Bats are in the news right now due to their capacity to host a high diversity of coronaviruses — and Ian Mendenhall ’99 is an expert on both the benefits and dangers they pose.
Current theory holds that the COVID-19 pandemic began when a novel coronavirus spread from bats to another mammal, and then into humans in China’s Wuhan province. Bats are also thought to be the source of the ancestral coronavirus that infected intermediate hosts, like civet cats, and subsequently caused the SARS outbreak in 2002, while MERS, another coronavirus disease, has a natural reservoir in camels, but these were likely infected by a bat coronavirus in the past.
A principal research scientist at Duke–National University of Singapore Medical School, Mendenhall has been studying zoonotic (animal-to-human) transmission of viruses from bats and other small mammals since 2011.
It all began his junior year at UW–Madison, when he took medical entomology with Professor Susan Paskewitz and parasitology with Professor Bruce Christensen. “Those classes opened my eyes to the burden that these agents place on humanity, and I found my calling,” he says. After graduation, he directed a mosquito control lab studying the West Nile virus and then got his PhD at Tulane University’s School of Public Health and Tropical Medicine.
Mendenhall says the danger of zoonotic disease is higher in modern times because we’re seeing more changes to the landscape and more unnatural relationships between wildlife and livestock and humans. We’re also detecting more diseases because of enhanced surveillance and laboratory techniques.
“Deforestation is a primary driver, with annual forest loss being approximately the size of the United Kingdom,” he says. “This results in more opportunities for viruses to infect different types of hosts and for spillover to occur.” Add to that our highly mobile society and the growth of dense urban areas, and it’s a perfect recipe to spread diseases once they have emerged.
Bats in particular are a successful reservoir and vector for disease, he says. “Bats are unique mammals. They are long-lived for their size, roost in very large numbers, are the only mammal capable of true flight, and rarely get sick, even though they are infected with viruses that are very dangerous to other mammals.” One theory about why bats can co-exist with viruses is that their immune system has had to adapt to the metabolic consequences of flight and the resulting inflammation. “It’s really taxing for an animal to fly,” he says. “Bat heart rates during flight can be over 1,000 beats per minute, and their metabolic rate can increase 15 times.” In response, he says, bats developed a robust anti-inflammatory response as well as DNA-repair mechanisms to deal with the generation of toxic compounds such as free radicals. This may also play a role in their response to virus infections.
On the flip side, though, bats can be very beneficial. Take, for instance, their huge appetite for insects such as moths and mosquitoes. Mendenhall says that one study estimated that bats save the United States almost $4 billion dollars a year in pest-control costs. And a Thailand study showed that bats prevent pests from destroying rice meals for more than 26,000 people annually. On top of that, they are important pollinators, responsible for pollinating more than 300 species of fruit. “If you like tequila [made with agave] or durian, you’ve got a bat to thank,” he says.
Mendenhall thinks that we can learn to co-exist with bats by creating policies that mitigate human activities such as deforestation, mining, and wildlife markets that contribute to disease emergence. But, while he advocates a healthy respect for the dangers, he also thinks people should give bats a break.
“Bats are so important to our lives, and most people just don’t recognize that because the media focuses on their role as virus reservoirs,” he says. “They are incredible animals that play a vital role in our ecosystems.”