Interactions between ecto- and endoparasites in trout Salmo trutta

Trout fry were experimentally infected with endoparasites (Anisakis sp.) and/or ectoparasites (Gyrodactylus derjavini). During the following 3 weeks the infection level of G. derjavini was significantly lowered in fish infected with live anisakids compared to fish injected with physiological saline, a corresponding amount of Anisakis protein sonicate or to untreated fish. Subsequent scanning electron microscopy (SEM) of recovered nematodes from the trout showed an extensive colonization of the worms with host macrophages. It is indicated that the activation of the fish immune system by infection with live anisakids influences the skin response of the host with a subsequent negative effect on the ectoparasites G. derjavini.