These Minnesota Department of Health (MDH) 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 MDH 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-seed-size 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 tick-borne 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, according to 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 MDH vector-borne diseases unit. “We know what’s going on because we’ve been out there.”
The work in Minnesota is unique because few places have the resources that the Minnesota Department of Health 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,” said Neitzel. 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 MDH 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, 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.”
The site, which boasts an extensive tree canopy, hefty shrub layer and leaf litter plus mammals such as mice that are important hosts for ticks and make an environment ideal for the critters, is one of MDH’s long-term monitoring sites. (The intermittent machine-gun fire — the National Guard trains at Camp Ripley — doesn’t seem to scare off the ticks, although Neitzel was once mistaken for the enemy in a training exercise and surrounded at gunpoint.)
“Hi, little tick!” says Bobbi Pritt, director of the Clinical Parasitology Laboratory in Mayo Clinic’s Department of Laboratory Medicine and Pathology, expertly grabbing her seventh tick with tweezers and slipping it into a vial. “I have to be really careful because the nymphs, being the size of a poppy seed, can look like just little dots,” she explains. “But then you look at them and they move — or they have little legs.”
The tick will be diced and its DNA extracted. Any pathogens it is harboring will be revealed.
Borrelia burgdorferi, the main bacterium that causes Lyme disease, is found in about 40 percent of adult ticks and 20 percent of nymphs, although researchers have found rates vary drastically by location. But the CDC report noted that nine new mosquito- and tick-borne germs have been discovered or introduced since 2004, including two discovered by Pritt’s lab that have been found only in the Upper Midwest. Those localized pathogens fall into a category of “non-notifiable” diseases that the CDC doesn’t track but that could affect people.
“I wouldn’t be surprised if more [localized pathogens] continue to be discovered,” says Pritt, possibly explaining cases where people are sick but no one knows why.
“Those are the cases that particularly interest me,” Pritt says. “A lot of my work is involved with trying to figure out what else is out there.”
After returning to her Rochester lab, she tests the ticks from the Camp Ripley drag for Powassan virus in addition to looking for the bacteria that cause Lyme, Ehrlichiosis, babesiosis, anaplasmosis and tick-borne relapsing fever. The Powassan virus, while rare, can be deadly. But Pritt suspects there are more patients who have a milder form of the disease. Pritt is developing a test for it; she was hoping to use some of the ticks from the drag in that work, but none tested positive for the virus.
Back at Camp Ripley, the researchers can shake their vials like snow globes after six hours to get a sense of the results: adult females, distinctive for the orange splotch on their backs, plain dark brown adult males, nymphs and larvae no bigger than the period at the end of this sentence. The official count confirms they’ve met most of their goals: The Mayo researchers captured 12 adult females, 26 adult males, seven nymphs and 128 larvae. The MDH folks collected 103 adults, 36 nymphs and 67 larvae.
The following week, on a tick drag in the northwestern portion of the state, the MDH team found enough ticks to confirm black-legged tick population in six counties. That brings the total number of counties with established tick populations to 54, up from 45 in 2015 — and nine in the 1990s.
The team has put 4,000 miles on a rental car since May 1, and they have another 1,500 to 2,000 to go this season. The summer field work will wind down soon, but “I feel like we’re still near the beginning of our quest for knowledge about ticks and tick-borne disease prevention,” Bjork says.