Which protozoan is responsible for freshwater white spot disease, also known as ICH?

The protozoan responsible for freshwater white spot disease, commonly referred to as ICH, is Ichthyophthirius multifiliis. This organism is a ciliated protozoan that infects the skin and gills of freshwater fish, leading to the characteristic white cysts that can be easily observed on the fish's body. The life cycle of Ichthyophthirius multifiliis involves a trophozoite stage that actively feeds on the fish, which causes irritation and can lead to significant stress and disease.

In contrast, Ichthyobodo is a flagellate protozoan that causes problems in fish, but it does not lead to white spot disease. Gyrodactylus salaris is a type of monogenean trematode and is responsible for damage to fish through a different mechanism, primarily affecting their skin and gills through attachment and feeding. Argulus refers to a genus of crustaceans known as fish lice, which also does not cause white spot disease but rather irritates fish in a different manner. Therefore, Ichthyophthirius multifiliis is distinctly recognized as the causative agent of ICH due to its specific life cycle, pathology, and the symptoms it produces in infected fish