Invasion of the Pathogens

How microbes, and invasive species spreading them, threaten the great outdoors

By Jennifer S. Holland

August 21, 2018


An Aedes albopictus mosquito | Photo courtesy of James Gathany 

While camping in Virginia recently, I found I couldn’t quite relax in the great outdoors the way I used to. I gazed at the tangled green flora beneath the trees, wondering which plant species were invasive. I squinted at my body’s landscape, searching for ticks. I glanced nervously up at the clouds of mosquitoes, my neck and ankles suddenly itchy.

I know too much—that cases of insect-borne diseases like Lyme are fast rising, aided by the spread of non-native hosts. It can be tempting to stay within tent walls or even on the couch at home.

I’m not alone. Many of the hard-core outdoorsy people I know say they now eschew certain places they used to go because of ticks, or they skip activities entirely to avoid risk. “Instead of running or biking these lovely woodland paths right outside our door, we drive to cleared locations,” a friend whose dogs have had Lyme disease told me. Another “won’t go near the woods.” A third recommends sailing as a relatively bug-bite-free activity. And so on. 

The official stats only stoke our fears: According to the latest report from the Centers for Disease Control and Prevention (CDC), insect- and other vector-borne-disease cases in the U.S. have nearly tripled in the last dozen years. Between 2004 and 2016, bites from mosquitoes, ticks, and fleas led to a reported 642,602 human cases of bacterial, viral, and parasite-related illness—with Lyme accounting for 82 percent of those cases. Importantly, nine of the diseases had never been reported in the United States previously. “Those are my biggest concern,” says CDC epidemiologist and report author Ronald Rosenberg, noting that seven of nine are carried by the seemingly ubiquitous tick. 

Why are vector-borne diseases on the rise? Lots of reasons, including the growth of global travel and international trade—which ferry diseases and their hosts across oceans—along with climate change and human alteration of habitats, which together affect everything from hydrology to wildlife composition to fire susceptibility. The effects of invasive species round out the picture of why ecosystems might be vulnerable to microbial outbreak. 

In fact, invasive species are an underappreciated factor in the emerging disease epidemic. Both the disease carriers that come from afar—say, a mosquito from Asia now in New York or a tick-ferrying rodent that’s invaded Hawaii—and the pathogens themselves that weren’t in the U.S. before, like the Zika virus, fit the official invasive species definition: a species that is non-native (or alien) to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health. Invasive species can themselves make us sick, plus they reduce the natural biodiversity that helps keep ecosystems healthy—including in wildlands and parks where people recreate—and that can give diseases an easier foothold in those special places.

The mosquito is a prime example of how an invasive species can introduce invasive pathogens. One of the world’s deadliest animals, it kills some 725,000 people a year via blood-borne disease. Two U.S. invaders, Aedes aegypti—which originated in Africa before spreading to tropical, subtropical, and temperate regions—and Aedes albopictus, the Asian tiger mosquito brought to the U.S. via trade and international travel, are the primary vectors for dengue, chikungunya, yellow fever, and Zika viruses, all now present in the United States. 

Ticks, too, are facilitating the spread of new diseases. In August last year an exotic species, H. longicornis (the long-horned tick), normally found in East Asia, Australia, and New Zealand, showed up in New Jersey—all over a local farmer. Although not yet reported to carry Lyme, it can transmit a number of other dangerous pathogens. “You never know how an invasive species will do in a new environment,” notes Pennsylvania Game Commission wildlife veterinarian Justin Brown. “Often they thrive unexpectedly. There’s a whole laundry list of diseases that we’re concerned about because of the potential of [this tick] serving as competent vector.”

The invasion of pathogens “is a complex problem,” notes Leigh Greenwood of the Nature Conservancy. “The life cycles of these parasites can be very Rube Goldberg,” she says, referring to the myriad stages and hosts that might be involved.  

Once an invasive species proliferates, it can have a cascade of effects; the European spotted knapweed is a case in point. Back in 2006, it was spreading like a virus across the American West. In Montana alone—where nature lovers spend $7.1 billion annually on outdoor recreation (a value that exceeds that of the state’s agriculture and livestock)—the spindly, pink-flowered plant had overtaken some 4 million acres of hill- and mountainside before biologists decided to import a natural foe of the weed, the gall fly, to control it. (The flies lay eggs inside the seed head of the plant; when the eggs hatch, the larvae eat the seeds.)

The flies failed to knock back the weeds. Worse, with a new winter supply of nourishing fly larvae, deer mice tripled in number, as did a nasty pathogen the mice carried. Hantavirus, which spreads via mouse urine and droppings, can cause a sometimes-deadly pneumonia-like disease in people. It killed three in 2012 in Yosemite National Park (others survived infection), prompting the closure of a rustic campground. Cases have been reported in 36 states, and more than a third of infected people have died.

Similarly, in Connecticut woodlands, invasive Japanese barberry, a red-fruited garden import now thriving in at least 31 states, supports a 10-fold increase in Lyme-carrying ticks. Not only is the plant perfect habitat for the insects, it also draws both deer and mice, efficient vectors for moving tick nymphs around. (The ticks also spread bacterial nasties like granulocytic anaplasmosis and babesiosis.) “It’s called invasional meltdown,” explains entomologist Michael Raupp of the University of Maryland, “when the presence of one invasive species contributes to the decline of an ecosystem by benefiting [other] invasive species.” When disease vectors are thrown into the mix, “it can really spell trouble.”

The same phenomenon occurs with Amur honeysuckle from Asia, which both bullies native plants and attracts white-tailed deer. These deer host the invasive lone-star tick, which spreads an emerging human bacterial disease called ehrlichiosis (and also a sugar that can cause a severe red meat allergy). Surveys in Missouri woodlands—favorites of hunters, birders, and hikers—found five times more deer where invasive honeysuckle was prevalent, translating to 10 times as many ticks infected by ehrlichiosis-causing bacteria than in woods without honeysuckle. 

Some of the most prolific invaders are themselves health hazards, like the noxious giant hogweed, a big-flowered transplant from Asia with a sap that causes blistering burns. But while we can learn to ID and sidestep harmful plants, ticks and other arthropods patiently wait for he who follows a bird off the trail or she who hunts thick in the weeds. Various flies, chiggers, roaches, lice, fleas, midges, and assassin bugs can also spread disease. Tropical sandflies that carry Leishmaniasis have shown up in Texas, for example. With the changing climate, experts say, by 2080 the flies’ range could stretch into Canada. 

These are all legitimate concerns. Especially when served up together, they make a strong case for taking up knitting.  

But let’s take a deep breath and a giant step back before stashing away our hiking boots. Because in reality, going out into nature is still one of the best things we can do for our health. And the risks of doing so, though they may seem great if you get caught up in outbreak news and government reports, are actually very small.

“Fear of the unknown drives panic around vector-borne disease,” says veterinary epidemiologist Danielle Buttke of the National Park Service. “And in truth, most people encounter these diseases in the media, not in nature.” Even with risk of the latter, “diseases are both rare and preventable,” she says, particularly those borne by ticks. “Not every tick is [a carrier], and there are simple steps we can take to protect ourselves,” especially making sure the parasites, if they do glom on, don’t have time to dig in for a meal. “They really need 36 or more hours to actually transmit disease,” she says.

Importantly, she adds, “when we keep ecosystems natural and healthy, we see less [disease].” For example, a robust population of natural predators keeps in check sick or weak individuals that might be heavy with parasites. Similarly, when invasive species are kept out, disease vectors find fewer hosts in new areas. Nature, when left to her own devices, she says, “helps stop the spread.”

The outdoor recreation industry—including hunting, fishing, boating, birdwatching, and hiking—remains a giant, contributing about 2 percent of the GDP in 2016, or $374 billion. Still, outdoor-based activity has been declining in recent decades.

Disease aside, there is already a very real phenomenon experts call “biophobia,” an unreasonable fear of the outdoor environment, that’s risen up in recent decades—especially in children. Even kids who express interest in nature conservation often report never having fished, hunted, or even hiked. News of pathogens lurking in the grass won’t help the cause.

This spells bad news both for nature-related businesses and conservation efforts, which need biophiles too. Consider hunters’ contributions. Through state agencies, fees for hunting permits and taxes on gun and ammo sales help pay for habitat protection, wildlife research, and other activities supportive of wild environs. The number of hunters has been trending downward, with fewer young people picking up the sport each year. Worries over disease could very well speed up that decline, further reducing the people and funds that support conservation efforts.

It doesn’t help that disease outbreaks like plague and relapsing fever can force temporary closures of recreation areas—stories that always make national news. 

Of course, “we will always have new viruses infecting humans; it’s just a fact of life,” says CDC’s Rosenberg. Vectors will continue to expand their ranges as we continue to rip down forests, travel, and live with a vacillating climate. New diseases will arise or be introduced. “And native diseases will behave badly,” says TNC’s Greenwood, “as climate change exacerbates the problem,” for example by giving vectors stymied by cold weather access to more northerly climes.

Science may have some answers, and there is progress being made against some of the most hazardous pests. A recent experiment in Australia to reduce disease-carrying A. aegypti mosquitoes (by releasing sterile males into the population), for example, knocked the bad bugs back by more than 80 percent in the study area. Such successes could have global implications down the line.

Ultimately, the health benefits of spending time outdoors far outweigh the real risk of contracting a disease while out there: Both mentally and physically, being in nature is good for us. A stroll through the woods, a hike in the mountains, a dip in the ocean—these can reduce stress, lower blood pressure, and boost our immune systems. Contracting disease is rare, and there are logical ways to reduce the risk, like holding back the flow of invasive species (e.g., by controlling the exotic animal trade) that provide disease routes and giving native habitats the chance to fight their own battles by leaving them whole. Avoiding inadvertently spreading invasive species—by cleaning our gear and not transporting firewood, for example—can also help keep diseases at bay. 

Protecting ourselves when we do venture out is the most obvious response, and one individuals can control. Whatever the pathogen and its hosts, wherever they hail from, the same rules apply that park managers and public health officials have been touting for decades. They include staying out of heavy brush and weeds, wearing protective clothing and appropriate insecticide, and most importantly, as the NPS’s Buttke noted, vigilantly checking ourselves, our kids, and our dogs for ticks and removing them promptly and properly.

“We want people to be aware of the risks, no matter how small, and to take protective measures,” says CDC’s Rosenberg. “What we don’t want is for them to stop going outdoors.” Because the vast majority of the time, venturing into the natural world, rather than endangering us, helps keep us healthy and happy.