Ichthyopthirius multifiliis is a species of ciliate protozoa which parasitizes freshwater fish. The disease it causes is usually called ick or white spot, and becomes especially serious in enclosed areas, where it spreads quickly from one fish to another. Ick is the disease responsible for the most fatalities in aquarium fish and can cause notable damage to aquaculture.

Ick infections are usually visible in the form of pustules or white spots on the side of the fish, though these are not always obvious. The fish may also show behaviours such as rubbing up against other objects. The pustules contain mature Ichthyophthirius cells, called trophozoites, which feed on the tissues of the host and may grow to 1 mm in diameter. Periodically these burst forth and encyst, then divide to form large numbers of swarmer cells, which subsequently infect new fish (or reinfect the old one). The entire life-cycle takes about two weeks to complete.

If ick is detected before it becomes too serious, a number of different treatments can be applied. These all target the swarmers, which only survive about three days in absence of a host fish. Other tactics such as keeping the tank in total darkness are sometimes mentioned as being helpful.

A similar disease in saltwater fish, sometimes called marine ick, is caused by the ciliate Cryptocaryon . Originally this was considered another species of Icthyophthirius, called I. marinus, but it belongs among a completely different group of ciliates. Ichthyophthirius is a hymenostome, now usually given its own family.

External link

• Life cycle and treatment of Ichthyophthirius in layman's terms.
• Diagnosis, treatment and prevention of Ick