What is St. Louis encephalitis?
St. Louis encephalitis (SLE) is a mosquito-borne viral disease that causes inflammation of the brain.

How do people get SLE?
By the bite of a mosquito (primarily the Culex species) that become infected with SLE virus.

What is the basic transmission cycle?
Mosquitoes become infected by feeding on birds infected with the SLE virus. Infected mosquitoes then transmit the SLE virus to humans and animals during the feeding process. The SLE virus grows both in the infected mosquito and the infected bird, but does not make either one sick.

Could you get the SLE from another person?
No, SLE virus is NOT transmitted from person-to-person. For example, you cannot get the virus from touching or kissing a person who has the disease, or from a health care worker who has treated someone with the disease.

Could you get SLE directly from birds or from insects other than mosquitoes?
No. Only infected mosquitoes can transmit SLE virus.

What are the symptoms of SLE?
Mild infections occur without apparent symptoms other than fever with headache. More severe infection is marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, occasional convulsions (especially in infants) and spastic (but rarely flaccid) paralysis.

What is the incubation period for SLE?
Usually 5 to 15 days.

What is the mortality rate of SLE?
Case-fatality rates range from 3% to 30% (especially in the aged).

How is SLE treated?
There is no specific therapy. Intensive supportive therapy is indicated.

Who is at risk of contracting SLE?
While the virus can affect anyone, it has its greatest impact on the very young and the very old because their immune systems are either in a state of development or decline.

Is there a vaccine for SLE?

How common is SLE in Connecticut?
There has never been a human case of SLE known to have been acquired in Connecticut.

How common is SLE in the United States?
SLE is the most common epidemic mosquito-borne viral disease in the United States (US). Since 1964 there have been 4,478 reported human cases of SLE, with an average of 128 cases reported annually. The outbreak of SLE in New York City in 1999 is the first recognized outbreak of SLE in the northeastern US since an outbreak in New Jersey in 1975.

What can you do to protect yourself from mosquito-borne viral disease?
The only way to protect yourself is to keep mosquitoes from biting you.

  • If you must be outdoors at dawn and dusk when mosquitoes are most active, wear long- sleeved shirts and long pants.
  • Use insect repellent with no more than 30% DEET, but use sparingly and with care. (Products containing 15% or less DEET are recommended for children, but products containing DEET should not be used on infants. Carefully read and follow directions on the container and wash treated skin when mosquito exposure has ended).
  • Take special care to cover up the arms and legs of children playing outdoors near swampy areas. When you bring a baby outdoors, cover the playpen or carriage with mosquito netting.

What can you do to reducing the number of mosquitoes around your home?

  • Mosquito breeding around the home can be reduced significantly by reducing the amount of standing water available for mosquito breeding.
  • Dispose of tin cans, plastic containers, ceramic pots or similar water-holding containers that have accumulated on your property. Do not overlook containers that have become overgrown by aquatic vegetation.
  • Pay special attention to discarded tires that may have accumulated on your property. The used tire has become the most important domestic mosquito producer in this country.
  • Drill holes in the bottom of recycling containers that are left out of doors. Drainage holes that are located on the sides collect enough water for mosquitoes to breed in.
  • Clean clogged roof gutters on an annual basis, particularly if the leaves from surrounding trees have a tendency to plug up the drains. Roof gutters are easily overlooked but can produce millions of mosquitoes each season.
  • Turn over plastic wading pools when not in use. A wading pool becomes a mosquito producer if it is not used on a regular basis.
  • Turn over wheelbarrows and do not allow water to stagnate in bird baths. Both provide breeding habitat for domestic mosquitoes.
  • Aerate ornamental pools or stock them with fish. Water gardens are fashionable but become major mosquito producers if they are allowed to stagnate. Clean and chlorinate swimming pools that are not being used. A swimming pool that is left untended by a family that goes on vacation for a month can produce enough mosquitoes to result in neighborhood-wide complaints. Be aware that mosquitoes may even breed in the water that collects on swimming pool covers.
  • Use landscaping to eliminate standing water that collects on your property. Mosquitoes will develop in any puddle that lasts more than 4 days.



This fact sheet is for informational purposes only. It should not be used for self-diagnosis or as a substitute for consultation with a health care provider. If you think that you may have this infection, or have questions about the disease described above, you should consult your health care provider.



For additional information on this disease, visit the Centers or Disease Control and Prevention website.

To contact the Epidemiology and Emerging Infections Program, please call 860-509-7994.