Strains of the flu virus, known as H1 and H3 are the most common to have crossed from animals to humans. These strains are especially efficient in attacking cells in the upper reaches of the respiratory system.
When they do infect humans it is often with fatal results, as immune systems are unable to recognise and counter the novel pathogen. According to the World Health Organisation (WHO) – of 348 confirmed cases of H5N1 avian flu in the last five years, 216 have died as a result.
There is fear among health officials of an emergence of a new H5 strain that can easily “jump” from birds to humans, potentially unleashing a pandemic on a massive scale.
Published in the British journal Nature, findings clarify scientific understanding of how viruses attach themselves to cells inside human lungs.
It has long been known among scientists that whether an influenza strain infects humans depends on the ability of a protein on the surface of the virus, called hemagglutinin, to bind to a sugar receptor in the respiratory tract.
Species jump possible
In humans, these receptors are known as alpha 2-6, and in birds are known as alpha 2-3.
Until recently, scientists believed it was a genetic switch in the virus that allowed it to bind to human rather than bird receptors, making the much-feared “species jump” possible.
But the study, which was led by professor Ram Sasisekharan of Massachusetts Institute of Technology (MIT), says that the big factor is the shape of the sugar receptors in human lung cells.
The human alpha 2-6 receptors have two shapes, one resembling an umbrella, and the other a cone. Researchers have discovered, that to infect humans, flu viruses must bind to the umbrella-shaped receptors.
Improved interventions for seasonal and avian flu
“Now that we know what we are looking for, this could help us not only monitor the bird flu virus, but it can aid in the development of potentially improved therapeutic interventions for both avian and seasonal flu,” said Sasisekharan.