First Equine Case of St. Louis Encephalitis Reported

A case of neurologic disease caused by St. Louis encephalitis virus (SLEV) has been found in a horse in Brazil. The horse died from the infection, which was thought to be the first instance of SLEV infecting an equine.

An outbreak of human encephalitis in St. Louis, Missouri, in 1933 was blamed on the virus, and more than a thousand cases were reported in the epidemic. While most human infections produce only mild illness, some individuals have flu-like symptoms or a high fever. In severe cases, swelling of the brain can cause neurological problems, altered consciousness, and headache. Damage to the central nervous system results in death in as many as 30% of human cases.

There is no preventive vaccine for the virus, which usually causes fewer than 130 diagnosed cases in the U.S. each year. The disease comes about most often in late summer or early fall, and can occur from Canada throughout the United States and south to Argentina. It has been detected in humans in Brazil for at least 40 years. Researchers said that the infected horse was carrying a newly isolated strain of the virus, which is in the same family as the West Nile and dengue fever viruses.

SLEV, which is spread to humans by mosquitoes, has been isolated in many species of migratory birds in Brazil. It has not previously been known to cause disease in other wild or domestic animals.