LANSING, Mich. — At least 114 cases of equine West Nile virus (WNV) have been reported in the United States in 2012, according to the U.S. Geological Survey’s (USGS) disease maps, and more than 1,100 human cases have been confirmed nationwide, according to the Centers for Disease Control and Prevention (CDC).

In Michigan, the first case of equine WNV was confirmed in an unvaccinated horse in Montcalm County Aug. 15, according to officials with the Michigan Department of Agriculture and Rural Development (MDARD) and the Michigan State University Diagnostic Center for Population and Animal Health (DCPAH). According to a report from the USGS, a second equine case has also been reported.

Last week, Michigan health officials reported the first human death in the state due to the virus. In neighboring Indiana, one human death has been reported and three cases have been confirmed in horses.

Thus far in 2012, 47 states have reported WNV infections in people, birds or mosquitoes, according to the CDC. As of Aug. 21, a total of 1,118 cases of WNV in people, including 41 deaths, were reported to the CDC. These cases thus far in 2012 are the highest number of WNV disease cases reported to CDC through the third week in any August since WNV was first detected in the United States in 1999.

Approximately 75 percent of the cases have been reported in five states – Texas, Mississippi, Louisiana, South Dakota and Oklahoma – and almost half have been reported from Texas.

“The unusually hot and dry summer has favored the mosquitoes that transmit WNV. These same conditions have led to a reduction in the number of ‘nuisance’ mosquitoes which are more abundant during wet summers, giving citizens a false sense of security,” said Dr. Corinne Miller, state epidemiologist with the Michigan Department of Community Health.

According to USGS disease maps, equine WNV has been reported in Alabama, Arizona, California, Colorado, Illinois, Indiana, Iowa, Kentucky, Louisiana, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, New Jersey, New Mexico, North Dakota, Ohio, Oklahoma, Pennsylvania, South Carolina, South Dakota, Texas and Wyoming.

To help protect horses, Michigan State Veterinarian Dr. Steven Halstead advises owners to have them vaccinated yearly, use insect repellants, put horses in stalls or barns during the prime mosquito exposure hours of dawn and dusk and eliminate standing water.

“WNV is spread from wild birds to humans, horses and, in some cases pets, through infected mosquitoes and causes encephalitis, or inflammation of the brain,” Halstead said. “Signs of WNV may include stumbling, limb weakness, facial paralysis, difficulty urinating and defecating, fever, blindness, seizures and struggling to get up.

“There is no specific treatment for WNV encephalitis, but supportive care can help horses survive until their natural defenses eliminate the virus.”

According to Halstead, MDARD partners with DCPAH for diagnostic testing whenever clinical signs indicate the animal could be suffering from a reportable disease.

“Our commitment to diagnostic testing for new and emerging zoonotic diseases such as West Nile virus strengthens partnerships with state agencies including MDARD, Department of Community Health and Department of Natural Resources, as well as national agencies like the Centers for Disease Control and Prevention,” said Dr. Carole Bolin, DCPAH director. “We conduct approximately 100 tests for WNV annually.”

The mosquitoes most likely to transmit WNV to humans lay eggs in small pools of standing water. Adult mosquitoes can hatch in 10 days in the warmest months of the summer. 

Mosquitoes become infected and transmit WNV after feeding on birds carrying the virus. 

Within 10-14 days, the mosquito can transmit the virus to humans and horses.

For more information about WNV in horses visit www.michigan.gov/emergingdiseases or DCPAH’s diagnostics at www.animalhealth.msu.edu

To be the first to know about an animal disease that may affect livestock or pets in Michigan, join the Animal Health Listserv online at www.michigan.gov/

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