2 horses in Delaware euthanized after contracting West Nile virus from mosquito bites

• Two horses in Delaware have been euthanized after contracting West Nile virus.
• The virus is carried by mosquitoes.
• The Delaware Department of Agriculture recommends vaccinations for horses and ways to try to prevent mosquito bites.

Two horses have been euthanized after the first cases of West Nile virus in horses this year were confirmed in Delaware.

The Office of the State Veterinarian in the Delaware Department of Agriculture announced the Kent County cases on Sept. 9.

A 4-year-old Standardbred gelding and a 7-year-old Standardbred mare were euthanized, said Michael Lewis, chief of community relations for the department.

The onset of symptoms in these cases occurred between Aug. 31 and Sept. 1.

West Nile virus is transmitted by mosquito bites, generally in summer and fall, with a peak period for disease transmission from mid-August to mid-October.  

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Signs of infection in horses include fever, although not always present, along with anorexia, head pressing, depression or behavior changes, wobbling or staggering, weakness, blindness, convulsions, hind-limb weakness and muscle spasms in the head and neck.

If owners notice any of these signs in their horses, the Department of Agriculture recommends contacting a veterinarian immediately.

Animal health professionals with formal training in diagnosing or recognizing animal diseases and pests of horses and livestock are required to report suspected or confirmed cases of West Nile virus and Eastern equine encephalitis to the Office of the State Veterinarian within 24 hours.

People can also be infected with West Nile virus, but transmission requires a mosquito bite. The virus cannot be directly transmitted between horses or between horses and people.

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Advice to protect horses from mosquitoes

Horse owners can take steps in the barn and around the farm to help protect horses from mosquito bites including:

• Keep horses inside during dawn and dusk, peak hours for mosquito activity
• Apply topical insect repellents labeled for use on horses
• Use fans in horse stalls to deter mosquitoes
• Dispose of old tires and containers with standing water
• Empty, clean and refill water troughs or buckets at least every two to three days to remove mosquito eggs or larvae
• Contact a veterinarian to ensure they add the West Nile virus and eastern equine encephalitis vaccines to their annual vaccination schedule for 2026

Neither disease has a specific drug treatment. Eastern equine encephalitis in horses is fatal in 70 to 90% of cases. The West Nile virus  in horses if fatal in 30% of cases.

Advice to residents for protection from mosquitoes

The Delaware Department of Agriculture reminds residents to continue taking precautions to avoid mosquito bites, including:

• Wear light-colored long-sleeved shirts and long pants when outdoors in mosquito-prone areas
• Apply insect repellent containing 10 to 30% diethyltoluamide (DEET) according to label instructions
• Avoid mosquito-infested areas and times of peak mosquito activity around dawn, dusk and night

The department recommends that residents eliminate standing water on their property that might have been there for four or more consecutive days because it can be a mosquito breeding habitat.

The mosquito season should end with the first hard freeze.

Who to contact if mosquito-borne illnesses are suspected

For questions about mosquito-borne illnesses in horses, along with calling a veterinarian, call the Delaware Department of Agriculture at 302-698-4500 and ask for the Poultry and Animal Health Section. 

If people suspect they may have West Nile virus or eastern equine encephalitis, along with calling a doctor, call the Delaware Public Health Office of Infectious Disease Epidemiology at 302-744-4990 Monday to Friday from 8 a.m. to 4:30 p.m., or after hours at 888-295-5196 or email reportdisease@delaware.gov.

Reach reporter Ben Mace at rmace@gannett.com.