Minimum infection rate of Ambylomma americanum (Acari: Ixodidae) by Ehrlichia chaffeensis (Rickettsiales: Ehrlichieae) in southern Indiana.

In 1994 and 1995, 8 cases of human monocytic ehrlichiosis were confirmed. These cases originated from southern counties where the putative tick vector Ambylomma americanum (L.) is well established. To confirm the presence of Ehrlichia chaffeensis in ticks in southern Indiana and to determine the minimum infection rate, specimens of A. americanum were collected from 5 counties (7 sites). Nucleic acid was isolated from 88 pools of ticks (430 individuals) using an optimized phenol/CTAB (cetyltrimethylammonium bromide) extraction procedure and subjected to polymerase chain reaction analysis using species-specific 16S rRNA gene bacterial primers. Twenty-one of 88 pools (a minimum of 21 of 430 individuals) were positive for the presence of E. Chaffeensis, yielding an average minimum infection rate of 4.9%. Minimum infection rates at individual sites ranged from 0 to 9.4%. These data extend the known distribution of the bacterium to 3 southern counties of Indiana and suggest a higher prevalence of E. chaffeensis than previously reported for Missouri, North Carolina, or Kentucky.