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Arboviruses: Cause of Encephalitis in Horses and Humans

Dr. Allan Paul
July 24, 2003

Recently, a case of eastern equine encephalitis (EEE) was diagnosed in a person in Illinois. In the eastern United States an unusually high number of cases have already been reported in horses for this time of year.

EEE is one of four main types of encephalitis (or inflammation of the brain) caused by arboviruses in the United States; the others are western equine encephalitis (WEE), St. Louis encephalitis (SLE), and LaCrosse (LAC) encephalitis. An arbovirus is a type of virus that is transmitted by arthropods (insects, spiders, etc.), and in the case of these four diseases the virus is transmitted by mosquitoes.

The majority of humans infected with these viruses either experience no symptoms or have a non-specific flu-like syndrome, with fever, headache, muscle aches, and malaise. Onset of the disease may be slow or sudden. In some cases, however, infection may lead to encephalitis, with a fatal outcome or permanent neurologic damage.

In horses arboviruses produce mild to severe signs, which may include fever, weakness, loss of appetite, incoordination, circling, paralysis, coma, and death.

EEE was first identified in the 1930s and currently occurs along the eastern seaboard, the Gulf Coast, and some inland Midwestern locations of the United States. In addition to humans, horses and some birds--such as pheasants, quail, ostriches, and emus--are susceptible to severe disease caused by the EEE virus. The virus's natural host is birds.

WEE was first isolated in California in 1930 and remains an important cause of encephalitis in horses and humans in North America, mainly in western parts of the U.S. and Canada. This virus also naturally resides in birds, such as the house sparrow. Children, especially those under one year old, are affected more severely and may be left with permanent damage.

SLE is the human disease agent most commonly transmitted by mosquitoes in the U.S. While periodic SLE outbreaks have occurred only in the Midwest and southeast, SLE virus is distributed throughout the lower 48 states. The disease is generally milder in children than in adults; the elderly are at the highest risk for severe disease and death. The virus that causes SLE is maintained in nature by a bird-mosquito cycle.

LAC was discovered in Wisconsin in 1963. Since then, the virus has been identified in several Midwestern and Mid-Atlantic states. In nature, the virus cycles between mosquitoes and mammalian hosts, such as chipmunks and squirrels. Severe disease occurs most commonly in children under the age of 16.

There are no commercially available human vaccines for these diseases, although vaccines against EEE, WEE, and Venezuelan equine encephalitis (VEE) are available for horses.

Arbovirus disease in humans can be prevented in two major ways: * through personal protective measures, such as reducing time outdoors, particularly in early evening hours; wearing long pants and long-sleeved shirts; and applying mosquito repellent to exposed skin; and * through public health measures to reduce the population of infected mosquitoes, for example, the spraying of insecticides to kill juvenile and adult mosquitoes.

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