from one of my favorite websites:
Is "Ich" always present in our aquaria?
There is a widely held belief in the marine aquarium hobby that "Ich" is always present in our aquaria and this belief is often repeated on marine bulletin boards. There is much information in the scientific literature that contradicts this belief.
C. irritans is an obligate parasite (Burgess and Matthews, 1994; Dickerson and Dawe, 1995; Yoshinaga and Dickerson, 1994). Obligate means the parasite can not survive without infecting its host, in this case, fish. Theronts have been shown to die if a suitable host is not found within the required time. Yoshinaga and Dickerson (1994) found that few theronts (0.34%) were viable 12.5 hours after excystment and Burgess and Matthews (1994) found that no theronts were viable 18 hours after excystment. Colorni (1985) found that some excysted tomites (=theronts) were observed to be moving weekly after 48 hours. While the life span of the theronts appears variable, it is limited and all will die without finding a suitable host.
If an aquarium has no fish in it, and there are no additions of fish, or anything else that could be carrying trophonts, tomonts, tomites or theronts for a period of 6 weeks or longer, all parasites will have died. An aquarium such as this is an obvious exception to "Ich" always being present.
Many fish collected for marine aquariums will not be carrying "Ich". Incidence of C. irritans in wild fish varies widely and may be geographically related. Some authors have found few infected fish, if any, in the areas they have examined (Puerto Rico: Bunkley-Williams and Williams, 1994; southern California: Wilkie and Gordin, 1969) . Others have found that low levels of infection are not uncommon (e.g. southern Queensland; Diggles and Lester, 1996c). Keeping multiple fish in holding tanks and at aquarium stores increases the chances of a fish carrying "Ich" parasites, but it is still possible to acquire a fish that is not infected with "Ich".
If new fish are quarantined for at least 6 weeks, any parasites on the fish will have gone through a number of life cycles increasing the number of parasites present. In the majority of cases, the increase in parasite numbers will result in full blown infection and fish can be treated to remove the parasites. Hyposalinity has been demonstrated to break the life cycle of "Ich" (Cheung et al. 1979; Colorni, 1985) and fish correctly treated with hyposalinity will be free from "Ich". Any fish that do not show signs of infection after 6 weeks are very unlikely to be carrying any parasites.
If fish that are free from "Ich" (either because they were not originally infected or because they have been treated with hyposalinity) are added to an aquarium that is free from "Ich", the aquarium will stay free from "Ich" and be another exception to "Ich" always being present.
Burgess and Matthews (1994) were attempting to maintain a viable population of C. irritans which could be used in later studies. To maintain the parasite populations, they needed host fish in order for the trophonts to feed and continue the life cycle. Each host fish was only used once in a process of serial transition such that none of the hosts would die or develop an immunity. While the procedure worked very well and enabled them to maintain populations for some time, the viability of the populations decreased with time and none of the 7 isolates they used survived more than 34 cycles, around 10 to 11 months. They suggest this is due to senescence and aging in cell lines is well recognised in Ciliophora.
The presence of aging cell lines in C. irritans suggests that an aquarium that has been running for longer than 12 months without any additions is unlikely to have any surviving "Ich" parasites, yet another exception to "Ich" always being present.
Whilst "Ich" may be present in some aquaria, it is certainly not present in all aquaria. Through careful quarantining and treatment, it is very much possible to establish and maintain an "Ich" free aquarium.
Does stress cause "Ich"?
Stress and poor water conditions do not cause marine "Ich", although they will lower a fish's resistance to infection and impair their immune system. If C. irritans is not present in a tank, it doesn't matter what how stressed a fish may be, it cannot get infected. In a tank where parasites are present, stressed fish are more likely to show signs of "Ich" before more healthy fish, but the healthy fish are just as likely to become infected as the numbers of parasites increase. Those fish species that are less susceptible to "Ich" or those individuals that have an acquired immunity, may show no signs and may not get infected.
