Why are Bats a Reservoir of Virulent Zoonotic Diseases? Inter alia, colonies can reach densities of 3000 bats/sq.m., in populations of up to a million individuals per roost

On the Evolution of Virulent Zoonotic Viruses in Bats. Frans L. Roes. Biological Theory volume 15, pages223–225(2020). https://link.springer.com/article/10.1007/s13752-020-00363-6

Abstract: Ideas formulated by Paul Ewald about the “evolution of virulence” are used to explain why bats, more often than other mammals, are a reservoir of virulent viruses, and why many of these viruses severely affect other mammals, including humans, but are apparently less pathogenic for bats. Potential factors contributing to bat viruses often being zoonotic are briefly discussed.

Why are Bats, More Than Other Mammals, a Reservoir of Virulent Zoonotic Diseases?

Bats harbor a significantly higher proportion of zoonotic viruses than all other mammalian orders (Olival et al. 2017, p. 646; disputed by Mollentze and Streicker 2020; see also Watson 2020). This is remarkable because, for instance, there are about twice as many species of rodents as there are species of bats, and rodents are more closely related to humans than bats are. Why are bats a reservoir of virulent viruses?

Many bat species are gregarious, some living in dense aggregations. Colonies can reach densities of 3000 bats per square meter, in populations of up to a million individuals per roost (Luis et al. 2013, p. 2). The theory of virulence implicates the close quarters of bats as a factor favoring increased virulence because bats roost so closely to each other that they can transmit infections to other bats even if they are immobilized by illness. A more virulent variant, making more copies of itself, will therefore spread. Note that it is not closeness per se that favors virulent diseases, but closeness favors transmission from animals that are not mobile.

To summarize: the extreme closeness of bats in many roosting sites allows the transmission of viruses from very sick hosts, favoring the more virulent variants in the population.

Why are many bat viruses also zoonotic? Several characteristics of bats seem to facilitate transmission to other host species. Bats are the only mammals with the capability of powered flight. This enables them to have a longer radius of action compared to terrestrial mammals and to have more direct or indirect contact with other animal species at different geographical locations. The mobility of bats probably allows bat viruses to be dispersed to humans and other mammals.

Whereas rodent species typically do not share communal nesting sites, roosting sites of bats can house diverse assemblages of multiple bat species (Luis et al. 2013, p. 3). This also may favor zoonosis. In the words of Ewald (pers. comm.): “The important point here is that multispecies populations may favor infection mechanisms that are not species specific and may thus allow for more frequent transmission across species including zoonotic transmission to humans.”

Finally, bats enjoy remarkable longevity for their body size. Some insectivorous bats can live up to 35 years (Wang et al. 2011, p. 650). Persistent infections may allow for prolonged release of viruses and thus greater exposure of humans (or other species) to the viruses.