Transmission of Ehrlichia risticii, the agent of Potomac horse fever, using naturally infected aquatic insects and helminth vectors: preliminary report.
Authors: Madigan J E, Pusterla N, Johnson E, Chae J S, Pusterla J B, Derock E, Lawler S P
Journal: Equine veterinary journal
Summary
# Potomac Horse Fever: Understanding Natural Transmission via Aquatic Insects Potomac horse fever (PHF), caused by *Ehrlichia risticii*, has long posed a diagnostic puzzle for equine practitioners, with transmission routes remaining incompletely understood until recent discoveries of the organism in aquatic trematodes and their insect vectors. Madigan and colleagues conducted experimental transmission trials in northern California's PHF-endemic region, exposing horses to infectious snail secretions through cutaneous contact, oral water ingestion, or feeding with naturally infected aquatic insects collected from the field. Only one horse—those fed mature caddisflies (*Dicosmoecus gilvipes*)—developed clinical PHF with characteristic haematological changes, with *E. risticii* subsequently isolated and confirmed via 16S rRNA gene analysis. These findings establish that incidental ingestion of infected caddisfly metacercariae represents a viable natural transmission route, suggesting horses acquire infection through grazing and drinking near contaminated water sources rather than solely through direct cercarial penetration. Whilst this preliminary work involved limited horse numbers and cannot exclude other aquatic insects or developmental stages as infection sources, it fundamentally reframes PHF epidemiology as a water-associated disease linked to aquatic insect populations—a consideration that should inform grazing management, water provision strategies, and seasonal risk assessment for horses in endemic regions.
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Practical Takeaways
- •PHF transmission occurs through ingestion of aquatic insects containing infected metacercariae, particularly caddisflies, suggesting horses grazing or drinking near endemic water sources are at risk
- •Clinical vigilance is needed in PHF endemic regions (northern California identified) with heightened awareness during seasons of high aquatic insect activity near pastures and water sources
- •Prevention strategies should focus on reducing horse access to or consumption of infected aquatic insects rather than preventing cercarial skin penetration, though transmission routes require further investigation
Key Findings
- •Only 1 of 10 horses experimentally challenged developed PHF after oral ingestion of infected caddisflies (Dicosmoecus gilvipes)
- •E. risticii was isolated from infected horse blood and confirmed by 16S rRNA gene characterization
- •Skin penetration with cercariae and ingestion of free cercariae did not result in infection in this preliminary study
- •E. risticii is maintained in aquatic ecosystems with transmission occurring through accidental ingestion of infected aquatic insects