Bats and Emerging Infectious Diseases

Bats are amazing.  It’s easy to forget about bats because we don’t usually see them, but they’re out there, performing important ecosystem services that we often take for granted.  If you don’t believe me, you can read this review by Kunz et al. (2011), which outlines some important services that bats provide for humans:

1. Pollination
2. Seed dispersal
3. Arthropod suppression
4. Guano for fertilizer
5. Tourism – caving, etc.
6. “Witches and sorcerers used bats in ancient magic to induce desire and drive away sleep.”  (Seriously, without bats, there would be no ancient magic.)
7. Bats are frickin’ cute (I added this.)

Bats are also reservoirs for many viruses that cause serious human illnesses.  These include viruses like SARS, Ebola, and some paramyxoviruses like the Hendra and Nipah viruses.  Because these viruses are such a big deal, there has been a lot of recent attention to bats and their potential as reservoirs for high-impact emerging zoonotic viruses.  Specifically, two major questions arise:

1. Are bats hosts for more zoonotic viruses than other wildlife?
2. If yes, what characteristics make bats such good reservoirs for these emerging zoonotic viruses?

In a recent meta-analysis of viruses of bats and rodents, Luis et al. (2013) found that on average, bat species host more zoonotic viruses than rodent species.  So, perhaps there is something special about bats that make them particularly good reservoirs!  Of course, comparing bats and rodents doesn’t fully answer Question 1, but it is a start to say that if we look at bats and another taxonomic group that shares many life history characteristics with bats, bat species host more viruses, on average. However, because there are more rodent species than bat species in the world, rodents host more total zoonotic viruses than bats.  Therefore, in terms of global human risk, bats don’t contribute more than rodents.

So, given that bats may be somewhat unique in their ability to host zoonotic viruses, what causes them to be such good hosts?  Good question!  At this point, no one can really say, but it’s probably a combination of some of these unique bat characteristics:

1. Bats have unique feeding ecology, where they tend to spit out their food. They suck on the fruit/flower of choice and swallow the nectar/juice, but then spit out the remaining material.  If another animal comes along and eats the pulp off the ground, it might ingest virus particles from the bat, and the virus will have the opportunity to jump/spillover into a novel host species.
2. Bats and humans tend to overlap in habitat, which provides opportunities for bat viruses to spillover into human populations.  This is particularly likely in places where humans are altering landscapes so that livestock operations and bat habitat get mixed together.  For instances, in places where livestock pigs have access to fruit that bats have spit out.
3. Bats can be gregarious, where they may roost in extremely high densities.  Furthermore, multiple bat species may share the same roost.  High densities of susceptible individuals provide a virus’ dream population.
4. Some bats migrate, and their long-distance travel may help them to spread viruses.
5. Some bats hibernate, and that reduced metabolic activity may be important for some viruses, like rabies.
6. Because bats are evolutionarily ancient, their viruses may have highly conserved cell-receptor proteins that are good at invading the cells of many mammal species.

The take home message is that we need to study bats and emerging infectious diseases more.  We know very little about how and why and when viruses spillover from reservoir hosts to novel species, but in this era of global change, understanding those spillovers is becoming crucial for human health.  And as Luis et al. (2013) found, the more we study a given host species, the more viruses we find that infect that species.  So, if we want to know which viruses bats currently harbor in order to asses which viruses might be most likely to spillover into human populations, we should invest in more bat research!

That brown thing is a tree.

References:

Kunz, T. H., E. Braun de Torrez, D. Bauer, T. Lobova, and T. H. Fleming. 2011. Ecosystem services provided by bats. Annals of the New York Academy of Sciences 1223: 1–38. (PDF link)

Luis, A. D., D. T. S. Hayman, T. J. O’Shea, P. M. Cryan, A. T. Gilbert, J. R. C. Pulliam, J. N. Mills, M. E. Timonin, C. K. R. Willis, A. a Cunningham, A. R. Fooks, C. E. Rupprecht, J. L. N. Wood, and C. T. Webb. 2013. A comparison of bats and rodents as reservoirs of zoonotic viruses: are bats special? Proceedings of the Royal Society B 280: 20122753. (PDF link)