I'm not saying it's not something that ever works, as referenced by the people on here who have had success with the method, but there are just as many posts on the various forum boards of the method failing for people. The usual response on the forum boards is that the person administering it did something wrong, when I think there's just as much of a chance that in many of the failed cases, the person was dealing with a variant of ich that had a greater halotolerance than their fish.
Papers - there's some earlier papers from the 70s that talk about halo-tolerance, but I' going to start with Colorni's work in the 80s as I think it's what really laid the foundation for understanding how to control C. irritans in culture:
Colorni, A. 1985. Aspects of the biology of Cryptocaryon irritans, and hyposalinity as gilt-head sea bream Sparus aurata. Diseases of Aquatic Organisms.
- In this paper it was found trophonts could drop off the fish and successfully encyst in to tomonts between 15-60ppt (1.011-1.045 SG), but only successfully incubated to produce tomites between 25-50ppt (1.019-1.038 SG). They found that temporary exposure (up to 48 hours) to even lower salinities didn't damage the tomonts, and they still went on to hatch after bringing the salinity back up. I think this is the main paper from which we got 'hypo' as a method as it seems to suggest that an extended exposure to reduced salinity will allow all trophonts to drop off, encyst, and then fail to mature if you leave them in there long enough. However, it's important to note that the author was using crypt samples that have become known in the literature as the 'Israeli isolates'. The author also noted that at reduced salinities, the tomonts could exhibit the longest incubation period, and the tomonts were much more resistant to damage when the change in salinity was gradual (ie, exactly how people start hypo), so the authors suggestion for control using hypo was:
"In the beakers in which salinity was reduced for 3h to 10% (10ppt or 1.008SG) 4 times at 3 d intervals, tomonts were de- stroyed after each treatment and the fish were free from infection in 5 to 8 d. Similar results were obtained when this procedure was applied to the 4500 and the 14000 l tanks."
This is not how we typically engage in hypo, and this test was done using Sea Bream, not super sensitive reef fish. They also talk about freshwater dips as a method to get rid of the parasites that are actively feeding on the fish:
"Herwig (1978) and Cheung et al. (1979) suggested hyposalinity as an alternative way to reach the trophonts and upset their osmotic balance. However, in the present study, embedded trophonts did not seem to be affected by a freshwater treatment of the host for as long as 18h."
basically, they found that freshwater dips had zero action against embedded C. irritans parasites on the host fish. They're too protected by the skin and mucous of the fish.
I'm pretty sure that this is also the first paper in which the tank transfer method was was noted to have 100% efficacy against the parasite, but in any case it's the best case for using hypo as we currently use it in the hobby.
Colorni, A., Burgess, P. 1997. Cryptocaryon irritans Brown 1951, the cause of ‘white spot disease’ in marine fish: an update. Aquarium Sciences & Conservation.
- A really good overall read on the parasite, it also acknowledges that what was known in 1985 when the last paper was published was only a tiny part of the story, and that ich is naturally present in a much wider range of habitats and temperatures than previously thought. They talk a lot about the extreme asynchrony of tomont excystment and how that's very likely an evolutionary adaptation (and also why it's so damn hard to get out of a system once it's in). They talk a lot about chemical control methods such as copper and formalin, and re-iterate that tomonts are most susceptible to rapid salinity changes, and re-state the hypo method that I quoted above. They also note that some tropical fish can't handle that kind of a treatment, and re-state that the tank transfer method is effective at eliminating an active infection in fish in 7-10 days. They also mention a 4th method that you don't see used very often that I'll explain more in a bit.
Diggles, B. K., Lester, R. J. G. 1996. Variation in the development of two isolates of Cryptocaryon irritans. The Journal of Parisitology.
- This is the first paper that gives really solid evidence that not all strains of ich are created equal. They looked at two variants from the Eastern coast of Australia, one from Moreton Bay near Brisbane, and one from 700km north off Heron island. life cycle, and morphologic analyses showed way more variation than they were expecting, suggesting that everything from timing, temperature tolerances, salinity tolerances, to size and host preference is variable within the species. The Moreton Bay strain came from waters with naturally low salinity (down to 25ppt, or 1.019 SG), and temperatures as cold as 15C.
Diggles, B. K., Adlard, R. D. 1997. Intraspecific variation in Cryptocaryon irritans. Journal of Eukaryotic Microbiology.
- Here they collected 16 isolates of C. irritans from Australia, Israel, and the US and compared them genetically. Their data suggested that there's warm and cold water variants of the parasite, and, importantly, that culturing parasites in laboratory conditions lead to significant temporal genetic variation (ie, the captive bred stuff looked different from the wild caught stuff over time). It looked like captive propagation for study was selecting for specific traits, suggesting that some of what's published about C. irritans in the literature might have more to do with the 'founder effect' of the particular variant that survived to be studied than the species in general. However, the most important part of this paper from a hypo perspective (:
"Tomonts of the Moreton Bay strain can survive salinities less than 10ppt [1.008 SG] (BKD, unpubl. data), whereas those of the Israel isolates undergo cytolysis at 10ppt [6]. Given that the Moreton Bay isolates originate from an estuary (normal salinity range 20ppt-35ppt), and the Israel isolates originate from the Red Sea (40ppt), it is plausible that these differences in halotolerance have evolved as a result of environmental selection pressures."
(Note: I changed the units in that quote because the symbol for ppt is a percent sign with two little italicized zeros after it and it doesn't copy and paste correctly. I also added the SG conversion for clarity).
It's important to note here that while the Moreton Bay strain is the 'cold water variant', they cultured them just fine at normal reef temperatures, so it would be wishful thinking to suggest that that strain hasn't entered the trade. Given the epidemiological soup that is the global fish supply chain, any one fish that makes it to a hobbyist's tank could be carrying multiple strains of ich from multiple parts of the world.
And finally:
Apolinario, V. Y., Yen-Ling S., Hung-Hung, S. 2003. Characterization of Cryptocaryon irritans, a parasite isolated from marine fishes in Taiwan. Diseases of Aquatic Organisms.
I've already typed too much so I'll just sum up the main points:
They cultured C. irritans successfully over many generations at the following salinities: 5ppt (1.004 SG), 10ppt (1.008 SG), and 35ppt (1.026)
The 5 ppt variant came from a near fresh water pond, the 10ppt variant came from cage raised brackish fish, and were 12th generation tomonts raised at that salinity, and the normal salinity variants came from wild, cage raised, and aquarium fish.
They go in to way more detail about the genetics and morphology of each of the variants, but the gist was that C. irritans is way, way more variable than previously known, with a much wider array of adaptations than previously thought. The super low salinity variant was also highly pathological:
"The occurrence of cryptocaryoniasis in inland sea bream ponds in Chiayi with a salinity of 5 ppt resulted in daily mortality of the cultured fish (about 20 dead fish d–1, each weighing ca. 300 g)."
and finally:
"The successful adaptation and continuous propagation of the Kaoshiung isolate at 10 ppt further attests to the need for immediate attention to out- breaks of the parasite at low salinity. It suggests that C. irritans is capable of adapting successfully to lower salinities and thus of causing damage over a wider salinity range, including brackishwater."
There are obviously way more papers about crypt out there, but those are the most relevant to hypo as a method. To summarize, the hypo method that's been developed in the forums is not a method that any of the people who's careers are made on this stuff have ever tested, and the one hypo protocol that is 'scientist approved' is never practiced by home aquarists as far as I can tell. Also, the hypo method that was tested was invented before most of what we know about ich's intraspecific variation was discovered, using a single variant of ich from one specific part of the world, and that likely had a naturally lower tolerance to low salinity.
So, the moral of the story is that it might work for you. Or it might not. It's worked for some, and it's not worked for others, and there's strong evidence to suggest that for some of the people it fails for, there's something more than them not sticking to the method properly going on. Like most things, there's no magic bullet (or magic salinity, in this case). You might have a strain that has a narrower halo-tolerance than your fish, or you might not. You will only find out if it works for you if you try, but when you're talking about completely nuking a mature system, I don't know if I would take that risk, but that's just me.
There is a 4th method that was suggested by Colorni that I will finish this post with, as maybe it's something you'd be willing to try. Again, it's highly, highly labour intensive, and for maximum effect I would likely re-do my rock work so that there weren't any places I couldn't reach, and I'd remove my sand bed. It has the added benefit of treating your whole system so you wouldn't need to fallow it for months, and is potentially over in 12 days. Logistically however, it seems difficult. You'd want to pay special attention to areas that you know your fish sleep, as that's likely where the tomonts will be:
"... a 1-2cm layer of fine sand is spread uniformly over the bottom of the treatment aquarium. Three days later, the sand is removed by suction and a new layer is deposited. This operation must be repeated four times at 3 day intervals. This treatment offers the protomont a substratum suitable for encystment, yet which is easily removable. The method is practical in aquaria in which fishes co-exist with sponges, corals and other delicate sessile invertebrates that cannot survive any drastic handling."
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