Temporal relation between Ixodes scapularis abundance and risk for Lyme disease associated with erythema migrans

Understanding the role that nymphal and female ticks, Ixodes scapularis, ha ve in the epidemiology of Lyme disease is essential to the development of s uccessful prevention programs. In this study, the authors sought to evaluat e the seasonal and annual relations between tick densities and patients gre ater than or equal to 16 years of age diagnosed with erythema migrans (EM), the rash associated with early Lyme disease. Ticks were collected weekly b y drag sampling throughout most of the year from 1991 to 1996 in Westcheste r County, New York. The number of EM cases was based on patients diagnosed at the Westchester County Medical Center using Centers for Disease Control and Prevention (CDC) criteria. No patients with EM were diagnosed from Janu ary through April, when only adult ticks were active. Correlation analysis between monthly tick densities and EM incidence was significant for nymphs (r = 0.87, p < 0.01), but not for adult ticks (r = -0.57, p > 0.05). There was a strong, although not significant, correlation between peak annual num ber of patients with EM and peak nymphal tick abundance (r = 0.76, p = 0.08 ). These data indicate that bites from adult I. scapularis only rarely resu lt in Lyme disease, and that annual nymphal tick abundance determines expos ure. This suggests that annual fluctuations in Lyme disease case numbers ar e largely due to natural changes in tick abundance and, therefore, that con trol of nymphal I. scapularis should be a major component of Lyme disease p revention efforts.