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West Nile Virus (WNV)

West Nile virus (WNV) is a mosquito-borne disease that can, in rare instances, cause serious illness and/or death. In Delaware, the virus was first identified in 2000 and the first human case was confirmed in 2002. The chances of becoming ill with WNV are small. You can reduce your risk of WNV by using insect repellent and wearing long-sleeved shirts and long pants to prevent mosquito bites.

What are the Symptoms of West Nile Virus?

No symptoms in most people.

  • Approximately 8 in 10 people infected with WNV do not develop any symptoms.

Febrile illness (fever) in some people.

  • Approximately 1 in 5 people infected with WNV develop a fever and other symptoms like headache, body aches, joint pains, vomiting, diarrhea, or rash.
  • Most people who develop symptoms recover, but symptoms can last for weeks to months.

Serious symptoms in few people.

  • Approximately 1 in 150 people infected with WNV develop serious symptoms. 
  • Serious symptoms occur when the virus affects the central nervous system and causes encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes around the brain and spinal cord).
  • Symptoms include high fever, headache, neck stiffness, altered mental status, disorientation, vision loss, paralysis.
  • People over the age of 50 and people who are immunocompromised are more likely to develop serious symptoms.  

How Does West Nile Virus Spread?

West Nile virus is most commonly spread by the bite of an infected mosquito. In nature, WNV cycles between mosquitoes (commonly Culex species) and birds. Some types of birds are considered amplifier hosts because they generate high levels of the virus in their blood and can infect other mosquitoes.

Sometimes mosquitoes with WNV can bite and infect humans, horses, and other mammals. However, humans, horses, and other mammals are considered “dead end” hosts because they don not develop enough virus in their blood to infect mosquitoes.

In less common cases, WNV can also be spread through blood transfusions, organ transplants, and mother to baby, during pregnancy, delivery, or breast feeding.




Cuxlex Life Cycle

Mosquitoes live part of their lives in water. After taking a blood meal, female Culex mosquitoes will lay their eggs on top of water in rafts. Some species of Culex mosquitoes will lay eggs in artificial containers like buckets, while others lay eggs in freshwater habitats with organic matter like roadside ditches and coastal wetlands. Within a few days (temperature dependent), these eggs hatch into larvae. 

The larvae feed on organic matter in water and need to come to the surface to breathe with their siphon. After the larval stage, the larvae turn into pupae and stop eating. After a few days to a week, the adult mosquito emerges from the pupal case. The entire life cycle can range from 4 days to as long as a month.



Healthcare providers diagnose WNV infection based on:

  • Symptoms
  • History of possible exposure to mosquitoes
  • Laboratory tests of blood or spinal fluid


Although there is a vaccine available for horses, there is no human vaccine for WNV. Rest, fluids, and over-the-counter pain medications may relieve symptoms. If you have severe symptoms, you may need to be hospitalized to receive supportive treatment.  

Avoiding Mosquito Bites

The best way to reduce your risk of WNV is to avoid mosquito bites.

  • Use an EPA-registered insect repellent with DEET, picaridin, IR3535, oil of lemon eucalyptus, para-menthane-diol, or 2-undecanone. Always make sure to follow label instructions.
  • Use the EPA’s search tool to find the insect repellent that is right for you.
  • Wear loose-fitting, long-sleeved shirts and pants. You can use 0.5% permethrin to treat clothing and gear. Permethrin is an insecticide that kills or repels mosquitoes.
  • For babies and children:
    • Cover strollers and baby carriers with mosquito netting.
    • When applying insect repellent to babies and children, do not apply repellent to their hands, eyes, mouth, cuts, or irritated skin. Adults should apply insect repellent to their hands and then use their hands to apply to a child’s face.
  • Minimize outdoor activity during dawn and dusk.

Mosquito Prevention at Home