Random Natural History: Ebola

through a glass, darkly

Currently there is a catastrophic outbreak of Ebola happening in West Africa. Over 1700 infections have been recorded with nearly 1000 deaths, making it the deadliest outbreak of ebola known. Infection results in a hemorrhagic fever, which starts out a bit like the influenza, but can result in bleeding from mucous membranes, organ failure, and ultimately death. But what is Ebola?

Ebola is a Filovirus. Filoviruses are a small group of viruses only known to infect mammals. They are so named because of their filamentous shape. They have tiny genomes, only ~19,000 base pairs in length, containing only seven protein coding genes and two regulatory regions. By contrast, the human genome is over 3 billion base pairs, contains around twenty thousand genes and has innumerable (by which I mean as yet unnumbered regulatory regions). Because of ebola’s simplicity, (as with all viruses), it cannot reproduce without commandeering the cellular machinery of its hosts. In the words of Cormac McCarthy, These anonymous creatures… may seem little or nothing in the world. Yet the smallest crumb can devour us.

The filovirus lineage is thought to be millions of years old, and genetic “fossils” of extinct filoviruses have been discovered in the genomes of a variety of mammals. It may sound surprising, but it is a fairly common occurence for fragments of viral DNA to become incorporated into the genomes of their host species and to be replicated and passed along with the host’s genes. All known living filoviruses, however, have descended from a common ancestor that probably existed only about 10,000 years ago.

Ebola virus infections are all a result of viruses in the genus Ebolavirus. There are five known strains (sometimes called species), four of which occur in Africa. Only three are known to cause fatal human infections. Notably, the Reston strain, found in the Phillipines can infect humans, but has no symptoms and has never resulted in a fatality (that we know of). The three that cause fatal infections vary in their severity, with mortality rates between 40 and 90%. The current outbreak is the the most deadly strain, known as Zaire ebolavirus.

Ebola falls into a class of diseases known as zoonoses. This means that human outbreaks of ebola typically begin by contact with animals carrying the disease, so that there is not a persistent population of infected humans (as in diseases such polio). In many known cases, outbreaks have originated by contact between humans and apes, typically when bushmeat hunters poach gorillas or chimpanzees. Infection probably occurs during cleaning of the animal.

Apes are not, however, the wild reservoir of the disease. They are also highly susceptible to it and their populations are often devastated where outbreaks occur. For the Zaire ebolavirus, it is currently believed that several species of fruit bat may be the natural reservoir. Several strains very closely related the Zaire strain were isolated from otherwise healthy bats in a survey of thousands of animals in the Congo basin. How fruit bats might transmit the disease to apes is not known, but it has been noted that ebola outbreaks often occur during the dry season. During these times, apes and bats may congregate at scarce fruiting trees, increasing the opportunity for contact.

It’s worth noting that wildlife poaching is an on-going crisis in many parts of equatorial Africa. Vast numbers of wild animals, including apes, are being harvested at utterly unsustainable rates in what’s known as the bushmeat trade. The meat is often transported to urban markets. While past ebola outbreaks have been in remote villages, the current one is urban, which is almost certainly part of the reason the death toll is so high. I have not yet read a definitive statement about how this outbreak emerged, but the bushmeat trade seems a likely suspect.

Leroy, Eric M., et al. “Fruit bats as reservoirs of Ebola virus.” Nature 438.7068 (2005): 575-576.

Taylor, Derek J., Robert W. Leach, and Jeremy Bruenn. “Filoviruses are ancient and integrated into mammalian genomes.” BMC evolutionary biology 10.1 (2010): 193.

Carroll, Serena A., et al. “Molecular evolution of viruses of the family Filoviridae based on 97 whole-genome sequences.” Journal of virology 87.5 (2013): 2608-2616.

Leroy, Eric M., et al. “Multiple Ebola virus transmission events and rapid decline of central African wildlife.” Science 303.5656 (2004): 387-390.

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