Delaware Confirms Initial Cases of West Nile Virus in Horses for 2025

DOVER, Del. (Sept. 9, 2025) - The Office of the State Veterinarian within the Delaware Department of Agriculture has confirmed the first two cases of West Nile Virus (WNV) in Delaware horses this year. The onset of symptoms in these cases occurred between Aug. 31-Sept. 1.

The cases include:

• 4-year-old Standardbred gelding, Kent County, euthanized
• 7-year-old Standardbred mare, Kent County, euthanized.

WNV and Eastern Equine Encephalitis (EEE) are diseases transmitted via mosquito bites, generally in summer and fall, with a peak period for disease transmissions from mid-August to mid-October.

Signs of infection in horses include fever (although not always present with WNV), anorexia, head pressing, depression or behavior changes, wobbling or staggering, weakness, blindness, convulsions, muscle spasms in the head and neck, or hind-limb weakness. If owners notice any of these signs in their horses, they should contact their veterinarian immediately. Humans can also be infected with WNV and EEE, but transmission requires a mosquito bite, and the virus cannot be directly transmitted between horses or between horses and people.

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 WNV and EEE to the Office of the State Veterinarian within 24 hours. Examples of animal health professionals include, but are not limited to, veterinary medical professionals, diagnostic laboratorians, biomedical researchers, public health officials, animal health officials, trained technicians, zoo personnel, and wildlife personnel with such training.

Horse owners can take several additional steps in the barn and around the farm to help protect horses from mosquito bites. Horses should be kept inside during dawn and dusk, peak hours for mosquito activity. Topical insect repellents labeled for use on horses may be applied. The wind generated by fans installed in horse stalls can also help deter mosquitoes. Old tires and containers should be disposed of, and standing water should be eliminated. Water troughs or buckets should be emptied, cleaned, and refilled every 2-3 days to remove mosquito eggs or larvae.

With the first hard freeze, the mosquito season should end; however, the State Veterinarian urges horse owners to contact their veterinarians to ensure they add the WNV and EEE vaccines to their annual vaccination schedule for 2026 to help protect their horses and other equines. Neither disease has a specific drug treatment, and EEE infections in horses are fatal in 70-90 percent of cases and 30 percent of WNV cases.

The public is reminded to continue taking precautions to avoid mosquito bites, including:

• Wearing light-colored clothing such as long-sleeved shirts and long pants when outdoors in mosquito-prone areas
• Applying insect repellent containing 10- to 30-percent diethyltoluamide (DEET) in accordance with label instructions
• Avoiding mosquito-infested areas and times of peak mosquito activity around dusk, dawn, and night.

Delaware residents and landowners should also eliminate standing water on their property that might have been there for four or more consecutive days, which can act as a mosquito breeding habitat.

For more information about WNV or EEE:

• To report suspected cases of human West Nile Virus or Eastern Equine Encephalitis Virus, contact the DPH Office of Infectious Disease Epidemiology (OIDE) at 302-744-4990 during regular business hours from 8 a.m.-4:30 p.m., Monday-Friday; 888-295-5196 outside of normal business hours; fax: 302-622-4194; or email [email protected]. • Animal health questions should be directed to the Delaware Department of Agriculture at 302-698-4500. Ask for the Poultry and Animal Health Section. • For mosquito biology/ecology and control services, contact the Delaware Department of Natural Resources and Environmental Control's Mosquito Control Section at 302-739-9917. For requests for mosquito relief in upstate areas from Dover to the north, contact Mosquito Control's Glasgow field office at 302-836-2555. For mosquito relief requests in downstate areas south of Dover, contact Mosquito Control's Milford field office at 302-422-1512.