‘A reality of the modern world:’ How infections like the coronavirus jump from animals to humans

The risk of zoonotic diseases crossing species is only growing, researchers say, as people move into previously wild areas.
Bats take to the sky at dusk from bat houses overseen by the University of Florida in 2014.
Bats take to the sky at dusk from bat houses overseen by the University of Florida in 2014. [ Times (2014) ]
Published Jul. 6, 2020|Updated Jul. 7, 2020

Among the scientific terms suddenly important in our pandemic lexicon is “zoonotic disease,” or an illness that jumps from animals to humans, such as the coronavirus.

The name may sound strange but the process is fairly common. Many new diseases come from animals, according to the U.S. Centers for Disease Control and Prevention, as do certain strains of the flu and tick-borne Lyme disease. The risk of viruses hopping species, researchers say, is only growing as people move into previously wild areas and move around, exposing a connected world.

“Any time we get closer to wildlife and displace them, that increases our risk of disease,” said Samantha Wisely, a professor in wildlife ecology and conservation at the University of Florida. Not all zoonotic diseases are unknowns like COVID-19. Perhaps the most familiar in Florida, she said, is rabies, which can spread from raccoons and foxes to household pets and people.

The coronavirus, scientists believe, could have originated in horseshoe bats, but it’s not known how it first passed to humans. A suspected culprit is a wildlife market in China, where someone may have touched or come into close contact with an infected animal. It’s possible the coronavirus spread directly from a bat to a person, or it could have passed through an intermediate creature like a pangolin, a scaly mammal that eats ants and is sometimes about the size of a small possum.

In combining species that would never interact in the wild, Wisely said, such a market acts as a “sort of virus laboratory.”

The pandemic has spotlighted not just the risk of people getting close to strange wildlife, said Vineet Menachery, a professor in microbiology and immunology at the University of Texas Medical Branch, but also the way diseases spread rapidly through population centers linked by travel.

“A virus like this may never have made it out of a small village before,” he said. “This is a reality of the modern world.”

Some notable outbreaks of the last two decades have come from zoonotic diseases, including Severe Acute Respiratory Syndrome (SARS), Middle East Respiratory Syndrome (MERS) and Ebola. Scientists describe a disease jumping as a “spillover event.”

Motorists wait in line for a coronavirus test outside Tropicana Field in St. Petersburg on Monday.
Motorists wait in line for a coronavirus test outside Tropicana Field in St. Petersburg on Monday. [ SCOTT KEELER | Times ]

Though COVID-19 is thought to have started in Asia, and other recent illnesses have spread from outside the United States, new diseases could certainly emerge here, Menachery said.

“It could have just as easily have come from wild game or a deer virus or something like that,” he said. “Animals everywhere harbor viruses.”

In a brochure on zoonotic diseases, the Florida Department of Agriculture and Consumer Services warns of dozens of illnesses that spill from animals to humans, including West Nile Virus, which comes from birds and is transmitted by mosquitoes, and brucellosis, which the CDC says can cause fever and pain and might pass through raw milk. The state urges people to wash their hands, clean up animal waste, use bug repellent and vaccinate their pets.

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Climate change, scientists say, is another reason for diseases spreading. As the world warms, researchers believe certain species will move, drawing closer to different populations. Some climate models, for instance, project Florida could one day house vampire bats that transmit rabies and are typically found in Central and South America.

The state is already home to a number of bat species, said Holly Ober, another University of Florida professor in wildlife ecology and conservation, but they are different from the type in which the novel coronavirus might have originated.

Besides, Ober said, bats are not a villain in the pandemic story. Her colleague, Wisely, noted how in Florida they eat prodigious amounts of mosquitoes — which also carry disease — and pollinate plants including mangoes and guavas.

They may spread rabies here, Ober said, and in some cases the buildup of guano (bat waste) supports a fungus that can cause a lung infection known as histoplasmosis. Both professors said people should mostly just avoid touching wildlife or feeding it around their homes. When bats roost in a house, Ober said, “it’s often in a place where forests were recently harvested” and the animals got displaced.

“If we were to do harm to our bats out of concern for coronavirus,” she said, “we might actually cause more harm to health.”

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