As of the week of August 27, 2012 the West Nile Virus epidemic in Texas has sickened hundreds and killed 66 people. Here is pertinent information from the California State Agricultural Board:
EQUINE WEST NILE VIRUS
West Nile Virus is a mosquito-borne virus that was first detected in the United States in 1999 in New York City area. The virus has since been identified in all 48 continental states. WNV can cause a wide range of clinical illness from mild flu-like symptoms to encephalitis (inflammation of the brain). The virus may be fatal to both humans and horses. While horses are susceptible to WNV, many horses infected with the disease will not develop any illness and will recover uneventfully. Currently, there is only supportive no specific treatment for WNV.
EQUINE WEST NILE VIRUS HITS CALIFORNIA
In 2003, one case of California acquired WNV was confirmed in a 20 year old Missouri Fox Trotter. But the story changed for 2004. The first cases of California acquired equine WNV were confirmed during June and July 2004 in San Bernardino, and Riverside counties. Since being detected in California the disease has expanded to the north. In 2005 cases were concentrated along the central valley.
What is West Nile Virus? (PDF) WNV is a virus that spreads to humans and animals by infected mosquitoes. Mosquitoes become infected with WNV when they feed on infected birds. However, humans and animals infected with WNV are not contagious.
Horses – While horses and birds are affected by WNV, horses are affected much more often than any other domestic animal. Livestock and poultry do not commonly show any illness if infected by WNV. Although horses are susceptible to the virus, few horses exhibit any signs of clinical illness. In the U.S. prior to the 2004 California outbreak, those horses that develop clinical illness, approximately 30% die or must be destroyed and another 17% have been shown to suffer from long–term related debilitation.
Vaccination – A vaccine for horses is available and horse owners should contact a veterinarian about timely vaccination.