Ebola has surfaced again. After a hiatus of over a year without any new identified outbreaks, the virus has reemerged in western Africa, in the first-ever multi-country outbreak of the Zaire strain of Ebola. As of this writing, there have been 122 suspected cases of the disease in Guinea (24 laboratory-confirmed per the WHO) and 80 deaths (66% mortality rate). Most of these cases have been in Guekedou and Macenta in rural Guinea, about 35 miles apart, but what’s really concerning is that at least 11 cases have also been identified in Guinea’s capital of 2 million people, Conakry. Conakry is over 400 miles away by the main road and contains the only major hospital in the country (and even that hospital is certainly less than ideal). A direct route from Guekedou to Conakry as the crow flies takes one right through Sierra Leone. It’s probably not too surprising then that this country has also reported 8 possible cases of Ebola, and another neighbor, Liberia, suspects 6 cases of the disease. Guinea’s northeast neighbor, Mali, has also just reported 3 potential cases, but these have not yet been confirmed. Senegal has closed its border to prevent importation of the virus.
All evidence points to Ebola having a reservoir in fruit bats. The virus can spread to other species when they come into contact with bats, and has caused massive great ape die-offs in addition to human outbreaks. Humans can become infected in a few suspected ways: direct contact with bats (such as visiting bat-infested caves or working in factories where large numbers of bats roost); butchering or consuming animals (particularly non-human primates, who can also acquire the infection via bats); consuming the bats themselves, as has been suggested may play a role in the current outbreak. Finally, once humans have been infected, human-to-human transmission can also spread the infection via contact with viral-laden body fluids (blood, saliva, feces).
Though Ebola has a very high mortality rate, the one good thing is that it’s really not easily transmitted between people. Though there have been some experimental evidence that it could be airborne, in epidemiologic studies during outbreaks, no airborne transmission has been confirmed. Instead, most people who contract it from another person have very close contact with that patient: they’re hospital workers, or caretakers of infected family members, or are preparing a body for burial and are exposed to fluids during cleansing rituals. It may also be transmitted via semen and has been found in breast milk. Casual contact doesn’t seem to readily spread the virus. That fact also makes the ongoing outbreak that much more tragic: new infections and deaths can be minimized if money, supplies, and education are provided.
Many of the more recent outbreaks in the Democratic Republic of Congo and Uganda have had much smaller numbers of cases than earlier outbreaks in these countries (Wikipedia has a nice summary here). Early detection and the use of basic personal protective equipment (gloves, etc.) and environmental disinfection stem transmission of the virus fairly effectively. Outbreaks in the 1970s and 1990s were amplified in hospital settings due to close contact between patients, shared/reused needles, and spread to healthcare workers who were inadequately trained and protected. Unfortunately, this is happening now as well, with 14 healthcare workers infected and 8 killed to date.
Though this is the first significant outbreak in West Africa, it’s not the first time Ebola has been found. One of the types of Ebola, Tai Forest ebola virus, originated in Ivory Coast after a primate researcher became infected while carrying out a necropsy on a chimpanzee. However, this is the only known case of that type of Ebola, and the current outbreak is caused not by the Tai Forest strain, but by the Zaire strain–which is the most virulent of the bunch. This strain has been most commonly found in Central Africa (Democratic Republic of Congo, Republic of Congo, Gabon), and based on analyses of these outbreaks, it was suggested that the Zaire strain had originated near Yambuku, DRC in the early 1970s, and had spread/diverged since then. However, it looks like this outbreak would be too distal for that model. Sequence data could clear that up but is not yet available.
While every outbreak of Ebola represents a golden opportunity to study this rare virus in nature, it’s an opportunity that no one wants or relishes. This one went almost 6 weeks before a definitive diagnosis was made, and now has the distinction of becoming the largest Ebola outbreak in at least 7 years–possibly more, depending on how quickly they can but the brakes on new cases. Unfortunately because these outbreaks start in rural, resource-poor areas, one thing we can be confident about is that we’ve not seen the last of this virus.