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.
4.2.14 – The current Ebola outbreak in Guinea is the first that’s been documented in West Africa since the disease’s discovery in 1976 . Historically, the Ebola virus [EBOV] has largely been confined to Central Africa; this is especially the case for the Zaire ebolavirus [ZEBOV] – the strain currently held accountable for #Ebola2014 [2, 3]. Investigations of past outbreaks have helped the greater scientific community come to several important conclusions about Ebola and how it’s transmitted. Index cases have frequently contracted EBOV due to the mishandling or consumption of an infected chimpanzee, gorilla, or duiker carcass . In recent years, however, researchers have come to the consensus that these terrestrial animals actually serve as intermediaries; instead, it’s likely 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 reservoirs overhead . The bats – unlike the terrestrial intermediaries – are asymptomatic when infected, making them ideal reservoirs for zoonosis – diseases that are transmittable across species .
Pictured above are the regions habitable by the three bat species. As can be clearly seen, two of the ranges extend into Southern Guinea. Given the forest-roosting nature of these species, it isn’t surprising that the habitable regions correspond closely with rainforest-cover in Central and West Africa :
With this understanding, it’s quite possible that two of these reservoir species have been living in Guinea with a population density similar to what we’d see in Central African countries. However, it’s probably safe to assume that there was little to no EBOV present among these bats 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 the Central African rainforest was under concession for industrial logging in 2007 . The region at large – and particularly, the DRC – contains the last frontiers for logging expansion on the continent . Could the growing amount of deforestation in ZEBOV-endemic countries have resulted in the habitat displacement of ZEBOV-infected bats?
Yes. Homeless, Ebola-infected bats may very well have migrated from Central Africa and relocated out West.
But just a handful of infected bats wouldn’t have been enough to cause an outbreak. Studies in Central Africa have found that about 5% of the reservoir species are infected during outbreaks in the human population. This drops to 1% after outbreaks, suggesting that infection rates among bats might be good indicator of whether or not humans end up getting it . So, how did so many bats get infected?
Because Guinea is considered a habitable environment for two of the three Ebola-carrying species, the infected migrant bats would likely have assimilated pretty quickly… And in their daily activities – namely, fighting and sexual contact – they would have been able to pass the virus on to the “local” members of their species, eventually meeting the 5% threshold deemed necessary for humans to contract the disease . 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 … And now that the virus has been introduced among local reservoir populations, it’s uncertain whether Guinea will ever truly be rid of it.
No outbreak is an island; it lives within an ecosystem that is much larger than the virus, the patients, and the communities affected. Managing #Ebola2014 and its aftermath will require significant capacity across multiple disciplines – not only from public health and medicine, but also from policy, zoology, and environmental science. The jury’s still out on exactly how Ebola got to Guinea, but deforestation and animal migration are compelling candidates that may be key to preventing further geographic expansion of this deadly disease.
—Maia Majumder, MPH