By Clare Booth, Winter 2021.
Ever since the novel coronavirus COVID-19 turned our collective lives upside down, people have been searching for something or someone to blame. Many have found that culprit in bats, which scientists believe were the viral reservoir from which COVID-19 moved to humans. In addition to COVID-19, many other human viruses have been linked back to bat species, including but not limited to Ebola, rabies, severe acute respiratory syndrome coronavirus (SARS-CoV), and Middle East respiratory coronavirus (MERS-CoV). This has caused many to wonder: are bats special in their ability to harbor and transmit viruses?
Scientists don’t currently know the full answer to this question. Some researchers have claimed that bats actually aren’t exceptional in the number of viruses they transmit to us. For example, a 2020 paper analyzing patterns of viral spread across groups of mammals and birds suggested that this could be simply down to the large number of species of bats. [1] They found that the number of species in a group of animals was a good predictor of how many viruses had jumped to humans. So, since bats are the second-largest group of mammals, perhaps they are not special for transmitting so many viruses to us. However, other work has shown that bats host more viruses per species than rodents do. [2] Thus, they do seem to be unique at least in the number of viruses they harbor in their bodies (if not how many they transmit).
The reason for this may be due to the differences in their immune systems. Even though bats generally harbor a relatively large number of viruses, they seem to be better able to deal with them than other mammals. Studies have shown, for example, that some bats may have more interferons (small molecules that interfere with virus replication) or more activation of these interferons, even when there aren’t viruses present in their bodies. [3] This means that a bat’s immune system may be more prepared to deal with viral intruders than other mammalian species.
In addition, many bats tend to be largely unaffected by the viruses that they carry ––they are more “tolerant” of diseases. Disease tolerance occurs when a host is infected with a virus, and, rather than waging an all-out war to eradicate the intruder, the host’s immune system simply tries to minimize the damage of the virus and otherwise carries on as normal. Sometimes the immune system overreacts when it encounters a new pathogen, causing more damage than the virus itself might have. This has been observed, for example, in some patients with severe reactions to COVID-19, who sometimes undergo a “cytokine storm” in which the body is flooded with cytokines, proteins secreted by the immune system that cause hyperinflammation and tissue damage. [4]
Fortunately for bats, their bodies actually seem to do the opposite of this. Studies have shown that bat immune systems may have reduced inflammatory responses. For example, one study of the greater mouse-eared bat (Myotis myotis) found that the production of a protein called interleukin-10 is increased, which suppresses inflammatory responses. [5] Another looked at immune cells from the black flying fox (Pteropus alecto) and found that activation of “inflammasomes”, protein complexes that activate inflammatory responses, was decreased compared to humans and mice. [6] This reduction in inflammatory response may be preventing bats from waging an “all-out war” against viruses and therefore allowing the viruses to remain in their systems.
Interestingly, it has also been speculated that there may be a link between bats’ adaptation to flight and their immune systems. Bats are the only mammals capable of true flight, and analysis of their genomes has shown that the evolution of this unique ability was accompanied by changes in their immune systems that allows for the higher metabolic rates needed to accommodate flight. [7] In particular, because they have such a high metabolism, bats produce a lot of reactive oxygen species, which are byproducts of the chemical reactions involved in producing energy. These reactive oxygen species can cause cellular damage and inflammation, so bats may have evolved milder inflammatory responses to counteract this increase in reactive oxygen species. In addition, some scientists have suggested that because of their high metabolic rate, bats have constantly elevated body temperatures, putting them in a fever-like state that makes them better at fighting off infections. [8]
So, given that bats may be uniquely capable of hosting viral pathogens and constitute a major source of zoonotic disease that could travel to humans, what can we do to prevent transfer of bat viruses to us? The solution is not to get rid of bats, though unfortunately some have suggested this. Not only are bats unique and fascinating mammals, but they also have an important ecological role, as they pollinate flowers and disperse seeds. In fact, there are over 530 species of plants that rely on bats for pollination, including mangoes, bananas, and agave, the plant used to make tequila. [9] Bats are also extremely useful to humans because they consume insect pests. In total, a recent study estimated that bats in North America provide agricultural services worth over $3.7 billion/year. [10]
Rather than eliminating bats, more surveillance could be done to monitor the viruses present in bat populations, especially those at the most risk of coming into contact with humans. [11] In addition, preservation of bat natural habitats is essential because when bats’ native habitats are destroyed by human development, they may move into closer proximity with us, increasing the probability of virus crossover events. And finally, perhaps we could learn something from bats, whose well-adapted immune systems could provide insight into treatments that help our own immune systems fight viruses such as COVID-19.
[1] Mollentze, Nardus, and Daniel G. Streicker. 2020a. “Viral Zoonotic Risk Is Homogenous among Taxonomic Orders of Mammalian and Avian Reservoir Hosts.” Proceedings of the National Academy of Sciences of the United States of America 117 (17): 9423–30. https://doi.org/10.1073/pnas.1919176117.
[2] Luis, Angela D., David T.S. Hayman, Thomas J. O’Shea, Paul M. Cryan, Amy T. Gilbert, Juliet R.C. Pulliam, James N. Mills, et al. 2013b. “A Comparison of Bats and Rodents as Reservoirs of Zoonotic Viruses: Are Bats Special?” Proceedings of the Royal Society B: Biological Sciences 280 (1756). https://doi.org/10.1098/rspb.2012.2753.
[3] Ehrenberg, Rachel. 2020c. “Why Do Bats Have so Many Viruses?” Washington Post. July 15, 2020. https://www.washingtonpost.com/science/why-do-bats-have-so-many-viruses/2020/07/10/0327f584-b65e-11ea-a8da-693df3d7674a_story.html.
[4] Fajgenbaum, David C., and Carl H. June. 2020d. “Cytokine Storm.” Edited by Dan L. Longo. New England Journal of Medicine 383 (23): 2255–73. https://doi.org/10.1056/NEJMra2026131.
[5] Kacprzyk, Joanna, Graham M. Hughes, Eva M. Palsson-Mcdermott, Susan R. Quinn, Sébastien J. Puechmaille, Luke A.J. O’Neill, and Emma C. Teeling. 2017e. “A Potent Anti-Inflammatory Response in Bat Macrophages May Be Linked to Extended Longevity and Viral Tolerance.” Acta Chiropterologica 19 (2): 219–28. https://doi.org/10.3161/15081109ACC2017.19.2.001.
[6] Ahn, Matae, Danielle E. Anderson, Qian Zhang, Chee Wah Tan, Beng Lee Lim, Katarina Luko, Ming Wen, et al. 2019f. “Dampened NLRP3-Mediated Inflammation in Bats and Implications for a Special Viral Reservoir Host.” Nature Microbiology 4 (5): 789–99. https://doi.org/10.1038/s41564-019-0371-3.
[7] Zhang, Guojie, Christopher Cowled, Zhengli Shi, Zhiyong Huang, Kimberly A. Bishop-Lilly, Xiaodong Fang, James W. Wynne, et al. 2013g. “Comparative Analysis of Bat Genomes Provides Insight into the Evolution of Flight and Immunity.” Science 339 (6118): 456–60. https://doi.org/10.1126/science.1230835.
[8] O’Shea, Thomas J., Paul M. Cryan, Andrew A. Cunningham, Anthony R. Fooks, David T.S. Hayman, Angela D. Luis, Alison J. Peel, Raina K. Plowright, and James L.N. Wood. 2014h. “Bat Flight and Zoonotic Viruses.” Emerging Infectious Diseases 20 (5): 741–45. https://doi.org/10.3201/eid2005.130539.
[9] “Bats Love to Pollinate.” 2018i. Bat Conservation International. June 18, 2018. https://www.batcon.org/pollinator-week/.
[10] Boyles, Justin G., Paul M. Cryan, Gary F. McCracken, and Thomas H. Kunz. 2011k. “Economic Importance of Bats in Agriculture.” Science. American Association for the Advancement of Science. https://doi.org/10.1126/science.1201366.
[11] Calisher, Charles H., James E. Childs, Hume E. Field, Kathryn V. Holmes, and Tony Schountz. 2006j. “Bats: Important Reservoir Hosts of Emerging Viruses.” Clinical Microbiology Reviews. American Society for Microbiology (ASM). https://doi.org/10.1128/CMR.00017-06.