On the Evolution of Virulent Zoonotic Viruses in Bats
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ORIGINAL ARTICLE
On the Evolution of Virulent Zoonotic Viruses in Bats Frans L. Roes1 Received: 6 May 2020 / Accepted: 21 September 2020 © Konrad Lorenz Institute for Evolution and Cognition Research 2020
Abstract Ideas formulated by Paul Ewald about the “evolution of virulence” are used to explain why bats, more often than other mammals, are a reservoir of virulent viruses, and why many of these viruses severely affect other mammals, including humans, but are apparently less pathogenic for bats. Potential factors contributing to bat viruses often being zoonotic are briefly discussed. Keywords Evolution of virulence · Hosts · Parasites · Reservoir of virulent viruses · Transmission mode · Zoonosis
Introduction Bats are exceptional in several ways. They are the only flying mammals, many species have echolocation, they enjoy extraordinary longevity for their size, and the viruses they host are exceptional. Every life form investigated has been shown to host viruses (Calisher et al. 2006, p. 542). Several viruses hosted by bats, like henipaviruses, coronaviruses, filoviruses, and the rabies-causing lyssaviruses have been shown to be transmissible from bats to humans (Wynne and Wang 2013, p. 1). In other words, they are zoonotic. Another remarkable aspect is that many of the viruses coming from bats are highly pathogenic for other mammals (including humans), but apparently less pathogenic for the bats themselves. The theory of Ewald (1993a, b), the “evolution of virulence,” is used here to answer three related questions. A. Why are some infectious diseases more harmful than others? B. Why are bats, more than other mammals, a reservoir of virulent zoonotic diseases? C. Why is it that many of the viruses that severely affect other mammals, including humans, are apparently less pathogenic for bats?
* Frans L. Roes [email protected] https://www.froes.dds.nl 1
Amsterdam, The Netherlands
Why are Some Infectious Diseases More Harmful Than Others? What follows is a concise description of Paul Ewald’s theory of the evolution of virulence (from Roes 2018, pp. 243–244): While suffering from a bout of diarrhea in the late 1970s, Ewald speculated that his body was being manipulated by some parasite or “guest.” The benefit to the pathogen causing the sickness was the potential transmission to other hosts. Much like the particulates expelled during coughing, diarrhea can be a means of distribution, for instance, when a sewer leaks into drinking water. The “transmission mode” or means of distribution of parasites to new hosts became the key variable in Ewald’s theory. If parasites depend on the movement of infected hosts to reach new hosts, they tend to be friendly to their current host, because they are interested in keeping this host healthy and mobile, so it can spread the parasite to new hosts. Somewhat benign host–guest relations are therefore expected when transmission to new hosts is impaired by illness. Quite the contrary holds when new hosts can be infected, even if the host is not healthy. This may happen when parasites
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