Symptomatic La Crosse virus infections of the central nervous system: a study of risk factors in an endemic area.

In most years, La Crosse virus is the most common cause of reported mosquito-borne illness in the United States. The authors conducted a case-control study to determine if behavioral and environmental factors influenced the risk of La Crosse virus illness. Data were gathered on 31 serologically confirmed cases and 60 age-, sex-, and geography-matched controls in West Virginia in 1987 and 1988. Univariate analysis revealed minimal elevation of disease risk (odds ratios (ORs) less than 2.0) with increased time outdoors, non-use of insect repellent, non-use of air conditioning, lack of screened windows, and not wearing protective clothing. Univariate and multivariate analysis indicated that the presence of tree holes significantly increased disease risk (OR = 8.5 for greater than or equal to 1 tree hole vs. 0 tree holes). The following factors may also increase disease risk, although the findings were not statistically significant: discarded tires (OR = 3.2 for greater than or equal to 10 tires vs. 0-9 tires); non-tire artificial containers (OR = 4.1 for greater than or equal to 6 containers vs. 0-5 containers); and close proximity of the house to the forest edge (OR = 3.2 for 0-49 ft (0-14.9 m) vs. greater than or equal to 50 ft (greater than or equal to 14.9 m)). The authors conclude that the presence of natural breeding sites (tree holes) is an important risk factor for La Crosse virus illness. These results may be important in guiding future efforts aimed at preventing infection with La Crosse virus.