How intrusion in animal habitats stirs up viruses
When the novel coronavirus made its way late last year from a bat to a human in Wuhan, China, as many scientists think it did, it was just the latest virus to make such a jump.
Diseases that come from animals, which include SARS, Ebola and HIV/AIDS — some of the most serious health problems the world has faced — are on the rise. Three of four emerging infectious diseases are now animalborne, according to the Centers for Disease Control and Prevention. A United Nations report estimates that one such disease pops up every four months.
A growing body of research is tying the increase to society’s unrelenting intrusion into the planet’s wild places. It’s a disruption that is reducing biodiversity and the health of natural ecosystems, and in doing so, stirring up and mutating deadly viruses. The latest study to draw the connection, published Tuesday by researchers at UC Davis, finds that the animals passing along the viruses are indeed those that humans are exploiting or encroaching upon, through hunting, trade or simply moving into their habitat.
“Viruses don’t cross over the species boundary very easily,” said Chris K. Johnson, professor of epidemiology at the UC Davis School of Veterinary Medicine and lead author of the new study. “The spillover from animals to humans is the direct result of our actions. It’s happening all over the world, and has for decades, as we modify the landscape in major ways.”
Johnson and her colleagues at UC Davis and the Melbourne Veterinary School in Australia examined existing scientific literature on infectious disease to evaluate which of thousands of species of mammals have been most responsible for passing along 142 viruses spread from animals to humans.
While domestic animals have shared the most viruses, with pigs, cattle, horses, sheep and dogs topping the list, the researchers found that when they accounted for population and proximity to people, primates, bats and rodents also were among the biggest sources.
A common thread they discovered among the animals with high risk of transmitting viruses is that most tolerate and even flourish around human activity. On the flip side, the researchers found that rarer animals shared very few viruses with humans.
The notable exception, though, is animals that are rare yet frequently come into contact with humans, perhaps being nudged out of a forest or hunted down and traded. When this was the case, the animal was found to pass along more viruses.
“Humans in close contact with wildlife is affording this opportunity for transmission,” Johnson said. “The spillover is causing some pretty major epidemics, and now that we’re connected globally, the risk is much bigger for all of us.”
Johnson's work was published in the journal Proceedings of the Royal Society B.
While rare, viruses cross from animals to humans in pretty much the same way people pass along viruses — through the air, through feces, through bodily fluids.
Rabies, the plague and dengue fever are believed to have made the leap centuries ago. HIV/AIDS is thought to have been spread from a chimpanzee to a person in the mid1900s. Ebola surfaced in the 1970s, likely from a bat, and SARS originated in the early 2000s, also likely from a bat.
The science community is still trying to figure out where the new coronavirus, SARSCoV2, came from and how humans were infected. The foremost theory is that it spread from a horseshoe bat, perhaps by way of another animal such as a pangolin, which is commonly sold in China for its meat and scales.
The first known death from COVID19, the disease caused by the virus, was a man who had shopped at a market in Wuhan. Hundreds in the region have since died.
Whatever shakes out as the source of the coronavirus, there’s little doubt that it’s related to man’s footprint on the environment, scientists say.
The environmental arm of the United Nations called on its members this month to recognize the relationship between humans and their natural surroundings. The agency said a combination of population growth and a reduction in wild places and biodiversity was creating “unprecedented” opportunity for pathogens to pass between animals and people.
“We are intimately connected with nature, whether we like it or not,” said Inger Anderson, executive director of the United Nations Environment Programme, in a statement. “If we don’t take care of nature, we can’t take care of ourselves.”
Rick Ostfeld, senior scientist at the Cary Institute of Ecosystem Studies in Millbrook, N.Y., who also studies viral spread between animals and humans, says critters with higher transmission risk tend to be not only those that are better adjusted to human activity but smaller.
“The species that are super resilient — the ones that don’t disappear when we’re destroying their habitat and often thrive — are little things like rats, mice and bats,” he said. “We’re destroying the bigger and protected animals, and we’re favoring these little nasties that carry disease and transmit it to us.”
Many of the small creatures carry viruses because their lives are short and their immune systems aren’t programmed to fight them off, Ostfeld said.
Ostfeld agrees that a better understanding of viral transmissions between animals and people can go a long way to reducing human infection and pandemics like the world is now experiencing.
“Lucky for us, human behavior is possible to manage,” he said. “If we can identify the behavior (behind the spread), we have mechanisms, called laws, that could prevent these things from happening.”