Flagellates. Oodinium ("Gold Dust" or "Velvet"), officially now called Piscinoodinium, is a genus of microscopic parasitic dinoflagellates. Call it "Oh-uh-DINN-ium," not "OOD-in-um" as I miscalled it for years! One species is P. pillularis, which gave the old-fashioned name of "pillularis" to these parasites. Oodinium is more insidious than Ich — the individuals are invisible at every stage. It's hardier and more resistant to treatment. Oodinium is widespread in aquarium fish, though "extremely rare" in koi ponds, according to Dr Erik Peterson at Koivet.
Flagellates are single-celled organisms that move around by thrashing their whiplike flagella. (A Latin flagellum is a whip.) Most are harmless organisms at the base of the trophic web. The photosynthesizing dinoflagellates form a huge class among the flagellate phylum and include Pfiesteria, the organism associated with the notorious "red tides" of polluted marine waters. Many species of flagellates are part of the normal intestinal fauna of fishes, and many more kinds of free-living flagellates make a living in the mucus of fish gills and skin, without attaching themselves or causing trouble, but even some ordinarily harmless ones can become pathogenic in stressed hosts.
Piscinoodinium is not harmless; it puts down a rootlike extension ("rhizoid") and can burrow into the skin or gill tissues. Don't ignore early warnings of Piscinoodinium, when fishes "flash" their bellies sideways in attempts to scrape their gills against stones, firm leaves or gravel. Fish react by producing copious mucus on skin and gills. They may begin to respire rapidly, hide or sink to the bottom and clamp their fins, in classic symptoms of malaise.
Oodinium moves fast, faster than Ich. If you're unwary, you might not realize the fish is being attacked by Oodinium until it begins to lose its glossy shine and seems to have patches of yellowish to golden-brown or rusty-colored varnish that gave it its old-fashioned name of "velvet disease". When the lights are on, the effect is hard to detect, but if you turn out all aquarium and room lights and use a flashlight, the point source of light will make patches of Oodinium more visible.
Longterm Oodinium infestation may result in darkened heads and backs of fishes. What you may not notice until it's too late is damage Oodinium may do to fishes' gills. Oodinium may first attack gills, where the parasites cause both localized and diffuse swelling of the gills and fusion of the gill lamellae. But it may be noticed first round the gill openings, and also near the base of fins. Later, it can even attach to the eyes. Badly infested fish in the late stages of Oodiniasis should probably be euthanized, so that you can concentrate your attention on the more likely survivors.
Don't expect to see the individual trophonts, the parasite's attached feeding form, which remain much smaller than Ich. The dinoflagellate that attaches to the skin contains photosynthesizing chloroplasts that supplement the nutrients it is going to be getting from the host fish. The chloroplasts combine chlorophyll with supplementary yellowish to brownish carotenoids, which mask the chlorophyll's green and give Oodinium communities their characteristic yellow-brownish cast. Any critter that could photosynthesize used to be classed as an alga, so if you hear about "parasitic algae," it's likely to be Piscinoodinium they're talking about. Oodinium can also settle out on the gills, where it sends down a rootlike extension into the gill lamella and dissolves cells with a histolitic action, to absorb their contents. This causes intense gill itch and swelling. If the infestation is bad enough, the gill-cover itself may seem swollen. But don't wait til the fish is at the surface laboring for oxygen. Fish can be killed by Oodinium in a few days, either directly from suffocation or from secondary bacterial infections of the abrasions.
An abstract of a 1998 article in the journal Diseases of Aquatic Organisms reported that Piscinoodinium was routinely found on wild-caught Brochis and Corydoras catfish imported to Great Britain from South America, where infected fish had trophonts of various sizes embedded in pits in the outer skin or enclosed by enlarged epithelial cells.
Like Ich, the Piscinoodinium trophont matures, then detaches and falls to the substrate, where it rounds up and encysts, in the phase called the tomont. (Ich passes through a comparable phase.) The encysted Oodinium tomont can spend three to five days at normal aquarium temperatures, dividing and dividing again as it forms tomites — as Ich does — but then, in an additional transformation, each tomite splits into many dinospores. There may be as many as 256 of these swarmers from each tomite. Spores or tomites can also mature within the trophont while it's still attached to the fish, I understand. They are tougher than Ich, more tolerant of salt and high temperatures, and they have a longer free-swimming period before they must settle out on a host. The University of Florida Cooperative Extension Service estimates the life cycle of Oodinium at aquarium temperatures as 10 to 14 days. The ZFIN website estimates the life cycle takes about two weeks at optimum temperatures (23-26°C).
During their free-swimming tomite stage, they sustain themselves with photosynthesis. This is why you're advised to darken the tank in which you're battling Oodinium. A Piscinoodinium infection of the gills may not be noticed for a couple more life-cycles. Advanced gill damage is not usually reversible; a fish may survive Oodinium but spend the rest of its days gasping for oxygen. Unfortunately Oodinium can also infest the intestinal tract, where it's comparatively safe from medication, unless you try the bioencapsulation technique, using brine shrimp. From its secure internal position the parasite is free to pass out with the feces in its spore-releasing stage. My understanding is that this isn't common.
Treatments. Oodinium derives some of its nutrition from its chloroplasts and starch reserves. So, whereas heat and salt may stress it like Ichthyophthirius, complete darkness is especially stressful for Oodinium; still, I wouldn't rely on promises that it can be killed with darkness alone. Though Oodinium is more salt-tolerant than Ich, sometimes a saltbath is recommended, at a strength of one teaspoon per 5 gallons, or a salt dip: one to three minutes in full-strength seawater (35 parts/thousand). Salt alone hasn't eliminated Oodinium for me. I combat Oodinium with increased heat and the combination of formalin and malachite green. Because formalin can only be used on ornamental fish that won't be eaten, the University of Florida Cooperative Extension Service's recommended treatment in food fish is quinine hydrochloride (the hydrochloride simply makes the quinine water-soluble). Several Ich cures combine malachite green with quinine hydrochloride. Kordon's Rid-Ich contains naphthoquinones. Atebrin combined with a salt bath is recommended at the ZFIN site. Acriflavine works for some aquarists. Oldtime recommendations of copper sulfate are too risky, especially in soft acidic waters, I feel. I got some of these details from Robert J. Goldstein's article in F.A.M.A., Feb. 1999, pp 52ff. I sure wish all the aquarium magazines were more forthcoming about posting archival material at their websites.
In 2006, an ionophoric polyether, salinomycin, experimentally incorporated in feed, successfully eliminated Piscinoodinium without stressing swordtails (Xiphophorus species). Salinomycin is widely used in chicken feed as an antimicrobial that promotes growth, but its high toxicity in concentration makes it unlikely that it will appear in any marketed aquarium pharmaceuticals, except perhaps incorporated in flake feed.
Prevention is better than treatment. Piscinoodinium is a parasite of waters that are too cool, inhibiting fishes' defenses, and high in dissolved organics. Sanitation and water changes, suitable warmth, lack of crowding, should make Oodinium rare in the aquarium.