Flowing Water: An Effective Treatment for Ichthyophthiriasis

Abstract

Ichthyophthiriasis, or ich, is a disease of freshwater fish that is difficult to treat chemotherapeutically because the causative agent, Ichthyophthirius multifiliis, is protected by the host's epithelium during much of its life cycle. In our experiments, a modified standard formalin treatment (25 mg/L for 4 h, 4 d/week) conferred partial protection but failed to prevent 40–70% mortality among channel catfish Ictalurus punctatus. A water velocity of 20.3 cm/min in 5-m-long raceways and a turnover rate of 2.5/h reduced mortality of ich-infected channel catfish fingerlings to less than 10%. A water velocity of 36.5 cm/min and a turnover rate of 4.5/h held mortalities to 7% and eliminated ich from the raceways. When turnover rate fell below 1.9/h (water velocity < 75 cm/min) in 24.4-m-long raceways, ich outbreaks occurred, but higher water velocities reversed the epizootics. Prophylactic increases in water flow (> 85 cm/min, turnover rate > 2.1/h) resulted in no losses caused by ich during 2 years at Little Grassy State Fish Hatchery, Illinois, where ich has historically caused 40–60% mortality of channel catfish fingerlings. Turnover rate may be more important than water velocity in controlling ich.