Sustained growth of Ichthyophthirius multifiliis at low temperature in the laboratory

Applied and basic research on the ciliate Ichthyophthirius multifiliis, an obligate parasite of freshwater fishes, requires passage on fish hosts to maintain laboratory stocks. However, continual repeated passage results in senescence of parasite clones over time. Because growth and development are directly correlated to water temperature, our objective was to grow the parasite at low temperature in order to extend the period that the organism remains on the fish, thus reducing: (1) the number of passages and (2) the number of fish required to maintain the parasite over time. Lowering of water temperature from 25 C to 9 C resulted in significant slowing of growth on channel catfish (parasites remained on fish for 20.4 days at 9 C, as compared to 5-6 days at 25 C), with no discernible changes in viability, antigenicity, or infectivity. Low-temperature growth is proposed as an improved method for stable maintenance of I. multifiliis cultures in the laboratory.