Infection by Ichthyophthirius multifiliis

Ichthyophthiriasis or white spot disease is caused by the protozoan ciliate Ichthyophthirius multifiliis. The disease is a major problem to both cultured and wild freshwater fish in all areas of the world. Most species of freshwater fish are considered susceptible. The clinical signs of I. multifiliis infection are characterized by white spots up to 1 mm that are often seen on the skin and fins of infected fish. The parasite has a direct life cycle and can multiply into hundreds of infective theronts with the developmental speed depending on ambient temperature. Within one to two weeks, the parasite can kill whole fish herds. Early detection and treatment strategies are crucial to control the spread of this disease.