Transmission of Ehrlichia risticii, the agent of Potomac horse fever, using naturally infected aquatic insects and helminth vectors: preliminary report

Ehrlichia risticii, the agent of Potomac horse fever (PHF), has been recent ly detected in trematode stages found in snail secretions and in aquatic in sects. Based on these findings, horses could conceivably be exposed to E. r isticii by skin penetration with infected cercariae, by ingestion of infect ed cercariae in water or via metacercariae in a second intermediate host, s uch as an aquatic insect, In order to test this hypothesis, horses were cha llenged with infectious snail secretions and aquatic insects collected from a PHF endemic region in northern California. Two horses stood with their f ront feet in water harbouring E, risticii-infected cercariae, 2 horses dran k mater harbouring E, risticii-infected cercariae, and 6 horses were fed po ols of different aquatic insects harbouring E, risticii-infected metacercar iae. In this preliminary study, only the one horse infected orally with mature c addisflies (Dicosmoecus gilvipes) developed the clinical and haematological disease syndrome of PHF, The agent was isolated from the blood of the infe cted horse in a continuous cell line and identified as E. risticii by chara cterisation of the 16S rRNA gene. Therefore, E. risticii is maintained in n ature in a complex aquatic ecosystem and transmission to horses can occur t hrough accidental ingestion of insects such as caddisflies containing infec ted metacercariae, At present, the small number of horses used in this stud y does not exclude other insects and free trematode stages as potential sou rces of infection.