Really like this article and I'd like to pose a couple of ideas regarding the data from this paper.
1st
@4FordFamily probably has it right on that each species of fish has varying degrees of susceptibility to infection and extent of infection that will occur. The authors cite their data as well as those from several other studies they referenced that provide evidence not all fish are equal (this study mostly done with emperors and groupers). There are probably fish that have more active innate immune responses, and other species have morphological adaptations that make it difficult or impossible for the parasite to attach or feed on the host. Our own personal observations in our tanks also tend to support this as well given surgeon fish and some other species we tend to like also tend to be much more susceptible to infection and probably also become much more heavily infected when an outbreak occurs and suffer the greatest morbidity.
2nd the authors data also suggest the quality of water (quality indicated by dissolved nutrients in the water) and amount of water exchange have a big impact on infection rates in wild populations. Estuaries tend to be Eutrophic, where as sand bars and coral reef environments tend to be oligotrophic and ultraoligotrophic respectively. Here again the data presented by the authors indicated areas with higher nutrient loads (estuaries) and limited nutrient export in the ecosystem also have higher numbers of infected fish. To bring this back to our tanks. Systems that have been running for long periods and have good nutrient control and or fauna that quickly process the nutrient load may also have less mortality and morbidity if/when they have a exposure to crypto in that environment, my guess and only an educated guess, is the parasite does require some nutrients in the water to successfully complete its life-cycle and may explain why even if an infected fish is introduced the parasite can't get a foot hold in system where the nutrients are limiting. When I speak of nutrients there is probably more to the game than the ones we think of being nitrate and phosphate but also many other small organic molecules some of which may be required for parasite development and maturation that are consumed by specific microbes that may not colonize readily or quickly into our systems. This may be a reason why newer tanks that are still developing a mature microbial fauna may have a tougher time managing an outbreak. I know some will ask about higher levels of water movement being the cause as the author did mention that, but they also mentioned studies that show crypto tends to excyst on a circadian pattern (mostly at night). This parasite does have to swim to a host and given it is a single celled cilliate who can't cover lots of distance evolution more than likely has selected those parasites that swim when the chance of finding a host within it's range is greatest i.e. when the fish has bedded down for the night and is in close proximity to the substrate where excystment occurs.
In addition to this the data also clearly show that wild fish do tend to be infected and carry a "parasite load", but I think the key take away message given fish mostly rely on the innate immune response (see the article posted by PaulB in this thread) is the parasite load at any one time averages about 8-10 individual tomonts (Diggle and Lester, 1996; quote above) that the innate response can keep in check. Key thing to remember about the innate immune response that differs from the adaptive immune response that mammals use more heavily is the innate system can be overwhelmed much more easily. If you didn't already know the innate immune system uses macrophages, phagocytes and granulocytic cells, which you might think of as a police force roaming the body looking for invaders. If the riot is to large they can only attack parasites as fast as they can physically make contact with them and have to be in the same physical location to do so, where as the adaptive immune response involves T and B lymphocytes that take messages from the innate response and then produce very specific antibodies to that particular invader and then leave behind memory B cells that are always ready to mount a massive response to any reoccurrence, they can be thought of more like the air force and the heavy smart bombers. Once the message comes in the B-cells begin synthesizing large number of antibodies and these circulate through the blood so the immune cells themselves do not need to be in physical proximity to do their job and the invader in essence gets carpet bombed and the innate system them mops up the remains. Fish don't do as good with the air force, this part of their immune system is not well developed compared mammals.
Bottom line fish deal with Crypto in the wild, but they have manageable parasite loads their immune systems and physiology can handle. In the closed environments we provide it's probably pretty easy for this balance to turn and why the tenets of ich management are all targeting keeping the parasite as low as possible, which may still doom some fish who bed down where excystment occurs and have little in the way of natural resistance to the parasites themselves as noted by
@4FordFamily and the Acanthurus genus which probably share morphology that makes them prone to infection. Ich management probably works because while you may not have success with these very susceptible species the parasite never gets to plague levels (i.e. concentration of free swimmers large enough) that cause massive infection and high mortality rates to a majority of fish species you keep it more in line with what is seen in the wild unless as noted by others an event occurs that shifts the balance to the parasites favor.
Sorry for the long winded comment to this thread, but found it to be a good read and being a biologist by trade I love that scholarly articles are used to support points, also found the articles quite informative given my specialty is mammalian immunology which does differ from that of our finned friends. Love to geek out on this stuff and share ideas :)
