West Nile Virus (WNV)

West Nile Virus (WNV) is a reportable disease in Oklahoma. WNV is part of a group of viruses called arboviruses and is the leading cause of mosquito-borne disease in the continental United States. West Nile Virus is primarily spread through the bite of an infected Culex mosquito (photo above). Mosquitos become infected when they feed on infected birds. The virus must circulate in the mosquito for a few days before they can pass the virus to other animals and humans while biting. There is no documented evidence of WNV being spread from person to person or from animal to person. In a very small number of cases, WNV has been spread through organ transplant, blood transfusion, and from mother to baby (pregnancy, delivery, or breastfeeding).

Persons of any age can become ill after being bitten by an infected mosquito, but those over the age of 50 are at greater risk of developing serious illness involving the nervous system. Most people (8 out of 10) infected with the virus never become ill or have noticeable symptoms. Symptoms can commonly include fever, headache, body aches, joint pains, vomiting, diarrhea, or rash. Those with illness recover completely, but fatigue and weakness can last for weeks or months. In more severe cases, persons can develop meningitis or other neurological disease.

It is helpful to know some key points about the differences in types of mosquitoes and the features of West Nile virus transmission:

The type of mosquitoes that hatch after severe flooding are primarily the species of mosquitoes classified as “nuisance mosquitoes”.  They bite aggressively and cause lots of itchy bites, but they are not typically involved with transmission of diseases.
Floodwater mosquito populations tend to die out 3 weeks after the rains stop and the sun dries out affected low lying areas.
Culex species of mosquitoes are the primary vector of West Nile virus.  This type of mosquito increases in numbers during mid to late summer when the temperatures climb, and the weather pattern is drier.
Since WNV was introduced into Oklahoma, previous outbreaks were characterized by higher-than-normal summer temperatures and drought.

The Oklahoma State Department of Health advises use of insect repellents—particularly those containing DEET, picaridin, oil of lemon eucalyptus (PMD), or IR 3535—when enjoying outdoor activities like gardening, yard work, camping, or other leisure activities. You can also reduce your risk of WNV by wearing long-sleeved shirts and long pants. The type of mosquitoes that transmit WNV are most active during early evening through early morning (dusk to dawn) hours, so it is important to take mosquito bite precautions during that time of the day.  It is also recommended to drain or treat standing water around your home with a mosquito larvicide to reduce areas where mosquitoes may lay eggs.

Persons are at greatest risk of exposure to infected mosquitoes from July through October in our state.  Persons of any age can become ill after being bitten by an infected mosquito, but those over the age of 50 are at greater risk of developing serious illness involving the nervous system.  Over 80% of people infected with the virus never become ill. 

West Nile virus is a reportable disease in Oklahoma.

Apply insect repellant containing an active ingredient like 10% to 30% DEET, Picaridin, or oil of lemon eucalyptus (PMD), to exposed skin and clothing when outdoors, especially during the evening and early morning hours. (Always follow labeled directions for proper use.)
Prevent items such as buckets and tarps from holding standing water.
If rainwater is collected, cover and seal containers when not collecting rain.
Empty your outdoor pet’s water bowl and refill daily.
Rinse, scrub, and refill birdbaths weekly.
Empty plastic wading pools weekly and store indoors when not in use.
Regularly maintain swimming pools and outdoor hot tubs.
Store boats covered or upside down.
For a water garden or ornamental body of water, use an environmentally safe product to kill the larvae, e.g., BTI, or stock with fish that eat mosquito larvae.
Regularly clean fallen leaves and debris from roof gutters.
Trim grass and weeds and dismantle brush to deprive mosquitoes of a habitat.
Repair or replace all broken or torn window and door screens.
Repair leaky lawn irrigation spouts.

If you believe you are ill with WNV, speak with your health care provider. They will be able to determine your diagnosis based on symptoms, history of exposure to mosquitos that carry WNV, and laboratory testing. Most persons who are infected with West Nile virus will have no noticeable symptoms or have an illness syndrome called “West Nile Fever” lasting two to 10 days. Common symptoms of West Nile Fever include headache, fever, and extreme muscle weakness, occasionally accompanied by vomiting or skin rashes. In some cases, West Nile virus infection will cause severe neurologic disease such as meningitis (swollen membranes in brain or spinal cord), paralysis (loss of muscle function), or encephalitis (swelling and inflammation of the brain). Symptoms of West Nile meningitis or encephalitis may be intense headache, dizziness, stiff neck, marked weakness, muscle tremors, disorientation, mental confusion, or convulsions.

Q: How do people get infected with West Nile Virus (WNV)?
A: The most likely way a human would become infected with WNV is through the bite of an infected mosquito. Some people have also become infected with WNV following receipt of contaminated blood or blood products, or transplanted organs from an infected donor. Mothers who are recently infected with WNV may also transmit the virus to their unborn child, or to their baby while breastfeeding.

Q: Who is at risk for getting West Nile encephalitis?
A: All residents of areas where WNV activity has been identified are at risk of getting West Nile encephalitis.

Q: What is the time from exposure to onset of disease symptoms for West Nile encephalitis in humans?
A: Usually 3 to 15 days.

Q: What are the symptoms of West Nile virus infection?
A: Most people who are infected with WNV will not have any noticeable illness, or have a mild form of illness called West Nile Fever. Persons with West Nile Fever typically experience symptoms of fever, headache, nausea, muscle weakness, and body aches lasting 2 to 6 days or longer. Sensitivity when looking at light and a skin rash appearing on the trunk of the body may also be present. Approximately 20% of persons infected with WNV will develop more severe neurologic disease that may be life-threatening. Adults over the age of 50 years are at greater risk of having serious disease. Potential symptoms of severe infection (West Nile encephalitis or meningitis) include intense headache, dizziness, severe muscle weakness, neck stiffness, vomiting, disorientation, mental confusion, tremors, muscle paralysis, or convulsions and coma.

Q: Is there a human vaccine against West Nile encephalitis?
A: No, but companies are working towards developing a vaccine.

Q: How can I reduce my risk of getting West Nile disease?
A: Practice the four“Ds”!
• Apply a mosquito repellant containing DEET or another approved active ingredient such as Picaridin, oil of lemon eucalyptus, or IR3535
• Avoid being outdoors between Dusk and Dawn
• Drain any standing, stagnant water observed in containers or artificial locations around your home and workplace
• Dress wearing long sleeves and long pants to shield skin from mosquitoes.

Q: What is the transmission cycle of West Nile virus?
A: Female mosquitoes become infected when they feed on infected birds. Infected mosquitoes can then transmit WNV to humans and animals while taking blood meals. The primary transmission cycle is bird-mosquito-bird, with incidental transmission occurring to people and horses.

Q: If I live in an area where birds or mosquitoes with West Nile virus have been reported and a mosquito bites me, am I likely to get sick?
A: It only takes one bite from an infected mosquito to transmit the disease; however, even in areas where the virus is circulating, less than 1% of mosquitoes are generally infected with the virus. Therefore, the chances you will become severely ill from any one-mosquito bite are extremely small. Persons who have repeated exposures to mosquitoes are at higher risk of acquiring illness from WNV.

Q: What types of animal develop illness from West Nile Virus infection?
A: Birds and horses are most susceptible to disease caused by WNV. Although numerous animals are bitten by infected mosquitoes, illness caused by WNV is uncommon in domestic pets or other wildlife. West Nile virus infections have been proven in a variety of animals, including squirrels, bats, dogs, cats, goats, skunks, domestic rabbits, and even alligators. However, WNV is NOT considered a significant health threat for dogs and cats; they appear to be very resistant to developing disease.

Q: Should I apply insect repellents to my pets?
A: In general, this is not a recommended practice. Because dogs and cats are at very low risk of becoming ill from WNV, the potential side effects of the insecticide outweigh the potential benefits. A more significant mosquito-borne disease threat to dogs and cats is heartworm disease. Safe and effective heartworm preventative medications can be obtained through your veterinarian. Application of insecticides to pet birds or “pocket pets” (hamsters, gerbils) is potentially harmful and should only be done under the advice of a veterinarian. Using insect repellants may help decrease mosquito exposure to horses, but vaccination is the primary method of WNV protection for horses.

Q: Are pet birds at risk?
A: The disease risk to a pet bird depends on the amount of exposure the bird has to mosquitoes. Domestic birds kept strictly indoors have minimal risk. Pet birds that are caged or perched outdoors for variable lengths of time are at increased risk.

Q: Can you get West Nile virus directly from birds?
A: There is no evidence that a person can get WNV from handling live or dead infected birds. However, persons should avoid barehanded contact when handling any dead animals. Dead birds can be double-bagged and placed in the regular trash for disposal.

Q: What are the symptoms of West Nile Fever in horses?
A: WNV primarily affects the brain and nerves. Therefore, symptoms may include a change in personality, hyperresponsiveness to sound or touch, muscle tremors or twitching, stumbling and falling, or circling. The illness may progress to more serious symptoms such as inability to stand, seizures, and death.

Q: How can I protect my horse from West Nile virus?
A: The most effective and preferred way of reducing a horse's risk of developing West Nile encephalitis is vaccination. Two WNV vaccination products are currently available from your veterinarian or over the counter. Initially, a horse must receive two doses of vaccine given three to six weeks apart to be fully immunized. Then an annual booster must be administered to maintain protection. Due to the longer mosquito season in southern states, your veterinarian may recommend more frequent boosters. Horses that are only vaccinated against “sleeping sickness” (Eastern equine encephalitis, Western equine encephalitis or Venezuelen equine encephalitis) are NOT protected against WNV.

Q: Can you get infected with West Nile virus by caring for an infected horse?
A: No. WNV is transmitted by infected mosquitoes. There is no documented evidence of animal-to-person transmission of WNV. However, precautions should be taken to avoid contact with the horse’s saliva because the symptoms of rabies and West Nile encephalitis may be indistinguishable. There is no need to isolate or quarantine a horse that is diagnosed or suspected of being infected with WNV.

Q: Can a mosquito transmit West Nile virus from an infected horse or human?
A: No. A human or a horse that is infected with WNV does not have a high enough concentration of the virus in its blood or tissues to be a source of virus for biting mosquitoes. This is why horses or people that develop illness from WNV do NOT need to be quarantined or otherwise isolated.

Q: Can a horse infected with West Nile Virus infect horses in neighboring stalls?
A: No. WNV is not transmitted between horses. Horses in the same vicinity are being exposed to the same mosquito population, though, so it is common to see more than one infected horse in a herd if they are not protected by vaccination.

Q: Are duck and other wild game hunters at risk for West Nile virus infection?
A: Because of their outdoor exposure, game hunters may be at risk if they hunt in mosquito-infested areas. Using insect repellants will decrease their risk of acquiring a tick or mosquitoborne illness.

Q: Can I get West Nile Virus from cleaning or eating wild game?
A: Two persons are known to have acquired illness from WNV following knife injuries sustained while harvesting organs from birds infected with WNV. Direct handling of infected birds or animal tissue is insufficient to spread the virus. Hunters are advised to wear gloves when handling and cleaning animals to prevent exposure to other disease agents, such as Salmonella. Game meat should be thoroughly cooked. Normal cooking temperatures will inactivate WNV.

West Nile Virus (WNV)

Who is at risk of developing illness from WNV?

What can I do to prevent WNV?

What should I do if I believe I am ill with WNV?

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