Natural and laboratory transmission of the marine myxozoan parasite Kudoa thyrsites to Atlantic salmon

Experiments were conducted to evaluate routes of transmission and subsequen t development of Kudoa thyrsites (Myxozoa: Myxosporea) in Atlantic salmon S almo salar. Atlantic salmon that were held for a 2-week exposure period at the seawater net-pen site and then transferred to freshwater tanks develope d K. thyrsites infections as evidenced by the presence of myxospores in the somatic musculature at 3 months postexposure. Attempts to transmit the par asite directly from fish to fish by intubation of fresh myxospores were uns uccessful. However, the parasite was transmitted to Atlantic salmon by intr aperitoneal injection of blood collected from a coho salmon Oncorhynchus ki sutch infected with K. thyrsites. The waterborne infective stage of K. thyr sites was not removed by filtration of seawater entering the tanks at the P acific Biological Station.