A horse stabled in Water Mill has died of West Nile viral infection, a mosquito-borne disease that is capable of infecting a wide range of species and has been a summertime threat for nearly 20 years.

Even though a West Nile vaccine has yet to be developed for people, there is one for horses. The 5-year-old mare had not been vaccinated. The horse was seen by a veterinarian on Aug. 20 and died the next day, the Suffolk County Department of Health reported late Friday.  

“Experimental studies have concluded that horses are dead-end hosts for West Nile virus, so this finding does not necessarily indicate an increased human risk for West Nile virus,” Dr. James Tomarken, Suffolk County’s health commissioner, said in a statement Friday. “Health department staff is monitoring the Water Mill area for mosquito activity and will report any finding of positive results for West Nile virus.”

Tomarken reminded owners to vaccinate their horses for West Nile virus and Eastern equine encephalitis, also known as triple E.

Since 1999, the first year West Nile was detected in the United States — emerging initially in Queens before rapidly spreading throughout New York and beyond — more than 25,000 cases of West Nile viral encephalitis have been reported in U.S. horses. The animals represent 96.9 percent of all reported nonhuman mammalian cases of West Nile disease, according to the American Association of Equine Practitioners.

The fatality rate for horses exhibiting clinical signs of West Nile infection is about 33 percent, data from the equine practitioners show.

In addition, 40 percent of horses that survive the acute illness caused by the infection exhibit residual effects, such as gait and behavioral abnormalities up to six months after diagnosis.

Along with the horse’s death, the health department disclosed that four new mosquito samples have tested positive for West Nile virus. The samples were all of the mosquito species Culex salinarius. One sample each was collected on Aug. 21 and Aug. 23 from Lindenhurst, Rocky Point, Aquebogue and Jamesport.

To date, the county has reported one horse, 93 mosquito samples and nine birds having tested positive for West Nile virus. There have been no human cases of West Nile virus in Suffolk County, Tomarken said. 


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