Ebola outbreak: is this the beginning, middle or the end?

The name ‘Ebola’ strikes fear at its mere mention; not just in areas where outbreaks are known to occur, but across the globe. Ebola is a lethal viral pathogen causing hemorrhagic fever, where case fatality rates average 78%. West Africa is currently experiencing the largest ever outbreak of the disease, as it moves its way through Liberia, Sierra Leone, Guinea, amd Nigeria. Last week Senegal reported its first case. As of 1st September 2014, the number of cases was reported to stand at around 3000, with 1500 deaths. The World Health Organisation (WHO) recently estimated that case numbers might rise as high as 20,000. But what do we really know about this Ebola strain, its transmission and how the outbreak is likely to play out over the coming months?

A recent paper in the journal Science, from researchers at Harvard Medical School and the Broad Institute in Boston, used modern sequencing technologies to try and characterise the current outbreak viral strains. The genomes of 99 Ebola virus strains were sequenced with a view to understanding the genetic variation within the outbreak and characterise transmission patterns. The currently circulating strain of Ebola appears to have arisen from central African strains and crossed from Guinea to Sierra Leone in May 2014. Normally Ebola outbreaks are associated with zoonotic (animal-to-human) transmission, usually from contact with Fruit bats or consumption of infected bushmeat (wild game sourced from the forest). However, it would appear there has been sustained human-to-human transmission within these strains, suggesting it has been around for a while in the population. It is hoped that these sequences can be utilised to aid in diagnostics, therapies and vaccine development. The authors of this paper also released all of the sequencing data immediately into the public domain to facilitate research and relief efforts; a great example for open science and the open data movement. Tragically five of the 50 co-authors listed on this paper lost their lives to Ebola whist collecting and preparing the samples for this sequencing effort, making the ultimate sacrifice for their science.

Data such as these are key to wider multidisciplinary efforts to understand the transmission of Ebola and how this outbreak may spread. Mathematically modelling Ebola has been difficult. Previous outbreaks have been relatively small, with the largest outbreak totalling 318 cases. This lack of prior data is hampering efforts to develop robust models. Epidemiologists have not seen much in the way of Ebola epidemics, so epidemiological models of transmission are not as developed as they might be. A news story, also in Science, discussed the efforts of modellers in this area. The article suggested that the key to solving the outbreak is ultimately down to what modellers refer to as the pathogens ‘effective reproductive rate’ (R_e), that is, how many people on average, an infected person subsequently infects. Quarantining people, increasing hygiene levels, and identifying cases early will help in reducing this rate, but in rural areas many cases may go unreported, with the scale of the outbreak potentially higher than anticipated. This is where modelling can help in identifying key transmission control points, almost in a classical John Snow scenario.

In the last couple of weeks there have been curfews introduced and soldiers on the streets in West Africa to try and stem the transmission rates. The WHO are also reporting that the number of new cases is increasing faster than the creaking health systems in West Africa can cope with. This suggests that the current Ebola outbreak is yet to peak; previous much smaller outbreaks have dissipated as fast as they have arisen.

The Wellcome Trust recently announced a significant funding package for research in this area, for which they are to be commended. There are on-going vaccine trials in a few laboratories but it is impossible to bring poorly tested and unlicensed vaccines to a population. So for now, it looks like this outbreak will continue to expand and without significant assistance from the rest of the world, we may still be in the early stages.

Gire, S. K. et al.(2014). Genomic surveillance elucidates Ebola virus origin and transmission during the 2014 outbreak. Science. DOI: 10.1126/science.1259657

Kupferschmidt, K.(2014). Infectious Disease. Estimating the Ebola epidemic. Science. DOI: 10.1126/science.345.6201.1108