A human virus

Klaus Osterrieder is a professor for virology at Berlin's Freie Universitaet and also holds a position at Cornell University in the United States. He spoke to Deutsche Welle about how viruses can be transmitted across species.

Deutsche Welle: How can viruses in pigs can make humans sick?

Klaus Osterrieder: With the new virus, I think more and more people are operating under the assumption that it is a human virus, which is transmitted, very efficiently apparently, between humans. What we are looking at now is not swine virus but a human virus, circulating in humans, transmitted from one individual to another.

The name swine flu automatically brings pigs to mind. Are we dealing with a virus that has been transmitted to humans from pigs or a purely human phenomenon?

I think it doesn't matter at this point. What we have is a virus that is transmitted from human to human, so it's a human virus.

It has sequences, such as hemagglutinin - a surface protein that helps a virus gain access into cells and destroy cells and ultimately make you sick - that are very similar to hemagglutinin that have been sequenced in swine viruses. That's where the name comes from, but how it ultimately got into humans and what happened on its way are very unclear at the moment.

Swine have been called "mixing vessels" of bird and pig viruses, which are then transmitted to humans. What is the process that makes this happen?

The mixing vessel has been proposed over and over again, and maybe it is true. But most influenza flu viruses originate from waterfowl. The birds usually do not come down with the flu, they harbor the viruses and they exchange virus components amongst each other and generate new viruses. This is what happens on a daily basis. Then these viruses can be introduced into other species, like swine and humans.

Then there is something happening in humans or swine that ultimately allows the virus or the viruses to travel within a species. Where this new virus comes from is unclear at the moment. Whether it really originated from the swine? Maybe. But when it gained its pathogenic potential - its ability to spread from human to human - this may well have happened in the human. The final mutation that made it so efficient could have happened in humans.

Are all animals capable of infecting humans with viruses?

We do not know. A few years ago, canines were pretty much considered resistant to the flu and now they can come down with flu and die of influenza. Theoretically, if you have the right virus at the right time at the right spot, we could be infected by influenza in other species.

But mostly we have assumed it's birds and pigs that give us influenza viruses - probably birds more so than pigs. Looking back, it's still largely unresolved how Spanish flu made its way into the human population and then turned into human flu. Textbooks will tell you it's mostly birds and swine.

What makes swine particularly adept at transmitting viruses to humans?

There are proteins that contain sugars and some of these sugars are important for an influenza virus to be able to infect cells. Birds and mammals have, by in large, different forms of these sugars. Pigs have both sorts, so viruses can utilize both these sugars. That's part of why the mixing vessel term originally garnered attention and has appeal. Influenza viruses coming from birds or mammals can deal with the pig really well because of the sugars that are there.

That's why pigs are considered the mixing vessel. But again, the definitive evidence that the virus circulating now comes from one pig is not there.

The virus seems to be relatively controllable outside Mexico. Do you think the possibility of a pandemic is a realistic fear at this point?

It's very hard to say. Fear always gives you bad advice. Be concerned and do what's necessary.

The cases that have been reported are, with the exception of Mexico, on the low side - and have all been traceable to Mexico. I would refer to the World Health Organization, which is on a level four (the WHO's ranks outbreaks on a scale of one to six -- eds.). We know it's human to human transmission and we have to be very careful. I think that's what's happening all over the world.

What should people do to protect themselves?

The good news is that laboratory tests have shown that the drugs Tamiflu and Relenza are still active against the new virus. So if something is wrong, go see a doctor.

Interview: Sean Sinico

Editor: Kate Bowen