St. Louis Encephalitis

St. Louis Encephalitis is a disease caused by the Culex mosquito borne St. Louis Encephalitis virus. It is related to Japanese encephalitis virus and is a member of the Flaviviridae subgroup. It mainly affects the United States and occasionally hits Canada and Mexico.

The name goes back to 1933 within five weeks in autumn an encephalitis epidemic of explosive proportions broke out in the vicinity of St. Louis, Missouri. Over 1000 cases were reported and the National Institute of Health was appealed to for epidemiological and investigative expertise. The NIH team isolated the previously unknown virus from monkeys and then in white mice.

Mosquitoes become infected from feeding on birds that are infected with St. Louis encephalitis virus. The mosquitoes then transmit the virus to humans and animals. The virus grows both in the infected mosquitoes and the infected bird but does not make either sick. The virus is not transmissible from one human to another.

Most infections result in mild illness and include fever and headache. More severe infections include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, and occasional convulsions. Fatality rates range from 3 to 30% and are higher for older aged people.

An average of 128 cases are recorded annually in the United States. In temperate areas of the US cases occur primarily in the late summer or early fall. Where the climate is milder encephalitis can occur year round. There are quite a few vaccines specifically for St. Louis encephalitis virus.