In light of the Ebola, Marburg, Hendra, Nipah, and Corona virus outbreaks, bats are an important reservoir to watch for emerging zoonotic diseases. A group of researchers recently compared bats to rodents as reservoirs of zoonotic viruses in the 2013 article “A comparison of bats and rodents as reservoirs of zoonotic viruses: Are bats special?”. The study compiled a list of viruses that have been discovered in each bat and rodent genus based on Web of Science, and compiled traits for each species, their conservation status, and phylogenetic correlation.
This study found that bats have more zoonotic viruses per species than rodents, but there are about two times as many rodent species as bat species. Each bat virus has a broader host range (4.51 bat host species per virus) than rodent viruses (2.76). Sympatry within a taxonomic order is the most important host trait associated with viral richness. There aren’t as many sympatric overlaps in bats compared to rodents, but when it does occur, sympatry seems to be of larger consequence in bats. Bat species with smaller litter size, greater body mass, longer life spans, and more litters per year tend to be more likely hosts of zoonotic viruses.
I hadn’t previously thought about the role of torpor in facilitating disease. Apparently in big brown bats in Colorado, torpor was identified as an important factor in the perpetuation of rabies. However, this study found that torpor was negatively correlated to viral richness.
Another surprising finding was that the study did not find that phylogeny had a significant effect, but can explain “residual variation” in the models used. The authors state that phylogeny is correlated to individual variables and that it may be more important than their models suggest. I’m not sure I understand why their model wouldn’t have been able to accurately account for this variable.
Also surprising was that although bats are evolutionarily ancient mammals, rodents are evolutionarily older and more closely related to humans. One would expect that cell receptors between humans and rodents would be more similar than cell receptors between humans and bats, which might better aid viral transmission.
The study measured the amount of interaction and removed specific species of rodents from the analyses, like Mus musculus, which has been extremely well studied in the lab, and has twice as many viruses known than for any other rodent species. How well rodents and bats have been studied would definitely impact the results of this study.
The authors conclude that their models can only account for about 43% of the variation seen zoonotic viral richness among hosts, but the majority of the variance remains unexplained. There are still a number of questions in understanding bat ecology and their ability to act as reservoirs for zoonotic disease. A lot of bat’s basic ecology remains unknown, such as the effects of pregnancy on the immune system, and roosting behavior and social structure of many species. Like all papers, this one concludes that more research needs to be done.
(Luis A. et al. “A comparison of bats and rodents as reservoirs of zoonotic viruses: are bats special?” Proceedings of the Royal Society” 7, Apr. 2013; 280(1756): 20122753)