Kudoa thyrsites (Myxozoa: Myxosporea) Infections in Pen-Reared Atlantic Salmon in the Northeast Pacific Ocean with a Survey of Potential Nonsalmonid Reservoir Hosts

Abstract

The seasonality of the infective stage of Kudoa thyrsites was investigated via natural exposure of Atlantic salmon Salmo salar held in seawater net-pens in Departure Bay near Nanaimo, British Columbia, an area for which this parasite is enzootic. Atlantic salmon were exposed for various 8-week periods throughout the year, and it was determined that the infections were readily contracted in the summer and fall but not through the winter and early spring months. The progression of K. thyrsites infections was followed in populations of Atlantic salmon held in either seawater net-pens or seawater tanks for periods of up to 20 months. The majority of postsmolts contracted infections within a few months after their transfer to the seawater net-pens, and sporulation of the parasite occurred 4–6 months posttransfer. Mature myxospores could be detected as early as 1,000 degree-days postexposure (p.e.); however, the majority of infections did not progress to sporulation until 2,000 degree-days p.e. The prevalence of infection, determined by detection of myxospores in muscle tissue with wet-mount preparations, indicated that the population almost completely recovered from the original infections. It also appeared that if Atlantic salmon are permitted to undergo sexual maturation, the prevalence of K. thyrsites rises dramatically within the net-pen population. A survey of nonsalmonid fishes was conducted to identify potential reservoirs for the infection. Nonsalmonid fishes were collected from the vicinity of seawater net-pens and during research cruises off the coast of Vancouver Island, British Columbia. Of 31 nonsalmonid fish species belonging to 27 genera examined for infection, 5 species were identified as hosts for the parasite: rock sole Pleuronectes bilineatus; lingcod Ophiodon elongatus; threadfin sculpin Icelinus filamentosus; tube-snout Aulorhynchus flavidus; and arrowtooth flounder Atheresthes stomias. The reports from lingcod and threadfin sculpin are new host records for K. thyrsites. A second, unidentified Kudoa species was also observed in lingcod collected in Barkley Sound, British Columbia.