Editor’s Note: This is part two of a two-part series on avian flu. Part one ran in Thursday’s issue.
For a pandemic to become a major public health problem, two things must happen: The virus has to become easily transmissible between humans, and it has to cause serious disease when it does.
In the March 16, 2006, issue of Sciencexpress, researchers reported that mutations in only two amino acids could enable one highly pathogenic strain of H5N1, isolated from a Vietnamese boy who died of the bird flu in 2004, to become more infectious to humans. (See BioWorld Today, March 23, 2006.)
First author James Stevens, senior research associate at the Scripps Research Institute, stressed to BioWorld Today that the mutations "would not make the virus more pathogenic, in terms of excess deaths."
The latter fact, though, is cold comfort, given that H5N1 already is highly pathogenic. The influenza virus uses its hemagglutinin (the "H" in H5) protein to enter cells. During the entry process, hemagglutinin protein is cleaved, or cut, by cellular proteases.
How serious most human H5N1 infections are is a matter of scientific debate. (See BioWorld Today, Jan 16, 2006.)
But one of the reasons H5N1 infection is so deadly to birds is that while most hemagglutinin types can be cut mainly by proteases found in lung tissue, H5 can be cut by a wider range of proteases, meaning that it infects other organs as well as the lungs.
The reason is that instead of the usual one, arginine, at which proteases cut, H5 has a cluster of them, which means it has "multiple potential cut sites" for proteases, Stevens said. And because of that, though the virus first infects the intestines of birds, it spreads rapidly from there to other organs.
"It basically blows up the bird," Stevens said.
Array Of Infectious Possibilities
The reconstituted 1918 Spanish flu virus also infected an unusually wide range of cells, though its mechanism was different. Studies with the resurrected 1918 virus published last October showed that it did not require a host protease to infect cells, in this case because of mutations in its N1 neuraminidase (the "N" in flu virus nomenclature.)
That ability may be part of what made H1N1 "Spanish flu" so deadly. Though autopsy tissues show that the Spanish flu virus damaged only the lung, the lung damage was more severe, and pneumonia more often followed influenza, than is the case with more typical annual influenza viruses.
"It might be that it had a propensity to replicate deeper in the airspaces of the lung," Jeffrey Taubenberger, co-author of the Sciencexpress paper, said at last October’s press conference, thus more easily causing the pneumonia that often kills influenza patients.
Asked whether the infection deep in the lungs is what makes H5N1 so dangerous for humans, Sciencexpress paper co-author Paulson said that "the pathogenicity of the 1918 virus isn’t solely due" to its affinity for sialic acid types that are deep in the lung. "That step is very important in initial infection," he said, "but what makes the virus so pathogenic are other proteins."
But if the virus is a serious problem when infection does occur, it has not become able to spread easily from human to human. In the nearly 10 years since it first surfaced as a potential problem in Hong Kong, there have been only about 150 confirmed human cases, and the vast majority of those appear to have caught the infection from birds, not their fellow man.
Taubenberger pointed out that the glycan array the researchers developed for their receptor-binding studies could be used to address a long-standing scientific question: whether all hemagglutinin subtypes can theoretically become adapted to humans and thus cause pandemics.
Of the 16 known types of hemagglutinin, types H1 through H3 definitely can become adapted to humans, and each subtype has been the culprit in one of the three 20th century pandemics. What is not currently clear is whether other hemagglutinin subtypes could become adapted to humans but haven’t, at least not since they have been tracked by molecular geneticists, or whether something prevents them from becoming adapted to humans in principle.
H5’s half-hearted binding preference switch from birds to humans in the current Sciencexpress paper sheds no light on that question; but Taubenberger said the array his colleagues have developed "can address this question systematically."
Stock Up On Tamiflu?
Last October, two teams that included Taubenberger announced they had resurrected the 1918 pandemic virus and sequenced its polymerase genes, respectively. (See BioWorld Today, Oct. 6, 2006.)
At that time, Taubenberger told reporters at a press conference that for a viral strain to cause a pandemic, changes must occur in each of the eight segments that make up an influenza virus, and that "the H5N1 strains in general only have one or two of these mutations."
Taubenberger told BioWorld Today that the fact that a mere two amino acid substitutions can increase H5N1’s affinity for human cells is "still in accord" with the idea that mutations must occur in all eight viral segments for a pandemic to occur.
An H5N1 preference switch to infecting human cells is "clearly one of the crucial changes that would have to occur for a pandemic," Taubenberger said.
But he said such a change probably would not be sufficient for the emergence of a pandemic strain. "Would it be enough to be biologically important? We don’t know."