[mikev]

(Not a disease case -- I actually did not have Ich in my tanks for 2.5 years... but a thought that occurred to me and I'd like your comments).

It is known that different species of fish show different levels of resistance to Ich; in some, the disease tends to be very mild, in some others it explodes very quickly, leading to tank wipeouts. A classic example here is the clown loach.

It seems to me that there is a correlation between the rate of the metabolism in fish and the intensity of the infection. For example, clown loaches are high-metabolism fish; otoh kuhlis (which may get ich, but not a disease explosion) are low. Ich in black-line rasboras is possible, but the disease tends to be mild, milder than for example in rainbows or danios; again, rasboras tend to be slow-moving, low-metabolism fish.

If this is true, the explanation would be simply that the Ich parasite fully relies on the host for the nutrients, and it can obtain much better nutrition from a high-metabolism host.

Does this make sense?

[plaamoo]

Might also be that these high metabolism, high strung, species are more prone to stress that weakens their immune system.

[Colin_T]

I'm not sure metabolism has anything to do with which fish develop Ich. It might as you stated, help provide more nourishment to the parasite when they feed from a host with a high metabolic rate. However, I have had tanks of fish, which were all the same type, and some developed whitespot while others didn't. And some developed whitespot at different rates to others. I have had this happen in rainbow tanks that contain several hundred young of the same species, and also in marine tanks with clownfish/ anemone fish. Some got the spots but others didn't.

I personally think it comes down to the overall health of the fish. Fish that are being bullied (subordinate clownfish) were more likely to develop spots, whereas the dominant pr didn't develop any spots.
Poor water quality/ dirty environmental conditions will also have an effect on who gets it. Basically the more stress a fish is under, the more likelihood of it developing the disease should the parasite get into the tank.

The toughness of the fish' skin might also play a roll in the development of Ich. The parasite cannot attach itself to crustaceans due to their exo-skeleton. The shell is too hard for the parasite to get a hold. The same might be true for certain fish. Fish with thicker skin (ie: fish that burrow in the substrate) will be less affected because fewer parasites will be able to attach themselves and start feeding.
There were studies done years ago on using slime coat enhancers to treat whitespot. There was a big thing about it back in the late 80s when people were recommending the use of products like "Stress Coat" and "Aquatan" when transporting fish. Apparently you add some of the stuff to the bag of fish and it helps prevent diseases.
The theory was the chemical increased the thickness of the slime coating on the fish and made it harder for disease organisms to infect the fish.
A similar thing has been noted in Discus & Uarus that were rearing young. Adult fish that had a decent slime coating for feeding their offspring, were significantly less likely to develop whitespot or other diseases even when diseased fishes were introduced into the tank. The thicker slime coat was thought to have prevented the pathogens from entering the fish’s skin and causing a disease.

[plaamoo]

Have you seen this?
http://www.fishchannel.com/fish-news/2009/12/07/ich-discovery-research.aspx

[mikev]

Now I did, thanks to you, interesting....

It may be worthwhile reading the actual article rather than the popular version, where they say, for example,