West Nile virus (family Flaviviridae, genus flavivirus WNV) is of concern in the US and Florida because the virus causes disease in humans and horses. Since 1999, there have been 23,925 clinical human cases of WNV in the United States (1999-2007). Prevention and reduction of cases requires a clear understanding of the WNV transmission cycle, but much of the needed information is lacking. It is still unknown which mosquito species transmit WNV to horses. This study integrated field investigations with laboratory studies to identify possible mosquito vectors of WNV to horses in north central Florida. The primary objectives of this research were to compare the abundance and seasonality of mosquito species collected near horses, and to characterize host preference of potential vectors. An additional aim was to evaluate extrinsic risk factors of WNV to Florida horses. The extrinsic factors of interest included farm management, farm ecology, and the entomological conditions associated with each farm. A questionnaire that focused on potential risk factors was mailed to the owners of all horses tested for arbovirus from 2001 to 2003. Vaccination was the factor most strongly associated with a protective effect for WNV disease outcome in horses. The factors that were associated with an increased risk of WNV in horses were fan use in the stable, mosquito activity, and dead birds on the property. Blood meal identification and virus screening were done in order to determine which mosquito species, if any, were involved in WNV transmission to horses. Mosquitoes were collected for a period of 26 months from a horse research area in north central Florida. DNA was extracted from the abdomen of the blood fed mosquitoes to test for the presence of avian, mammalian, and reptilian blood using PCR with different primer sets. The blood meals were confirmed with sequencing. The non-blood-fed mosquitoes were sorted into pools of up to 50 mosquitoes and screened for WNV, SLEV, and EEEV by Real-Time quantitative RT-PCR. A total of 45,851 mosquitoes (twenty three species) were collected, 252 of which had visible blood meals. Twelve mosquito species (fifty eight individuals) were positive for horse DNA. St. Louis encephalitis virus was detected in one pool of Mansonia titillans collected on September 26, 2006. This study was able to identify several mosquito species feeding on horses and risk factors associated with WNV disease. The vaccine can protect horses against WNV disease if administered two weeks prior to exposure and if a booster is administered yearly.