Ichthyophthirius multifiliis

Ichthyophthirius multifiliis is an ectoparasite of freshwater fish which causes a disease commonly known as white spot disease, or Ich. Ich is one of the most common and persistent diseases in fish. It appears on the body, fins and gills of fish as white nodules of up to 1 mm, that look like white grains of salt. Each white spot is an encysted parasite. It is easily introduced into a fish pond or home aquarium by new fish or equipment which has been moved from one fish-holding unit or pond to another. When the organism gets into a large fish culture facility, it is difficult to control due to its fast reproductive cycle and its unique life stages. If not controlled, there is a 100% mortality rate of fish. With careful treatment, the disease can be controlled but the cost is high in terms of lost fish, labor, and cost of chemicals. The protozoa damages the gills and skin as it enters the tissues, leading to ulceration and loss of skin. Severe infections rapidly lead to loss of condition and death. Damage to the gills reduces the respiratory efficiency of the fish, reducing its oxygen intake from the water. This causes the fish to become less tolerant to low oxygen concentrations in the water. Contrary to popular belief, white-spot is not present in every aquarium or pond. The ich protozoan goes through these life stages: