Fighting Tick-Borne Disease with Computer Science

Ecologists Tempest McCabe and Michael Dietze develop computational tools to study the complex interactions between plants, animals, and climate. They want to find ways to predict ecological changes, such as fluctuations in tick populations, which could affect human health. Photo by Jackie Ricciardi

Lone star ticks, prevalent in the southeastern US, have “unusually long mouthparts,” according to military records from the 1940s. They produced painful bites that left soldiers scarred and, in some cases, with unhealed wounds for as long as four years after removal of the tick.

These ticks bring other dangers to soldiers as well. They carry tularemia, also called rabbit fever; it’s rare but can be deadly. They also carry ehrlichiosis, which causes fatigue and aches. Between 2000 and 2008, the number of cases of ehrlichiosis in the US more than quadrupled, from 200 per year to 961, with most cases in the Southeast, according to the Centers for Disease Control and Prevention. As with Lyme disease, common in the Northeast, these bacterial infections can be treated with antibiotics, if detected early, or prevented by checking for ticks after outside activities.

But for US military soldiers on southeastern bases, who spend hours crawling on their bellies, training in the woods and grasses where ticks live, such precautions may not be so practical. “As someone who did fieldwork in the Southeast, the thought of doing army drills in that habitat makes me cringe,” says Michael Dietze, a Boston University associate professor of Earth and environment.

The military takes the threat of tick-borne disease seriously and already manages the land to keep ticks in check. But climate change and an influx of invasive grasses are altering the landscape in ways that could make existing management techniques, such as controlled fires, more risky. So Dietze, an expert in predictive forecasting of ecosystem changes, and colleagues are using a five-year, $2.45 million grant from the Strategic Environmental Research and Development Program, which is funded by the US Department of Defense, Department of Energy, and Environmental Protection Agency, to learn more about how southeastern pine forests—and the ticks that live in them—are changing.

The interdisciplinary team, including Dietze and experts who study grasses and ticks, will venture into the field to figure out how climate change, invasive grasses, and fires are affecting habitat, and, in turn, how habitat is influencing ticks and tick-borne diseases. Using that information, they will build a predictive tool that will help land managers on military bases decide when and how to keep ticks in check.

“Military land managers need more information about how to respond as these landscape changes interact and feed back on one another,” says Dietze. “They want to know how to get ahead of this problem.”

Currently, the US military uses fire to manage the landscape on army bases. Controlled burns preserve pine trees while curbing the growth of unwanted vegetation. A possible side benefit of fire and other land management tools, such as weeding and herbicides, is that they kill the grasses ticks live in and drive away the deer that carry them.

But climate changes and invasive grasses are causing hotter burns that are harder to control. To understand how fire, climate, and invasive grasses are interacting, a team of ecologists led by Luke Flory, a plant ecologist at the University of Florida, will visit over a dozen pine forests on military bases in the Southeast with different histories of fire, plant invasion, and other disturbances, such as drought. In these sites, scattered across Louisiana, Mississippi, South Carolina, Georgia, Alabama, and Florida, the team will gather data about populations of native and invasive grasses, white-tailed deer, and ticks.

To collect and count ticks, they’ll attract them with carbon dioxide—the ticks think it’s coming from a potential host—and trap them with sticky tape. Since the researchers know how far a tick travels in a given amount of time, the method gives a pretty good measure of tick density in the area. They’ll also collect ticks the old-fashioned way. “We just drag a piece of white cloth through a grassy area and the ticks jump onto it,” says Flory.

The ticks get sent off to the lab of principal investigator Brian Allan, an entomologist at the University of Illinois. Allan’s team will assess the relationship between ticks and disease-causing pathogens. One question is whether changes in climate and habitat will alter disease levels. “We’re concerned about the potential impacts of climate change on human risk of exposure to diseases, including those transmitted by ticks,” says Allan.

Each piece of data will be linked back to the landscape’s fire history, climate trends, and plant and wildlife composition. “We have to connect all of these pieces to understand why we have certain tick densities and pathogens,” says Flory.

Those connections happen back in Dietze’s lab, where first-year graduate student Tempest McCabe (GRS’21) will use the data to build mathematical models of how the grasses, wildlife, and ticks respond to different conditions. She is building on an existing model that already predicts changes in a given landscape, such as a southern pine forest, based on temperature and rainfall trends caused by climate change.

Part of McCabe’s job will be to introduce a problematic invasive grass, called cogongrass, into the system. She’ll start by modeling cogongrass as a generic grass, but as data flows in, she will fine-tune how cogongrass grows, spreads, and responds to different climate conditions and disturbances, such as fire. “With more information, the representation of the grass will get better and better,” she says.

Ultimately, Dietze’s team will put representations of the grasses, deer, and ticks into one system that will project how their populations will change under different climate scenarios. First, they’ll predict habitat changes in response to varying climate conditions, fire, and land management scenarios. Then, given the predicted habitat changes, they can project tick populations and tick-borne disease risks.

The team’s final step will be creating a useful tool for land managers on military bases and elsewhere by making it interactive and accessible via the web. They envision land managers using the tool to test different land management scenarios, such as schedules of controlled fires, under current and changing climate and habitat conditions. “The idea is to make tools that are directly relatable to the environmental problems people are grappling with, and to help them anticipate problems and manage them,” says Dietze.