an invasive disease of many species of fish caused by the ciliate Ichthyophthirius.
Ichthyophthyriosis is often found on fish farms in eastern and western Europe, Canada, and the USA. All age groups offish are susceptible to the disease, but the most severe ichthyophthiriosis occurs in fry and in breeders older than three years. The source of infestation is other fish affected by the disease. The parasites produce inflammation of the skin, fins, gills, and eyes. The affected fish become less active, keep to the shore, and hardly react to external stimulation. Farms affected with ichthyophthiriosis are subject to quarantine restrictions and their ponds and fish stock are disinfected. The diseased fish are treated in saline and other baths.