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
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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

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