In the forests of Minnesota, researchers take aim at ticks
CAMP RIPLEY, MINN. — It’s a picture-perfect summer day in the woods of central Minnesota: 71 degrees, humidity about 73 percent, sunshine dappling the trees and glinting off glimpses of the Mississippi River.
But as five scientists pull on white painter suits and start ducttaping the cuffs to their hiking boots, no one is certain if the conditions will be ideal enough to complete their task for the day: catching about 300 ticks, both adults and 150 nymphs.
These Minnesota Health Department researchers are teaming up with scientists from the Mayo Clinic for this “tick drag,” gathering samples to bring back to their labs to add to surveillance records and test for disease pathogens, both of which help determine the risk that black-legged ticks pose to people.
Ticks prefer at least 85 percent humidity, and they tend to come out to feed in slightly hotter conditions. And weather aside, tick collection is never predictable, says Jenna Bjork, an epidemiologist with the department’s vector-borne diseases unit.
Fingers figuratively crossed, the scientists gather the supplies they’ll need for the day: reading glasses (to see the sesame-seed-size ticks and poppy-seedsize nymphs), fine-tip tweezers (to grip their heads), vials (to preserve them) and the “drags” themselves — large pieces of white canvas attached to dowels with rope. Then they head into the woods.
It’s prime time for illnesses carried by ticks. A recent Centers for Disease Control and Prevention report, which found that the annual incidence of vector-borne diseases tripled from 2004 to 2016, generated a new level of attention for Lyme and other tickborne diseases.
The report also criticized vector-control organizations — state health departments and mosquito-control districts — noting that about 80 percent “lack critical prevention and control capacities” for mosquitoes. The percentage is probably higher for ticks, because few state agencies or vector-control organizations do any surveillance or control work related to ticks, said Rebecca Eisen, a research biologist in the division of vector-borne diseases at the CDC.
That leaves yawning gaps in data and unanswered questions: How high is the risk to the average person? Who is most at risk? When? Where? What are you exposing yourself to when you go for a walk through the woods? In most places, the honest answers are “We don’t know.”
“Field work gives us a chance to really speak from experience,” says Dave Neitzel, supervisor — or Tick Boss, according to the patch on the fanny pack that holds his tick-dragging supplies — of the Minnesota department’s vector-borne diseases unit. “We know what’s going on because we’ve been out there.”
Each year, about 30,000 cases of Lyme disease are reported to the CDC by state health departments and the District of Columbia, although many are unreported and the CDC estimates the actual number at about 300,000 or more. Minnesota reported 1,300 cases in 2016.
The work in Minnesota is unique because few places have the resources that the Minnesota Health Department enjoys, employing staffers twice a week during tick season to do tick drags across the state.
“My colleagues that are only doing human case surveillance for tick-borne and mosquito-borne diseases are missing the whole field-ecology side of the disease-risk picture,” Neitzel said. That can lead to inaccuracies.
Minnesota is on the leading edge of tick habitat in the United States; Neitzel, who has been doing tick drags here since about 1989, has watched the slow but steady establishment of black-legged ticks into most of the state.
The old lore was to watch out for ticks and the bull’s-eye rash if you were at your cabin in east-central Minnesota from mid-May to July. Now, the message from the Minnesota Health Department is simpler: Anytime you’re in the woods anywhere in the state when the temperature is above freezing, take precautions, but especially from mid-May to mid-July.
Accuracy is one reason the CDC is developing surveillance guidelines for states, Eisen says.
“State and local health departments know their areas better than anyone else,” she says. “As much boots-on-the-ground work” that they can do, the better for everyone, she says.
Fortunately, the Minnesota researchers generally enjoy the ground work. At Camp Ripley, they drag their white cloths for 20 meters (about 65 feet), then stop and search for tiny smudges on the canvas. When ticks are ready for a blood meal (they take two or three over the course of their two-year life), they crawl to the tips of grass stems to more easily latch onto a host, making it easy for the cloth to catch them. But for every 100-meter transect, the researchers are finding no more than five ticks.
“We’re having to work for ‘em today,” Neitzel says. Then he brightens. “Here’s a nice little nymph. That’s what I like to see,” he says in triumph as he picks three nymphs off his cloth. “That’s more like the Camp Ripley I know and love.”