What Makes Them Tick

Researchers in the college investigate tick behavior and molecular biology with a goal of preventing tickborne illnesses.

Erika Machtinger doesn't mind having a few ticks on her desk. She keeps them there--confined to a covered lab dish--to observe their behavior.

Machtinger, assistant professor of entomology, is interested in sustainable biological control methods for ticks. "I want to learn more about the ecology and behavior of ticks so we can figure out what causes them to do what they do," she says. "We know they respond to heat and chemicals released from the skin, but why do they choose a particular person? Can we avoid that selection process? I'm interested in what odors attract ticks. Can we use pheromones as attractants or repellents, either to get them to go where we want or to get them off people?"

Erika Machtinger

So in informal experiments, Machtinger puts a cotton ball that ticks have been sitting on for several weeks in a lab dish. Then she puts other ticks in with the cotton ball to see whether they'll be attracted to tick pheromones on the cotton ball. Watching what they do could shed light on the pheromone question.

According to Gary Felton, professor and head of the Department of Entomology, this work is especially important in Pennsylvania, which is ground zero for many tickborne diseases. "If you look at the maps from the Centers for Disease Control and Prevention, Pennsylvania is the heartland for disease outbreaks of Lyme disease and other tick-transmitted diseases," he says. "We are at growing risk not only for Lyme disease, but for other serious diseases, such as anaplasmosis, spotted fevers, tularemia, ehrlichiosis, and Powassan virus disease."

In conjunction with the growing incidence of these diseases, new tick species inhabiting Pennsylvania, such as the Asian longhorned tick, will bring new risks to livestock, pets, and humans, Felton adds. "The factors contributing to the rise of tick populations and tickborne diseases are poorly understood," he says. "Research is desperately needed to understand these factors and to develop effective management strategies to minimize tick populations and the diseases they spread."

Tick Control

Machtinger's work directly addresses these problems. She and her colleagues are looking at individual and combined efforts to reduce either the number of ticks in the environment or the infection status of mice, which is one of the most important hosts. Both of these efforts would reduce tickborne disease.

To determine large-scale methods to reduce ticks and the diseases they transmit, Machtinger has been working on a project in collaboration with the USDA focusing on tick control methods for suburban schools in Maryland that have athletic fields or playgrounds bordering wooded areas. The goal of the research is to develop a plan that schools can implement to reduce the numbers of ticks in their area.

Although the research focuses on all species of ticks--there are 26 known species in Pennsylvania--Machtinger is paying special attention to the blacklegged tick. Also known as the deer tick because it was thought that deer were the primary host, the blacklegged tick is the vector of Lyme disease. In Maryland, the researchers trapped mice adjacent to six schools and determined the mice's infection status for Borrelia burgdorferi, the bacterium that causes Lyme disease. They found that among the various school areas, 30 to 80 percent of the mice were infected.

"We don't like those numbers because these areas are next to kids," she says. "We can never get rid of all the ticks, but with tick control methods we can envision a safety bubble around areas where people work and play--and we continue to look for ways to create those safe areas."

Current landscape-level methods for tick control rely mostly on chemical treatments that target the two primary host animals that ticks feed on: deer and mice. One method is a baiting station for deer, developed by the USDA. The station features cotton rollers treated with the pesticide permethrin. As deer come to eat from a corn-filled trough in the baiting station, they have to tilt their heads to get to the corn. As they do, they rub their heads on the rollers, essentially treating themselves for ticks. The other chemical method is a baiting device for mice. Mice attracted to food inside the device must pass through a wick treated with a pesticide that kills ticks that bite the mice.

Although these host-targeted methods have proved effective in some cases, Machtinger wants to explore landscape-level biological control methods to reduce pesticide use and wildlife baiting. One chemical alternative that shows promise is a naturally occurring soil fungus that kills ticks several days after they come in contact with it. It's effective in killing ticks but doesn't harm nontarget insect populations, such as pollinators.

Biological control methods will likely become more important as ticks develop pesticide resistance. Ticks have a much longer life cycle than insects--two years as opposed to two to four weeks for houseflies, for example--and therefore develop resistance more slowly because they're not creating many generations every year. However, with constant applications of pesticides in the environment, Machtinger believes it's likely we'll begin to see pesticide resistance in tick populations. In fact, resistance to some compounds has already been seen in brown dog ticks in the southern United States.

Another recent project involved determining the infection status of rodents that were captured in exterior habitats versus interior habitats. "We want to see how the immune status of mice with higher or lower tick burdens are affected," says Machtinger, "and whether the mouse host itself and its immune function influences tick attraction."

Tracking Ticks

While Machtinger studies how hosts and parasites interact, Joyce Sakamoto, assistant research professor of entomology, assesses populations of blacklegged and other tick species, tracking where they've been and where they're going.

Joyce Sakamoto

Sakamoto and her colleagues recently completed a historical documentation of tick populations, cataloging 117 years' worth of data on ticks submitted from across Pennsylvania. In the 1960s, the late Robert Snetsinger, who retired in 1999 as professor emeritus of entomology, put out a statewide call for residents to submit tick samples. "He launched a campaign," Sakamoto says. "He used radio, TV, newspaper ads, asking everyone to submit samples, and he got several thousand. We've compiled all that data, and it's ready to be published."

Historical information on ticks is important because the past may provide clues to predicting future tick behavior, Sakamoto says. "What's happening right now seems like the most important thing ever, but historical context can be valuable. Early in the 1900s, for example, we know the population of blacklegged ticks crashed because of massive deforestation. The forests were decimated, along with all the wildlife that depended on them. But over time, with the introduction of reforestation, and more deer, there was a slow recovery. Observing this process can help give us the big picture."

As well as the historical study--called passive surveillance--Sakamoto uses active surveillance to monitor current blacklegged tick populations: going into brushy, wooded, and grassy areas and collecting ticks with a specially designed drag cloth. "The blacklegged tick is an ambush hunter," Sakamoto says. "They will climb onto vegetation and wait for a suitable host. They have their arms out--you can see them. When you're dragging this cloth, they are waiting for vibrations and when they sense it they grab hold. From the tick's point of view, if it's making that much noise it must be food."

Adult ticks thrive on large animals, especially deer, that allow them to feed and produce offspring. Female ticks drop off their hosts to lay eggs on the ground. The eggs hatch into larvae that feed on mice and other small mammals such as voles, shrews, and chipmunks throughout the summer and into early fall. Larvae then become inactive until spring, when they molt into nymphs, which also feed on small mammals through late spring and summer. Nymphs emerge as adults in the fall.

It's typically during the larval and nymphal stages of the life cycle when ticks become infected with Lyme disease bacteria as they feed on small animals that are infected with Borrelia. The bacteria remain in the tick throughout its life cycle, and infected nymphs and adult ticks then bite and transmit the bacteria to other small rodents, larger animals, and humans.

Sakamoto notes that tick populations are expanding into habitats where they weren't found before. In the United States, populations of several tick species are expanding northward and westward on the East Coast (and eastward from the West Coast). And they're moving around worldwide as well, being transported globally by animal or human hosts. For example, last November, a tick species native to Asia was reported in the eastern United States. First discovered on a sheep in New Jersey, the Asian longhorned tick spread to at least six other states, including Pennsylvania.

"We don't know how this species will adapt to North American conditions and animals and which hosts it will select, so it's important we remain vigilant," she says. "Rather than focusing on preventing the spread of ticks, we need to find the best methods to deal with them."

Preventing Disease

In conjunction with landscape- and population-level tick studies, college researchers conduct basic science at the microscopic level to develop tools to manipulate ticks at the genetic level. Once developed, these techniques could potentially be used to eliminate a molecule within a tick that would prevent it from becoming infected by a bacterium. Jason Rasgon, professor of disease epidemiology, and his colleagues are modifying a gene-editing technique called CRISPR to more easily manipulate the genomes of ticks and other organisms.

Jason Rasgon

CRISPR (which stands for Clustered Regularly Interspaced Short Palindromic Repeats) is a tool that enables either specific elimination or insertion of genetic material within an organism's genome. But it's been problematic to use this tool in ticks. "Right now, genetic engineering by CRISPR involves injecting genetic material into the eggs of an organism, and with ticks, it's really hard," Rasgon explains. "Their eggs don't take well to being poked with a needle, so no one has been able to do genetic engineering with ticks by injecting eggs."

To overcome the problem of injecting eggs, Rasgon's group has developed CRISPR technology that will allow them to inject gene-editing material directly into the pregnant female. "Then we can edit those eggs inside the tick's body before she lays them," he says. "We identified small protein molecules that will specifically bind to receptors on the developing eggs, which lets us target the gene-editing materials directly to those developing eggs. The female tick will then lay eggs that develop into genetically modified offspring. We're excited about this technology, which has important implications for research on pathogens that infect ticks, but can also be applied to lot of other organisms."

A Quick Look at Lyme Disease

Tick populations are expanding into habitats where they weren't found before. In the United States, populations of several tick species are expanding northward and westward on the east coast.

Pennsylvania leads the nation in the number of confirmed cases of Lyme disease. About 9,000 cases were reported in the state last year, but the Centers for Disease Control estimates that, when unreported and undiagnosed cases are included, that number should be multiplied by about 10. First identified in the 1970s in the United States near Lyme, Connecticut, Lyme disease is one of the most commonly reported tickborne diseases.

Blacklegged ticks, also called deer ticks, are found in all Pennsylvania counties and are the only known vectors of Lyme disease. The Borrelia burgdorferi bacterium was identified as the cause of the disease in the early 1980s through DNA analysis of rodents. Blacklegged ticks pass the bacterium from small animals to humans. Between 40 and 60 percent of blacklegged ticks tested carry the Lyme disease pathogen, though that doesn't necessarily mean all of them can transmit the disease.

Adult blacklegged ticks can be active from fall through spring if temperatures are warmer than 28°F. Ticks in the nymphal (immature) stages are active in May, June, and July and attach to mice, chipmunks, birds, and other small animals. Adults typically attach to white-tailed deer or other large mammals.

The larval and nymphal stages of the blacklegged tick are about the size of a pinhead, and adults are slightly larger. Ticks most often transmit Lyme disease to humans during the nymphal stages--most likely because nymphs are small enough to go unnoticed on a person's body, thus they have more time to feed and transmit the infection before they are detected.

Although Lyme disease is the primary concern for Pennsylvanians because of the large blacklegged tick population, blacklegged ticks can transmit other pathogens as well. "A tick bites you and you automatically think Lyme disease, but other pathogens may be co-infecting people," says entomologist Erika Machtinger. "That can make diagnosis and treatment difficult."

That's why careful symptom monitoring is important after a tick bite. Lyme disease can cause symptoms such as rash, fever, stiff neck, muscle aches, and headaches. It can usually be treated with antibiotics, but if left untreated the disease can cause worse symptoms such as paralysis, meningitis, or cognitive disruptions such as memory loss or mood changes.

By Krista Weidner