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What is encephalitis?

Encephalitis is an inflammation of the brain and spinal cord usually caused by viral infection. Diseases such as rabies, polyomelitis and herpes encephalitis are all caused by viral infections that affect the brain and spinal cord and are transmitted in a variety of ways. Arboviral encephalitis refers to similar maladies that are transmitted by arthropods, mainly mosquitoes. Although the majority of cases of arboviral encephalitis infection are asymptomatic or have only very mild symptoms, the disease can sometimes damage nerves and can cause lasting damage and even death. Symptoms include sudden fever, headache, vomiting, unusual visual sensitivity to light, stiff neck and back, confusion, drowsiness, clumsiness, difficulty walking, and irritability.

Arboviral encephalitides are maintained in nature in complex life cycles involving a nonhuman primary vertebrate host and a primary arthropod vector and which usually do not include humans. Humans and domestic animals can contract the disease when the virus escapes the cycle and infects a secondary host. This can happen because of ecological or demographic changes, or due to population changes in the primary vector, host, or both. Many arboviruses that cause encephalitis have a variety of different vertebrate hosts and some are transmitted by more than one vector.

There are five major types of arboviral encephalitis in the United States:

• St. Louis encephalitis (SLE) — This virus causes the most common mosquito-borne disease in the United States. Like most types of viral encephalitis, it's transmitted to mosquitoes by birds. The mosquito vector of St. Louis encephalitis breeds in areas of standing water, including such places as discarded tires, polluted pools, roadside ditches, and containers such as birdbaths and flower pots.
• Eastern equine encephalitis (EEE) — This is the most serious encephalitis virus in North America. As the name suggests, eastern equine afflicts horses, but it also can affect humans. Eastern equine encephalitis outbreaks occur most commonly in the eastern United States. This virus infects birds that live near freshwater swamps.
• Western equine encephalitis (WEE) — Like eastern equine encephalitis, this virus affects horses and humans. Most cases of western equine encephalitis are reported in the central and western plains of the United States. This virus flourishes in birds that live near irrigated fields and farming areas.
• LaCrosse (LAC) encephalitis has been identified in several midwestern and mid-Atlantic states. During an average year, about 75 cases of LAC encephalitis are reported to the CDC. Most cases of LAC encephalitis occur in children under 16 years of age. Vertebrate primary hosts in include chipmunks, tree squirrels and other small vertebrates that live in forest habitats.
• West Nile encephalitis (WNE) — This virus first appeared in the United States in 1999. It is normally found in Africa and the Middle East and in parts of Europe, Russia, India and Indonesia. The virus is very similar to the St. Louis virus in that birds are its main animal hosts.

Florida Medical Entomology Laboratory

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Volusia County, Florida.