By David N. Leff
Now that the president of the U.S. has declared war on terrorism worldwide, it's time to take a fresh look at one of the most terrible terrorists of them all — the influenza virus.
Like Saddam Hussein, this pathogenic artful dodger hides, changes and shifts its bioweaponry, one step ahead of international flu-control agencies. Its principal perpetrators are birds, particularly Chinese chickens and ducks.
These fowl carry the avian version of influenza virus, which they pass on to their human victims.
Now, a six-nation consortium of virologists has revealed a secret intermediary in this deadly traffic — the swine flu virus.
They report their two-year viral search in the Journal of Virology for September 1998, in an article titled "Molecular basis for the generation in pigs of influenza A viruses with pandemic potential."
Virologist Robert Webster, a senior co-author of this paper, told Bioworld Today, "It's been proposed that pigs may be important as the intermediate host in transmitting avian influenza viruses to humans. The key issue is, does the pig have the right receptors for avian influenza viruses and human influenza viruses? And the answer is: Yes!
"So, it is scientifically proven that the pig could be described as the potential mixing vessel," Webster continued. "The evidence we present supports the role of swine as a source of potentially hazardous influenza A viruses, arising through classical genetic reassortment or a novel adaptation to human virus receptors, or perhaps through both mechanisms."
That demonstration calls for action, Webster said. "We'd better get with it," he declared, "and do some more surveillance of pigs in the world for influenza. It is quite clear that not enough is being done in terms of such surveillance now."
Asked whether these receptors might complicate the xenotransplantation of porcine donor organs and tissues into humans, Webster replied, "In the sense that the human recipient would then be carrying those viral receptors, this is a potential complication indeed. It would facilitate the transmission of influenza viruses to humans. A good point."
Webster is chairman of virology and molecular biology at St. Jude Children's Hospital, in Memphis, Tenn., and professor of pathology at the University of Tennessee.
He cited the crucial importance of the mixed avian/human receptor in the pig, "because this is the big puzzle about the influenza outbreak in Hong Kong late in 1997. That was a huge puzzle because it broke the rules." (See BioWorld Today, Jan. 16, 1998, p. 1.)
Virologist Yoshihiro Kawaoka, the paper's other senior author, explained why:
"Before the Hong Kong outbreak," he told BioWorld Today, "we knew that avian influenza virus does not transmit to humans directly. But the Hong Kong epidemic changed that notion. Now we know that pigs may serve as mixing bowls for the generation of hybrid avian/human virus, though they may not be required for the virus to transmit from birds to people. Also, we know that pigs can serve as hosts that convert avian-like virus toward human-like. The virus changes during that replication.
"We need to be prepared for any avian subtypes that may appear in pigs," Kawaoka warned. A professor of virology at the University of Wisconsina at Madison, he focuses on researching the influenza and Ebola viruses.
Six people died of influenza during the Hong Kong flare-up, and another 12 survived their infections. No new cases arose after the island government preventively slaughtered more than 1.5 million chickens and ducks on its territory.
Virus Goes For The Throat — Except In Ducks
In humans, of course, the flu is a respiratory ailment; the virus homes in on cells lining airways and lungs. In ducks, curiously, the guiltiest vector of the disease, avian virus prefers intestinal to airway cells.
The co-authors discovered the mixing-bowl effect in porcine trachea, with those throat linings harboring both human and avian influenza viruses. They also found that with continued replication inside pigs, avian viruses changed from bird-type to human-type.
What the consortium determined was which viral amino-acid residue was important for the virus to change its spots from avian-like to human-like. That alteration took place in hemagglutinin, the viral protein ligand that attaches to the receptor on the cell surface. That molecule contains upward of 500 amino-acid residues, Kawaoka said, but the crucial changes involved fewer than 10.
"We use the term 'reassortment,'" Kawaoka explained, "when a single cell is infected with two different viruses. Influenza, unlike other viruses, contains segmented genomes — not a single sequence, but eight pieces of core genomic RNA. Of varied lengths, they total 15.1 kilobases of RNA, and encode the various viral proteins.
"So, theoretically, you can have 256 different combinations — two to the eighth," he pointed out "of those two different viruses — one human, one avian — coming into that single pig tracheal cell. There they mix together, creating new and possibly more dangerous viral strains."
Kawaoka is now "working on the exact receptors, and the mechanism by which the virus changes in pigs. We found those changes, but we don't know what makes it do it."
His goal is to develop an antiviral therapeutic specific to the influenza virus. "We are working on protease," he said. "The viral hemagglutinin has to be cleaved into two pieces for the virus to become infectious. And that process is mediated by the host's protease enzyme. It should be a good drug-discovery target, but no company I know of is working on it.
"We've worked on this protease process for a long time," he concluded, "and I'm interested in finding somebody who's interested." *