Sometimes I read papers that are so cool that I’m not sure how to go about blogging them. I recently read one such paper, and this post is my attempt to describe to you the awesomeness of said paper. But I would encourage you to just skip this post and go straight to the paper so that you can experience the full awesome, rather than my diluted version of it. Here’s the link to the PDF.
If you’re still here, you might want to take a little trip in the Parasite Ecology time machine to this 2013 post based on Luis et al. (2013). Luis et al. (2013) showed that rodents harbor more total zoonotic viruses than bats, probably because there are more total rodent species than bat species on the planet (or the universe, for that matter). But there are more zoonotic viruses per bat species than there are per rodent species, on average, which suggests that there might be something special about bats that causes them to be extra important hosts for zoonotic viruses. If you want a full list of those potential special somethings, you can visit my previous post.
Here’s one possibility: bats might be really good at sharing their viruses with other species. Unfortunately, we can’t just go out and quantify interspecific transmission among bat species and rodent species to see if bats are better at hosting and transmitting viruses…yet. But we (or, well, Luis et al. 2015) can look at published accounts of viruses in bats and rodents to better understand how viruses are shared among species.
Ok, awesome methods time. Networks are a great way to quantify how pathogens are shared among individuals in a population. It turns out that they’re also a fantastic way to quantify how pathogens are shared among species in a community or a larger taxonomic grouping. Using published accounts of zoonotic viruses in bats and rodents, Luis et al. 2015 made networks where the nodes were individual bat or rodent species, and the edges (the connecting lines) were weighted to represent the number of viruses shared among two species. Using those networks, Luis et al. 2015 could calculate all kinds of network metrics that quantify things like the number of host species that any given host species shares viruses with (degree) and the number of shortest paths through the network (via edges) that involve a given host species (betweenness). One big take home message was that bat networks are more connected than rodent networks; individual bat species share viruses with more host species, on average, than rodents do.
What determined how many viruses bats shared with other bat species? Several things. Notably, bat species that had been studied the most (as indicated by citations on Web of Science), were known to have and share more viruses with other bat species. That tells us that we need more bat virus survey data! After accounting for study effort, fruit-eating bats hosted more viruses than bats with other diets, either because the weird fruit-eating strategy that bats use spreads viruses to other species, or because fruit-eating bats share some other trait. Bat species that live in the same geographic locations as many other bat species also had more zoonotic viruses, like we discussed last week. Gregarious bats that roost in high densities also had more zoonotic viruses, because host density is an important driver of pathogen transmission. And finally, bats that migrated had particularly high betweenness – they linked different parts of the network together.
Luis et al. 2015 could use their global bat virus database and the corresponding network to look at clusters of bat species (i.e., communities) that tended to share more viruses within the community than with species outside of the community. There were 10 of these viral sharing bat communities, some of which covered huge regions, like all of the Americas or a big region from Europe to Australia. Identifying these communities could be really helpful during future emerging epidemics. If you know what community one infected bat species belongs to, you now know where to look for other bat species that might be important reservoirs.
And now, for my favorite part! Last week, we talked about a model that explained the number of viruses shared between humans and bats based on bat species richness and anthropogenic influences like human population sizes, domesticated pig abundances, and bushmeat practices. In that paper and in this one, the authors found that Europe and India have many zoonotic bat viruses, even though neither Europe and India are hot spots for bat species richness. The model from Brierley et al. (2016) would suggest that anthropogenic factors like large human population sizes result in high viral sharing between humans and bats in those areas. The model from Luis et al. (2015) might suggest a different possible mechanism: the bats in Europe and India are part of the giant green and purple bat communities that include regions with much higher bat species richness, like East Asia. So maybe areas with low bat species richness are still exposed to high viral diversity due to connections with more species rich areas. These are both really cool hypotheses that deserve further study!
(Bats use hands-free electronic equipment, obviously.)
Brierley, L., M.J. Vonhof, K.J. Olival, P. Daszak, and K.E. Jones. 2016. Quantifying global drivers of zoonotic bat viruses: a process-based perspective. The American Naturalist.
Luis, A.D., T.J. O’Shea, D.T.S. Hayman, J.L.N. Wood, A.A. Cunningham, A.T. Gilbert, J.N. Mills, and C.T. Webb. 2015. Network analysis of host–virus communities in bats and rodents reveals determinants of cross-species transmission. Ecology Letters 18(11): 1153–1162.