H7 Viruses Adapting to Humans: Study

May 29, 2008, Toronto, Ont. – North American avian flu viruses of the H7 subtype seem to have adapted to more easily invade the human respiratory tract, a new American study suggests.

Experts say the findings underscore the fact that H7 flu viruses pose a significant pandemic threat and that surveillance for cases in wild birds, poultry and people ought to be a high priority.

“I think this is certainly amongst the most dangerous (avian flu) viruses out there,'' said virologist Dr. Ron Fouchier, with the Erasmus Medical Centre in Rotterdam, the Netherlands.

“And I think we need to continue to develop vaccines for H7 just as well as H5(N1).''

Fouchier was commenting on a study published May 26th by the journal Proceedings of the National Academy of Sciences. He was not involved in the work.

Scientists from the U.S. Centers for Disease Control reported on their study of a number of H7 viruses. Among them were viruses recovered from the two people in British Columbia who became infected during an H7N3 outbreak there in 2004.

They tested the various viruses to see which types of receptors they can latch onto – those typically found in the guts of birds, the natural host of influenza viruses or those found on the cells of the lining of the upper respiratory tract of humans.

Human flu viruses that circulate every winter have adapted to be able to bind to the receptors that predominate in the human respiratory tract, known as alpha 2-6 receptors. Avian viruses, on the other hand, prefer the alpha 2-3 receptors found in the guts of wild birds and poultry but which are scarce in the human upper respiratory tract.

It is assumed that an avian virus would need to make this kind of adaptation – learning to latch onto the human receptors – before it could transmit easily to humans and then among them.

H7 viruses from North America that have been isolated from about 2002 onwards seem to have developed an increasing affinity for the human-type receptors, said Dr. Terrence Tumpey, the CDC scientist who led the work.

“These viruses are partially adapted to recognize the receptors preferred by human influenza viruses, but not completely,'' he said in an interview from Atlanta.

“It needs to be adapted further. But I think it shows that potentially that these viruses are changing.''

“Because we can look at an older North American H7 or Eurasian H7s or H5s and they have the characteristic avian influenza binding properties. Whereas these seem to be different and possibly changing.''

At this point it is unclear what additional changes would be needed for an H7 virus to fully adapt to a human host. And whatever the changes are, scientists cannot say whether the virus could ever acquire all of them.

When H7 viruses have caused human cases, the ensuing disease has typically been mild, with people suffering conjunctivitis (pink eye) and-or mild respiratory symptoms. There is one exception – a veterinarian infected with an H7N7 virus died during an outbreak in the Netherlands in the spring of 2003.

The mildness of the disease may have lulled some people into a sense of complacency about H7 viruses, said Dr. Danuta Skowronski, an influenza expert at the British Columbia Centre for Disease Control.

But she insisted the fact that the virus doesn't induce the life-threatening disease seen in H5N1 infection doesn't mean it
shouldn't be taken seriously as a pandemic threat.

“H7, with its mildness, may be more – I hate to anthropomorphize – but more devious. Because through surreptitious spread – because it's milder, it's unrecognized, people might dismiss it more – it may actually have more opportunity to adapt to the human respiratory tract,'' she said from Vancouver.

“And even though it may be mild today, even thought it may not transmit easily today, the potential is always there for it to change. And basically we don't want new (flu) subtypes in the human population. We've got enough to deal with the humanized strains.''