Every once in a while I find myself having a "grass is greener" moment in science.  I sit at the computer thinking about all of the annoying things in lab that aren't working and wonder, "If I could answer any question, given unlimited resources, what would I choose to study?"  This, my friends, is where I prove my unhealthy obsession with fish.

Credit: Colorni and Burgess, 1993

If money, fame, and science groupies weren't a priority, I'd spend my time and resources trying to find a cure for Cryptocaryon irritans, which is better known in the tropical fish industry as white spot disease, Ich, or Crypto.  Crypto is a nasty little bug.  It's a protozoan ectoparasite that lodges itself in the epidermis of fish where it grows until it drops off to mature into an Aliens inspired cyst that spits out 300 new little bastard parasites that go on to infect more host fish.  In the open ocean, this guy isn't so terrible because its chances of infecting a host are minimal considering the massive amount of water a parasite has to travel through to find a new host.  This is a completely different story at an aquaculture facility or in the home aquarium where fish are typically stocked to capacity in relatively small volumes of water.  A single cyst can quickly turn into a fish killing epidemic.

Trophont in fish epidermis Credit: Colorni and Burgess, 1993

The life cycle of crypto is simple.  It has 4 stages.  The on fish stage is called the "Trophont."  This stage lasts anywhere from 3-7 days with the height of observable parasitic whitespots being day 4.  During this stage, the parasite fattens up on fish cells and other yummies found in the epithelium to prepare for the expansion stage of the life cycle.  Once the trophont has matured, it falls off of the fish and at this stage is called a "Protomont."  The protomont floats around until it bumps into something that it can stick itself to.  Usually this is a rock or a piece of sand.  There's evidence that the Protomonts have a hard time sticking to glass and plastic, so many experts recommend treating fish for this disease in a bare bottom aquarium decorated with PVC pipes.  Once the protomont finds a nice place to rest, it secretes some goo that allows it to be stuck in place.  At this point the parasite is considered a tomont and encysts (covers itself with a hard candy shell).  Over the next 3 - 60 days, the cyst matures with approximately 300 tomites growing inside.  These parasitic particles then eat their way out of the cyst and become "Theronts" which actively hunt down fish hosts using their cilia to propel them through the water.  These things are creepy looking by any standard, and I'm super happy they can't infect humans.  When the theront finds a host, the life cycle starts over again resulting in the rapid exponential production of more nasty parasites.

Theront Credit: Colorni and Burgess, 1993

The problem with Crypto is that its really only vulnerable to treatment at two of the 4 stages.  The Trophont has been said to be susceptible to Quinone during the the period when its lodged in the fish epithelium and the Theront (the swimming creeper) is highly sensitive to copper, low salt, and formalin.  The problem with all of these treatments is that many of them are toxic to the fish and highly toxic to invertebrates (corals, snails, crabs, shrimp etc).  So the only real way to treat for the disease in a home aquarium is to remove the fish from the tank and treat them for 8 weeks, while leaving the main aquarium containing the invertebrates free of fish for the same amount of time.  This is because the cysts can take up to 60 days to mature and hatch out their Theronts.  Without a fish host, the Theronts die within 48 hours and can't complete the life cycle.

Finding a specific inhibitor of infection that does not require the removal of fish from a system or the treatment of them with heavy metals or carcinogens would be ideal.  There have been a few recent studies looking at alternative cures for Crypto.  One of these includes a vaccine which was tested by injecting grouper with formaldhye killed Theronts.  It was found that only injection with a large amount of killed theronts resulted in protective immunity, while injection with killed tomonts and trophonts offered little protection (Bai et al 2007).  Another study looked at the susceptibility of 8 saltwater fish to the parasite and found that one fish in particular, the rabbitfish (Siganus oramin) was resistant to infection and a specific antiparasitic protein was isolated from the fish's serum and found to have a strong parasite killing effect when administered to Theronts growing in a test tube (Wang et al 2010).  Maybe one day we will be able to vaccinate new fish against the parasite or cover them in a protective protein infused slimecoat.

However, neither of these developments seems practical for a home aquarium user.  It'd be much better if we could find a specific small molecule inhibitor of the parasite that could be fed to the fish during an outbreak without affecting the rest of the aquarium ecosystem.  In vitro culturing techniques for the parasite have been developed and it seems likely that a plate assay could be created using recently hatched theronts.  I imagine theront viability could be determined using a flourescence assay to detect the viability of the parasites making the entire screening process speedy and high throughput.  The candidate drugs could then be used to screen for toxicity in both fish and invertebrates.  Of course this simplistic description of drug development isn't exactly a proposal, but anything is possible given unlimited time and resources.

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References:

Angelo Colorni and Peter Burgess. 1993. Cryptocaryon irritans Brown 1951, the cause of ‘white spot disease’ in marine fish: an update. Aquarium Sciences and Conservation Volume 1, Number 4, 217-238, DOI: 10.1023/A:1018360323287.

Jian-Shan Bai, Ming-Quan Xie, Xing-Quan Zhu, Xue-Ming Dan and An-Xing Li. 2007. Comparative studies on the immunogenicity of theronts, tomonts and trophonts of Cryptocaryon irritans in grouper. Parasitology Research Volume 102, Number 2, 307-313, DOI: 10.1007/s00436-007-0766-6

Wang FH, Xie MQ, Li AX. 2010. A novel protein isolated from the serum of rabbitfish (Siganus oramin) is lethal to Cryptocaryon irritans. Fish Shellfish Immunol. 2010 Jul;29(1):32-41. Epub 2010 Feb 1.