St. Louis encephalitis outbreak in Louisiana in 2001.

A description of the St. Louis encephalitis (SLE) outbreak in Northeast Louisiana is presented. In the fall of 2001 there were 63 cases of St. Louis encephalitis in Monroe and West Monroe and seven additional cases in the neighboring parishes of Richland, Morehouse and Franklin. All cases had a clinical presentation of encephalitis and were confirmed serologically. Clinically most cases presented with fever, meningitis syndrome with altered mental status. Tremors were common (56% of cases). There were three deaths. Age-group distribution showed a predominance among 45 and older. The epidemic curve based on diagnosis date showed an explosive outbreak starting August 8, 2001, reaching a peak by the second week, and progressively slowing down. The curve showed that by the time the first case was diagnosed, 60% or more of the cases were already infected. Most of the cases come from low socio-economic areas. Houses were often run down, many with screens in disrepair. Backyards were usually large, with heavy brush and many trees. There was an abundance of sources of mosquito larvae, particularly for Culex quinquefasciatus which is the main vector. Mosquito pools confirmed the presence of SLE virus. As soon as the first case was reported, a campaign of health education and increased mosquito adulticiding were implemented.