This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Marburg Virus. Note distinctive "Shepherd's crooks". CDC/ Dr. Erskine Palmer, Russell Regnery, Ph.D., CDC Public Health Image Library #275; Public Domain. Click for link.
Back in 1994, Richard Preston scared the bejeesus out of everyone with his eye-opening non-fiction thriller "The Hot Zone". It was a gripping read, and teenage me couldn't put it down. In it, Preston documented the depredations of filoviruses -- a family of wildly contagious filamentous RNA viruses that cause horrible, gory, swift deaths by multiple organ failure. You may be bleeding from any number of orifices when this happens.
Death rates for the two classic filoviruses -- Marburg Virus and Ebola Virus -- hover between 30 and 90%, and for Ebola, consistently hit the higher end of that range. In fact, death is so swift that experts have theorized that it actually limits the success of the virus among humans. It so far has failed to escape either Africa or the labs in which it is studied in other continents. So far.
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One of the central mysteries of "The Hot Zone" was just where the viruses were coming from. Although it was clear that some apes and primates could also get sick, they, too, died from it. They did not seem to be hosting the virus during quiet periods in which the disease disappeared. Based on studies of victims who hadn't gotten sick by eating contaminated bushmeat, suspicion fell on caves, and perhaps bats, but nothing conclusive could be shown.
In the last two decades, evidence has accumulated that bats are, indeed, filovirus's true hosts. I covered one such revelation in a blog post here. Up 'til now, it has seemed as if bats infected with Marburg or Ebola were perfectly healthy, however, and only when the viruses escaped into primates did they become lethal. It now appears that that isn't always the case.
It flew under the radar last fall, but in October scientists announced the discovery of the first filovirus found not in Africa or Asia, but in Europe. It's not time to hit the panic button yet, though. In the case of the new virus, it seems like it's the bats we should be worried about.
The slinky, sinuous, and frighteningly efficient killer Ebola Virus. Note the branches in this filament. CDC/ Cynthia Goldsmith. CDC Public Health Image Library #1832. Public Domain. Click image for link.
Sometime in 2002, Schreiber's bats (Miniopterus schreibersii) across France, Spain, and Portugal died by the score. In Spanish caves, several bat colonies were wiped out in less than 10 days. The dead bats looked normal, but their lungs were filled with immune cells while their spleens were depleted of them. The signs pointed to viral pneumonia.
A team of Spanish and American scientists, who reported their results to PLoS Pathogens last year, tested dead bats found in Lloviu Cave (that's pronounced "Yo-view", I think) in Spain for a variety of viruses, and found evidence of filovirus DNA in the expired bats. Healthy bats showed no sign of infection. Although the researchers took pains to note that they had not demonstrated causality, the evidence seems pretty damning for the virus in question, which they named Lloviu Virus.
When they sequenced up their virus, they found it to be a new type that displayed characteristic filovirus traits: a mere seven open reading frames (basically, genes) on a single strand of negative-sense RNA. Analysis of its RNA polymerase -- the enzyme it uses to replicate its genome -- indicated it is more closely related to Ebola virus than to Marburg virus, but that it's clearly not nested within the Ebola viruses and is in a group of its own. (See Fig. 3 in the paper)
Though bats seem easy prey for this virus, so far there seem not to have been any human infections, in spite of human visitors to virus-infested caves. Perhaps that has something to do with the fact that the filoviruses that are pathogenic to humans seem to co-exist happily with bats, while Lloviu virus bumps off bats. Or perhaps not.
In any case, I think it is the bats we should be worried for. Given the devastating losses we've seen in North America thanks to the fungus Geomyces destructans, the cause of White Nose Syndrome, it's unsettling to hear about another emerging pathogen of bats. I can't find any information on whether these die-offs have continued since 2002 or not, or whether they threaten European bats the way that White Nose looms here.
However, the authors of the study note with some concern that the geographic distribution of the affected bat M. schreibersii in various locations throughout the sub-tropical Old World (though interestingly totally outside filoviruses' traditional African stomping grounds) and the tendency of known filoviruses to have a stable host-parasite relationships may mean that there are many more bats at risk. Though homely, bats help keep the world running smoothly. The last thing they need is another pestilent disease -- especially one who comes from a virus family with such a nasty reputation.