As #Ebola2014 continues to spread throughout West Africa, many have been wondering: “How did it get there in the first place?”
4.17.14 – Please see this post for more current information regarding this topic.
3.28.14 – The current Ebola outbreak in Guinea – and likely, neighboring countries Sierra Leone and Liberia – is the first that’s been documented in West Africa . Historically, the Ebola virus [EBOV] has largely been confined to Central Africa. This is especially the case for Zaire Ebolavirus (ZEBOV) – the viral strain currently suggested by the CDC as the cause of #Ebola2014 . In fact, in order for a virus to be classified as Zaire Ebolavirus, it must be endemic to Central Africa – namely, the Democratic Republic of the Congo, Gabon, and/or Republic of the Congo .
Investigations of past outbreaks have helped the greater scientific community come to several important conclusions about Ebola and how it’s transmitted. Most commonly, index cases have contracted EBOV due to the mishandling or consumption of a contaminated chimpanzee, gorilla, or duiker carcass . In recent years, researchers have come to the consensus that these species actually serve as intermediaries; instead, it’s probably that the natural reservoirs for EBOV are three particular fruit bat species: Hypsignathus monstrosus, Epomops franqueti, and Myonycteris torquata [4, 5]. The intermediaries often contract EBOV by eating pieces of infected fruit dropped by the bats overhead . All three bat species are forest-dwelling and preferentially roost in rural communities. These bats – unlike the intermediary species – exhibit no pathogenesis of the disease and are entirely asymptomatic when infected, making them ideal reservoirs for zoonosis – diseases that are transmittable across species .
Pictured above are the regions habitable by Hypsignathus monstrosus [6 – Allen 1861], Epomops franqueti [7 – Tomes 1860], and Myonycteris torquata [8 – Dobsen 1878]. As can be clearly seen, two of the three species can live in Southern Guinea, Sierra Leone, and Liberia. Given the forest-dwelling nature of these species, it isn’t surprising that the habitable regions correspond closely with forest-cover in Central and West Africa :
Given this understanding, it might very well be that these three species of bat have been living in Guinea with a population density similar to what we’d see in habitable Central African countries. However, it’s probably safe to assume that there was little to no EBOV present among these three bat species in West Africa until fairly recently – resulting in the outbreak we’re seeing currently. So, how did the virus get to the bats living in Guinea?
Deforestation & Animal Migration
According to a study published in Science, about 30% of forest in Central Africa was under concession for industrial logging in 2007 . Central Africa at large – and particularly, the Democratic Republic of the Congo – contains the last frontiers for logging expansion on the continent . With this in mind, it’s possible that the growing amount of deforestation in ZEBOV-endemic countries has resulted in the habitat displacement of ZEBOV-infected bats.
Homeless, Ebola-infected bats may very well have migrated from Central Africa and relocated out West.
Given that Guinea is still considered a habitable environment for two of the three Ebola-carrying species, the infected migrant bats would likely have assimilated quickly. In their daily activities – namely fighting and sexual contact – they would have been able to easily pass the virus on to the “local” members of their species … But just a handful of infected bats wouldn’t have been enough to cause an outbreak.
In Central Africa, about 5% of the three reservoir bat species are infected during outbreaks in the human population. This drops to 1% when outbreaks are not occurring, suggesting that infection rates among bats is a good indicator of whether or not humans end up getting it. It’s likely that this 5% bat infection threshold has been met in Guinea; however, this alone would not be enough to result in transmission to human beings . However, when paired with common cultural practices of consuming bats and bushmeat, it becomes clear why the population was particularly vulnerable to this brand of zoonosis .
Climate Change & What’s Ahead
While it’s still unclear how long it will take for this outbreak to be contained, the future for Ebola in West Africa doesn’t look promising and should be monitored closely. As it stands, climate change models suggest that while Central Africa’s rainforests will get wetter in the future, West Africa will experience increasing dryness . Dry seasons in the region often yield increases in great ape poaching, as well as interaction among intermediary and reservoir species, due to food scarcity. In this way, drier climates result in higher EBOV transmission rates across species . Now that the virus has been introduced among local reservoir populations, there may be some dire consequences ahead for Guinea and its neighboring countries.
Without question, managing #Ebola2014 and its aftermath will require significant capacity across multiple disciplines. Today, it’s a highly infectious, deadly zoonotic disease that demands the attention of animal and human medical specialists, as well as public health and policy experts. But it’s also the product of deforestation and animal migration – processes that need to be better understood so we can prevent further geographic expansion of Ebola and its transmission agents. And in the future, climate change may cause its resurgence – making Ebola a permanent fixture in West Africa’s horizon. Working at this intersection will be challenging, uncertain, and vital to limiting #Ebola2014’s immediate and impending damage. Whether we succeed or not? We’ll have to wait and find out.
—Maia Majumder, MPH