“Bats and Viruses” Book Review by Merlin Tuttle

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Bats and Viruses, edited by Lin-Fa Wang and Christopher Cowled, provides the first summary of current knowledge on how bats and viruses interact. It is an invaluable resource for all who are concerned about bats, whether from a public health or a conservation perspective. Given the rate of viral discovery it is commendably up-to-date.

 

Viral discoveries, distribution, potential for zoonoses, best practices, research biases and areas in need of further investigation are thoroughly covered.

Bats appear to serve as reservoir hosts for several of the world’s deadliest diseases. However, as noted, transmission to humans or their livestock is rare, and in most cases can be easily avoided. Advice not to eat bushmeat, handle unfamiliar animals, mix unquarantined wildlife in markets or plant fruit trees where flying foxes can be lured into close proximity to livestock is appreciated.

Numerous biases and possible misinterpretations are explained. Viral reservoirs cannot be confirmed based on mere presence of viruses or antibodies, and those found in bat guts or feces may come from insects or other foods. Also arthropods such as mosquitoes can simultaneously infect more than one species with identical zoonotic viruses, giving a false impression of transmission between incidental hosts.

Since 2010, unprecedented funding has triggered a disproportionate global hunt for viruses in bats, leading to exaggerated perceptions of bats as uniquely rich sources. The very discovery of a new virus in a bat often makes headline news with sinister implications, though hundreds of new viruses can be discovered in a single human or virtually anywhere we look. Viral taxonomy is in its infancy, with the vast majority of species yet to be discovered. And of course all life is related at some level.

Thus, the announcement of a new virus related to one of the few deadly ones, can be cause for premature panic, especially when associated with poorly understood bats. In our minds, the very word virus, connotes disease and possible death. The fact that most bat viruses are likely benign or even essential to our survival is seldom considered. Even those viruses most closely related to deadly ones may turn out to be more helpful than harmful when further investigated.

This first-of-its-kind review notes that “None of the recently discovered novel viruses, first identified in bats, have yet been shown to be of significant public health concern.” Yet, even among many virologists, the flood of recent speculation about new bat viruses has led to the erroneous belief that bats are unique in harboring more viruses and more deadly ones than other mammals. Direct investigation of such hypotheses reveals serious bias.

Careful analysis finds no evidence that bats host more viruses per species than rodents or primates, nor do they have more significant zoonoses. In fact when mammalian orders were ranked according to emerging infectious disease host status, and use of human-modified habitat, “it was found that rodents, primates and carnivores, all had higher odds ratios than bats.”

Several times, chapter authors themselves appeared to fall victim to widely repeated but erroneous speculation. For example chapter 13 claims without documentation that bats host more viruses than other mammalian orders, while chapter 11 cites studies with opposite conclusions. MERS is listed as a bat virus in the preface as well as in chapter 11, though the authors of chapter four cite clear documentation that it came from dromedary camels. Also, chapter 9 begins with a statement that bats can “carry and transmit rabies,” while chapter 3 cites clear evidence against a carrier state. Though bats can transmit rabies, 99 percent of human cases come from dogs. Transmission from bats is rare and easily prevented.

Disproportionate focus on bats is especially tempting, given that colonial species (the ones predominantly accused of harboring zoonotic viruses) can be sampled rapidly in far larger numbers than any other group of mammals. Such narrow sampling can prove highly misleading. The search for Middle East Respiratory Syndrome (MERS), eventually traced to dromedary camels, was misdirected and likely delayed due to a premature focus on bats.

Despite intense efforts to link Ebola to bats, including the examination of large numbers of individuals and species, there is still “no specific evidence of direct bat-to-human infection”, and numerous efforts to isolate the virus from bats have failed. Nevertheless investigation of the most recent index case focused almost exclusively on bats and again failed to find Ebola virus. If bats are indeed the primary reservoir for Ebola, it is difficult to explain the scarcity of outbreaks among the large numbers of people who hunt, butcher and eat bats over wide areas. A broader, comparative approach is needed. In the case of bats and viruses, science appears to have suffered from too many attempts to prove rather than test hypotheses.

Premature, high-profile speculation, combined with use of terms like “emerging” disease is unfortunate. Reviewed studies show that most bat viruses are ancient, simply so rare that they are only now being discovered. Furthermore, despite the admitted rarity of human outbreaks from any bat-borne virus, even scientific journals have often presented unsubstantiated speculation in a manner that is too easily presented by popular media as grossly exaggerated fact, leading to unintended and counterproductive consequences.

In the preface, Bats and Viruses cautions that modern communications have the ability to spread fear and panic to millions, that studies to date should be considered preliminary, and that much more work is needed to achieve a complete understanding.

The essential contributions of bats to healthy ecosystems and human economies are repeatedly stressed. And we are warned that culling bats, even evicting them from buildings, can exacerbate disease risks, since stress increases susceptibility, and displaced bats are more likely to come into contact with humans and domestic animals.

Nevertheless, in my decades of experience conserving bats, people kill what they fear regardless of advice to the contrary, and premature speculation too often has led to highly counterproductive eradication efforts. For example, at the site of the presumed index case for the recent Ebola outbreak a nearby bat roost was burned killing many bats. And people were almost certainly put at greater risk from immunocompromised, now homeless bats that were stressed and forced into greater contact with humans.

Despite making up roughly a quarter of all mammal species, it is true that bats have been largely over-looked relative to their actual and potential roles in the health of humans and ecosystems upon which we depend. Greater funding for research on bats is indeed necessary, but without greater balance in investigating both risks and benefits the results could prove counterproductive.

Studies of bat ‘super immunity’ could prove crucial in preventing pandemic disease outbreaks of the future, and enhanced use of bats in bio-control of insect pests could aid significantly in reducing public health risks, both from disease and toxic pesticides. The positive side of bats and viruses provides many exciting possibilities for advancement of human wellbeing, a fact that must be presented in greater perspective if public support is to continue.

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