Zoonosis, Ebola, and the Elusive Reservoir Host

There’s a really interesting article in the upcoming (July 2015) issue of National Geographic entitled “Stalking a Killer“. Using the most recent outbreak of Ebola in West Africa as a case study, the article looks at the nature of a variety of unusual and rare diseases caused by filoviruses. Ebola is not a virus that evolved to infect humans; if it was, it would be much more prevalent and much more mild in its effects. Instead, periodically humans pick up the virus from a “reservoir host”, an animal species that harbors the virus without suffering grave consequences as a result of being infected. What is interesting about Ebola is that its reservoir host has not been found. Several bat species have been shown to harbor the virus, and in some outbreaks (including the most recent one of 2014) there is circumstantial evidence linking bats to human infection. But the prevalence of Ebola in bat populations is so low that it is theoretically unlikely that bats are the primary reservoir host for Ebola. Some researchers are looking at insects, which might harbor Ebola more prevalently and then transfer it to mammal species, as a primary reservoir for Ebola. If some species of insects do harbor the virus more prevalently, that would suggest that humans are “two steps away” from the real source of the virus. Unfortunately, the combined forces of difficult logistics and lack of funding have prevented serious and comprehensive research into the origin of Ebola from being conducted.

Beyond providing a great overview of what is known and not known about the source of Ebola, the article also has some pretty strong things to say about the role of culture in the epidemiology of Ebola. No less than four images depict cultural practices in West Africa that make the outbreak and spread of Ebola more likely:

1. Exorcism rituals;
2. Bush-meat hunting;
3. Sleeping in close proximity to recently-deceased relatives; and
4. Living in dense, crowded urban centers.

I do not have any way to assess how intensely each of these cultural practices contribute to the spread of diseases like Ebola, but I found it really interesting that National Geographic presented these practices so prominently. There is an interesting question here about the coevolution of cultural practices with diseases that have an ecological origin. A lot of our cultural practices seem to have been ‘limited’ in the past by the communicable diseases we faced.

Zoonotic diseases are fascinating and scary. They become problematic when two very different ways of human living interact and combine. You need people living in very rural areas to come in contact with animals that carry the disease for there to be any outbreak, but the outbreak only becomes epidemic if it can be spread to dense urban centers. In a globalized world, this rural-urban contact becomes more prevalent, so in order to prevent the spread of these sorts of diseases we need to create new cultural norms in order to avoid seeding new epidemics.