Descriptions, development and pathogenicity of myxozoan (Myxozoa: Myxosporea) parasites of juvenile cyprinids (Pisces: Cyprinidae).

Approximately 5000 young of the year (0+) cyprinids comprising roach, chub, dace, minnow, bleak, bream, barbel and gudgeon were examined histologically for the presence of myxozoan infections. Thirteen myxozoans were identified to species, the majority being Myxobolus spp. In addition, two species of Myxidium and of Sphaerospora were recorded. All organs were examined, with the majority of infections being found in the gills, musculature and kidney. However, isolated spores were occasionally found in other tissues. Whilst roach contained the highest number of myxozoan species, it was chub that showed the greatest host response to sporogonic forms. Data are provided on spore morphology, pathogenic responses and tissue and host specificity of the myxozoans recorded.