Thursday, November 17, 2016

Ebola surprises again

Ebola virus (CDC)
New research published this week reveals the surprising discovery that the ebola virus doesn't always make people sick.  I've had a keen interest in ebola for a long time, after hearing a lecture in the 1990s from a CDC researcher who worked on the Reston outbreak (his name long forgotten).  As the story goes, in 1989, macaques imported from the Philippines exhibited the nasty symptoms of hemorrhagic fever, namely diarrhea, high fever, and bleeding.  US Army researchers studying the dead monkeys discovered that the monkeys tested positive for ebola virus.  Up to that point, ebola was known only from Africa, and it was well-known to be a potent and terrible killer.  During the documented ebola outbreaks up to that point, hundreds of people had died.  This was the first ebola virus (now called Reston virus) known to come from Asia, and most surprisingly, humans seemed to not show the symptoms it caused in the monkeys.  Several individuals who had worked with the sick monkeys tested positive for antibodies to ebola, and one researcher studying a monkey who had died was accidentally contaminated with the virus but did not develop the disease.  So Reston virus, a close relative of the deadly ebola of Africa, did not cause the same disease in people.

Why did this spark my imagination?  Because I'm very interested in the question of death, the Fall, and where bad things come from.  I've written on the topic of natural evil many times (see this post, for example).  Viruses are one particularly challenging aspect of natural evil.  After all, I can imagine ways that a lion or bark beetle could exist without causing death and destruction (although it might take a pretty wild imagination to do that), but viruses are much more challenging. First up, they're not really alive in the traditional sense.  Viruses only replicate when they hijack other critters' cells, usually destroying those cells in the process.  That's why viruses make us sick.  Ebola is an especially awful virus.  The most recent and most tragic outbreak saw more than 28,000 cases with more than 11,000 deaths.  It was a far greater outbreak than any other ebola outbreak on record.  Why is ebola so deadly?  What's it doing in a good creation?

This week comes new research in the journal PLoS Neglected Tropical Diseases from Richardson et al. that reveals individuals from Sierra Leone that tested positive for ebola antibodies but who never contracted the disease.  They evaluated 187 individuals from a village that was a hotspot during the most recent outbreak, all of whom could have been exposed to the virus but had not contracted the disease.  Fourteen tested positive for ebola virus.  Two had reported fevers that never developed into full-blown ebola virus disease, but the other twelve denied any symptoms at all.  That means 7.5% of the villagers who did not get ebola virus disease actually had the virus in them.  This is consistent with a previous report this year that found other evidence of ebola infections that did not cause ebola disease.  Both studies estimated that about a quarter of ebola infections in people could cause no symptoms!

That's kind of an astonishing result for a virus with a reputation for being such a killer, and it raises a range of really interesting questions about viruses.  At its most basic, the fact that human beings can be infected with ebola virus without even getting sick shows that ebola is not always a killer.  Maybe the new killer ebola comes from a change in the virus?  Or maybe a change in us?

But where did ebola originally come from, and what's the point of ebola in God's good creation?  I've mentioned before that bats are known to carry the virus generally without symptoms, and that some mammals actually have what appear to be functional ebola virus genes in their genomes.  One of the more intriguing ideas I've heard about viruses came from a paper by Liu and Soper that suggested that our modern viruses (at least retroviruses) might be features that were supposed to be part of our own cells but somehow escaped, possibly after the Fall (I reviewed this idea here).  It's a very, very speculative idea, but I like the creativity of turning the evolutionary concept on its head.  Evolutionary biologists would say that we have virus genes in our genomes because of past infections.  Liu and Soper suggest that it's the other way around: our "virus genes" were the originals, and the viruses have escaped from us.  Here we have ebola viruses that don't cause damage.  The question now is what could the viruses be doing that might be constructive or helpful, if anything?  Or are they irrevocably broken something-or-others escaped from otherwise helpful cellular genes?  I don't know the answers to these questions, but the ebola story continues to fascinate.

Richardson et al. 2016. Minimally Symptomatic Infection in an Ebola ‘Hotspot’: A Cross-Sectional Serosurvey. PLoS Neglected Tropical Diseases 10(11): e0005087.

Dean et al. 2016. Transmissibility and Pathogenicity of Ebola Virus: A Systematic Review and Meta-analysis of Household Secondary Attack Rate and Asymptomatic Infection. Clinical Infectious Diseases 62 (10): 1277-1286.

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