Members of the Filoviridae, a family of viruses to which Ebola and its relative, Marburg virus, belong, are between 16 and 23 million years old – far older than previously thought, according to a new study led by Prof Derek Taylor of the University at Buffalo.
Transmission electron micrograph of an Ebola virus virion. Image credit: Frederick Murphy / CDC.
The family Filoviridae (filoviruses) includes three genera – Cuevavirus, Ebolavirus, and Marburgvirus – and is included in the order Mononegavirales.
Marburg virus (named after a city in Germany) was the first virus of the family to be discovered.
It was in 1967 when a number of lab workers in Yugoslavia and Germany, who were handling tissues from green monkeys, developed hemorrhagic fever. A total of 31 cases and 7 deaths were associated with these outbreaks.
After this initial outbreak, the virus disappeared. It did not reemerge until 1975, when a traveler, most likely exposed in Zimbabwe, became ill in Johannesburg, South Africa.
A few sporadic cases and 2 large epidemics of Marburg hemorrhagic fever – in the Democratic Republic of Congo (DRC) in 1999 and Angola in 2005 – have been identified since that time.
Ebola was first identified in 1976 when two simultaneous outbreaks of Ebola hemorrhagic fever occurred in Nzara, southern Sudan, and in northern Zaire (now DRC). The latter occurred in a village near the Ebola River, from which the genus takes its name.
The outbreaks involved what eventually proved to be two different species of Ebola virus; both were named after the nations in which they were discovered. Both viruses showed themselves to be highly lethal, as 90 percent of the Zairian cases and 50 percent of the Sudanese cases resulted in death.
Currently, there are five species assigned to the genus Ebolavirus, four of which are known to cause disease in humans: Ebola virus (Zaire ebolavirus); Sudan virus (Sudan ebolavirus); Taï Forest virus (Taï Forest ebolavirus, formerly Côte d’Ivoire ebolavirus); and Bundibugyo virus (Bundibugyo ebolavirus). The fifth, Reston virus (Reston ebolavirus), has caused disease in nonhuman primates, but not in humans.
Since 1976, Ebolavirus have appeared sporadically in Africa, with small to midsize outbreaks confirmed between 1976 and 1979.
Large epidemics of Ebola hemorrhagic fever occurred in Kikwit, DRC in 1995, in Gulu, Uganda in 2000, in Bundibugyo, Uganda in 2008, and in Issiro, DRC in 2012. Smaller outbreaks were identified in Gabon, DRC, and Uganda.
The current outbreak in West Africa – first cases notified in March 2014 – is the largest and most complex Ebola outbreak since the Ebolavirus was first discovered.
There have been more cases and deaths in this outbreak than all others combined. It has also spread between countries starting in Guinea then spreading across land borders to Sierra Leone and Liberia, by air to Nigeria, and by land to Senegal.
In a new study published in the open-access journal PeerJ, Prof Taylor’s team concluded that filoviruses likely existed in the Miocene epoch, and at that time, the evolutionary lines leading to Ebola and Marburg viruses had already diverged.
The study adds to developing knowledge about filoviruses, which experts once believed came into being some 10,000 years ago, coinciding with the rise of agriculture. The new study pushes back the family’s age to the time when great apes arose.
It does not address the age of the modern-day Ebolavirus. Instead, it shows that Ebola and Marburg are each members of ancient evolutionary lines, and that these two viruses last shared a common ancestor sometime prior to 16-23 million years ago.
The scientists found filovirus-like ‘fossil genes’ – chunks of genetic material that animals and other organisms acquire from viruses during infection – in various rodents.
One fossil gene, called VP35, appeared in the same spot in the genomes of four different rodent species: two hamsters and two voles. This meant the material was likely acquired in or before the Miocene Epoch, prior to when these rodents evolved into distinct species some 16-23 million years ago.
In other words: it appears that the known filovirus family is at least as old as the common ancestor of hamsters and voles.
“These rodents have billions of base pairs in their genomes, so the odds of a viral gene inserting itself at the same position in different species at different times are very small. It’s likely that the insertion was present in the common ancestor of these rodents,” Prof Taylor said.
The genetic material in the VP35 fossil was more closely related to Ebola than to Marburg, indicating that the lines leading to these viruses had already begun diverging from each other in the Miocene.
“Understanding the virus’ ancient past could aid in disease prevention,” Prof Taylor said.
“Knowing more about filoviruses in general could provide insight into which host species might serve as ‘reservoirs’ that harbor undiscovered pathogens related to Ebola and Marburg.”
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Taylor DJ et al. 2014. Evidence that ebolaviruses and cuevaviruses have been diverging from marburgviruses since the Miocene. PeerJ 2: e556; doi: 10.7717/peerj.556
