample taken from the environment.

Six of the genes came from avian H9N2 viruses. The closest origin of the other genes in 95.2 percent of those analyzed was found to be from duck H7N3 influenza viruses.

The rest of the H7N9 virus isolates were closest to H4N9 or H11N9 influenza viruses that came from ducks and environmental samples from the Dongting Lake region.

The analysis shows the novel H7N9 virus is a mix of internal genes from one donor. But there are hemagglutinin (HA) and neuraminidase (NA) proteins from several other donors that are the result of gene reassortment - the process used by different influenza viruses to swap genes and share some, but not all of the same properties.

An additional finding from the researchers is that the virus seems to have developed the ability to infect humans in the same way regular influenza attacks humans. The scientists discovered a characteristic of H7N9 that is similar to human influenza virus.

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Expert insight

Richard A. Stein, M.D., Ph.D., from the Department of Biochemistry and Molecular Pharmacology, New York University School of Medicine told EmaxHealth in an e-mail that the finding highlights the "urgency" of understanding how viruses can mutate to infect other species, an important task that allows us to better visualize the "host-pathogen interface".

Stein notes that the newly identified H7N9 virus is particularly unique because of "its wide prevalence in the environment and multiple live poultry species” and, also, due to the fact that “several mutations illustrate its exceptional ability to adapt to humans."

He adds that "it is important to appreciate that, generally, avian influenza viruses do not easily infect humans, due to the existence of a species barrier. However, under certain conditions, such as when it acquires specific mutations, a virus may cross this barrier and cause disease in a different species”.

One of the mutations in this new virus, intimately linked to virulence and lethality in humans, is a lysine substitution at position 627 in one of the viral proteins.

“This mutation, which was identified in all three human H7N9 isolates analyzed by Shi and colleagues, was previously shown to enhance the ability of the virus to replicate in the upper respiratory tract of humans, where the temperature is lower than in birds.

Influenza viruses that were established during each of the three pandemics of the 20th century contained this mutation,” says Stein.

Stein points out that it would have been extremely challenging to uncover the genetic and genomic features of the virus in 1977, when sequencing a thousand base pairs took approximately a year to complete, but thanks to advances in biotechnology, "genomes can currently be sequenced within hours or days, at incomparably lower costs than before”.

What the avian flu genome discovery means for public health is that researchers can obtain information early on to develop drugs and other interventions in a timely fashion. Identifying the unique characteristics of this new virus at this early stage has been possible thanks to major advances in biology and biotechnology that, Stein says, make us "...much better equipped to monitor emerging outbreaks, obtain actionable information, and implement timely and effective public health interventions”.

Reference: Shi J Z, Deng G H, Liu P H, et al. Isolation and characterization of H7N9 viruses from live poultry markets—Implication of the source of current H7N9 infection in humans. Chin Sci Bull, 2013, 58, doi:10.1007/s11434-013-5873-4.

This page updated April 27, 2013