Avian influenza viruses can be transmitted from birds to humans, but transmission among humans is limited. The reason may be an eggshell-like mineral layer that the virus acquires due to the high calcium concentration in the intestines of birds.
As reported by Chinese researchers in the journal Angewandte Chemie, these mineralized viruses are significantly more infectious, robust and heat stable than the native viruses.
Avian flu is a highly infectious disease among birds that has developed into a serious threat to human health. Close contact with diseased birds or their feces is considered to be the primary source of infections in people. Person-to-person transmission is often limited, however, which indicates that these viruses cannot directly infect humans.
Previously, it was assumed that these viruses crossed the species barrier as a result of mutation or recombination with another virus strain. More recent results demonstrate that avian flu viruses isolated from infected people have the same gene sequences as those from birds.
Scientists have wondered how humans catch the disease from birds. Researchers working with Ruikang Tang at Zhejiang University in Hangzhou, China, claim that it occurs because the viruses acquire a mineral “shell” in the birds' intestines, becoming mineralized under calcium-rich conditions. The digestive tract of birds — the primary location of avian flu viruses — provides just such a calcium-rich environment so that the birds can make egg shells.
Experiments with a solution that imitates the bird's intestinal environment allowed the researchers to demonstrate that 5-6 nm shells of a calcium phosphate mineral formed around the H9N2 and H1N1 viruses. In both cell cultures and mice, these mineralized viruses proved to be significantly more infectious — and deadlier — than the native viruses.
In humans, avian flu viruses infect the airways and are then found in bodily fluids, where the calcium concentration is too low for mineralization.
The mineralized shell changes the electric surface potential of the viruses. This causes mineralized viruses to adsorb much more efficiently onto the surfaces of future host cells. The mechanism for uptake into the host is also different. Normally, the virus docks at receptors on the cell surface and is then brought into the cell. The mineral layer inhibits this — but clearly stimulates very efficient uptake on its own. Within the cell, the mineralized viruses enter into lysosomes, whose slightly acidic environment dissolves the mineral shell and releases the viruses.
This new information may explain why people are more likely to catch avian flu from birds than from other people and may help in the development of new approaches to battling avian flu, the researchers said.