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The research team travels to the woods near Middlebury in search of ticks
MIDDLEBURY, Vt. – Biology professor David Allen and his intrepid crew of researchers come out in the morning. in the woods, trolling for ticks.
"I do not like having to handle live ticks – I mean, I'm used to it," says Robert Cbadidy, 19, a major in conservation biology, who admits that by the way hours to search the woods for ticks, "the feeling is very much like schadenfreude, where you get excited when you find ticks … but you know it's good for no one if you find a lot ticks. "
2018 marks the third summer of Allen's continuing study on the ecology of Lyme disease. Allen, a forest ecologist with a strong background in mathematics, conducts fieldwork, lab work, and develops a mathematical model to help badyze, determine, and quantify the factors responsible for the continued increase in transmitted diseases. by ticks.
"Tick-borne diseases are interesting because we have a very complex system where many factors interact: the climate, the host community, the vegetation," Allen said. "And all of these things can affect both the vector population, the tick, but also the persistence of bacteria in the system, so I'm trying to see how abiotic drivers, like climatic and biotic drivers, affect this. vector-borne disease. "
The first step is to count the ticks.
Every morning, Allen and his team dress in the light Once there, the researchers separate, each armed with 39, a white canvas square one meter long mounted on a wooden peg
.Low tech, but it's fast, it's cheap, and it works, "note Allen, of the team's slide-counting technique
Each site is carefully subdivided, marked by small red and yellow flags, and each subplot indexed to the type and density of vegetation. The sites extend from low altitudes to high altitudes es (uses Allen & # 39; elevation as a replacement for the climate) and s & # 39; range from 62 to about 42,000 acres, with large and small high and low altitude sites.
To catch ticks, a researcher slowly and methodically walks a secondary plot, dragging the place of the canvas along the forest floor. Every 10 meters, it stops, spreads the canvas and inspects it closely. Some kneel and peers; some lay the canvas against a tree trunk to catch the light. All find and count each tick. Nymphs and adults are placed in small plastic vials, labeled and brought back to the laboratory for further research. The larvae are stuck on a piece of ribbon and counted.
Allen's technique to distinguish larvae from dirt stains? "Blow on it and see if it crawls." Ticks respond to movement, heat and carbon dioxide.
Afternoons are spent in the lab. Each tick is put under a microscope, pierced in the intestine (where Borrelia burgdorferi resides) with a surgical needle, and tested for bacteria causing Lyme disease. Other tests then reveal which of the 19 strains of Borrelia found in the Northeast, an infected tick was carrier. Many strains can be correlated to specific host animals; some are more virulent for humans than others.
In the field, not any old tick will do the trick. Allen and his team search only Ixodes scapularis (commonly known as blacklegged tick or deer tick), the tick that spreads Lyme disease in the northeastern, central and northern states of Wisconsin . Minnesota
"It takes a bit of time to learn to identify them quickly, but after that, it's pretty simple," said Maisie Anrod & # 19; major biologist
In Vermont, the life cycle Ixodes takes two years to complete. In the first year, the larvae hatch, feed and pupate. In the second year, nymphs feed and moult as adults; the new adults then find a host and a food. Most often, adults mate on the host, males die after mating and females spend the winter laying their eggs the following spring. . . and the cycle starts again. At every stage of life, a tick only feeds once. In Vermont, larvae tend to hatch in late summer. Nymphs – which transmit the most Lyme to humans because they are more difficult to detect – are the most active from early to mid-summer. Adults are the most active in the fall.
Larvae are born free of Borrelia; To be vectors of Lyme disease, ticks must acquire Borrelia host animals. Larvae and nymphs prefer to feed on small animals, such as mice. Adult ticks prefer large mammals, such as deer. Some animals are better Borrelia hosts and transmitters than others. The upper dog in the northeast is the white-footed mouse, Peromyscus leucopus, which is 1.5 times more likely to transmit B. burgdorferi than striped chipmunks, six times more likely than eastern gray squirrels, and a whopping 35 times more than opossums, according to some studies. (Possums, oddly enough, emerge as unlikely heroes in the battle against Lyme, because in addition to low incidence for transporting bacteria, they are also great groomers and therefore much more likely to bring the tick life cycle to a mordant end.)
In 1975, Lyme emerged as the number one public health enemy.
"The intersection of public health and conservation is really interesting," said Harper Baldwin, conservation biology specialist. with the rest of the Allen research team, appreciates the real-world applicability of the Lyme Ecology Project. Evan Fedorov, a future sophomore specializing in molecular biology and biochemistry, noted that he was interested in research on ticks because of his interest in infectious diseases: "Dave's research is so relevant to Vermont and public health, "
Tick-borne diseases, fleas and mosquitoes have tripled since 2004, according to a report published in May 2018 by the Centers for Disease Control and Prevention. Lyme disease is by far the most common, accounting for 82% of all tick-borne diseases and 63% of all vector-borne diseases. While only 30,000 cases of Lyme are reported each year in the United States, the CDC estimates the actual numbers at around 300,000.
Allen and other scientists continue to ask, "Why?"
Possible culprits include a climate and a forest heating fragmentation, resulting in loss of biodiversity. While mice, for example, do well in fragmented ecosystems, many others do not. Fewer foxes or owls mean a lot more mice-more ticks and more diseases
But the picture is far from complete.
Since 1975, Lyme has spread mainly from northern Connecticut to Maine (now upper state for the incidence of Lyme, with Vermont just behind), and Wisconsin and Minnesota. But what part of the seasonal temperature variation had previously limited the range of Ixodes ? Cooler summers? Warmer winters? Both? What is the share of rainfall, humidity and other weather conditions? Last summer, one of the rainiest days ever recorded, Allen counted ticks twice; this year, the numbers have dropped to previous levels. Allen and his team are building "ticking hotels" this summer to see how ticks behave when they are exposed to different levels of moisture: desiccation can mean mortality. And what about the "quest" behavior, the way ticks hang on the vegetation when they come out of the litter, hidden by a host?
The student discovered last year that unlike literature, ticks favored dead leaves. Was it because of the record rain? Allen discovered that ticks do not like ferns. Why? A recent graduate has studied Borrelia in two identical species of mice. Taking mouse ear clips to genetically identify each species, she determined that deer mice had an even higher infection rate than white-footed mice. Similarly, previous studies ignored the mouse doppelganger.
Understanding the Ecological Complexities, Allen said, will give humans better tools to predict, control and prevent the spread of Lyme disease
"We try to distinguish these potential covariates." Allen said, noting that "ecology is a very complicated place to do modeling, very small systems and very large systems," like those studied by biologists, physicists and astronomers, "are more orderly. do not follow the rules often. "
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