State’s First 2025 Detection of West Nile Virus Found in DNREC’s Sentinel Chickens

No Human Cases of WNV Reported to Date in State; Equine Owners Urged to Vaccinate Their Animals

West Nile Virus (WNV) has been detected in Delaware for the first time for 2025 in sentinel chickens that help the Department of Natural Resources and Environmental Control (DNREC) monitor the state for mosquito-borne illnesses.

The DNREC Mosquito Control Section reported WNV detection at three different sentinel chicken flock locations in New Castle County that were sampled Aug. 11, with virus-positive confirmation of the chickens Aug. 13 from the Delaware Division of Public Health (DPH) laboratory. “The date of these detects is slightly later than our typical first detection date, historically the last week in July or first week in August – but is consistent with other WNV activity thus far this summer elsewhere in the mid-Atlantic and Northeast,” said DNREC Mosquito Control Section Administrator Tom Moran.

The Mosquito Control Section deploys small sentinel chicken flocks throughout the state during summer and early fall as surveillance strategy for certain mosquito-borne illnesses. Weekly blood samples taken from these chickens are tested for WNV and Eastern Equine Encephalitis (EEE) antibodies by the DPH lab. Sentinel chickens bitten by mosquitoes carrying WNV or EEE develop antibodies to these diseases but are otherwise unaffected.

A positive detect informs the Mosquito Control Section about which virus is circulating in the vicinity of the detection and is evidence that infected mosquitoes in the area have passed the virus to the sentinel chickens. DNREC then conducts further surveillance on which, and how many, mosquito species are present in the area of detection, as only certain species can transmit WNV and/or EEE to humans and horses. This surveillance will in turn determine any control measures to be taken by the Mosquito Control Section.

While there have been no reported WNV cases yet in humans for 2025 in the state, Delawareans are reminded that the possibility of contracting mosquito-transmitted diseases, including WNV and EEE, will continue until colder autumn temperatures arrive in mid-October. The DPH Disease Prevention and Control Section reports that Delaware had one probable case of neuroinvasive WNV in 2024 with no human WNV cases to date this year.

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. EEE is not as prevalent as WNV but can present more severe symptoms in humans and horses.

According to the Delaware Department of Agriculture (DDA), there were six equine cases of WNV in 2024, and no EEE cases, with no cases of either disease to date recorded this year. The State Veterinarian’s Office within DDA urges horse owners to have all their equine animals vaccinated by veterinarians against both WNV and EEE. While vaccines for the mosquito-borne illnesses are highly effective in minimizing disease, horses and equines that have been vaccinated in the past will need an annual booster shot. Neither disease has a specific drug treatment, and infections in horses are fatal in 70% to 90% of EEE cases and 30% of WNV cases.

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

Spraying to reduce mosquito populations in areas where WNV or EEE is detected may be initiated by the DNREC Mosquito Control Section as warranted, based on factors to include mosquito population levels and mosquito species present in affected areas. Residents can also contribute to lessening the chance of mosquito-borne disease transmission by eliminating mosquito habitat – by draining or removing outdoor items that collect water, such as discarded buckets or containers, uncovered trashcans, stagnant birdbaths, open rain barrels or cisterns, old tires, upright wheelbarrows, flowerpot liners, depressions in boat tarps, clogged rain gutters, corrugated downspout extenders, and unused swimming pools.

Delawareans also are reminded to take common-sense precautions to avoid mosquito bites, including wearing light-colored, long-sleeved shirts and 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 times of peak mosquito activity around dusk, dawn and at night.

More information about mosquitoes and mosquito-borne diseases in Delaware is available from the following resources:

• For mosquito biology/ecology and control, contact the DNREC Mosquito Control Section office in Dover at 302-739-9917, with more information at gov/mosquito
• For requests for mosquito relief in upstate areas from Dover north, contact Mosquito Control’s Glasgow field office at 302-836-2555.
• For requests for mosquito relief in downstate areas south of Dover, contact Mosquito Control’s Milford field office at 302-422-1512.
• For animal health questions, contact the DDA Poultry and Animal Health Section at 302-698-4500.
• To report suspected cases of human WNV, call the DPH Office of Infectious Disease Epidemiology toll-free at 888-295-5156.

For more information on West Nile virus or eastern equine encephalitis, visit the Centers for Disease Control website.

About DNREC
The Delaware Department of Natural Resources and Environmental Control protects and manages the state’s natural resources, protects public health, provides outdoor recreational opportunities and educates Delawareans about the environment. For more information, visit the website and connect with @DelawareDNREC on YouTube, Facebook, Instagram, X (formerly known as Twitter) or LinkedIn.

Media contacts:
DNREC: Michael Globetti, michael.globetti@delaware.gov
DDA: Stephanie Alexander, stephanie.alexander@delaware.gov
DPH: Sean Dooley, sean.dooley@delaware.gov

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