The mature trophont is large, oval to round, dark in color (due to the thick cilia covering the entire cell) and measures 0.5 to 1.0 mm in size. This stage also has a horseshoe- or C-shaped macronucleus that may be visible under 40x magnification (Figure 7). The parasite moves slowly in a rolling, sometimes amoeboid motion and, with practice, is easily recognized. The immature, free-swimming theronts (Figure 5) are smaller, pear- or spindle-shaped, translucent, and move quickly, continuously spinning on its longest axis as it swims. Theronts can resemble other parasites (especially Tetrahymena), so if only this juvenile stage is seen, prepare a second slide and examine it closely for the trophont stage to confirm the diagnosis. One Ich organism will produce up to 1024 individuals in one generation. Therefore, if even a single Ich parasite is seen, fish should be medicated immediately because the fish may not survive as the infection advances, even with treatment.
Once an outbreak of Ich is detected, it is important that a treatment protocol be started immediately. Control of this parasite can be difficult because of its complex life cycle, multiple protected stages, and large number of off-spring from a single individual. The role of water temperature in determining the timing of treatment application is also critical and is discussed in more detail below. Of the life stages shown (Figure 2), only the free-swimming theronts are susceptible to chemical treatment. This means that the application of a single dose of a treatment will only kill theronts that have emerged from the tomont cyst and have not yet burrowed into the skin or gills of a host fish. This single treatment dose will not affect organisms that emerge after the chemical has broken down or been flushed from the system. Appropriately timed, repeated treatments, however, will continually kill the juvenile, infective theronts, preventing continuation of the infection. The disease outbreak will be controlled as more adult trophonts drop off the sick fish, encyst, and produce theronts that cannot survive the chemical treatment in the water. In a tank or vat, this process can be greatly enhanced if organic debris is removed following treatment. Because the sticky cyst of the tomonts may attach to organic material, cleaning this debris will help remove many cysts from the environment, further decreasing the number of emergent theronts. When removing debris, it is important that the debris or water not be discarded on the floor or anywhere that it could spread the parasites to a different tank or system. Any dead fish should be removed as soon as they are seen because mature trophonts will quickly abandon a fish once it has died and begin reproducing in the environment. 781b155fdc