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Skip to 0 minutes and 14 seconds MARTIN HIBBERD: Ebola is a member of the Filoviridae family, named after the filamentous shape. The family consists of three broad groups of virus, including the Ebola, the Marburg and the newly discovered Cueva, which was discovered in bats. The Ebola group consists of five different species with the Zaire and Sudan and the Reston viruses being the most common viruses. So, the current strain of virus is highly related to the previous outbreak strains of Ebola. We know this from the whole genome sequencing both of this outbreak strain and also the previous outbreak strains. Despite the fact that these outbreaks have been some distance in time between them, we can see that highly relatedness between them.

Skip to 1 minute and 4 seconds We can also show from the sequencing that the new outbreaks have not come out of the previous outbreaks but in fact from some other reservoir of these viruses. And we believe this other reservoir, probably bats, is where all the evolution of these viruses is actually occurring. So while there’s an outbreak, we can show there’s a lot of diversity that’s occurring during this outbreak. That diversity is then lost at the end of the outbreak. The current outbreak of Ebola is obviously carrying on much longer than the previous outbreaks of Ebola, and this does allow increased amount of genetic variation within the virus this time compared to previous times.

Skip to 1 minute and 51 seconds And that may allow some selection of the viruses to occur at some point during this outbreak, selecting for viruses that are presumably more transmissable and so more fit for the transmission process. This selection process may have various potential effects on the human disease component of this. And in fact there are a number of possibilities that the virus might end up selecting for. One is that you might have increased viral load, and this would then allow a greater transmission process. And the increased viral load might be associated with more severe disease, for example. However, if you look at the other species, they have less viral load and are able to transmit more because they infect for a longer period of time.

Skip to 2 minutes and 48 seconds So both of these possibilities are there. However, I would argue that this would likely take a very long period of time for this type of selection to occur, possibly years of transmissions before we would see any evidence of this. The viruses, at the end of the outbreak, these mutations will disappear unless they can be transmitted back to the reservoir species. So whatever happens, as long as the outbreak is contained, these aren’t likely to have a long-lasting effect. I think there is no chance of the virus becoming airborne at this stage. None of the other Filoviridae viruses are airborne, and I think the chances of it changing are extremely small.

Skip to 3 minutes and 37 seconds In fact, if you look at our experience from any other viruses, we don’t observe transmission changes in those viruses either. So I think we can pretty much rule that out.

Where has Ebola come from?

The Ebola virus causing the outbreak in West Africa is the same species that caused outbreaks in the Democratic Republic of Congo (formerly Zaire). There is no reason to think that the size of the West African outbreak is due to any differences in the virus. Although there is some genetic change that takes place within each outbreak, at the end of each outbreak this is lost, as there is no transmission back to the reservoir.

Ebola is a member of a family of viruses known as Filoviridae, and is related to Marburg virus and the Cueva virus (identified in bats). There are five species of Ebola virus, of which three have caused large outbreaks in Africa. All of these viruses have similar structures and genetic makeup.

Image of graph showing the number of cases and deaths in different species of Ebola

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Similarity with other Ebola outbreaks

The Ebola species causing the current West African outbreak is Zaire Ebola, the same as in the first known outbreak. Using whole genome sequencing, the similarity in the genetic code of the virus can be compared between patients with Ebola in this outbreak and between those in different outbreaks.1 This shows that over time the diversity between patients within each outbreak increases, but there is no transmission from humans back to the reservoir, so any changes that occur are lost at the end of each outbreak. Each new outbreak begins as a new zoonosis, and not from a previous outbreak.2 All outbreaks of Zaire Ebola in humans have been highly related to each other genetically, despite some large time intervals between them.1

Image of graph showing sporadic nature of previous outbreaks

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The reservoir

The similarity between the strains in the different outbreaks tells us there is a non-human reservoir that is maintaining the population of viruses, and because there is evolution of the virus over time within the reservoir species, the virus in one human outbreak is not genetically identical to the previous one, although it is similar. It’s worth noting that evolution does not imply that the virus is becoming more or less virulent; most genetic changes that occur will have no effect on how the virus behaves.

The reservoir is thought to be species of bats, because they can be infected without getting any symptoms of disease. In the wild, a number of other animals can be infected, particularly primates, but they become ill and die from Ebola so are unlikely to be the natural reservoir.

Will Ebola mutate and become airborne?

Concerns have been expressed that since the virus is undergoing multiple passages through humans in a way never seen before, it could change and become more or less virulent. However, natural selection is likely to take some time, probably years of sustained transmission between humans. Note that none of the Filoviridae are airborne viruses and there is no precedent for a virus changing its route of transmission and becoming airborne when it was not before.

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Ebola in Context: Understanding Transmission, Response and Control

London School of Hygiene & Tropical Medicine