PRODUCTION AND CHARACTERIZATION OF EHRLICHIA-RISTICII, THE AGENT OF POTOMAC HORSE FEVER, FROM SNAILS (PLEUROCERIDAE, JUGA SPP.) IN AQUARIUMCULTURE AND GENETIC COMPARISON TO EQUINE STRAINS

We report on the production and characterization of Ehrlichia risticii , the agent of Potomac horse fever (PHF), from snails (Pleuroceridae: Juga spp,) maintained in aquarium culture and compare it genetically t o equine strains. Snails were collected from stream waters on a pastur e in Siskiyou County, Calif., where PHF is enzootic and were maintaine d for several weeks in freshwater aquaria in the laboratory, Upon expo sure to temperatures above 22 degrees C the snails released trematode cercariae tentatively identified as virgulate cercariae, Fragments of three different genes (genes for 16S rRNA, the groESL heat shock opero n, and the 51-kDa major antigen) were amplified from cercaria lysates by PCR and sequenced. Genetic information was also obtained from E., r isticii strains from horses with PI-IF, The PCR positivity of snail se cretions was associated with the presence of trematode cercariae, Sequ ence analysis of the three genes indicated that the source organism cl osely resembled E., risticii, and the sequences of all three genes wer e virtually identical to those of the genes of an equine E. risticii s train from a property near the snail collection site. Phylogenetic ana lyses of the three genes indicated the presence of geographical E., ri sticii strain clusters.