Infection of aquatic insects with trematode metacercariae carrying Ehrlichia risticii, the cause of Potomac horse fever

We provide evidence of Ehrlichia risticii Holland, the agent of Potomac hor se fever, in trematode stages found in aquatic insects collected from a pas ture stream in northern California, using nested polymerase chain reaction (PCR) amplification and sequence analyses of the 16S rRNA, 51 kDa major ant igen and groEL heat shock protein genes. E, risticii was detected in metace rcariae found in the immatures and adults of the following insects: caddisf lies (Trichoptera), mayflies (Ephemeroptera), damselflies (Odonata, Zygopte ra), dragonflies (Odonata, Anisoptera),and stoneflies (Plecoptera). The pre valence of E, risticii was 31.9% (n = 454 individuals) in aquatic insects ( 13 of 17 species were positive). Prevalence within orders was as follows: 4 3.5% (n = 207) in caddisflies, 15.2% (n = 92) in mayflies, 13.9% (n = 115) in damselflies, 10.0% (n = 10) in dragonflies, and 80.0% (n = 30) in stonef lies. This study demonstrates a broad intermediate host range for trematode s that act as vector for E. risticii. Insects are likely to play an importa nt role in the epidemiology of this disease.