Fish rust, or Velvet, is an infection of fish caused by dinoflagellates.
Many tropical fish have been reported to exhibit clinical signs characteristic of velvet disease. Anabantids (Siamese fighting fish, gouramies), cyprinids (goldfish, barbs), and cyprinodontids (killifish) are most frequently affected. Tilapia also has experienced epidemics (Ramesh et al., 2000). Temperate species, e.g., common carp (Cyprinus carpio), tench (Tinca tinca), and even larval amphibians (Amblystoma mexicanum, Rana temporaria, R. arvalis) also have been reported to be susceptible.
Dinoflagellates are a diverse group of aquatic protozoans. Their most prominent role is as important components of the food chain, where they typically function as either free-living autotrophs (primary producers) or heterotrophs (grazers). Some are important endosymbionts of invertebrates, e.g., photosynthetic corals, whereas others are parasitic. Although most of the 150 or so parasitic species infect invertebrates, those belonging to the genera Oddinium; Amyloodinium spp, Pisicinoodinium, and Crepidoodinium infest the skin, gills, or both of marine, brackish, or freshwater fish.
These parasites adhere to their host using flagellum, then forms rod pseudopodia which penetrate the skin and soft tissues of the gills. The pseudopods destroy the cells and feed on the nutrients inside. After feeding and maturing, the parasite drops off the fish and divides into dozens of cells that are released into the water to seek hosts. They must find a host within 24 hours, or die.
Oödinium produces white pustules on the fish that are much finer than the spots seen in Ich. In fact they are so fine they are often not seen before the fish perishes. Like Ich, Oödinium is present in most commercial tanks, but only becomes a problem when the fish are stressed by poor quality water, changes in the water temperature, or being transported.
The parasitic dinoflagellate caused a chronic infestation that typically developed over the course of 2 mo. Visible lesions began as a light golden dusting in oblique light and then progressed to more severe infestation intensity associated with dense white dusting of the skin; other clinical signs included dyspnea, lethargy, cachexia, localized secondary infections, and eventually mortality.
Initially the fish rub against hard objects trying to dislodge the parasites. As the disease progresses the fish becomes lethargic, fins are held close to the body, appetite is reduced and the fish loses weight. A key symptom is difficult breathing, resulting in rapid gilling.
Perhaps the most telltale symptom is the appearance of a velvety film on the skin that resembles gold or rust colored dust. The film may be difficult to see, but can be more easily detected by directing a beam of a flashlight on the fish in a darkened room. The parasite is most often seen on the fins and gills.
Velvet attacks all fish and will even affect fry that are only a few days old. Anabantoids, danios, goldfish, zebrafish, and killifish are particularly susceptible to velvet disease.
- Raise water temperature
- Dim lights for several days
- Add aquarium salt
- Treat with copper sulphate for ten days
- Discontinue carbon filtration during treatment
Because Velvet is highly contagious and usually far advanced before being diagnosed, it is important to take steps to treat it as soon as possible. Treatment is targeted at the free-swimming stage of the parasite.
Copper sulphate is the treatment of choice. It should be used according to the manufacturers instructions for a full ten days to ensure that the parasite is completely eradicated. Atabrine (Quinacrine hydrochloride) is another medication that can be used to treat Velvet. Because Oödinium is dependant on light, dimming the aquarium lights aids in eliminating the infestation. Increasing the water temperature to 82°F will speed the process, and adding salt to the water will ease the labored breathing caused by destruction of gill tissue. As with any treatment, activated carbon should be removed from the filter, as it will remove the drugs from the water. Prevention:
- Quarantine new fish for two weeks
- Maintain high water quality
- Provide fish with a nutritionally balanced diet
Velvet usually only arises when poor aquarium conditions prevail and is highly infectious. Quarantine of new fish for two weeks will greatly reduce the likelihood of contaminating a healthy established aquarium. Any fish that appear to be ill should immediately be removed and kept in a hospital tank to avoid the spread of the parasite.
- Geus, A. (1960) Nachtragliche Bemerkungen Biologie des Fischpathogenen Dinoflagellater Oodinium pillularis Scha¨perclaus. Aquarien Terrarien Zoologica 13: 305–306