This is a difficult question to answer really. What we know is that in
one study (Colorni and Burgess 1997), it took
up to 72 days for all the theronts to be released from a group of tomonts. To my knowledge, these results have never been duplicated - not even close. In other studies, 7-14 days has been more the "norm" for theront release with the entire life cycle completing in just 2-4 weeks. So, one must assume that this "72 day strain" is quite rare.
However, I have seen some say that they went fallow for 72+ days, QT'd & treated everything by the book, and yet ich still comes back once the fish are reintroduced into their DT. Two possible explanations for this:
- A strain (or multiples) of ich exists which takes longer than 72 days for theronts to be released. This is very possible, as true scientific studies on ich are not often done. Perhaps this strain exists or one that can lie dormant for a period of time without needing a fish host, but has not yet been discovered.
- Operator error. I can tell you from experience that you must dot every i and cross every t when QT'ing fish and especially when going fallow. If you make a single mistake, even a small one, the disease you are battling will most likely capitalize on it. The possibilities are endless: cross contamination (wet hands/feeding apparatus going from fallow DT to QT or vice versa), aerosol transmission (not observing the 10 foot rule), not QT'ing corals/inverts and adding them directly to fallow DT, not testing Cu level in QT on a daily basis, not treating in QT for long enough, etc. etc.
There is also the possibility that certain strains of ich are building up a resistance to copper based medications, especially if they have been treated with such multiple times. Many collectors/wholesalers/vendors treat with copper (for a brief period of time - often not long enough) before you ever receive the fish.
Hypo resistant strains of ich have already been proven to exist (study done by Yambot in 2003.)
In conclusion, while I do believe there is some anecdotal evidence to question the "72 day rule" - at the end of the day, it is still the best scientifically grounded information we have regarding theront release and that particular aspect of ich's known life cycle. However, I see no harm in going fallow
for longer than 72 days for those who have the patience to do so.
Actually, it's 76 days now anyway when you consider these new calculations (see below):
Let's do the math and tweak the fallow period for ich using the parasite's known life cycle & worst case scenario:
- Let's say a trophont drops off the last fish you catch before going fallow. We know that the protomont can only crawl around 18 hours max before beginningthe encysting process. The process itself takes no longer than 12 hours until it has hardened around what is now called a tomont. 18+12=30 hours, but I'm just gonna call it 2 days to err on the side of caution.
- The longest known time period it took for theronts (free swimmers) to be released from a group of tomonts is 72 days. However, I want to make it clear that this 72 days has only been encountered once; one study involving a single strain of ich. In most other studies, 7-14 days has been "the norm" for theront release.
- Once released from it's tomont, a theront must find a fish host to attach to within 48 hours (2 days) or it dies. Thus ending ich's presence in your fallow tank. Although in actuality, a theront's infectivity is greatly reduced just 6 - 8 hours after it leaves the cyst. It's non-infective after just 8 hours, but still able to move for up to 48 hours. So again, to err on the side of caution, we're gonna say 2 days for this "final phase" of it's life cycle.
So, let's add it all up. 2+72+2 = 76 days. That would be the true maximum fallow period for ich. Probably a bit of overkill, but I will make the necessary adjustments to the stickies and start recommending 76 days from this day forward.
