#855
One of the barriers that has prevented H5N1 from becoming a pandemic strain is believed to be its ineffective binding to human receptor cells. It may not be the only barrier, but it is a major one.
With apologies to scientists and virologists worldwide, what follows is a simplified explanation of how a virus binds to human receptor cells.
Avian species have a-2,3 receptor cells in their guts, their primary route of infection, while humans predominantly have a-2,6 receptor cells in their upper respiratory system. These receptor cells are different enough that a virus attuned to binding to one, has difficulty binding to the other.
That has been, thus far, our good fortune.
The H5N1 virus, being an avian virus, is adapted to easily bind to the a-2,3 receptor cell. While we have some of these receptor cells in our respiratory system, most are suspected to be deep in the lungs, and harder for the virus to reach. Our upper airway passages are primarily a-2,6 receptor cells, which human adapted influenza viruses readily bind to.
The news yesterday out of Indonesia, that the H5N1 virus may be acquiring the ability to more easily infect humans, suggests a transition of the virus towards affinity to the a-2,6 receptor cell. I say `suggests', simply because very little scientific information was provided with this announcement.
"Virus samples from poultry cases have increasingly shown a similarity in their amino acid structure to virus samples extracted from humans," Wayan Teguh Wibawan told Reuters.
"This makes it easier for the virus to attach to human receptors," he said, referring to receptor cells lining the human throat and lungs.
The H5N1 virus has spikes on its surface, that it uses to attach to receptor cells. These spikes are like keys, and the receptor cells in our airways are like padlocks. If the key doesn't fit the lock, it doesn't open. It takes only small amino acid changes, however, to modify the key to fit new locks.
Since Indonesia has been withholding virus samples since the beginning of the year, scientists around the world are kept in the dark as to what changes are occurring with the virus.
If the virus is acquiring an affinity for the a-2,6 human receptor cell, then one of the barriers preventing greater spread of the virus may be toppled. That would not be good news. It doesn't, however, mean a pandemic is imminent. There may be other barriers to efficient transmission we don't know about.
This year, despite growing skepticism over the official numbers, Indonesia has only reported 24 cases of human H5N1 infection. Worrisome, but hardly indicative of greater transmissibility.
There have been persistent rumors, however, that the number of cases is higher than has been admitted, and some of these reports have even made it into the local press. Whether these are true, or simply a bit of media sensationalism is impossible to know.
The veil of secrecy surrounding the virus samples, and what is really going on in Indonesia, does little to reassure those of us watching the situation.
Yesterday's announcement could be legitimate, and something to concern us, or it could simply be another attempt to up the ante by Indonesian officials in their vaccine negotiations. I honestly don't know.
For now, at least until we see a spike in human cases or independent laboratory confirmation of a change in the virus, I remain concerned, but not alarmed.
My status is subject to change, however.
