DOVER, Del. (October 26, 2023) – The Office of the State Veterinarian announced Delaware has had five confirmed cases and one suspected case of West Nile Virus (WNV) recently reported in Delaware horses. These are the first Delaware cases of WNV in horses in 2023, with the cases occurring between September 9 and October 16, 2023.

All of the recent cases involve horses that were not vaccinated, including:

• 1-year-old Standardbred mare, Kent County, confirmed case, recovered
• 3-year-old Standardbred mare, Kent County, suspected case, recovered
• 6-year-old Standardbred mare, Kent County, confirmed case, euthanized
• > 20-year-old Percheron mare, New Castle County, confirmed case, recovered
• 1-year-old Standardbred colt, Kent County, confirmed case, recovered
• 14-year-old Standardbred mare, Kent County, confirmed, outcome unknown

Signs of infection in horses include fever (although not always with WNV), anorexia, head pressing, depression or personality change, 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.

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.

WNV and Eastern Equine Encephalitis (EEE) are diseases transmitted to horses via the bites of mosquitoes. 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.

Most people infected with WNV do not develop symptoms, but about 20% can develop a mild illness, including fever, body and muscle aches, headache, nausea, vomiting, and rash symptoms. A small number of people can develop serious illness involving neurological problems, paralysis, and possibly death. The Division of Public Health Disease Prevention & Control Section reports that Delaware had four confirmed cases of neuroinvasive WNV this year.

The public is reminded to take precautions to avoid mosquito bites, including wearing light-colored clothing of long-sleeved shirts and long pants when outdoors in mosquito-prone areas; applying insect repellent containing 10 to 30% diethyltoluamide (DEET) in accordance with label instructions; and avoiding mosquito-infested areas and at times of peak mosquito activity around dusk, dawn and at night.

With the first hard freeze, the mosquito season should come to a close; however, the State Veterinarian is urging horse owners to contact their veterinarians to ensure they add the WNV and EEE vaccines to their annual vaccination schedule for 2024 to help protect their horses and other equines. Neither disease has a specific drug treatment, and EEE infections in horses are fatal in 70 to 90 percent of cases and WNV in 30 percent of cases.

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, if possible, to remove any mosquito eggs or larvae.

For more information about WNV or EEE:

• To report suspected cases of human West Nile Virus, contact the DPH Office of Infectious Disease Epidemiology (OIDE) at 302-744-4990 during normal business hours from 8 a.m. – 4:30 p.m., Monday – Friday; 1-888-295-5196 outside of normal business hours; fax: 302-622-4194; or email reportdisease@delaware.gov.
• Animal health questions should be directed to the Delaware Department of Agriculture at 302-698-4500. Ask for the Poultry and Animal Health Section.
• Questions about the state’s mosquito control program or mosquito biology should be directed to the Delaware Department of Natural Resources and Environmental Control’s Mosquito Control Section at 302-739-9917.

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