What is arboviral encephalitis?

Encephalitis is an inflammation of the brain that can be caused by arboviruses (viruses carried by arthropods, such as mosquitoes and ticks) or by other types of viruses. In Illinois, arboviruses are primarily transmitted to humans by the bites of infected mosquitoes.

St. Louis encephalitis, West Nile virus and LaCrosse (California) encephalitis are the arboviral diseases most likely to occur in Illinois. These infections are most common in June through October when mosquitoes are active. Although they produce similar symptoms, these viruses vary in severity and, typically, affect different age groups. Fortunately, these types of viruses are transmitted by only a few types of mosquitoes and, usually, only a small proportion of those mosquitoes actually carry the virus.

How is arboviral encephalitis spread?

Infection with an arbovirus occurs only through the bite of an infected insect. These diseases are not transmitted from person to person. In Illinois, the primary vector (carrier) of St. Louis encephalitis and West Nile virus is the northern house mosquito (Culex pipiens). The northern house mosquito breeds in small stagnant bodies of water and receptacles--such as discarded tin cans, flower urns, old tires, buckets and other containers--that hold water. The mosquitoes are infected with the St. Louis encephalitis and West Nile virus when they bite infected birds.

The mosquito that transmits LaCrosse encephalitis is the tree-hole mosquito (Aedes triseriatus). Found in many wooded areas, the tree-hole mosquito breeds in water-filled discarded tires and other containers and in tree holes. LaCrosse encephalitis infection in mosquitoes occurs when they bite small mammals 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 asAedes 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 only. Approximately 1 percent to 2 percent will develop recognizable symptoms. The symptoms of West Nile Virus, St. Louis encephalitis and LaCrosse encephalitis are similar. Some persons may have mild symptoms, such as a fever and headache. 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, West Nile Virus and St. Louis encephalitis 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.

LaCrosse encephalitis most often occurs in children. Symptoms are generally milder than those of West Nile Virus and St. Louis encephalitis, and fatalities rarely occur. However, studies indicate children with LaCrosse encephalitis may experience some reduction in intellectual ability.

Infection with an arbovirus provides 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. A follow-up blood test two or three weeks after the onset of symptoms is almost always necessary to confirm an arbovirus diagnosis. A physician will attempt to relieve symptoms of the illness, but there is no specific medication available to treat or cure the disease.

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 doughnut-shaped Bti briquets (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 more DEET a product contains, the longer the repellant can protect against mosquito bites. However, concentrations higher than 50 percent do not increase the length of protection. For most situations, 10 percent to 25 percent DEET is adequate. Apply repellents to clothes whenever possible; apply sparingly to exposed skin if label permits. Consult a physician before using repellents on young children.