August, 1976. A new infection was causing panic in Zaire. Hospitals became death zones, as both patients and medical staff succumbed to the disease. Reports of nightmarish symptoms trickled in to scientists in Europe and the US, who sent investigators to determine the cause and stem the epidemic. Concurrently, they would find out, the same thing was happening hundreds of miles to the north in Sudan. In all, 284 would be infected in that country, and another 358 in Zaire–over 600 cases (and almost 500 deaths) due to a mysterious new disease in just a few months’ time.
The new agent was Ebola, but remarkably, the outbreaks were unrelated, at least as far as any direct epidemiological links go. No one had brought the virus from Sudan to Zaire, or vice-versa. Molecular analysis showed that the viruses causing the outbreaks were two distinct subtypes, subsequently named for their countries of origin, Ebola Zaire and Ebola Sudan.
While Uganda is currently battling another outbreak of Ebola Sudan, rumors in the past week have suggested that this virus may have spread to former Zaire (now the Democratic Republic of Congo), where Ebola has reappeared 4 additional times since the first discovery there in 1976. It’s now been confirmed that Ebola is again present in the DRC, with an (unconfirmed) 6 deaths. However, it’s not related to the Uganda outbreak. Reminiscent of 1976, the strain that’s circulating currently in the DRC is the Bundibugyo subtype, which was first identified in Uganda in a 2007-8 outbreak in that country, rather than the Sudan type causing the current Ugandan epidemic. Interestingly, every previous outbreak of Ebola in the DRC has been caused by the Zaire type of Ebola, so the appearance of Bundibugyo is a first–though not altogether surprising given that the outbreak province borders Uganda.
Is this just coincidence that Ebola has twice now broken out in two different places at the same time, but with different viral subtypes? Hard to say. Though we can now say it’s fairly likely that bats are a reservoir host for Ebola and other filoviruses, we can’t say for sure that bats are the *only* reservoir. Indeed, we know that some outbreaks have occurred because the index case was in contact with an infected ape or their meat–were these animals originally infected by a bat, or by another source? How does the ecology of an area affect the chances of an outbreak occurring? Were there reasons that humans might be increasingly exposed to the virus in these different areas–Zaire and Sudan in 1976, DRC and Uganda in 2012–at the same time? Weather conditions? Trade/industry? Host migration or dispersal? We know with another bat-borne virus, Nipah, that changes in farming practices led to increased proximity of fruit bats and farmed pigs–allowing pigs to come into contact with virus-laden bat guano, become infected with Nipah, and subsequently transmit the virus to farmers. Things that may seem completely inconsequential–like the placement of fruit trees–can actually be risk factors for viral emergence. Is there a common factor here, or just bad luck? Only additional hard-won knowledge of filovirus ecology will be able to tell.