The equine monocytic ehrlichiosis in the region South of the Rio Grande do Sul has demonstrated to be important in the creations of Crioulo horses. It has been reported as cause of diarrhea in equine not surround and as cause of losses with treatments and death of the animals, being pointed as a limited factor in the range breeding in some regions. The way of transmission for the oral way, intermediated for trematodes in aquatic environments, has been supported. Freshwater snails are involved as intermediate hosts of trematodes and harbor of the Neorickettsia risticii. A total of 16,846 Heleobia snails had been collected in the cities of Arroio Grande, Rio Grande, Palmares do Sul and Santa Vitoria do Palmar, 92.2% of which had been found in the roots of aquatics plants (Eichornea spp.). The frequency of trematodes present in the snails varied of 2.3 to 12.8% in the collected regions. Three types of cercariae were found, morphology type 1, morphology type 2 and morphology type 3, and two morphologic type of metacercariae of the snails. A total of 357 insects of the Odonata order were collected, stages of metacercariae had been found in the tissues of suborder Anisoptera with 5.3% of frequency. More studies are necessary for identification of the joined larval phases, as well as knowing the host definitive and identifying the adult parasite and the relation of its cycle of life with the occcurrence of equine monocytic ehrlichiosis.
trematodes; Heleobia spp.; Neorickettsia risticii.