Infection rate of Ehrlichia risticii, the agent of Potomac horse fever, infreshwater stream snails (Juga yrekaensis) from northern California

Juga yrekaensis freshwater snails were tested for trematode stages and for Ehrlichia risticii DNA using a nested PCR assay. Snails were collected mont hly from two Potomac horse fever (PHF) endemic locations in northern Califo rnia (Montague and Weed). The trematode infection rate varied between 40 and 93.3% in large snails (s hell size > 15 mm) and between 0 and 13.3% in small snails (<15 mm). The hi ghest trematode infection rate for large and small snails was recorded in S eptember and the lowest infection rate for large snails was recorded in Jun e (Weed) and October (Montague). The E. risticii PCR infection rate among s mall snails from both sites was similar and varied monthly between 0 and 3. 3%. The PCR infection rate for large snails from Weed was high in May (20.0 %) and decreased progressively until November (10.0%). The PCR infection ra te for large snails from Montague was 5.0% in May, 26.3% in August and 16.7 % in October. PCR-positive snails were always related to the microscopic de tection of trematode stages (virgulate cercariae). This study provides evidence that J. yrekaensis are infected with trematode cercariae that harbor E. risticii. The number of snails harboring trematod e stages and the number of PCR positive snails varied with the size of the snails, the month of collection, and the geographic origin. (C) 2000 Elsevi er Science B.V. All rights reserved.