Ichthyophthirius Multifiliis
(White Spot) Infections in Fish - Part 1
Ruth Francis-Floyd and Peggy Reed
Ichthyophthirius multifiliis is a ciliated protozoan which causes "Ich" or "white spot disease." This disease is a major problem to aquarists and commercial fish producers world wide. Ichthyophthirius is an important disease of tropical fish, goldfish, and food fish. The disease is highly contagious and spreads rapidly from one fish to another. It can be particularly severe when fish are crowded. While many protozoans reproduce by simple division, a single "Ich" organism can multiply into hundreds of new parasites. This organism is an obligate parasite which means that it cannot survive unless live fish are present. It is capable of causing massive mortality within a short time. An outbreak of "Ich" is an emergency situation which requires immediate treatment: if left untreated, this disease may result in 100% mortality.
The Parasite"Ich" is the largest known parasitic protozoan found on fishes. Adult organisms are oval to round and measure 0.5 to 1.0 mm in size. The adult is uniformly ciliated and contains a horseshoe-shaped nucleus which can be seen in older individuals.
The breeding stage of the parasite encysts between the layers of the host skin. When mature, it leaves the fish and produces large numbers of free swimming young. These must find a host within 48 hours (at water temperatures of 75-79 F) or they will die. The life cycle of "Ich" is shown in Figure 1 .
Disease Signs
The classic sign of an "Ich" infection is the presence of small white spots on the skin or gills. These lesions look like small blisters on the skin or fins of the fish. Prior to the appearance of white spots, fish may show signs of irritation, flashing, weakness, loss of appetite, and decreased activity. If the parasite is only present on the gills, white spots will not be seen at all, but fish will die in large numbers. In these fish, gills will be pale and very swollen. White spots should not be used as the only means of diagnosis because other diseases may have a similar appearance. Gill and skin scrapings should be taken when the first signs of illness are observed. If the "Ich" organism is seen, fish should be medicated immediately because fish which are severely infected may not survive treatment.
Diagnosis of "Ich"Diagnosis of "Ich" is easily confirmed by microscopic examination of skin and gills. Remove several white spots from an infected fish, then mount them on a microscope slide with a few drops of water and a cover glass. The mature parasite is large, dark in color (due to the thick cilia covering the entire cell), and has a horseshoe-shaped nucleus which is sometimes visible under 100 x magnification ( Figure 2 ). The adult parasite moves slowly in a tumbling manner and, with practice, is easily recognized. The immature forms (tomites) are smaller, translucent, and move quickly. The tomites ( Figure 3 ) closely resemble another protozoan parasite called Tetrahymina ( Figure 4 ). Tetrahymina usually does not require treatment, so it is important to recognize the difference between the two parasites. If only tomites are seen, prepare a second slide and examine it closely for the adult parasite to confirm the diagnosis. Observation of a single organism is sufficient to make treatment necessary.
Figure 2.
Figure 3.
Figure 4.
Inflammation
Prevention of "Ich" is preferable to treating fish after a disease outbreak is in progress. All incoming fish should be quarantined for at least three days when temperatures are 75 to 83 F. At cooler temperatures a 3-day quarantine will be inadequate for "Ich" because of its lengthened life cycle. For this reason, and to prevent introduction of other diseases which have incubation periods greater than 3 days, a longer quarantine is strongly recommended. Three weeks is generally considered a minimum period for adequate quarantine of new fish.
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