A type of avian flu that is common in birds and rarely caught by humans, the North American Avian H7 influenza virus, is acquiring transmission properties similar to human influenza, according to a new study by US researchers at the Centers for Disease Control and Prevention (CDC), based in Atlanta.

The study is the work of lead author and CDC researcher Dr Jessica Belser, and colleagues, and is published in the Proceedings of the National Academy of Sciences (PNAS).

Most cases of avian flu in humans come from contact with infected birds or objects they have contaminated. But viruses are constantly changing, which is why scientists are always tracking them. Belser said she and her co-investigators found that some strains of North American avian influenza A H7 virus have developed characteristics that could increase their potential to infect humans, and also spread amongst them.

Avian H7 viruses from both Eurasia and North America have caused outbreaks in poultry since 2002, wrote the researchers, and have been known to infect humans in outbreaks in the The Netherlands, British Columbia, and the United Kingdom.

Most H7 infections in humans end up as self-limiting conjunctivitis, and spread among humans is very rare.

Influenza viruses infect humans by attaching themselves to sugar molecules that dock onto receptors on the surfaces of cells in the human body's respiratory tract. The ability of a virus to do this varies from virus to virus. The ones that do it well are the ones that are most likely to spread from human to human, for example through cough or sneeze droplets.

Belser and her co-researchers used glycan microarray technology to investigate the receptor-binding preference of Eurasian and North American lineage H7 influenza viruses and their ability to spread in birds and also in ferrets, which, like mice, have similar inflluenza transmission properties to humans.

The results showed that: Highly pathogenic H7N7 viruses from The Netherlands in 2003 kept the classic avian receptor-binding preference and were not readily transmissible in ferrets, as has been observed for highly pathogenic H5N1.
But H7N3 viruses isolated from Canada in 2004, and H7N2 isolated in 2002-2003 from the northeastern United States, showed a binding preference for the receptor linkages found prominently on human tracheal epithelial cells.
A low pathogenic H7N2 virus isolated from a man in New York in 2003, A/NY/107/03, replicated efficiently in the upper respiratory tract of ferrets and was capable of direct contact transmission in this species. The authors concluded that:

"H7 influenza viruses from the North American lineage have acquired sialic acid-binding properties that more closely resemble those of human influenza viruses and have the potential to spread to naïve animals."

In other words, in terms of its ability to gain a foothold in the upper respiratory tract and therefore spread to uninfected humans, the H7 bird flu virus has changed to a form that more closely resembles the human flu virus itself.

Belser said that:

"The results of this study underscore the importance of continued influenza virus surveillance."

Although the H7 viruses are rarely dangerous to humans, studies like this are important because what happens in one strain can shed light on what happens in another strain. Experts believe it is only a matter of time before the much deadlier bird fllu virus, the H5N1, mutates into a human to human form.

"Contemporary North American influenza H7 viruses possess human receptor specificity: Implications for virus transmissibility."
Jessica A. Belser, Ola Blixt, Li-Mei Chen, Claudia Pappas, Taronna R. Maines, Neal Van Hoeven, Ruben Donis, Julia Busch, Ryan McBride, James C. Paulson, Jacqueline M. Katz, and Terrence M. Tumpey.
PNAS, 2008 105: 7558-7563.
Published online on May 27, 2008.
DOI: 10.1073/pnas.0801259105

Click here for Abstract.

Source: CDC, PNAS.

: Catharine Paddock, PhD