Distribution Model of the American Dog Tick, EP Vector

Equine piroplasmosis (EP) is a tick-borne disease detected in isolated outbreaks in the United States within the last few years. Predicting where and when equines have a greater chance of becoming infected is difficult because of the complex interactions among the environment, tick vectors, and equine populations. For this reason, the USDA Animal and Plant Health Inspection Service (APHIS) Veterinary Services Center for Epidemiology and Animal Health (CEAH) analysts are using geospatial methods (applying statistical analysis and other informational techniques to geographically based data) to determine the potential distribution of the American dog tick (Dermacentor variabilis), a natural and experimental vector of EP within the continental United States. The objective is to establish correlations between tick presence and environmental factors to help identify areas where tick transmission of EP to horses could occur.

The CEAH developed a distribution model for the American dog tick using the MaxEnt program. This free software program models tick species distribution from tick collection records. Historical tick presence data* collected from 1960 to 1999 were combined with environmental data including mean temperature, precipitation, land cover, and topography. These data were used to define the tick species' ecological requirements and then used to predict the relative suitability of the habitat for this tick species across the contiguous United States.

The results show that the presence of the American dog tick is not uniform across the country. The environmental conditions surrounding the presence of the American dog tick varied between the Western and Eastern regions of the United States; thus, these regions likely represent different ecological niches for this tick species. The largest per-state percentage area of suitable habitat for the American dog tick was in the Eastern region of the United States in Delaware, Florida, New Jersey, and Virginia. Other states with a high percentage of suitable habitats were California, Minnesota, Missouri, North Dakota, Ohio, Oregon, South Dakota, and Washington. Suitable environmental conditions were either absent or present only in localized areas in the Rocky Mountain regions of New Mexico, Arizona, and Colorado.

Environmental variables having the greatest contribution to the model were elevation, temperature, and the amount of vegetation moisture present in the months of May and August. Data results show the American dog tick is more likely to be found at lower elevations and in areas with warmer temperature and higher humidity.

In assessing the likelihood of a horse encountering the American dog tick, it is necessary to combine information on the presence of suitable habitat, tick abundance, and the number of horses exposed in specific tick habitats. For example, an area with ideal tick habitat and frequent horse visits--either while a horse is feeding or through human or horse activities--provides a greater chance of disease transmission via ticks to a horse than an area with limited tick habitat and infrequent horse exposure. Understanding the factors necessary for ticks to thrive in a given region will aid in targeted EP surveillance activities, customization of horse management practices, and effective tick control on a regional basis. Currently APHIS's recommendations are adequate for managing and controlling ticks on horses to minimize EP transmission via ticks.

*Data obtained from the Smithsonian's United States National Tick Collection and the tick surveillance program of the Veterinary Service's National Veterinary Services Laboratories.

CONTACT: Drs. Angela M. James and Jerome E. Freier, 970/494- 7001, Angela.M.James@aphis.usda.gov, Jerome.E.Freier@aphis.usda.gov, USDA:APHIS:VS:CEAH, Fort Collins, Colorado

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd's, London, brokers, and their Kentucky agents.

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Equine Disease Quarterly

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