How do I treat Dermocystidium

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A reader worries about her diseased Cardinal Tetras, Dr Peter Burgess advises.

Q: A couple of the Cardinal tetras in my community tank seem to be infected with Dermocystidium. They have been infected for several weeks and one of them has several ‘spots’ that have yet to burst. Another has a definite worm-like parasite in a fin. I’ve tried treating with esHa Exit with no success. Any advice on what to do would be welcome. Should I quarantine the affected fish?

JULIE WILKINSON, VIA EMAIL

A: Peter says: Dermocystidium is a cyst-like skin parasite that has been recorded several times in Cardinals over recent years, as well as other tetras and some other fish groups. 

In Cardinals, Dermocystidium generally manifests as one or more bubble-like skin cysts reaching 5mm in diameter, each typically having a whitish worm-like structure inside. Hence it is easily confused as a skin-parasitic worm infection — and it’s certainly fooled me a couple of times!

So, if you see one or more protruding cysts on the body and/or fins of your Cardinals, each looking like a clear bubble, and some containing a white ‘worm-like’ tube inside, as you mention, then it most probably is Dermocystidium. Proof would be to burst a cyst (although I’m not suggesting you actually do this!) onto a microscope slide and look for thousands of tiny spores under high power of a microscope. These are the infective ‘zoospores’ that swim in the water in search of a new host.

Affected fish may be symptomless; others may show notably lethargy, colour loss, and/or bursts of erratic swimming. It’s possible the Cardinals were already carrying the parasites when you purchased them and may have shown no symptoms at the time. It can take several weeks for the cysts to develop.

Dermocystidium is a fungus-like parasite and for many years no one knew how to classify it. We know there are 20+ species of Dermocystidium and between them they affect a wide range of fish (and amphibia) including tetras, cichlids, goldfish, Koi, and salmon. These parasites are very hard to identify to species level, making it difficult to gauge which species of fish each type of parasite infects. It is likely that some affect just one or a limited number of fish species. 

And this brings us to your Cardinals: you mention not seeing the infection in the other fish, and this is borne out by a couple of scientific reports that point to the Dermocystidium species in Cardinals having a limited aquarium host range — perhaps restricted to a few tetra species. If so, this is good news. On the flip side, one paper suggests it is Dermocystidium salmonis that affects Cardinals, which if true means this particular parasite can affect a wide host range.  

Unfortunately, Dermocystidium is untreatable, and I don’t know how long the zoospores remain infective in the water. But given these zoospores are actively motile, they must be consuming energy at a fair rate, and given they don’t seem to feed in the spore stage then they must die fairly quickly if unable to locate a host. I’d reckon the zoospores remain infectious for a few days at most. 

There are really two choices here. You could move the Cardinals to another aquarium and house them in isolation for the rest of their lives. Alternatively, you could keep them in the main tank but don’t buy any more Cardinals or other tetras until the present Cardinals and any other tetras sharing the tank have all died, and then wait a couple of weeks for any free-wandering zoospores to perish. But this assumes my theories about host specificity and zoospore survival are correct.