YES! While it is true that the incidence of West Nile Virus in horses has decreased, it is still a very prevalent and deadly disease. West Nile Virus (WNV) first entered the United States in 1999. 2002 was the peak year for horses with 15,257 cases nationwide. The number of equine cases has continued to go down each year to 109 US equine cases in 2010. The decrease in the number of cases has been attributed to vaccination and to natural immunity from exposure. 70 to 90% of non-vaccinated control horses become viremic and 90 to 100% develop serious signs of encephalomyelitis. There is no vaccine available for humans, and while the incidence has decreased in humans, it has not decreased at nearly the rate of horses where we have a very effective vaccine.
The case fatality rate for horses exhibiting clinical signs of WNV infection is approximately 33%. Data have supported that 40% of horses that survive the acute illness caused by WNV still exhibit residual effects, such as gait and behavioral abnormalities, 6 months post-diagnosis. Thus vaccination for West Nile virus is recommended as a core vaccine and is an essential standard of care for all horses in North America.