Every year, at least 30,000 people — and possibly 10 times that — are infected with the bacterium that causes Lyme disease, most in the Northeast and upper Midwest. Symptoms can include fatigue, joint pain, memory problems and even temporary paralysis. In a small minority of cases, the malaise can persist for many months.
So it’s worrisome that in recent decades, Lyme cases have surged, nearly quadrupling in Michigan and increasing more than tenfold in Virginia. It’s now the “single greatest vector-borne disease in the United States,” Danielle Buttke, an epidemiologist with the National Park Service in Fort Collins, Colo., told me, and it’s “expanding on a really epic scale.”
What’s behind the rise of Lyme? Many wildlife biologists suspect that it is partly driven by an out-of-whack ecosystem.
Lyme disease is transmitted by bites from ticks that carry the Lyme-causing bacterium, Borrelia burgdorferi. Ticks get it from the animals they feed on, primarily mice and chipmunks. And rodents thrive in the fragmented, disturbed landscapes that, thanks to human activity, now characterize large sections of the Northeast.
They would help keep down the number of deer, which, although they don’t carry the Lyme-causing bacterium, probably encourage its transmission.
A largely unsung conservation triumph in the Northeast is the regrowth of its forests. New England has more trees than at any time since the Colonial period. But the forests should have a robust understory — grasses, shrubs and other plants. In the Northeast, abundant deer have depleted this ground cover. Taal Levi, a biologist at Oregon State University, speculates that the diminished understory has limited the recovery of some small predators from the weasel family that hunt rodents. Dr. Levi suspects that more deer may have meant fewer rodent hunters, and more rodents.
Wolves and cougars may also control one predator that has settled in the Northeast over the past century — and that counterintuitively may have worsened the Lyme problem. The coywolf, as it’s sometimes called, is a hybrid coyote that’s one-quarter wolf, two-thirds Western coyote and also part dog. Dr. Levi thinks that this Eastern coyote has exacerbated the Lyme problem by killing or scaring off another rodent predator — the fox — without reducing the deer population.
As with much of ecology, these connections are hypothetical. And predators are not the only important factor; overall diversity matters, too. Consider the fastidious opossum, which, because it grooms obsessively and is expert at removing ticks, functions as a tick death trap. More opossums in the ecosystem might mean fewer ticks, reducing the chance of getting bitten by one that carries Lyme.
But there is evidence to support the predator theory. On California’s Channel Islands, off the coast of Santa Barbara, scientists found that, once variation in rainfall and island size was accounted for, those islands with the greatest number of predator species had the lowest prevalence of hantavirus, a nasty rodent-borne disease that kills 36 percent of the people it infects. “Predators can really regulate infectious disease, and actually protect us,” Dr. Buttke said.
As it happens, after more than a century, large predators are already trying to return to the Northeast on their own. In recent years, a number of wild wolves have been killed in the Northeast, said Linda Rutledge, a research associate at Princeton. They were probably migrants from Canada or around the Upper Great Lakes, and were most likely mistaken for coyotes. And in 2011, a cougar was hit by a car and killed in Connecticut. The animal had traveled from South Dakota’s Black Hills, some 2,000 miles away.
Wolves need uninterrupted space, so Dr. Levi thinks that, even if they established a breeding population, they’d be restricted to remote areas like the Adirondacks. But cougars are another matter. In theory, they could live on the outskirts of New York City, as they do around Los Angeles and other Western cities. “I think the best chance to make Lyme disease go away would be recolonization by cougars,” Dr. Levi told me.
The question is: Will people allow it?
Over the years, scores of eastward-bound cougars have been killed in the Midwest, including in Chicago proper. Large cats are scary; long ago, they probably routinely ate people, said William Stolzenburg, author of “Heart of a Lion,” a book about that Connecticut cougar. And when attacks occur today — like a recent case in which a Colorado mother wrestled her 5-year-old son from a cougar’s jaws — they make headlines. But fewer than 30 people have lost their lives to cougar attacks in the past 125 years, Mr. Stolzenburg pointed out. “These are just animals trying to reclaim the half of the country we drove them out of,” he said. “We ought to make a little bit of room for them.”
The argument isn’t just sentimental. Accommodating these animals might improve human health and save money. A recent analysis by scientists at the University of Washington concluded that, by controlling the deer population, cougars in South Dakota’s Black Hills prevented $1.1 million in collision damage annually. If cougars recolonized the East over 30 years, the scientists calculated, collisions with deer might decline by nearly one-quarter, preventing 21,000 injuries and 155 deaths, and saving $2.13 billion.
As far as I’m aware, no one has crunched the numbers for Lyme, but the same principle applies. How many days spent ill with Lyme disease might cougars prevent? How much suffering?
The relationship between the health of ecosystems and humans extends beyond Lyme. Over 60 percent of emerging infectious diseases, including Ebola, SARS, the Nipah virus and hantaviruses, originate in animals. The major killers of history — smallpox, measles and the plague — also came from animals. Yet the emergence of these zoonotic diseases seems to have accelerated.
A rapidly growing human population pushing into former wilderness might explain this uptick. Another possibility, though, is that many places we think of as wild — like the Northeastern forests — aren’t as wild as they could be. And degraded ecosystems may harbor more pathogens.
The first animals to go are usually the large predators. The last ones standing are often small rodents, bats and their ilk — the very animals that serve as reservoirs of disease. It’s true that large predators can take livestock, eat pets and even occasionally attack people. But, by preventing disease, they may ultimately help far more of us than they harm.