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DNREC's Mosquito Control section sprays via helicopter to control aquatic immature mosquitos. The public and horse owners are being reminded about the risk of mosquito-borne West Nile virus and Eastern equine encephalitis after West Nile was detected in sentinel chickens for the first time in the 2021 season.

West Nile virus (WNV) has been detected in Delaware for the first time in 2021 in sentinel chickens, DNREC announced this week. The first finding occurred in northern New Castle County, at a sentinel chicken station sampled by DNREC’s Mosquito Control section on July 19.

While there have been no reported WNV cases in humans this year in the state, Delawareans are being reminded that the possibility of contracting mosquito-transmitted diseases, including WNV and Eastern equine encephalitis (EEE), will continue until colder autumn temperatures in mid-October or later.

Blood samples are collected by the Mosquito Control section each week from early July into October from the state’s outdoor-caged sentinel chickens that are humanely housed and handled at 20 monitoring stations statewide, officials noted. The blood samples are tested for WNV and EEE antibodies by the Delaware Public Health Laboratory.

Sentinel chickens bitten by mosquitoes carrying WNV or EEE develop antibodies to these diseases but are otherwise unaffected by them. WNV and EEE can be transmitted by mosquitoes to humans and horses.

Most people infected with WNV do not develop symptoms, but about 20 percent can develop a mild illness, which may include 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.

DNREC reminded the public to take common-sense precautions to avoid mosquito bites, including wearing long-sleeved shirts and long pants when outdoors in mosquito-prone areas, applying insect repellent containing 10 to 30 percent diethyl toluamide (DEET) in accordance with label instructions, and avoiding mosquito-infested areas and times of peak mosquito activity around dusk, dawn and at night.

Spraying to reduce mosquito populations in areas where WNV or EEE is detected may be initiated by the Mosquito Control section as warranted based on factors including mosquito population levels and mosquito species present.

To reduce mosquito-breeding habitat and chances of disease transmission, residents should drain or remove outdoor items that collect water, such as discarded buckets or containers, uncovered trashcans, stagnant birdbaths, unprotected rain barrels or cisterns, old tires, upright wheelbarrows, flowerpot liners, depressions in boat tarps, clogged rain gutters, corrugated downspout extenders and unused swimming pools.

The state veterinarian within the Department of Agriculture urged horse owners to contact their veterinarians as soon as possible to have horses and other equines vaccinated against WNV and EEE. Neither disease has a specific drug treatment, and infections in horses are fatal in 70 to 90 percent of EEE cases and in 30 percent of WNV cases.

More information about mosquitos and mosquito-borne diseases is available:

  • For mosquito biology/ecology and control, contact the DNREC Mosquito Control section office in Dover at (302) 739-9917.
  • 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 Delaware Department of Agriculture’s Poultry & Animal Health Section, at (302) 698-4500.
  • To report suspected cases of human WNV, call the Division of Public Health Office of Infectious Disease Epidemiology toll-free at 1-888-295-5156.
  • For more information on West Nile virus or Eastern equine encephalitis, visit