No, I haven’t turned into one of those fear mongering pundits who make a handsome living scaring us into catatonic despair. But we should be prepared to deal with the problem .
So what’s the problem? It is the latent viruses that have been hiding in the genomes of animals for millions of years, waiting for the opportunity to strike when circumstances are right. Something like an Al-Qaida sleeper cell.
The fascinating story of the zebra finch
In the recent issue of PLOS Biology two researchers, Cedric Feschotte and his postdoc Clement Gilbert from Texas University at Arlington, reported that they found DNA fragments of Hepatitis B buried deep in the genome of the lovely Zebra Finch. We are all familiar with this devastating virus; so what’s the big deal about finding it in a bird?
First, a word about viruses. These are really mysterious organisms; nobody knows where they came from or how many of them there are. We do know that when they come for a visit they integrate themselves into the genome of the host at random places, and depending on the site of integration they can be either harmless or quite deadly. An arms race between the invaders and the poor host ensues, and eventually after thousands of years of evolutionary war, the invaded species prevails by evolving defense mechanisms. Only the smoldering remains of the battle remain, in the form of fragments of the virus integrated into the victorious host.
The Hepatitis B virus was thought to be about 6000 years old-a newborn baby in evolutionary time scale. Feschotte and Gilbert discovered 15 Hepatitis B fragments strewn around 10 sites in the genome, and wondered how old could these fragments be. They hit upon an ingenious solution.
They selected five bird species that are related to the zebra finch and that split off from the common ancestor millions of years ago. They reasoned that if they find the virus fragment in the same place in the genome as in the zebra finch then the split happened after the invasion, and if they don’t find any virus fragments, then the split occurred before the invasion. Lo and behold, the dark-eyed junco (I am watching a pair right now in the backyard) had the virus fragments at exactly the same place as their distant cousin the finch. The olive sunbird had no fragments. Now, it was established that the junco split off from the common ancestor 25 million years ago, and the sunbird about 35 million years ago. Voilá, the invasion of the virus occurred between 25 and 35 million years ago.
More evidence of ancient battle royales.
HIV: Until recently it was believed that HIV was around for only a few hundred years. Recently it was discovered that the ancestor of this virus, SIV (simian immunodeficiency virus) has been lurking in monkeys and apes for 32,000 years. During all this time humans were happily butchering and feasting on monkeys. What happened a few hundred years ago is that SIV, causing a mild monkey disease, mutated into a highly transmissible virus that caused one on of the most devastating epidemic in human history.
Ebola virus: this virus burst on the scene only four decades ago in Africa. It causes a dreadful syndrome of uncontrollable bleeding from every orifice in the body, necrosis of the liver, and death in up to 90% of cases. We now know, based on fragments of viral genes found in the DNA of bats and wallabies that relatives of the Ebola virus began infecting mammals tens of millions of years ago.
The emerging threat.
The story how Feschotte came to look for Hepatitis B fragments is instructive. As he tells it, late one night he was browsing the DNA sequence database GenBank. On a whim, he typed in the amino acid sequence that makes up the hepatitis B virus. “I was just playing,” he says. Now think of it, if hidden virus DNA sequences was a rare phenomenon, the odds that he would get a hit on the first try are as good as winning the grand prize in the lottery. It raises the probability that there is a vast reservoir of virus DNA in all those animals, waiting to be discovered.
The picture is taking shape: all animals harbor ancient viruses that are lying dormant until the right mutation and the right host happen to come together. So why are we getting an increasing number of those outbreaks now?
- One theory is the increasingly closer proximity of humans to the habitats of wild animals.
- The world getting smaller: air transportation brought the Ebola virus from Africa to Europe in a matter of hours, Likewise the West Nile virus travelling from Israel to New York in 8 hours.
- And we cannot forget climate change; as the world gets warmer tropical species are slowly migrating north, introducing new diseases to the northern hemisphere.
Have you noticed from the list above (and believe me, the list is much longer; remember the SARS virus? the HANTA virus?) that these viruses cause especially deadly diseases? This is not accidental; when these viruses finally gain entry into humans, we are caught with our pants down -we have no history of immunity to them.
The end is nigh…Not!
Can we handle the emerging infections? I believe so. Our modern biological tools, vaccine manufacturing, coupled with modern Epidemiology will be able to withstand such outbreaks. Witness the very effective way the world dealt with the H1N1 virus.
So no reason to despair; the story is actually quite hopeful.