Detection of Ehrlichia risticii, the agent of Potomac horse fever, in freshwater stream snails (Pleuroceridae: Juga spp.) from northern California

Abstract

Ehrlichia DNA was identified by nested PCR in operculate snails (Pleuroceridae: Juga spp.) collected from stream water in a northern California pasture in which Potomac horse fever (PHF) is enzootic. Sequencing of PCR-amplified DNA from a suite of genes (the 16S rRNA, groESL heat shock operon, 51-kDa major antigen genes) indicated that the source organism closely resembled Ehrlichia risticii, the causative agent of PHF. The minimum percentage of Juga spp. harboring the organism in the population studied was 3.5% (2 of 57 snails). No ehrlichia DNA was found in tissues of 123 lymnaeid, physid, and planorbid snails collected at the same site. These data suggest that pleurocerid stream snails may play a role in the life cycle of E. risticii in northern California.

FIG. 1

Pleurocerid snails collected from stream water fed by the Shasta River in Weed, Calif. (Siskiyou County). Bar = 1 cm. (Photograph courtesy of Jerry Fields.)

FIG. 2

E. risticii nested PCRs performed for the 16S rRNA, groEL (of groESL), and 51-kDa antigen genes by using DNA from the Shasta snail-1 (lanes 1) and Shasta snail-2 (lanes 2) pools and positive (lanes +) and negative (lanes −) E. risticii DNA controls. Lane φ, φX174 replicative-form DNA HaeIII digest (molecular size marker).

FIG. 3

Deduced amino acid sequences of GroEL segments. Periods indicate conserved positions relative to the E. risticii sequence.

FIG. 4

Deduced amino acid sequences of E. risticii 51-kDa major surface antigen segments. Periods indicate conserved positions relative to the E. risticii sequence.

FIG. 5

Phylogenetic tree generated by GrowTree from a DNAML-based alignment of 16S rRNA gene sequences (length, ca. 1,400 bp), showing the relationship of the Shasta snail-1 and Shasta snail-2 ehrlichiae to other rickettsiae. ER, Ehrlichia risticii.