Temperature- and salinity-dependent development of a Nordic strain of Ichthyophthirius multifiliis from rainbow trout

During the twentieth century evidence was presented which suggested the presence of various strains and races of the parasite Ichthyophthirius multifiliis Fouquet. However, ecological profiles of various parasite isolates from different climatic zones are sparse. Such stringent characterizations of parasite development at defined abiotic conditions could provide valuable criteria for the different races; profile comparison from various localities is one way to differentiate these strains. Baseline investigations were therefore performed on the associations between abiotic factors (temperature/salinity) and the development of theronts in tomocysts of I. multifiliis isolated from rainbow trout in a Danish trout farm. It was shown that tomocyst formation and theront development took place between 5 and 30°C. Development rates and sizes of theronts were clearly affected by temperature: theronts escaped tomocysts already after 16–27 h at 25°C and 30°C, whereas this process took 8–9 days at 5°C. Likewise, theront size decreased steadily from a maximum of 57.4 × 28.6 μm at 5°C to 28.6 × 20.0 μm at 30°C. This size variation was only partly associated with the number of theronts that appeared at different temperatures. The lowest number of theronts escaping from one tomocyst was indeed found at 5–7°C (mean 329–413). At 11.6, 17.0 and 21°C, the highest number of theronts appeared (mean 546–642). However, at 25 and 30°C, the number decreased (458 and 424, respectively). Additional studies on the salinity dependent development of the parasite (at 11.6°C) showed that salinities above 5 p.p.t. totally inhibited development. Even at 5 p.p.t. the developmental time significantly increased and the number of theronts produced from one tomocyst decreased.