Landscape Terrain Provides New Angle for Measuring Tick Abundance
By Andrew Porterfield
Ticks have long been problematic vectors of various diseases that affect humans and animals. Controlling their growth and spread has also been a challenge to public health officials. A new study shows that understanding the effect of geographical terrain on tick populations may help in controlling them.
The lone star tick (Amblyomma americanum) has been behind recent infections from Heartland and Bourbon viruses and a red meat anaphylaxis reaction. The range of A. americanum, which already covers much of the eastern and midwestern United States, has been expanding, driving greater interest in understanding how the tick is distributed.
Solny Adalsteinsson, Ph.D., a staff scientist at Washington University in St. Louis, and her colleagues discovered that, in the forested Missouri Ozarks, a temperate mountain range, A. americanum nymphs and adults were found more often in valleys and on north-facing hills. Nymphs appeared less often in the areas of greater temperature variability, while adults were less prevalent with increased elevation. Knowledge of this tick distribution, reported last week in the Journal of Medical Entomology, can help health officials predict tick locations and tailor their control efforts.
Thomas Van Horn, lead author on the study and an undergraduate research fellow at Washington University at the time the research was conducted, and fellow researchers surveyed tick populations in a 29.7-acre portion of the Tyson Research Center Plot, part of the Smithsonian Center for Tropical Forest Science—Forest Global Earth Observatory Network, about 40 miles away from St. Louis. The area is mainly a hilly, oak-hickory forest, with undergrowth of buckthorn, spicebush, and paw paw. The most common tick in the area is A. americanum. The team used carbon dioxide-baited traps that captured the ticks with double-sided carpet tape. Trapping took place in June and July of 2015, for 120 nights in total. The researchers also compared leaf litter, woody stem plants, slope and direction of terrain, elevation, temperature, and soil types. Data from the Smithsonian ForestGEO database was helpful for collecting this information—a resource that wouldn’t have been available in most other areas.
A total of 2,089 ticks were collected: 1,009 A. americanum nymphs, 999 A. americanum adults, 1 A. americanum larva, 72 adult American dog ticks (Dermacentor variabilis), and eight larval blacklegged ticks (Ixodes scapularis).
A number of statistical models were used to compare various terrain features with tick abundance, and the best model for nymphs focused on aspect, slope, and temperature variance, with north-facing slopes showing 7.55-16 nymphs per trap, versus south-facing slopes showing 0.64-2.7 nymphs per trap. Nymph abundance decreased with greater fluctuations in temperature and with increases in slope.
For adults, aspect, elevation, and slope provided the best predictor of tick abundance. More ticks were found on northeast- and north-facing slopes (4.56-12.42 adults per trap) versus south slopes (0.71-3.16 ticks per trap). Increases in slope and elevation resulted in decreases in ticks per trap. Also for adults, more were collected in valleys than on ridges.
The study is unique in that it looked at terrain changes in a more temperate U.S. geographic area, with mountain ranges that were more gentle (650 feet) in elevation than those studied in European models. In addition, the undergrowth and oak forests make up a model forested area that is common to many areas of the United States. And while it is not practical to alter slope and elevation to control tick populations, related issues such as moisture, soil quality, and leaf litter can be controlled.
The researchers point out, however, that the study only explains about half of the variation in A. amblyomma abundance. This finding was a surprise, Adalsteinsson says. “We had expected to find greater influence of the biological variables (e.g., leaf litter and plant stem density) than we did.”
“The remainder of variation was unexplained by physiognomy or biological variables,” Adalsteinsson and her colleagues write. “Regardless, that easily collected landscape characteristics were predictive of a substantial amount of variance in A. americanum abundance … is encouraging for land management purposes.”
Journal of Medical Entomology
Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.