Experimental Spironucleus infections in Atlantic salmon (Salmo salar) : Hidden secrets of the life cycle of Spironucleus salmonicida revealed

Abstract: We have performed experimental infections of Atlantic salmon (Salmo salar) from the Baltic Sea region with the protozoan parasite Spironucleus salmonicida. By this infection trial we have been able to show that the life cycle may use an alternative route of transmission. Instead of the commonly used faecal-oral route, the parasites can also be excreted directly into the surrounding water from the mucous layer of the skin or from an ulcerated skin lesion. Three different stages of the infection were identified: one intestinal, one blood stage and one tissue stage. New putative life-cycle forms of S. salmonicida cells detected by ex-vivo cell-cultures. Similar cells were also observed in imprints from skin lesions sampled during the experimental infection trials. The cells differed morphologically, from normal trophozoite cells and indicate an alternative cell-cycle when exposed to salmonid host cells.