Useful Information

St. Louis Encephalitis

September 13, 1999

Dear Doctor,

St. Louis Encephalitis

There has been report of an ongoing outbreak of St. Louis encephalitis in New York City in the United States of America. As of 12 September 1999, there have been reports of 9 confirmed cases with 3 deaths and another 89 suspected cases.

St. Louis encephalitis is an acute viral infection involving the central nervous system. It is caused by flaviviruses through an infective mosquito vector (primarily the Culex species). The main natural host is wild birds. Human cases occur mainly in North America and are rarely reported in other parts of the world. The disease is NOT transmitted from human-to-human.

Incubation period of the infection ranges from 5 to 15 days. The majority of human infections are asymptomatic or may result in a non-specific flu-like syndrome. A small proportion of infected persons progress to encephalitis which is marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions and spastic paralysis. Diagnosis can be confirmed by serological tests. Infection can occur in all age groups. The reported case-fatality rates range from 3% to 30%; higher among the elderly. There is no specific therapy and treatment is mainly supportive. There is currently no vaccine available against the infection.

Doctors should be on the alert when seeing patients coming back from areas with reported increased incidence of St. Louis encephalitis. Travellers going to these areas should be advised to take precautionary measures to prevent mosquito bite. These include:

• wearing long sleeved clothes and trousers, and to use insect repellents over exposed parts of the body when outdoors,
• avoiding unnecessary outdoor activity during evening and night hours, and
• using mosquito screens or nets when the room is not air-conditioned.

Yours sincerely,

(Dr. L Y Tse )

for Director of Health