What is a common clinical sign seen in fish infected with Ichthyophthirius?

A common clinical sign seen in fish infected with Ichthyophthirius, commonly known as "ich," is the presence of white spots on the skin and fins of the fish. These white cysts are actually developmental stages of the parasite, and they can create a characteristic "salt-like" appearance on the fish's body. The white spots result from the trophozoites (the feeding stage of the parasite) invading the epidermis and creating cysts, which can lead to irritation and discomfort for the fish.

Infected fish may exhibit additional signs such as increased respiration rate, lethargy, and attempts to scratch themselves against surfaces in their environment due to irritation. However, the hallmark sign of Ichthyophthirius infection is the visible white spots, which make it a key diagnostic feature of this common parasitic disease.