Ichthyophthiriasis, caused by the protozoan parasite Ichthyophthirius multifiliis (fig. 9.16), is considered to be one of the most detrimental diseases in pond culture of fresh- and brackish-water fish. All species of pond-cultured fish
including common carp, Chinese carp and trout, are susceptible to the disease. I. multifiliis has a round or ovoid body (0.5–1.0 mm long)with a small rounded mouth. Longitudinal rows of cilia can be found on the surface of the body, converging at the anterior end. The large macronucleus is horseshoe-shaped and has numerous contractile vacuoles. This species multiplies on fish by repeated binary fission. The mature parasites (trophonts) break the white epithelial tubercle that covers them and enter the water. They settle at the bottom and attach themselves to submerged objects. On attachment, the parasite becomes enclosed in a gelatinous cyst and multiplies. One trophont divides into as many as 2000 ciliated bodies. They emerge into the water by dissolving the cyst which encloses them, with the enzyme hyaluronidase. They swim free for two to three days and if a host is found during that period, they penetrate under the skin, grow and mature. If they do not find a host, they will die. The optimal temperature for development is 25°–26°C. Outbreaks of the disease usually occur in spring and summer at high water temperature and under conditions of over-crowding that facilitate the spread of the disease. In light infections, the fish show rest-lessness and gather in groups near the water inlet. In heavy infections in yearlings, more severe symptoms can be noticed, including acute restlessness, the fish rubbing against the bottom and sides and collecting in masses near the inlet. Small tubercles occur on the body and the fish stop feeding and cease reacting to stimuli. In advanced cases, the fish swim at the surface and rush around swallowing air. Small white tubercles cover the entire body, and severe lesions of the cornea and blindness may also occur.
The fact that the non-parasitic stages of Ichthyophthirius are very sensitive to environmental factors makes it somewhat easier to prevent infection.The destruction of carrier fish is an essential aspect of prevention. Disinfection of contaminated water or equipment is recommended. Even dilute solutions of salt (0.5 per cent) will kill encysted parasites and the ciliated bodies. The ciliated bodies can also be killed by drying the ponds or other rearing facilities. A pH below 5 and oxygen concentrations of less than 0.8 mg/l are reported to be
fatal. Even though vaccines are not available for immunization of fish against ich, repeated infections apparently provide relative immunity. In ponds, treatment with 0.1 ppm malachite green or 15 ppm formalin once, twice or three times, has been reported to be effective.