As autumn approaches, some owners are ready to start thinking about phoning their veterinarians to set up their horses’ fall vaccinations. And because the number of West Nile virus (WNV) cases confirmed in U.S. horses continues to rise slowly, consider requesting a WNV booster.

According to preliminary data released Sept. 5 by the U.S. Department of Agriculture Animal Health Monitoring and Surveillance, 64 cases of WNV have been confirmed in 23 states thus far in 2013. Some recently confirmed cases include:

California—As of Aug. 16, the California Department of Food and Agriculture (CDFA) reported that two additional horses in that state had tested positive for WNV. The affected horses are "a 1-year-old unvaccinated Andalusian colt in Sacramento County and a 4-year-old Morgan mare of unknown vaccination status in Placer County. Both horses are recovering."

The CDFA’s statement also indicated that a total of eight horses have been confirmed positive for WNV in 2013. They were located in Kern, Placer, Riverside, Sacramento (3), and Stanislaus (2) Counties. Four of those were euthanized.

In 2012, California confirmed 22 equine WNV cases.

Delaware—The Delaware Department of Agriculture announced Aug. 30 that a Kent County horse had tested positive for WNV, the first case in that state since 2003. Delaware State Veterinarian Heather Hirst, DVM, MS, said in a statement that the horse was recovering well.

Michigan—On Sept. 5 the Michigan Department of Agriculture and Rural Development reported WNV in two horses, one residing in Kent County