Is the Ebola epidemic really coming to an end?
Posted: April 16, 2014 Filed under: virology | Tags: ebola, Ebola virus disease, virology 6 CommentsThe initial announcement of the latest outbreak of Ebola was on 25 March, in Guinea – part of West Africa, where the virus has never been known in the population, meaning that they would have been totally unprepared for such an epidemic to occur. As of 14 April, a total of 202 clinical cases of Ebola virus disease (EVD) – which includes confirmed and suspected cases – have been reported in Guinea, Liberia, Mali and Sierra Leone, including 128 deaths. Researchers have also determined that it is genetically similar to Ebola Zaire, but it is a brand new strain. This is significant because it shows that this Ebola epidemic has arisen separately from the previous epidemics in Central Africa. The Ministry of Health of Guinea has announced that with the decrease in the number of new cases, this latest epidemic of EBV is coming to an end; however, the fact that the latest wave of cases involved people in Conakry, Guinea’s capital with an airport, is slightly worrying.
Ebola is a fairly lethal virus, which is actually part of the reason why it hasn’t turned into a pandemic yet; it kills its hosts too quickly – up to 90% of the infected will be dead in a couple of weeks after manifesting symptoms – for them to pass on the virus very far. The other reason is because it is spread via contact with an infected victim’s fluids, unlike the flu, which can spread via air. And that is the next fear – that the virus will mutate such that it can be transmitted through the air, which was suspected to have happened with another species of Ebolavirus known as Reston virus, of the book The Hot Zone fame. On the bright side, it seems that the more people the Ebola virus infects, and the more it adapts to humans, the less lethal it gets.
viirulentscience 
Thanks for your interesting post. It got me thinking – surely Ebola virus is a case whereby lethality and transmissability are directly linked? Given that the virus is transmitted via broken skin, blood, and mucous membranes, it benefits from the very symptoms which make it so lethal – haemorrhaging. With this in mind, how can it be explained that the virus is adapting to humans to become less lethal? Thanks again!
I’m not sure whether lethality and transmissability in Ebola are linked – I’d have to go look that up again, but it’s an interesting point! Lethality from viruses in the general typically comes about via two mechanisms – 1) because of the virus itself bursting out from the host cells after they have replicated (as I elucidated in one of my Virology 100x posts) + other virus-related reasons, which are currently not well-understood, and 2) because of the host immune system reacting violently to the foreign invaders, causing things like inflammation and worse, a cytokine storm, and other destructive mechanisms meant for flushing out the invaders. So lethality and transmissability could be linked in Ebola, or it could be /not/ linked.
The virus particles are transmitted via blood, true, but if the virus can’t go very far because its human host has died i.e. maximum lethality, it won’t be able to transmit itself to new human hosts because of the direct contact element required. So it is actually evolutionarily advantageous to Ebola to tone down on its lethality so that it can continue to circulate in the human community and continue infecting more people. It may cause haemorrhaging, but it has to be of the non-lethal variety – that is to say, the type of haemorrhaging that most people can recover from.
It is my understanding that infected individuals are not contagious during the asymptomatic incubation period (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2870608/ – it’s not the primary paper that suggests this, but if you really want that you can find it!). This suggests two things to me: 1) the virus can not have symptoms so severe that there are insufficient opportunities to be passed on before an individual dies/recovers, and; 2) the symptoms may be in some way related to transmission (but not necessarily). However, it is interesting to note that men can pass on Ebola in their semen for a long period after symptoms have been lost. Beyond that, although I’ve had a brief read, I’ve not been able to find anything suggesting either way.
While it’s true that the infected individuals are not contagious during the asymptomatic incubation period, how does this suggest to you that the virus cannot have symptoms so severe that there are insufficient opportunities for the virus to be passed on before an individual ceases to be contagious?
EVD typically runs its course from 14 to 21 days (http://jvi.asm.org/content/77/18/9733), like most haemorrhagic fevers. While the infected individual is technically contagious at any time during this period, transmission only occurs with contact with the infected person’s blood. However, massive blood loss resulting from EVD is atypical – suggesting that not a lot of blood actually exits the infected individual in such a manner that another person will come into contact with it. Sufficient to be passed on to a small number withing a small perimeter, perhaps, but not enough to cause a pandemic – this is comparing it to viruses that can be spread rapidly through the air like influenza and coronaviruses. Furthermore, this blood loss/expulsion usually only occurs at the peak of the disease – a time period of a few days at the most. Couple the relatively small amount of blood loss and the short period of actual infectivity leads to a less efficiently spreading virus overall.
… sorry for the wall of text but does this kind of answer your question?
The reason that I made my first point is that the epidemic would not have occurred if the symptoms were severe enough that insufficient transmission events occur prior to death. Think of it in terms of the basic reproduction number (R0 – I shan’t clog up these comments even more with an explaination for those that don’t already know what this is, but Wikipedia explains this metric very well: http://en.wikipedia.org/wiki/Basic_reproduction_number) – in order for this outbreak to have occurred at all, it must be above 1.
That is not to say, however, that transmission cannot be improved, and undoubtedly the virus can evolve to be transmitted better between humans (increasing the R0 further). My first point wasn’t a point which supports my argument, rather one which is important when considering my second point.
There is one further point that would be interesting to discuss – other than through semen, is the Ebola virus still transmissable after an individual has lost all symptoms? If not, from the perspective of the virus recovery of the host is almost equivalent to the death of the host (discounting sexual transmission) – a dead-end. As such it may benefit from being more virulent during the symptomatic stage to maximise transmission before the host can launch an adaptive response. In fact, it may even be that a dead host which still has a high viral load may be more infectious than a receovered host – in countries where infrastructure is poor, and where the culture is to touch the dead, dead people have been suggested to a large source of infection. It is certainly advised to avoid contact with those that have died with Ebola (http://www.who.int/ith/updates/20140421/en/).
To be clear on my stand point, I could be convinced either way but have stuck to the opposing view because I’m quite enjoying the debate!
Reblogged this on Going Viral (and other Pathogens) and commented:
An thought-provoking post on Ebola from what seems to me an interesting virology blog. Check out the comments section too where we’ve begun to discuss the future evolution of the virus. Please feel free to join in!