It is almost too difficult to remember a time without the hashtag “coronavirus” in it. The protagonist of the current pandemic, SARS CoV-2, the causing agent of COVID-19, has spread like wildfire and as of June 1st infecting about 1.8 million people worldwide and leaving behind a massive death toll that recently surpassed 100 thousand in the United States alone. From stay-at-home orders to quarantines, and a high expected death toll, COVID-19 has changed our everyday lives for good, and knowing how all this originated could shed light into future prevention of outbreaks.
As you are likely aware, misinformation circulating around this pandemic has been rampant, and generally if you ask around many feel certain that the SARS CoV-2 virus spilled over from bats to humans. Still, this quick assertion has not been thoroughly examined and research has led to wide misinterpretation leading to exaggerated and often unsubstantiated disease warnings against bats. But what is the origin of COVID-19 and what prompted this quick blaming of bats?
Some recent genetic studies have linked the SARS CoV-2 virus as a close relative of RaTG-13, a bat virus isolated in 2013 from Pu’er, China and not known to cause disease in humans. While the close relationship of these two viruses is undisputed, this does not account for the specific wildlife origin of SARS CoV-2. To dissect this, let us look at recent phylogenetic (i.e. evolutionary relationship) evidence. In a recent preprint study, Bonni et al. [1] used genetic data and phylogenetic methods to generate an estimate of the time when SARS CoV-2 and RaTG-13 viruses split as compared to other samples from different wildlife. Their analysis confirms that indeed SARS CoV-2 and RaTG-13 are closely related to each other. However, by also quantifying the rate of change between the two viral genomes they conclude that the most likely date when they last shared ancestry ranged between 1948 and 1982 (that would be at least 40 years ago!). Simply put, despite the close relationship, it is unlikely that the current SARS CoV-2 directly came from the RaTG-13 bat virus; so this tells us that there must be a missing intermediary host that has not been sampled and scientists must keep searching to pinpoint the likely origin of this pandemic.
Scientific advancements and the current ability to sequence DNA from nearly any type of sample have increased the pace at which we identify and catalog biological diversity across the tree of life. These methods have become the standard for new species discovery, including viruses. Viral discovery studies have surged nearly exponentially in recent decades perhaps because of their importance in human disease (see Fig. 1 in [2]). As scientists rushed to document viral diversity, headlines such as “optimizing viral discovery in bats”, “many zoonotic viruses that have emerged recently are thought to have their origins in bats”, and “bat origins of human coronaviruses” [3,4,5] are a cause of concern. Evidently, the language often used has led to the unsupported notion that bats are a primary source of human disease. In a recent study, Mollentze & Streiker [6] took a stab at examining 429 virus–host associations to identify whether novel human viruses disproportionately originate from specific birds or mammals. They found that the proportion of viruses that infect humans has a large variation across all birds and mammals studied and that the number of human infecting viruses increases proportionally with the number of species within each animal group. Bats are the second most diverse group of mammals with 1421 species documented. Despite that, the proportion of zoonotic viruses in bats is no different than that in rodents and primates, respectively the first and fifth most diverse mammal groups, and it is less than what we see in birds (see Fig. 1 in [6]). These findings support that bats, in particular, have often been mistakenly portrayed as a main reservoir of viruses by recent press regardless of other additional and perhaps equally important wildlife reservoirs.
Bats have a secretive nocturnal lifestyle and provide many services that we benefit from, like natural insect pest control, pollination, and seed dispersal, all under the cover of darkness. Notwithstanding, the basic biology of most bat species is not well understood, which in turn generates an air of mystery surrounding these important yet imperiled mammals. By undermining the importance of the services they provide and magnifying the bad press we are leaving bats defenseless as they fall under attack, feared and often despised by many, and putting their conservation at risk. Evidence shows that we can benefit and learn more from bats than we realize. As we move forward during this pandemic, it is important to not lose sight of parallel conservation issues at stake in the equally important race to curtail the current global biodiversity crisis.
Written by Dr. Angelo Soto-Centeno, FCW guest writer
Angelo Soto-Centeno is an evolutionary biologist and Assistant Professor of Biology at Rutgers University, Newark. He is interested in mammal biodiversity, particularly bats, and their biogeography. Angelo’s recent work tries to shed light into bat extinctions and the factors that contribute to bat population loss.
References:
- Boni, et al., BioRxiv https://doi.org/10.1101/2020.03.30.015008 (2020).
- López-Baucells, et al., Mammalian Review 48:62 (2018).
- Young & Olival, PLOS ONE 11(2): e0149237 (2013).
- Wang, et al., Journal of General Virology 96:2442 (2015).
- Hu, et al., Virology Journal 12:221 (2015).
- Mollentze & Streiker, PNAS 117(17): 9423 (2020).