Bats and Human Health
In “Give Bats a Break” (Issues, Spring 2017), Merlin D. Tuttle argues that limited scientific evidence supports the degree to which the media sensationalize the role of bats as hosts of significant human viral pathogens other than rabies-causing lyssaviruses. He is correct in assessing the total annual number of human deaths due to bat-borne viruses as low. And like him, I am appalled by the bad reputation that bats have received over the past decade based on limited or misinterpreted scientific data, leading to measures to destroy entire bat populations for no reason. Tuttle emphasizes his frustration with the unanswered question: why are there so few outbreaks of highly lethal diseases caused by coronaviruses or filoviruses every year given the abundance and geographic distribution of their presumed bat hosts? Indeed, my favorite phrase in his article is: “small samples have been mined for spurious correlations in support of powerful pre-existing biases [in regard to bats], while researchers ignored evidence that pointed in the opposite direction.”
However, Tuttle takes the pendulum and swings it too far into that opposite direction. He correctly cites my speculation that arthropods or fungi could be the hosts of Ebola virus. This statement, however, does not mean that I am certain that bats have to be excluded from the Ebola virus host search. Although no evidence unambiguously supports bats as harboring Ebola virus, scientific data suggest that bats may be exposed to this virus on a relatively regular basis. Thus, an arthropod or fungus on a bat may be the Ebola virus host—and to examine such a hypothesis, bats would have to be sampled.
Tuttle also minimizes the fact that Marburg and Ravn viruses, very close relatives of Ebola virus and equally if not more lethal to humans, have been isolated repeatedly from Egyptian rousettes, or Egyptian fruit bats, sampled in caves associated with human deaths. In experimental settings, these bats can be subclinically infected with Marburg viruses, and the infected bats shed the viruses orally and in their excreta for sustained periods. Further, under experimental conditions, these bats have been shown to transmit the viruses to other bats. Thus, though it’s possible that Egyptian rousettes may not be the major host of Marburg viruses, the bats certainly are a host of all known Marburg viruses and therefore their role in disease transmission ought to be studied.
Tuttle is right about MERS coronavirus being harbored in dromedary camels rather than in bats, as was hypothesized when the virus was discovered. However, he omits the accumulated scientific evidence that this virus is nested deep in a branch of bat-borne coronaviruses on the coronaviral phylogenetic tree. The question is not only from where a human contracts a virus, but also how this virus emerged. The current scientific evidence strongly points to a bat-dromedary camel transmission event in the past—and this hypothesis then brings forth the question: under which circumstances do bat viruses evolve to become human health threats? Consequently, the phrase used to introduce the article, “Searches for new viruses in bats are unlikely to contribute substantially to human health,” should not have been used.
Ultimately, the correct path lies somewhere in the middle: scientific exploration of the bat virome and the role of bats in human disease ought to be performed in the least disruptive and destructive manner possible. The incredibly important role of bats in mosquito control and plant pollination ought to be taught more effectively than in the past, and scientific sensationalism of any kind ought to be stamped out. Still, a single introduction of Ebola virus into the human population in 2013 ultimately led to more than 11,000 human deaths. Thus, if bats were involved in this unlikely, typically rare, and yet very impactful event, shouldn’t we have an eye on them?
Jens H. Kuhn
It has been more than 20 years since a new war on bats has been waged. In its current form, the new outbreak is being waged primarily by scientists, but it has been picked up by decision makers and even sometimes the public, leading to a series of misunderstandings, myths, unsupported statements, and partial truths that have been interwoven to present a picture that bats are the most dangerous, filthy, pathogen-harboring organisms on earth. Few voices are rising in defense of bats, and Merlin Tuttle, speaking through his article, provides one of the most prominent, presenting real evidence against the case.
I concur with his arguments one by one. The alarmistic tone employed every time a “new” emerging disease is reported makes it sound as if that is the end of civilization—but that is very far from the truth. On the basis of conjectures and misinterpretations of inexistent evidence, bats are blamed time and again, from Ebola to SARS to MERS. By knowingly, intentionally attaching the adjective “deadly” to a virus, the alarm is raised even more. And once the alarm is raised, health officials and other government leaders start paying attention and obviously more money is thrown at the “deadly problem.”
Furthermore, the emerging infectious diseases community is knowingly and intentionally promoting this false, unfair, destructive reputation of bats. Viruses and bacteria themselves are unfairly treated. The overwhelming majority of viruses and bacteria are beneficial, and the very balance of life on earth depends on their presence and interactions with other living things. I can draw on a number of lines of research to support this case. For example, it has been learned that one milliliter of seawater contains as many as 10 million viral particles, yet no one is saying we should dry up the ocean. Similarly, one kilogram of marine sediment contains one million different viral sequences, and no one is fighting to keep humans away from the sea. Finally, the human navel has been found to contain at least 2,368 bacterial phylotypes. If we employed the same rhetoric and flawed reasoning that Tuttle points out, the consequences would be devastating for the ocean, for our lifestyles, and for our belly buttons.
So it is time to set the record straight and let bats be what they are: some of the most beneficial organisms on the planet for human and natural interests equally.
Rodrigo A. Medellin
During the past decade, concern about the role of bats on spreading diseases has increased ferociously due to the last SARS and Ebola outbreaks. I will not repeat the multiple facts that Melvin Tuttle has already provided to counter claims that are unsupported by robust empirical evidence, rising concern for the future of bats. Unfounded fear can result in excessive demands for wildlife disease management, with detrimental results such as weakened legal protection for animals and unnecessary animal deaths.
Human societies have been transforming the landscape of the planet so intensely that we are now living what we call “Global Change,” which includes a massive destruction and fragmentation of natural habitats, the elimination of numerous species, and a decline in many ecosystem services on which we rely. This new situation is now posing numerous challenges, including some threats to human health. And this is the point where bats become part of the story. Unfortunately, as Tuttle mentioned, they are continuously identified as the main virus reservoirs and described as an extraordinary threat for human health even though the evidence of their role is often open to scrutiny.
Research on this topic should be sensitive to the fact that human-bat relationships are extremely complex, involving factors ranging from the importance of ecosystem services to the myths, legends, and fears surrounding bats. This affects not only what research is performed but how it is communicated to the public.
Although further research to assess the real disease risk is advisable, greater attention must also be paid to science communication to avoid misinformed risk perception that could undermine long-term conservation efforts. Whereas fear is easy to create and difficult to eliminate, it requires time and persistence to inculcate love and respect for nature. Thus, in any publication, scientific or not, it is not enough to superficially mention some of the ecosystem services bats provide. Benefits need to be given enough attention to provide a comprehensive picture of the human-bat relationship
We should never forget the lasting consequences of our messages and how the journalists/public will interpret our words. In a world experiencing the rise of social media as the most powerful tool for science communication, it is time for scientists to make an extra effort to consider the social implications behind our discoveries. We can no longer ignore the public response.