THE SALMONID ONCORHYNCHUS MYKISS AND BRYOZOAN FREDERICELLA BOREALIS SERVE AS ACTIVE HOSTS FOR THE NORTH AMERICAN...

The myxozoan parasite responsible for proliferative kidney disease (PKD), Tetracapsuloides bryosalmonae, alternates between salmonid fishes and freshwater bryozoans to complete its life cycle. However, no active hosts (species that shed viable, infectious spores) for the North American (NA) strain have been identified. We used fish-to-bryozoan and bryozoan-to-fish transmission experiments to assess the active host status of steelhead trout [Oncorhynchus mykiss] and the bryozoan Fredericella borealis. Specific pathogen-free F. borealis colonies were first exposed to effluent from naturally infected O. mykiss and monitored weekly using stereomicroscopy for overt T. bryosalmonae infections. Malacospores from infected colonies were then introduced to naïve O. mykiss, which were held at 16 C and sampled for polymerase chain reaction (PCR) and histology 2 and 8 wk post-exposure. Additionally, statoblasts and hatched zooids from overtly infected F. borealis colonies were screened with stereomicroscopy and quantitative real-time PCR (qPCR) to investigate transmission of T. bryosalmonae from zooids to statoblasts. Our results demonstrate that subclinically-infected O. mykiss shed viable fish malacospores of the NA strain of T. bryosalmonae, as evidenced by the development of overt infections in exposed F. borealis. Malacospores shed from the bryozoan subsequently induced clinical PKD in naïve O. mykiss, which exhibited kidney swelling upon necropsy, extrasporogonic stages of the parasite in histology, and PCR detections of T. bryosalmonae DNA. We found no evidence of vertical transmission within F. borealis. Given the widespread rearing and stocking of O. mykiss across North America, these findings highlight the importance of pre-release surveillance in susceptible stocks to prevent dissemination of T. bryosalmonae in habitats where suitable host populations may overlap.