Although cooler temperatures have arrived in Florida, horses in the Sunshine State are still at risk for contracting potentially fatal mosquito-borne diseases, such as West Nile virus, University of Florida veterinarians and state officials warn.

“The National Weather Service is projecting a warmer than normal winter, so horse owners should not become complacent and make sure they vaccinate their horse,” said Michael Short, DVM., equine programs manager for the Florida Department of Agriculture and Consumer Services’ Division of Animal Industry.

While state officials report no equine cases yet this year, a new single-dose vaccine recently tested in horses by a University of Florida infectious disease specialist may reduce the overall occurrence of the cyclical virus because the product can be administered any time of year, with almost immediate protection. Known as PreveNile, the vaccine began reaching veterinarians in late September.

“Horse owners who have not vaccinated their animals already should do so as soon as possible,” said Maureen Long, DVM., an associate professor of equine medicine at UF’s College of Veterinary Medicine and a nationally recognized expert on West Nile virus. “We want horse owners to vaccinate if they haven’t, because since there is no cure for West Nile Virus, prevention is really the only tool we have for controlling this ongoing threat.”

As of Oct. 31, the disease has been reported in 3,752 people nationwide and in 939 horses this year. In its most serious manifestation, West Nile virus causes fatal inflammation of the brain, and it also occurs in a variety of domestic and wild birds, including crows. Nationwide, more than 23,000 cases have been reported in horses since its initial appearance in 1999, with more than a third of these animals dying, including more than 1,000 in Florida.

West Nile virus cycles between birds and mosquitoes, and mosquito bites are the only way a horse can become infected. Horses and humans infected with the disease cannot infect other horses and humans, experts say. Compared with most states, Florida has a year-round mosquito season, but the insects are most active in the summer and fall.

“Vaccination is a very important component of horses’ health, and the arboviruses, West Nile virus and Eastern equine encephalitis, are two diseases we strongly urge horse owners to have their horses vaccinated for,” Short said. “Many horses die every year from these two diseases and those we report are just confirmed cases. There probably are a lot more out there that we don’t hear about.”

PreveNile is marketed by Intervet Inc. and received approval from the U.S. Department of Agriculture for commercial use in July. Long and her staff provided immune protection studies for the product, the first live-virus vaccine to prevent West Nile virus in horses.

PreveNile provides 12 months of immunity and may be used even if other products have been administered within the past year. Other vaccines previously on the market required two doses before becoming effective.

“The other vaccines are labeled only for protection against viremia, or the presence of virus in the blood,” Long said. “This is the only market vaccine that is labeled for protection against disease itself because of the way in which we tested the product in horses.”

Some 19,000 humans have been infected with the virus, and nearly 800 people have died from it, according to the USDA’s animal and plant health industry surveillance program.

“There is intense interest in developing vaccination strategies for humans,” Long said. “A similar product is currently being tested in humans by Acambis Inc., the human vaccine company that constructed this product originally. Work in horses is invaluable for assessment of this type of vaccine for use in humans.”

Horse owners with questions about vaccination protocols and options should contact their veterinarian