La Crosse Encephalitis 2002

Table 1. La Crosse Encephalitis Cases by Race and Sex, Indiana, 2002

  2002 1998-2002
Cases Rate* Cases
Total 4 .06 12
   White 2 .04 6
   Black 1 .19 1
   Other 1 .66 1
   Not Reported 0   4
   Male 3 .10 9
   Female 1 .03 3

*Rate per 100,000 population based on the U.S. Census Bureau’s population data as of July 1, 2002

La Crosse encephalitis virus is endemic in Indiana. The disease is found primarily in the eastern United States where hardwood forests exist. The disease is maintained in nature in a cycle between the tree-hole mosquito, Ochlerotatus triseriatus, and small woodland mammals such as squirrels and chipmunks.

During the five-year period 1998-2002, 12 cases of La Crosse encephalitis were reported in Indiana, for an average of 2.4 cases/year. Figure 1 shows the number of reported cases per year from 1998-2002.

The female to male ratio was 1:3. Cases ranged in age from 4-11 years, with over half of the cases in the 5-9 years age group (Figure 2).

Cases were diagnosed in seven counties; however, no county reported five or more cases. Historically, cases have been concentrated in two areas, Allen and Decatur Counties.

Clinically recognized infections occur mainly in children under 16 years of age. Symptoms include headache, fever, nausea, vomiting, drowsiness, and disorientation. Severe cases may result is seizures or coma. Cases are rarely fatal but may result in learning disabilities in recovered individuals. It has been estimated that for every symptomatic case there are 1,500 asymptomatic cases.

You can learn more about La Crosse encephalitis by visiting the following Web site: