Ich has three life-stages. We can begin with the feeding stage that has settled under your fishes' outer skin: the trophont (or trophozoite). The trophont is the
only feeding stage (its name contains the same Greek
troph="feeding" element familiar in "heterotroph" or "trophic level"), yet it has no mouth. Instead it secretes histolytes to break down neighboring host cells, in order to absorb their contents. The histolytes cause the host fishes' epithelium to thicken, so that the host's own immune reaction provides a safe haven for this "adult" or "mature" stage, where it's protected from medication. Constantly rotating inside its pustule, the trophont swells to 50 times its original size, large enough to appear to the naked eye, grayish-white, round to oval, as big as a grain of salt. In a few days or much longer, depending on temperature, it is ripe. It sheds its cilia, grows a thickened gelatinous outer shell, lets itself be shed into the fishes' mucus, and drops away as a "tomont." The trophont does not need to become completely mature. A
lab study by T.A. Nicholl and M.S. Ewing at Oklahoma State found that most of the embedded trophonts left the host within four hours of death: it's worth noting that the corpse of an Ich-infested host is a major source of infection.
The released tomonts swim for 2 to 6 hours before settling on a substrate. (Nicholl and Ewing found that a light substrate was preferred to a dark one.) Some biologists count this brief interval as a fourth life stage (in which it is susceptible to medication, by the way, according to Dr. Peter Burgess, the resident "fish doctor" at
Practical Fishkeeping magazine). Quickly it attaches to a substrate and encysts, as the reproducing stage. This life-stage doesn't eat. Its metabolic clock is now ticking; it is spending its stored energy to divide and divide again within the short-lived cyst.
The tomont's time-span remains temperature-dependent: at common aquarium temperatures it's a matter of hours to days. (In a chilly koi pond in early spring, the cyst may persist longer.) Ultimately hundreds of mobile tomites burst from the cyst, even as many as 2000. They quick sprout cilia and start actively swimming about in search of a host. The fully developed "swarmers" are now called theronts (Greek
ther- denotes a critter). The tomites'/theronts' metabolism is also temperature-dependent, but they must find a host within a very few days or perish: at 68oF none survived after 55 hours, according to Schaperclaus.
The gelatinous thin-walled cyst can't survive being completely dried out, an incentive to let your nets dry out completely, if there is Ich anywhere among your tanks. Only this third life-stage, the free-swimming tomite/theront, is susceptible to medications. Only the trophont can persist "dormant" in the aquarium, though it's never free-living but always attached inconspicuously to a host, perhaps on a gill surface.
Ich is all too easily identified with the naked eye at the final full development of the trophont. (In fact
Ichthyophthirius multifiliis is the largest known single-celled parasite on fishes.) As a result, many aquarists don't want to believe Ich is attacking their fish till they actually
see the white spots. This is an error. Often a spot or two pass unnoticed, since newly-settled trophonts are too small to see anyway, and especially since early infestations are likely to attach to the gills, where they stay invisible as they grow. If you wait until later stages, badly infested small fishes may be too weakened to save.
The other common error, once a medication regime is begun, is that it's often ended as soon as no more mature "white spot" trophonts are visible in obvious places on the fish. It's absolutely essential to
keep on medicating till the last encysted theront has released its tomites, and the last tomite has been eliminated by the medication. Remember, it only takes a single tomite successfully settled into the fishes' epithelium to inaugurate a whole new cycle. As the University of Florida puts it, once more, "Uncontrollable or recurrent infestation with ciliate protozoans are indicative of husbandry problems."
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