For the record, it is also noted that the process works both ways. Diseases specific to humans can cross over and infect animals as well. In that case the process is referred to as anthroponosis. Quammen refers to the work of the biologist Patricia Reed.
Being human, we are more concerned about zoonotic processes. And that is Quammen’s main topic.
Quammen is concerned that man, in his roaming and thrashing about, is disturbing ecosystems and increasing the probability that diseases can transfer from animals to humans. He suggests that such transfers have become dangerously common.
"If you assembled a short list of the highlights and high anxieties of that saga within recent decades, it could include....Machupo ....Marburg (1967), Lassa (1969), Ebola (1976)....HIV-1 (inferred in 1981, first isolated in 1983), HIV-2 (1986), Sin Nombre (1993), Hendra (1994), avian flu (1997), Nipah (1998), West Nile (1999), SARS (2003), and the much feared but anticlimactic swine flu of 2009."
The evidence that Quammen compiles indicates that bats are often the animal that harbors the virus that transfers to humans either directly or via another species. The SARS virus was ultimately determined to have been transferred from a species of bat to a species of mongoose and then on to humans in China. What is it about bats that makes them such a prolific disease transmitter? Quammen explains that we have only begun to study bats, their ecology, and their immune systems. While there are interesting conjectures, there is no definitive answer yet. That is also the conclusion of this academic paper: Ecology of Zoonotic Infectious Diseases in Bats: Current Knowledge and Future Directions. This paper indicates that bats have been identified, or tentatively identified, as the source of the virus for rabies, SARS, Hendra, Nipah, Ebola, and Marburg. It also provides this warning:
Quammen provides the more interesting attempt at an explanation. We will return to his discussion.
To understand bats and their issues, some basics about disease epidemics are required. Consider a given population and the introduction of a disease causing virus. Some fraction of those infected will die and some will survive and become immune. Some fraction of those infected will pass on the virus and infect one or more others. If the population affected is too small, the disease will burn out because it will infect people and run its course faster than the population can produce new candidates for infection. If the population is large enough, new candidates can be produced at a rate that can sustain the viral infection within the population. Given the characteristics of the disease and its transmissibility, the size of the population required to sustain it can be calculated.
Clearly, accumulating large populations in a small area is good way to perform experiments with infectious diseases. While humans have begun to experiment with some rather large and dense populations, we will never be able to match bat colonies.
How close is close? Very close!
Any virus looking for a host would seem to find a bat colony an ideal place to land. While bat habits would seem to be rather unhealthy from an infectious disease point of view, bats seem to represent one of the more successful evolutionary strains.
Quammen suggests that the issue we have to deal with is not that we are ignorant of bat epidemiology, but that we, as humans, are altering our ecology as well as that of the bats. He issues this warning:
Quammen provides a theory for why the disease Hendra might have suddenly become a danger to humans. Hendra has emerged in Australia as a disease transmitted from bats to horses to humans.
Before humans began altering the environment, bats were widely distributed and lived in relatively small colonies distributed somewhat evenly along the terrain—as were their sources of food. These small populations could not support an epidemic for very long. Bats migrated from one colony to another quite frequently, and disease could be reintroduced frequently. Frequent infection means there are few new individuals in the population who can become ill and the intensity of the disease is low.
Along came humans who cut away the natural forest habitat and replaced it with orchards and farms and parks and cities. And bats have adapted to this new environment by coming in closer contact with humans.
Larger populations with less interchange of individuals leads to less frequent re-infections, but more intense epidemics when they do occur. Both the decreased separation between humans and bats, and the increased likelihood of a highly infected population at the time of re-infection enhances the probability of disease transfer to humans.
Quammen provides an interesting and a cautionary tale about the discovery that Marburg is a disease present in bats. There is a cave in Africa that came to be called "Python Cave" because pythons took up residence there and fed well on the bats residing there. Tour guides thought tourists might be interested in seeing the snakes and began taking the occasional visitor. Two women visitors were subsequently diagnosed with Marburg. They made the news because they carried the virus back to their home countries. Astrid Joosten, a Dutch woman died from the disease. Michelle Barnes brought her illness back to the United States and managed to survive even though her sickness was not accurately diagnosed until after the illness had subsided. These events sent scientists scurrying to that cave where they detected Marburg infection in the resident bats.
A few lessons are to be learned from this and other tales provided by Quammen
If someone asks you if you want to crawl into a cave filled with bat droppings and bat urine to watch a snake eat bats—tell them you will wait for the movie to come out.
If you encounter a dead animal—call a professional to deal with it. It is unlikely that it died of old age.