In a recent study, University of Florida researchers evaluated the effectiveness of three commercially available equine West Nile virus (WNV) vaccines. While all vaccinated horses, irrespective of the vaccine administered, did not develop viremia and all survived a severe challenge model of WNV-induced encephalomyelitis (in this case, WNV was introduced directly into the spinal fluid), only the chimera-vaccinated horses did not develop any clinical signs of WNV-induced disease. Kathy Seino, DVM, MS, a PhD student at the university, presented the study’s findings.

Pharmaceutical companies responded quickly to the invasion of WNV by developing vaccines to protect horses from this virus. To date, there are three types of vaccines on the U.S. market. These include a killed virus (or inactivated) vaccine, a modified-live canarypox-vectored virus vaccine, and the recently approved chimera
vaccine (see www.TheHorse.com/emag.aspx?ID=7569).

Study design Researchers divided healthy horses that tested negative for WNV into three groups of eight receiving either the killed virus vaccine (produced by Fort Dodge), the modified-live canarypoxvectored
virus vaccine (Merial), or the live chimera vaccine (Intervet). Appropriate nonvaccinated controls were used.
Vaccines were administered in accordance with the manufacturers’ labels, and horses were challenged after one month.

Horses were challenged with a model that reproduced clinical signs of WNV. Within seven days of infection, all six of the control horses had increased rectal temperatures and clinical signs of WNV, and none survived to the end of the study period