With high fatality rates and disease symptoms that seem to echo scenes from apocalyptic movies, the current Ebola outbreak has managed to capture much debate and controversy this past month. The death toll is now up to 1013 (as of August 11) with West African countries on a high alert. With so much media coverage of the current outbreak, it seems there are some fundamental questions that are consistently left unanswered; I’ve put together three important questions mostly centered around treatment, vaccine and the pathology for the current Ebola outbreak which may be helpful to keep in mind when sifting though the onslaught of news reports:
1. Why is the current outbreak so much worse than previous outbreaks?
In spite of being classified as the worst outbreak in history, the current strain has a mortality rate of about 50%; still high, but significantly lower than previous Ebola strains. To put it in relative terms, the current strain of Ebola is not as highly contagious as influenza; the basic reproductive number R0 (i.e., how many secondary cases each infected person will produce on an average over the period of infection) for Ebola strains from previous outbreaks was ~ 1.3 while influenza can be up to 3. It is possible that the lower mortality rate of the current strain has been advantageous for its transmission; more virulent strains limit their spread by killing off the host faster than lower virulent counterparts. Many believe that the reduced mortality is because patient treatment has significantly improved; but poor healthcare system and cultural factors are playing a role in the rapid spread of this outbreak, summed up here. You might be aware that experimental medicines are currently being considered as an option to treat those who are infected. But many researchers including scientists who have designed the vaccine are not too optimistic about a accelerated or early stage clinical trial for the vaccine.
2. How did the Central African Zaire strain end up in West Africa?
The biggest surprise with the outbreak came in April when it was reported that the strain was actually of Central African origin rather than the expected Tai strain of Ivory Coast. This raises the concern that fruit bats are spreading the virus (or if we are encroaching more bat territories) giving a chance for transmission. However, direct bat-human transmission is yet to be documented, despite claims that villagers are eating dead bats and other bush meat, thereby contracting the disease. Why bats? As it turns out, bats have a unique evolutionary history of positive selection on genes that are related to resist oxidative damage and immunity, which lends to their ability to harbor diseases; read more about this in Science and New Scientist. Although it has not been comprehensively tested in experiments, there are several lines of evidence supporting the notion that bats can resist the “cytokine storm” produced by the immune system in response to an acute viral infection making them asymptomatic carriers for a number of viruses including Ebola.While there still is no definitive answer to this question, there are groups currently tracking the bat reservoir in Sub-Saharan Africa and the path of Ebola’s outbreak to get insights into this problem.
3. How old is Ebola?
You may be familiar with the issue that HIV is difficult to prevent because of it’s high mutation rate; this isn’t the case with all viruses. Ebola belongs to the negative sense single-stranded RNA (ssRNA) family filoviridae. Because of RNA polymerase’s lack of a proof-reading mechanism, and because there is no DNA intermediate involved (e.g. HIV), the estimated mutation rate in the virus genome is a million fold higher than it is for HIV. The catch here is that there are no major sequence dissimilarities between the various strains (dating back to 1976). This has led to a thought that the virus has jumped recently from a non-human species (1976) and the outbreaks are essentially its continuation into newer territories (a wave phenomenon). The other school of thought essentially contradicts this idea by saying Ebola has existed for a long time in a reservoir and its mutation rate helps it jump species occasionally into vulnerable populations (some medical researchers have proposed that Ebola could have caused the plague of Athens in 430 B.C). Both groups agree that fruit bats are reservoirs but there could be a third unknown reservoir which we have no idea about. It will be important to keep an eye on studies that answer these questions in coming years based on data from this outbreak. With luck, the vaccines and treatments would protect us then and Ebola would only be studied for its intriguing biology and not due to its potential for devastation.