THERE HAS BEEN something of a controversy, or at least confusion, over the origins of Ebola, specifically on the so-called ‘reservoir’ species. Which species is “the” one from which humans become infected.
David Quammen has popularized the word “spillover” to refer to the transfer from species in nature to humans – and his book “Ebola:The Natural and Human History of a Deadly Virus” is a must-read for anyone interested in the on-the-ground stories of potential sources of Ebola outbreaks. emerging zoonotic diseases. Beautifully written, this book is a excerpt of Quammen’s larger book, “Spillover“, which would be an excellent addition to the library of anyone interested in the larger problem of zoonotic diseases.
The excerpt, which was published at the height of the 2014 epidemic, tells the tale of tracking potential sources of Ebolavirus in the wild. Of particular interest will be the incidence of an infection very likely to have occurred that took the life of a female tourist named Astrid Joosten. She and her husband descended into a cave housing Egyptian fruit bats. This particular stop at Python Cave in Maramagambo Forest, Uganda was an afterthought. But in terms of nailing down the likely migration of Ebola with the bats, scientifically, it turned out to be rather important. Their stop led to the discovery of a radio collar which had been used to tag bats in another region, proof of the migration of the bats.
Unfortunately, proof of the migration of the virus came in the form of a lethal infection of the woman, who had likely slipped and accidentally touched some guano (bat droppings), becoming infected with the Marburg virus, a relative of Ebola viruses.
The report is also of high scientific merit because, according to those involved in the visit, and in follow-up studies of the cave, the floor of caves are sometimes covered in dead and dying bats.
This particular observation helped me formulate a hypothesis that ebolavirus and its relatives may be a plant defense mechanisms – specifically, they may act to reduce the fruit-eating bat populations. Fruiting trees often adopt various strategies to insure their reproductive success. Oak trees, for instance, in North American flower and produce pollen in turn, and therefore avoid gamete wastage. They also, of course, produce tons of acorns. This is called “masting”, and it thought to be a strategy designed to overwhelm seed predators.
Could ebolavirus be a strategy some fruit trees use to reduce the size of the bat population? The high mortality in bats suggested by the descriptions in Quammen’s book, suggest that the mortality of the virus is non-trivial in bats. Being social mammals, their physiology and patterns of close contact are also suggestive. The fruit they dropped has killed 1/3 of the gorillas and chimpanzees in the wild since the mid-1970s. Thus, we see that ebola is likely circulating, in a haphazard manner, in the wild. The deadly virus does not survive well in mammalian populations – so where does it keep originating? I would not be surprised if genomic analysis of some species of fruiting trees does not find a copy of the ebola genome in the DNA of that plant. Alternatively, it could circulate (without harm) in insects that pollinate or even feed on the plant juices, an opportunistic, mutual relationship between plant and virus known as commensalism. A final suggestive observation is the observation that ebolaviruses can infect plant tissues. These possibilities are explored a bit in my own book “Ebola: An Evolving Story“. But David’s book partly inspired these thoughts, so, thank you David!
Here is a link to the Over Astrid Foundation, which provides educational opportunities to children in Uganda. Their aims is to insure education for all children in Uganda.