Encephalitis Transmission in Florida

Eastern equine encephalitis virus (EEEV) is a disease problem from Maine to Florida. This virus is carried by passerine songbirds that serve as the reservoir hosts, then transmitted by mosquitoes to other birds and mammals (including horses and humans). With no human vaccine currently available, the human fatality rate approaches 35% and Florida accounts for 25% of the human fatalities.

While there is EEEV transmission year-around in 64 of Florida’s 67 counties, most horse and human cases occur in June and July.

Mosquito surveillance programs, using sentinel chicken flocks and mosquito testing, are ongoing in Florida due to the prevalence of multiple mosquito-borne viruses, such as EEEV, West Nile virus and St. Louis encephalitis. Because the mosquito vector is a fresh-water species, risk of exposure along coastal areas is not as prevalent as in more inland areas.

A recent study has developed a predictive map of EEEV transmission risk. Areas with documented horse cases were evaluated using five individual risk variables in the model:

Local habitat: The highest density of equine EEEV infection occurred within low-density residential areas, and the next highest occurred in crop-pastureland habitat.

Wetland proximity: More than half the cases occurred within 400 meters (1/4 mile) proximity to wetlands; most of the cases occurred within 1,500 meters (~1 mile).

Wetland composition: The Everglades have high wetland proximity and composition values.

Tree plantation/coniferous forest proximity: More than half the cases occurred within 400 meters of tree plantation/ coniferous forests; almost all within 1,500 meters.

Tree plantation/coniferous forest composition: The Florida Panhandle, North and Central regions have the highest risk values for tree plantation/ coniferous forest proximity and composition.

Based on assigning values to these five parameters, the study “was able to assess risk from a regional perspective and at more localized scales of individual farms.”

In summary, the study stated: “The results of the model show considerable amounts of high risk locations in the upper three regions of the state and relatively little in the South.”

The model made accurate predictions for the North, Central and Panhandle regions, but was less accurate for the south.

Inclusion of additional variables such as climate—temperature, water table depth, rainfall—could improve the risk assessment prediction. Because these variables change daily, it is suggested that the model be run more frequently to improve accuracy. This risk assessment model for horses could potentially be applied to evaluate EEEV risk for humans. Positioning chicken sentinel sites in high-risk areas is another means of improving targeted surveillance. In addition, knowledge of high-risk areas enables agencies to focus more efficient efforts on mosquito control while decreasing the costs of implementing these measures.

Citation: Vander, Kelen P.; Downs, J.A.; Unnasch, T.; Stark, L. A risk index model for predicting equine encephalitis virus transmission to horses in Florida. Applied Geography (2014), 48:79-86.

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