There was a certain kind of quiet hopefulness when, in late April this year, the last Ebola patient of the West African epidemic – a two-year-old boy – walked out of a treatment facility in Monrovia, Liberia. With the smouldering embers of the outbreak fading, there was cause for celebration. But there remains the impotent fear of the unseen: Ebola is still out there, lurking. We just don’t know where it’s hiding or when it will be back.
If we’re going to stop Ebola in the future, we have to find its hiding places. Ebola is a zoonotic disease, meaning that it can spread between animals and humans. It burns hot and fast through people.
Its ruthless nature means that we are often the end of the line for the virus: a host like us that gets too sick too fast, that dies too quickly, cuts down the virus’s ability to jump into a fresh body. To remain a threat, Ebola needs a safe house in which to lie low and hide.
Such a long-term host, the quiet refuge of a pathogen, is known as a reservoir species. If a reservoir species is Ebola’s safe house, we are its luxury retirement property, a place for it to live out its last days with a bang. The trouble is that we aren’t sure where the safe house is. If we are going to be vigilant against Ebola’s re-emergence, we need to find it.
Searches so far have focused on forested parts of Africa, the home of a number of possible reservoirs. Classically, bats have been considered the most likely culprits, given that they overlap with humans geographically and can carry Ebola infection without symptoms. Based on research that has tested a wide variety of small mammals, bats, primates, insects and amphibians, several species of fruit bat have emerged as possible candidates.
A 2005 study published in Nature and helmed by Eric Leroy tested more than 1, 000 small vertebrates in central Africa and found evidence of symptomless Ebola infection in three species of fruit bat, suggesting that these animals – which are sometimes hunted for bushmeat – might be Ebola’s reservoir. An editor’s summary ran alongside the paper, titled simply: ‘Ebola virus: don’t eat the bats’. But not everyone is convinced that fruit bats are to blame.
Some researchers, like Fabian Leendertz of the Robert Koch Institute in Berlin, are working with circumstantial evidence that points to the insectivorous bat Mops condylurus. The first – or “index” – case of the 2014 Ebola epidemic was traced to a two-year-old boy in Guinea who may have spent time inside a large hollow cola tree near his house before falling ill. The tree was a known roost for these bats and a popular neighbourhood play spot. The boy died in December 2013, and by the following March, officials were alerting the public to the brewing outbreak. However, by the time researchers arrived in April to examine the tree and its inhabitants, it had been burned down.
Still others are looking elsewhere for Ebola’s home, sceptical that bats are to blame. Virologist Jens Kuhn of the US National Institute of Allergy and Infectious Diseases at Fort Detrick, Maryland, has told Nature that he thinks bats live much too close to humans: if they were the reservoir, it would be curious that there have been so few emergences of Ebola since we first discovered the virus 40 years ago. Instead, he believes insects or fungi could be possibilities. As he told National Geographic in 2015, he’s betting on finding Ebola in a “strange host”, explaining that perhaps the virus is hiding in a tick or a flea that intermittently bites bats, which only sometimes initiates the virus’s move from the wild into human communities.