In most cases, it is assumed the the index human infection in each of Africa’s Ebola outbreaks have come as the result of exposure to an infected animal, most commonly through the hunting, preparing, or consuming of bush meat.
Bats are believed to be the primary host of the four known African species of Ebola (plus Marburg), but other mammals may serve as intermediate hosts as well.
Today we’ve an open access study that attempts to map those geographic regions of Africa where Ebola virus transfer from animals to humans is most likely to occur. This identifies areas where an index case might appear, but does not address where the virus could spread through human-to-human transmission.
David M Pigott, Nick Golding, Adrian Mylne, Zhi Huang, Andrew J Henry, Daniel J Weiss, Oliver J Brady, Moritz U G Kraemer, David L Smith, Catherine L Moyes, Samir Bhatt, Peter W Gething, Peter W Horby, Isaac I Bogoch, John S Brownstein, Sumiko R Mekaru, Andrew J Tatem, Kamran Khan, Simon I Hay
Ebola virus disease (EVD) is a complex zoonosis that is highly virulent in humans. The largest recorded outbreak of EVD is ongoing in West Africa, outside of its previously reported and predicted niche. We assembled location data on all recorded zoonotic transmission to humans and Ebola virus infection in bats and primates (1976-2014). Using species distribution models, these occurrence data were paired with environmental covariates to predict a zoonotic transmission niche covering 22 countries across Central and West Africa. Vegetation, elevation, temperature, evapotranspiration, and suspected reservoir bat distributions define this relationship. At-risk areas are inhabited by 22 million people; however, the rarity of human outbreaks emphasises the very low probability of transmission to humans. Increasing population sizes and international connectivity by air since the first detection of EVD in 1976 suggest that the dynamics of human-to-human secondary transmission in contemporary outbreaks will be very different to those of the past.
Of the 22 million people who live in `at-risk’ regions, the vast majority live in the DRC (formerly Zaire), which unsurprisingly is where the virus was first discovered in 1976, and the home to the most outbreaks.
The authors talk about some of the factors behind the size and spread of Ebola outbreaks in recent years:
In all countries at risk we show that since the discovery of EVD in 1976, urban and rural populations have increased and have become more interconnected both within and across national borders. During the last 40 years the increasing size and connectivity of these populations may have facilitated the subsequent spread of EVD outbreaks. These factors underline a change in the way in which EVD interacts with human populations.
They also argue that Ebola Virus Disease (EVD) is likely under reported in Africa.
The remote and isolated nature of Ebola zoonotic transmission events, paired with the relatively poor diagnostics and understanding of the disease transmission routes in early outbreaks, mean that underreporting of previous outbreaks is probable. An increasing understanding and description of a broader range of symptoms used in case definitions of EVD (Feldmann and Geisbert, 2011; Leroy et al., 2000) also increase the possibility that past outbreaks may have been misattributed to different diseases (Tignor et al., 1993). This poor detectability of EVD also clearly limits capacity to accurately identify the locations and transmission routes of index cases (Baize et al., 2014; Heymann et al.,1980).
The entire 40 page PDF file is available for download, albeit in draft form.