Ebola is one of about 177 pathogens that are considered to be newly emerging diseases. These diseases are generally zoonotic diseases. Animal diseases that have jumped species. A familiar example of a zoonotic disease is measles, which is derived from dog distemper.
For the past thirty years, forty-one previously unrecognized human infectious diseases have jumped from their normal animal hosts to human beings. This has frequently occurred in Africa, as human beings settle into new areas. Ebola is a member of a small, recently recognized family called filoviruses, named for the filamentous or threadlike appearance of the virus on electron microscopy.
Like all viruses, these are far too small to be seen with a regular microscope. The first recognized outbreak of a filovirus occurred in 1967, when there were thirty-one cases of a hemorrhagic disease in vaccine production workers in Marburg, Germany, who had contact with blood, organs, or cell cultures from a batch of imported African green monkeys from Uganda. Ebola first emerged in Africa in 1976 in three simultaneous outbreaks and was initially called green monkey fever.
The disease also devastated populations of chimpanzees and gorillas. More human outbreaks occurred – and then ended. Intrepid researchers trekked through the jungle trying to find out where the virus went between outbreaks.
In his best-selling book, Spillover: Animal Infections and the Next Human Pandemic, David Quammen details how scientists caught, killed, dissected, or took samples of blood and internal organs from more than one thousand animals, including 679 bats. The bats were identified as one important “reservoir.” In reservoir animals, the virus can proliferate and be shed without sickening or killing the animals.
Some animals domesticated by humans, notably pigs and dogs, can be infected with Ebola virus and serve as reservoir animals. Bats are especially important. These very diverse animals – of which there are more than a thousand species – have a long association with RNA viruses. They can disseminate a disease widely because they fly hundreds of miles in a season between their summer and winter roosting sites. Humans can be infected by contact with their droppings, or in the course of hunting and butchering them as a food source. For many Africans, “bush meat” is an important protein source.