Before this year, the largest-known outbreak of Ebola comprised 318 cases, in 1976. That was the year the virus first became known to scientists. This October the number of likely cases of infection reported by the World Health Organisation in Guinea, Sierra Leone and Liberia has exceeded 8,000, of which more than 4,000 have resulted in death.
The number of cases is growing exponentially in these regions: the outbreak is out of control. The estimated doubling period for the number of infections ranges from 15 days in Guinea to 30 days in Sierra Leone. Based on these estimates, unless something changes it is expected that there will be 20,000 cases by the end of November.
The medical services are overwhelmed. Sierra Leone and Liberia, the two worst-hit countries, have a total of 191 doctors for a population of about 10 million. The current outbreak has stretched resources well beyond their limits and has the knock-on effect of reducing the care available for other medical conditions.
There have been about 20 outbreaks since the Ebola virus was first recognised in 1976, but none of them resulted in such a large number of infections. What makes this year different?
Previous outbreaks were in rural areas; this is the first time the virus has spread to densely populated urban areas. The isolated location of previous outbreaks provided an almost natural quarantine, and the outbreak burned itself out quite quickly. This outbreak has spread to new countries in west Africa, where the virus wasn't seen before, and has reached the capital cities of Sierra Leone, Guinea and Liberia, where it continues to spread.
Epidemiological work by the staff of Kenema Government Hospital in Sierra Leone traced the origin of the outbreak in that country to an individual who attended the burial of a traditional healer in Guinea.
As the virus grows, new mutations arise in its RNA (similar to DNA). These changes are inherited by the descendent viruses, so it is possible to trace the connections between the viruses in infected individuals through the sharing of these characteristic mutations. This information is used to construct a family tree – called a phylogenetic tree – of the viruses, showing the relationships.
Outbreak traced to funeral
Using this approach, an international team of scientists, including researchers in Sierra Leone, University of Edinburgh, and Harvard, examined the relationships of viruses sequenced from 78 patients in Sierra Leone. Their work is reported in the journal Science. Through an evolutionary analysis of the new mutations in the RNA of the virus, Stephen Gire and colleagues were able to show that the outbreak in Sierra Leone was linked to 13 people who attended the funeral.
By counting the number of changes that have occurred in the viral genome and using information on how quickly these changes arise, it was possible to calculate that the virus probably entered Sierra Leone in May, and that it entered human populations in Guinea in February.
Even though this virus has been recognised for nearly 40 years, there remain large gaps in our knowledge. For example, it is still not certain what the natural reservoir of the virus is. Viruses cannot survive out in the open for long, but must live inside other cells. Where does the Ebola virus live in the long gaps between human outbreaks? The current best hypothesis is that it is a natural infection of certain fruit bats. These bats are often eaten, thus providing a route of infection into humans.
David M Pigott and colleagues, writing in the journal eLife, have combined several sources of information to identify locations at high risk of future outbreaks. Among the contributory factors, the distribution of the fruit bats and the increasing human population in an area increases the chances of sustained human transmission of the virus.
Ebola is not airborne and is only transmitted through direct contact with body fluids; thus it is significantly harder to catch than, say, flu. My cousin Fiona McLysaght, country director in Sierra Leone for the aid agency Concern, says that “poverty, myths, misconceptions and cultural practices are all drivers for the spread of Ebola”.
The best hope for containment of this deadly virus is to equip health staff and to educate the population at large about how to avoid infection.
Aoife McLysaght is a professor in genetics at Trinity College Dublin, where she leads a research group focusing on identifying and interpreting the evolutionary patterns in animal genomes