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Science & Environment
Mon August 13, 2012
It Takes an Ecosystem
In the first installment of our series, Sean Corcoran reports on researchers' newest understanding of tick ecology -- how they travel, how they live, and how they survive.
Part 1 of 5
There's growing awareness about the dangers of Lyme disease, as it and other tick-borne illnesses become more common. Disagreements about treatment and diagnosis are important and well-documented. But in the fight against Lyme, it's crucial to know where the pathogen lives when it's not inside us, and how it makes its way under our skin.
At Native Earth Teaching Farm in Chilmark, on Martha's Vineyard, there's a secret weapon being used in the fight agains the tick. They're called guinea hens, and they're often called "African pheasants," because of their origins, and "the farmers' watchdog," because of the sounds they make when spooked.
They also can be called big-time consumers of ticks.
"I almost never find a tick when staying in the open areas that they patrol,” Gilbert said, adding that they've had them on the farm for about 15 years. “And there was a couple years in there where we didn't have them, and we could really notice the difference in the ticks, between having them and not having them. I've personally seen a guinea fowl walk past an ant, look at a spider, and then see a tick and go for a tick.”
With tick-borne diseases becoming more prevalent, guinea hens are one of just a handful of biocontrol methods researchers are looking at as they search for innovative ways to take the fight directly to the tick.
And if the Guinea fowl eat ticks as advertised, Barnstable County's entomologist Larry Dapsis is up for giving them a try, though he has his doubts.
"I'd love to run a trial like that," he said. "I'd love to release 10,000 of those in Nickerson State Park and see what happens."
People on Cape Cod say there are more ticks than ever before, and the old-timers say that when they were young, they didn't see any black-legged ticks, more commonly known as deer ticks.
The truth is, no one really knows when black-legged ticks arrived and in what numbers. Scientists only began paying attention to tick numbers on the Cape and Islands after they were well-established and people already were sick.
The longest-running tick research project in the world is underway at the Cary Institute in Millbrook, New York -- research that one day could be used to fight tick-borne diseases on the Cape and Islands.
Peter Rockerman is the lead assistant on what's called the Mouse Project at the Cary Institute. Each morning, Rockerman and his a crew of four researchers put on baggy, white jumpsuits, they tuck their pants in their socks and head into the woods. There, they check rows of safety-deposit-box-sized traps for mice, chipmunks and squirrels -- small mammals known to carry ticks and Lyme Disease.
Rockerman sits on the ground and takes out a scale, some tools and a reinforced plastic freezer bag. He shakes the chipmunk out of the trap and into the bag; he snags it by the scruff of the neck and notes such things as its sex and weight, and he counts how many ticks are on its head and ears. It's the same information that's been collected from trapped animals here for more than 20 years.
"Check it out, right here," he said, "that little spot, right at the tip of the tweezers there just, maybe two millimeters in diameter there, that little circle spot, that's a larva. So we have to pick through their hair and find all those little first stage tick. So this tick is getting its first blood meal right now."
The tick also is likely contracting Lyme disease at the same time. Because one answer to the chicken-and-egg riddle: 'Which came first, the tick or the Lyme Disease?' could be, 'The mouse.'
Ticks are born disease free. They typically acquire Lyme disease from their favorite meal -- the white-footed mouse. Mice are excellent hosts because there are lots of them on the forest floor and they are very poor groomers. Research shows that between 75 and 95 percent of ticks feeding on mice will contract Lyme disease.
"Right now we're sort of between two peak tick seasons," Rockerman says. "The Nymphal peak usually happens from May to June. And we expect to see a larval peak in August."
The fact that researchers have been surveying this same plot of land -- keeping track of temperatures, seed production, the abundance of deer, small mammals, and of course ticks -- for more two decades, means that all of the information can be put together to explain fluctuations in tick numbers and the importance of biodiversity.
Richard Ostfeld is one of the world's leading researchers in Lyme Disease Ecology, and a senior scientists at the Cary Institute. He says that much of what people assume is true about ticks is actually wrong. They don't jump or leap from trees. Even the common name "deer tick" is a misnomer.
"By calling it the 'deer tick'," he said, "you are trying to perpetuate this notion that there's a tight linkage between deer numbers and tick numbers and that all you need to know about Lyme disease risk is how many deer there are, and all you need to do to control Lyme disease is to control the deer herd. And those are ideas that have been largely discredited."
The conventional wisdom about warm winters being good for ticks, and hot, dry summers being bad? Also untrue, Ostfeld said.
"We find very little impact of winter temperature or precipitation or summer temperature and precipitation on tick numbers," he said. "Tick numbers are driven largely by white-footed mice, and secondarily chipmunks that are in the woods."
Back in the woods, Rockermen and his crew set and trap small mammals five days a week. They also check nets that collect seeds that fall from the trees. Tick populations are directly affected by how many acorns and other seeds are available for the mice, chipmunks and shrews.
"The project has been running approximately 20 years," he said, "and it's looking at the interaction between oak seed production -- masting seeds -- the mouse populations and the tick populations. So there's really a cool interaction between all three things."
In years when there are a lot of acorns -- such as in 2010 -- the following year will see a spike in the mouse population, as mice are better able to survive the winter. And that's what happened. After the enormous acorn crop of 2010, 2011 saw a surge in mice numbers. That also meant that when the new baby ticks hatched last summer, they found lots of mice to feed on.
"First year you see a lot of oak seeds, second year you see an increase in the mouse population," Rockerman said. "And then in the following year, two years after a big oak masting, we see a lot of Lyme disease-infected ticks."
That's this year -- 2012. And as predicted, researchers say more nymphal ticks on the ground than usual, and they're monitoring what percentage are infected with Lyme. Researchers expect that the risk of infection will go down next year.
"That's what I'm interested in," Rockerman said. "The whole interaction between seed production, the exploding mice population following the seeding years. And the year after that, the whole big giant spike in infected nymphs. That's cool."
As thinking about tick ecology has evolved, it's become clear that seemingly small ecological changes can bring large, sometimes unexpected results. Ostfeld says that knowing the rolls acorns and animal diversity play in the lives of ticks is paramount to finding ways of stopping them before they reach us.
Science & Environment