New maps predict where medically important ticks are found in Florida

A female Ixodid scapularis (type of tick).
A female Ixodid scapularis. (Image courtesy of Claudia Ganser.)

This spring, the Florida Department of Health will have a new tool to investigate cases of tickborne illness: maps created by University of Florida medical geographers that predict where three medically important tick species are likeliest to occur in the sunshine state.

The good news is that these ticks aren’t everywhere, says Greg Glass, a geography professor in UF’s College of Liberal Arts and Sciences who led the mapping effort. Ticks that have the potential to carry Lyme disease, for example, are found in a smaller region of the state than expected — though they are found very reliably in some areas.

The maps were recently published in the Journal of Medical Entomology. The study looked at three tick species: the lone star tick (Amblyomma americanum), deer tick (Ixodes scapularis), and the American dog tick (Dermacentor variabilis). These species are vectors for tularemia, human ehrlichiosis, heartland virus disease, Bourbon virus disease, Southern tick-associated rash illness, Rocky Mountain spotted fever, babesiosis, anaplasmosis, Powassan virus disease and Lyme disease.

Ensemble map for Ambylomma americanum, also known as the lone star tick.
Ensemble map for Amblyomma americanum, also known as the lone star tick.

“Based on some citizen science work and anecdotal reports, there was an impression that some of these species may have been more widespread than we found them to be,” Glass says. “If you live in northeast Florida, these ticks are very prevalent. But west of Tallahassee and south and east of Tampa, their presence declines dramatically.”

The maps’ spatial detail is highly refined compared to national-level maps that tend to treat whole counties as either having ticks, or not. “Ours are down to the 100-meter resolution, which is football-field sized,” Glass says. “Which, when you are talking about practical investigations is far more useful and efficient.”

Predictive power

The project grew from past tick mapping efforts by the Southeastern Center of Excellence for Vector Borne Diseases that surveyed Florida to document tick distribution. The center is one of five across the nation that were established in 2017 by the U.S. Centers for Disease Control and Prevention to address issues concerning infectious diseases transmitted by insects and arthropods. Glass leads the tick program in the southeastern center and is the first author to the most recent mapping paper.

“We had a lingering question as to whether our original maps merely described our survey data, or if they had any real power for predicting where these ticks may be,” says Glass, who is also a member of UF’s Emerging Pathogens Institute. 

Ensemble map for Ixodes scapularis, also known as the deer tick or black-legged tick. This is the vector for Lyme disease.
Ensemble map for Ixodes scapularis, also known as the deer tick or black-legged tick. This is the vector for Lyme disease.

Glass and his colleagues previously used CDC-recommended protocols to survey the State of Florida for ticks, as part of an effort to create standardized data sets spanning the nation. But in their most recent effort, they used ensemble modeling to see where five different models held the most agreement to predict where these medically important ticks would be. They then tested these predictions by both resampling old survey areas and sampling new survey areas that they’d not previously visited — but where their maps and the ensemble modeling indicated the ticks would be.

“Turns out, our maps do very well at predicting where these ticks are,” Glass says.

The team surveyed 250 transects at 43 sites (25 new and 18 old) every other month in 2019, performing a total of 1,450 surveys.

Green areas of the maps represent areas where the ensemble modeling held low agreement for the presence of ticks, and red areas show where they held high agreement. Survey sampling from the transects validated the models. The researchers note that the ensemble models performed better at predicting where ticks would be absent than they did in predicting exactly where they are.

Lyme disease and citizen science

The deer tick, also called the black-legged tick, from the Ixodes genus is known to transmit the spirochete-shaped bacteria, Borrelia burgdorferi, which causes Lyme disease.

“Northeast up around Jacksonville, and across northcentral Florida, are great areas for Ixodes. If I were investigating Lyme disease, that’s where I’d look,” Glass says.

Ensemble map for Dermacentor variabilis, also known as the American dog tick or wood tick.
Ensemble map for Dermacentor variabilis, also known as the American dog tick or wood tick.

There are popular opinions that lots of different ticks can transmit the spirochete that causes Lyme. He thinks multiple vectors are unlikely.

“Next, we’re going to compare the maps produced by citizen science and see how well they align or not with ours, now that we’ve validated ours,” Glass says. “How different are they? Are they biased? If so, how?”

Projects that collect data from citizens tend to be less costly than time- and resource-intensive sampling, and which can be attractive for research efforts with tight funding. If citizen science is found to be a reliable method for surveying ticks, it could benefit cash-strapped states. However, it’s not clear whether attempts to generalize tick distribution from citizen science may potentially suffer from biases that lead to incorrect conclusions.

Despite all the time spent surveying for ticks, their absence made just as much of an impression on Glass in some regions of Florida as did their presence.

“After covering 40,000 miles in four years, I can tell you that you can find places in the right season where you can find lots of ticks, but most of the time, you’re just walking and not picking up anything.”

Written by: DeLene Beeland