Arboviral Encephalitis

What is arboviral encephalitis?

Encephalitis is an inflammation of the brain that can be caused by an Arthropod borne virus (arbovirus). Arthropods include mosquitoes and ticks. In Illinois, arboviruses are transmitted to humans by the bites of infected mosquitoes.

Encephalitis due to St. Louis virus (SLE), West Nile virus (WNV) and California group viruses are the arboviral diseases that occur in persons in Illinois. WNV is the most commonly reported of the three diseases. Other viruses are in the California group; Lacrosse virus is the most common in Illinois.

Arboviral infections typically occur June through October when mosquitoes are active. Only a few types of mosquitoes carry and transmit these arboviruses and, usually only a small proportion of those mosquitoes actually carry the virus. Another virus, Eastern equine encephalitis (EEE) has been found in birds and horses in the state on rare occasions.

How is arboviral encephalitis spread?

Infection with an arbovirus occurs only through the bite of an infected arthropod such as a mosquito. These diseases are not transmitted from person to person. In Illinois, the primary vector (carrier) of SLE and WNV is the northern house mosquito (Culex pipiens). The northern house mosquito breeds in small stagnant bodies of water (like ditches, street “catch basins”) and receptacles - such as discarded tin cans, flower urns, old tires, buckets and other containers - that hold water. Mosquitoes become infected with the St. Louis encephalitis virus and WNV after biting infected birds.

The mosquito that transmits California (LaCrosse) encephalitis is the tree-hole mosquito (Aedes triseriatus). Found in wooded areas, the tree-hole mosquito breeds in water-filled discarded tires and other containers and in tree holes. California (LaCrosse) encephalitis virus infection in mosquitoes occurs when they bite small mammals that carry the virus or when an infected female mosquito transmits the infection to her offspring.

About two weeks after a heavy rain, large numbers of floodwater mosquitoes (such as Aedes vexans) can emerge from river floodplains and flooded woods. Although they can be a major nuisance problem for several weeks, floodwater mosquitoes have not been significant disease carriers in Illinois.

What are the symptoms of encephalitis?

Most persons bitten by an infected mosquito will experience no symptoms of the disease or will have very mild symptoms including fever and headache. Approximately 1 percent to 2 percent will develop recognizable symptoms. Symptoms of WNV, SLE and LaCrosse encephalitis virus are similar. Severe infection may produce a rapid onset of severe headache, high fever, muscle aches, stiffness in the back of the neck, problems with muscle coordination, disorientation, convulsions and coma. Symptoms usually occur five to 15 days after the bite of an infected mosquito.

Who is most susceptible to arboviral encephalitis?

Although anyone can be infected with an arbovirus, WNV and SLE usually occur in persons older than 50 years of age. Most patients recover fully, although severe infection may, infrequently, result in neurologic damage or death.

California (LaCrosse) encephalitis virus infection most often occurs in children. Illness is generally milder than those due to WNV and SLE, and fatalities rarely occur. However, studies indicate some children with California (LaCrosse) encephalitis virus may experience persistent neurologic problems. Infection with an arbovirus may provide some immunity to that specific virus, but not to other arboviruses.

How is encephalitis diagnosed and treated?

An arbovirus infection is usually diagnosed through a blood test or testing of cerebrospinal fluid. A physician will attempt to relieve symptoms of the illness, but there is no specific treatment or cure for these diseases.

How can arboviral encephalitis be prevented?

Because the mosquitoes that transmit arboviruses breed in small pools of water, removing potential breeding places is the most effective form of disease prevention. Here are a few suggestions:

  • Remove or empty water in old tires, tin cans, buckets, drums, bottles or other places where mosquitoes might breed. Be sure to check clogged gutters and flat roofs that may have poor drainage. Make sure cisterns, cesspools, septic tanks, fire barrels, rain barrels and trash containers are covered tightly with a lid or with 16-mesh screen.
  • Empty plastic wading pools at least once a week and store indoors when not in use. Swimming pools should be properly maintained; if not used, pools should be drained and kept dry during mosquito season.
  • Change the water in bird baths, plant saucers and trays weekly.
  • Store boats covered or upside down, or remove rainwater weekly.
  • Empty your pet's water bowl and refill daily.
  • Level the ground around your home so water can run off and not collect in low spots. Fill in holes or depressions near your home that accumulate water.
  • Fill in tree rot holes and hollow stumps that hold water.
  • Stock ornamental water gardens with fish (e.g., minnows, "mosquito fish" or goldfish) that eat mosquito larvae
  • Small pools of water can be treated for mosquito larvae with "Bti," a bacterial insecticide. Many hardware stores carry Bti briquettes (such as donut-shaped Mosquito Dunks ®) for this purpose. Be sure to follow the insecticide label exactly.
  • Keep weeds and grass cut short; adult mosquitoes look for these shady places to rest during the hot daylight hours. If adult mosquitoes are present in high weeds along the edge of a yard, that location can be sprayed with an appropriately labeled insecticide.
  • Be sure screens in homes and buildings are intact and tight-fitting to prevent the entry of mosquitoes. Use a flyswatter or household spray to kill mosquitoes, flies or other insects that get into buildings.
  • Some mosquito control methods are not very effective. Bug zappers and anti-mosquito buzzers (or sound devices) are not effective in controlling biting mosquitoes. Various birds and bats will eat mosquitoes, but there is little scientific evidence that this reduces mosquitoes around homes.
  • Some communities conduct community-wide mosquito abatement programs. Whenever possible, the primary effort of such programs should be identification of mosquito-breeding sites, followed by removal or treatment of these sites with an insecticide used for control of mosquito larvae.
  • If participating in outdoor activities when mosquitoes are biting, wear protective clothing (shoes, socks, shirt and long pants). For additional protection from mosquitoes, use an insect repellent. The Environmental Protection Agency (EPA) registers products for use as mosquito repellents. Products containing DEET, picaridin, oil of lemon eucalyptus (PMD), or IR3535 typically provide reasonably long-lasting protection when applied to skin and clothing. The CDC also recommends the use of permethrin-containing products on clothing, shoes, bed nets, and camping gears. The efficacy and duration of protection may vary among products and types of mosquitoes. If you start to get mosquito bites, reapplication maybe needed.

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Illinois Department of Public Health
535 West Jefferson Street
Springfield, Illinois 62761
Phone 217-782-4977
Fax 217-782-3987
TTY 800-547-0466
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