I’ve read several popular books about diseases recently: Rabid, The Hot Zone, Smallpox, Disease, Spillover, and Deadly Outbreaks. More books on a single topic than usual, but not with any project in mind. It just happened, the way it happens to folks who start to read books by Michael Connelly or Rex Stout. Several focussed on diseases that leap from animals to humans (“zoonotic” diseases). No curious person could resist daydreaming about some of the unsolved mysteries one encounters when reading about diseases like Ebola and Marburg. You play detective in your brain and then you almost certainly come up with your own crackpot theory, just like I did.
Ebola and Marburg belong to a strange family of virus called “filoviruses.” As their name suggests, they look like filaments rather than spheres or rods. Like other viruses, we can’t see them without an electron microscope. The first practical electron microscope was built in 1938; that’s when the study of viruses began in earnest. Scientists only created the family of filoviruses in 1982. Because the research is so new, there’s a lot we still don’t know.
The filoviruses cause horrible, usually fatal, diseases in humans. It may take a week or so for a human to show symptoms after exposure, but once they do they probably won’t live long enough to see any green bananas they just bought ripen. The virus can leap from animals to humans via blood contact, or by humans consuming food or liquids contaminated by animal feces or urine. There may be other ways, no one’s sure. The diseases jump to humans from a very few species: chimps, gorillas, and probably fruit bats. We can’t prove that it’s happened often, at least so far, but the diseases are so terrible scientists are pretty worried.
Over generations, a bacteria, virus, parasite, or fungus can develop a working relationship with its host. It’s bad business to kill off your host, at least before you’ve had time to reproduce. Rabies solves this ingeniously: the disease makes the host want to bite anything or anyone near it. The bite spreads the disease, so it doesn’t harm the virus “colony” to kill off the dog. It’s already thriving in the next animal.
Ebola and Marburg viruses kill humans, gorillas and chimps too quickly, from the virus’s perspective. If you kill off everyone in the village before they can infect other villages, you’re going to put yourself right out of the virus business. Other viruses have adopted a slick strategy: they infect a species but don’t kill it. In such a “reservoir” species, the virus can reproduce happily without bothering the host at all. The virus survives and multiplies for years without touching a more fragile group. If it happens to kill off a village of humans along the way by sheer coincidence, no problem. The huge population of viruses in the reservoir species doesn’t even notice. We suspect Ebola does this too, we just can’t prove it yet.
We’ve studied dozens of species looking for that reservoir species for Ebola, but have come to no consensus. The likeliest suspect right now is fruit bats, because several incidents of human infection can be traced back to bat caves. But examination of hundreds of fruit bats reveals that only a tiny percentage contain Ebola antibodies (meaning they’ve been exposed to the virus at some point) or live virus. None of the fruit bats examined had both, which seems a paradox. And none had any evidence of symptoms. One fruit bat that tested positive for live virus survived for over a year with no symptoms at all. Maybe fruit bats are the reservoir species. But the virus shows up so rarely in them, many people aren’t convinced.
Once scientists locate the origin of a virus leaping to humans (perhaps a cave, perhaps a clearing where a villager found a dead gorilla) they descend on that location to collect samples of every critter they can catch. They take these back to the lab and examine them for clues. If we can identify the reservoir species, we’ve got a lot better chance of predicting outbreaks and curing the disease. But it’s hard. No outbreak is reported the instant it occurs, the detective work to find the origin is tedious, the examination takes time. Outbreaks are unpredictable. Ebola springs into existence overnight, burns through an area, then disappears completely only to appear a few years later, miles away.
That seems like a fascinating clue to me. What species appears, then disappears for years, then reappears? My mind came to rest on cicadas. Could cicadas be the reservoir species for filoviruses? They spend most of their lives, sometimes several years, invisible underground. One North American variety stays underground for 17 years before emerging. Another has a 13 year cycle. They erupt in a horde, live for only a few weeks as flying bugs (some only three weeks) then lay eggs and die. Their bodies disappear quickly. They are powerful flyers once they warm up, but sluggish and easy to catch when chilly. Although they don’t normally travel great distances, a wind could easily carry them the distances required. Plus, most animals find them tasty. Even fruit bats sometimes eat a bug; if they don’t, they die of a Vitamin B-12 deficiency because you can’t get B-12 from fruit. Zoos and scientists couldn’t keep fruit bats alive in captivity until they figured this out. Seems reasonable a bat might choose a fat, juicy, and clumsy bug sitting right next to his favorite fruit high on a tree, rather than chase a cricket through the underbrush. Fruit bats spit out fruit pulp and pass food very quickly through their digestive systems but we don’t know many of the details. No one does.
I started reading about cicadas online in a casual way. Investigators did find some cicadas at the scene of one Ebola outbreak. They collected five of them and found no trace of filoviruses. For perspective, they collected 600 fruit bats.
There are over 2,000 species of cicada in the world that we know of, including 400 in Africa where these diseases have popped up. According to cicada expert Kathy Hill, hundreds of species remain unknown in Australia alone. Animals and humans love to eat them, but there are no believable reports of them carrying any disease. They use their proboscis to suck sap from trees, not gorillas. Plus, most sources say that the only “periodical” cicadas live in North America. Everywhere else in the world, they seem to emerge briefly every year. After more research, it looks like that is probably not true, although the periodical aspect of foreign cicadas isn’t well documented. It also appears that, although they don’t seek out animals or humans, if they land on one that isn’t moving, they’ll poke their snouts through its skin just to see if it might be tasty.
I asked my library to get a book on cicadas written in 1929 (via inter library loan. Thank you Worldcat.org) that remains important to (and beloved by) cicada enthusiasts. The book, “Insect Singers: A natural history of the cicadas” by John Golding Myers is out of print, but probably available if you look hard enough. Everyone still refers to it. I read it over a few days.
The guy who wrote it, J.G. Myers, is pretty funny; he is pretentious and opinionated and brooks no sloppy science. (I talk about authors who seem like fictional narrators in the present tense. Don’t send me an angry letter just because you brook no changes of tense). He quotes things that Aristotle wrote about cicadas (Aristotle wrote a LOT about cicadas) in the original Greek, with Greek letters and all, but no translation anywhere. We’re clearly supposed to be competent at Greek, and it’s our own damn fault if we’re not. He quotes Galileo, who also wrote a lot about cicadas, but in Italian, I think. Couldn’t understand a word of it. He quotes French guys, and German guys and Latin guys, sometimes for a whole paragraph, all with no explanation or translation. He would never stoop to use the word “butterfly” when he can rise to cast out the word “Lepidoptera.” He’ll mention some French scientist from 1850 and say basically, well, that story’s been told to death, hasn’t it? So no need for us to repeat it here. Which makes me wonder what the story was. Then in shame, I realize that if he repeated the story it would be in French and I’d be even more embarrassed.
He discusses cicada physiology, reproduction, diet, vision, and psychology. He documents their role in history and mythology. In the final chapter, he translated the songs of various species into sheet music, most of which would be boring to play– one note, in a specific rhythm. But a few had cool melodies I could read and hear in my brain. One species sings A-minor chords that resolve to F-major chords. I’m glad there was no quiz, but I actually enjoyed the book. I identified with the author more than I wish I did, and he cracked me up fairly often, mostly by being so pretentiously opinionated. I now know more about cicadas than most folks ever even dream of knowing. Alas, most of it will have disappeared from my brain by the time you read this.
Most interesting tidbit: In 1859, Thomas Huxley (who also coined the term “agnostic”) discovered little sacs in the eggs of aphids that fascinated him. He called them “pseudoviellus” and thought they might be a “supplementary yolk.” In 1899, Richard Heymons followed these little sacs over time, from the egg of a cicada all the way into the abdomen of a hatched nymph. In 1910 and 1911 Pierantoni and Sule independently decided these little organelles in the cicadas’ abdomen (they called them “mycetomes”) seem to have no purpose at all except to house symbiotic fungi and bacteria. But even earlier, in 1833, a guy named Dufour had described them in great detail in cicadas. In order to give proper credit to Dufour for discovering them, the author of our book, Doctor Myers (I’m just guessing he liked to be called “doctor”) actually stooped to translate Dufour’s lengthy description of these little sacs. Dr. Myers believed they housed symbiotic bacteria and fungi. And, he hastens to add, probably the invisible new “filterable viruses” (all the rage back then) which we now call simply “viruses.”
Some cicadas squirt an irritating fluid from their tail end, which may contain (and spread) these symbionts. Dufour observed this squirting and determined that it came from these little globs, these mycetomes. Dufour counted “fifty or sixty” mycetomes in one cicada. I bet Dufour’s vagueness on the precise count caused Dr. Myers actual physical pain.
So I did more research online. Turns out we still talk about mycetomes, but mostly because insects that suck blood (or consume some other single food source — like a cicada’s tree sap diet) have them. Blood, as a food, is low in some B vitamins so things like head lice, mosquitoes, and tsetse flies maintain colonies of bacteria in mycetomes to manufacture the vitamins they need. Mycetomes are designed to carry around bacteria you want to take advantage of but don’t really want loose in your other organs. Less valuable pests can take advantage of this nice safe harbor, too. It looks like the bacteria responsible for Rocky Mt Spotted Fever and the parasite responsible for sleeping sickness reside in mycetomes within the tick or tsetse fly. Within the last decade we’ve been studying mycetomes in leeches as well, and have discovered some brand new bacteria lurking in there.
The mycetomes in a cicada’s abdomen might be the perfect hiding place for a filovirus.
The idea of cicadas being a reservoir species for deadly viruses is almost certainly wrong. I realize that, and you should too. I have absolutely zero proof; I just wanted to share an interesting idea. Sometimes one bizarre idea sparks a curiosity in someone else that leads to a third person manufacturing a new thought. Every now and then something valuable hatches from that nest that looks much different from the original goofy egg. It’s not a waste of time, any more than writing a novel is, but I do not proclaim anything.
I merely say, “Interesting. Very interesting.”
Here’s a Youtube video of cicadas singing, sucking and squirting. The description is wrong about how they produce sound though. They don’t use their wings and legs like a cricket, but have a special sound producing organ:
Nice list of links to cicada sites and sites with recordings of cicada songs at
Largest site for cicada enthusiasts (you know who you are):
Info on cicadas of South Africa by Martin H. Villet
More filovirus resources
Long but informative article about Ebola by Oxford Journal:
Links to many articles about filoviruses here: