By Drs. Foster & Smith Aquatic Veterinary Staff
Marine velvet disease is one of the most common diseases that affects marine aquarium fish. It is known by a variety of names including; amyloodiniosis, marine oodinium disease, oodiniosis, and gold dust disease. The scientific name of the infecting organism is Amyloodinium ocellatum. Amyloodinium is a one-celled organism called a dinoflagellate because it has whip-like structures (flagella) which help it move. It is highly adapted to parasitism. There are many free-living dinoflagellates present in most aquatic environments, but this particular species is one of the few that will actually cause disease in fish. This disease is widespread and can cause serious illness and death in aquarium fish if not recognized and treated quickly and properly. This article will give a description of the disease and offer treatment and prevention strategies.
The number of organisms: Amyloodinium is present in a free-swimming and infective form in many aquatic environments. The organism is extremely hardy and can withstand a wide variety of salinity and temperature fluctuations. In the wild, the number of infective organisms that are found in the water in any particular area is very small. In aquarium environments, there is the potential for large numbers of organisms to be found in a very confined space.
The immune system of the fish: As with almost all parasitic infections, most fish can fight off minor infections providing their immune system is strong. Many fish that are caught from the wild, however, and then placed in an aquarium are very stressed and their immune systems are not able to fight off minor infections let alone the greatly increased numbers that may be present in an aquarium environment. Amyloodinium can infect any fish at any time, but it appears to be much more of a problem when new fish are brought into an aquarium. Adding a new fish to an aquarium is obviously very stressful for the new fish and can be stressful for the existing tank inhabitants as well. Fish that are properly quarantined and fed are not as stressed and are much less likely to become infected with the disease and to create an outbreak when transferred to the existing display tank.
The symptoms of marine velvet usually involve the skin and lungs. Mild infections will usually only infect the gills and the fish may show minimal symptoms. As the infestation becomes more severe, the gills will become inflamed, bleed, and the lung tissue will begin to die. The fish will show signs of irritation and distress, with rapid breathing and lethargy. As the inflammation increases, the fish will lose its ability to transport oxygen across the gill membranes resulting in a fish that shows symptoms of suffocation, and if treatment is not initiated, death will often result.
The skin is the site of attachment for the organism and in severe infections, small gold-colored spots will cover the skin, which can progress to create a "velvet" appearance which gives the disease its name. By the time the gold-colored velvet appears, however, the gills may be so infected that treatment is usually too late. Many fish die from this disease without ever showing any visible skin changes. It may be possible to visualize early forms of the infection on the skin by using indirect illumination. This works best on dark fish and can be done by shining a flashlight on the dorsal surface of a fish in a darkened room. Viewing infected fish against a dark background may also be helpful.
- Trophont on skin of fish
- Trophont detaches and becomes a tomont
- Tomont divides
- Tomont releases dinospores that will attach to the fish
The life cycle of Amyloodinium
is very similar to Ichthyophthirius multifiliis, which causes freshwater ich, and is composed of multiple stages. The free-swimming organism is called a dinospore. When it attaches to the fish's skin, it is called a trophont. The trophont feeds on the fish for several days and then detaches. Once detached, it is called a tomont. The tomont divides and produces between 64 and 256 motile infective dinospores that attach to the fish and become trophonts starting the life cycle all over again.
Diagnosis is usually made based upon history and symptoms, but scrapings of the skin can be performed and the trophonts identified. Another technique is to place the infected fish in fresh water for several minutes and the organisms will drop off and float to the bottom. The surface water can be poured off and the sediment can be examined microscopically.
Initiate treatment immediately because the disease has such a high mortality level if not treated quickly. Treatment for this disease is almost exclusively with
copper. There has been some success reported with the use of the antimalarial drug chloroquine diphosphate but the drug is expensive, difficult to obtain, and therefore not a common treatment option. Copper comes in several forms including ionic and chelated forms. The chelated forms are supposed to be safer but the total amount used is higher, possibly offsetting the safety benefits.
Copper is very effective but can be ineffective or toxic if not used at very specific levels. Choose a high quality product, follow the directions closely, monitor the copper levels in the water, and adjust accordingly. When copper is used, the water should be tested twice a day for copper levels for the first few days, and then daily for the rest of the treatment period. Most sources recommend an ionic copper level of between 0.15 and 0.2 parts per million for a minimum of 14 days. Alkalinity and the presence of carbonate-containing substrates can impact the absorption and release of free copper in the water.
Copper is extremely toxic to invertebrates and should not be used in tanks where invertebrates are, or will ever be, housed. It is always best to use copper or any treatments in a quarantine tank and to only treat the infected fish. If Amyloodinium does develop in a reef tank with invertebrates, it will be very difficult to rid the tank of the Amyloodinium as long as fish are present. Removing the fish to a separate tank and allowing the tank to run fish-free for a month is probably necessary. This will allow the organism to run through its life cycle and die out due to the lack of a host.
If Amyloodinium does strike your tank, you will want to make sure that the water quality and nutrition are at the highest possible levels. Do everything possible to reduce the stress level of your fish to allow their immune systems to fight off this disease. There is evidence that fish that contract Amyloodinium and recover develop some lasting immunity to the disease.
There are several very important things to remember when treating this disease:
- Stressed fish are much more likely to develop the disease.
- This disease is highly contagious.
- The key to treating this disease is early detection and prompt treatment.
- Most fish that show the severe skin form are probably too sick to respond to treatment.
- Only the free-swimming dinoflagellate form of the organism (the dinospore) is susceptible to treatment.
- Properly sized UV sterilizers will also kill the dinospores.
- The encysted form is not susceptible to any treatment.
Prevention is the best way to treat marine velvet disease.
Prevention is truly the best way to treat this disease. We know that this disease is extremely contagious, widespread, and targets stressed fish. This disease most commonly shows up in newly purchased fish but will then often spread to other fish in the tank. Remember that adding a new fish to a tank is stressful for the existing inhabitants as well, and if a new fish comes in with an existing case of marine velvet disease, many of the other fish in the tank could develop the disease. Quarantining new fish is one of the best things you can do to maintain the health of your tank and is critical in preventing outbreaks of marine velvet disease. A quarantine period of a few weeks in a properly functioning quarantine tank will allow the aquarist enough time to ensure the new fish is not harboring velvet or ich. In addition, quarantined fish can be hand fed, isolated from aggressive fish, and treated, if necessary, allowing them to get through this high stress time in the best possible condition. The first several weeks after entering the new home is the highest period of mortality and disease in marine fish. It is impossible to be too careful or spend too much time or effort during this critical time period.
Amyloodinium is a common and deadly disease, but if proper quarantine procedures are followed coupled with early recognition and treatment, you can hopefully avoid the damage caused by this disease.
Faifield, T.A. Commonsense Guide to Fish Health. Barrons, New York; 2000.
Fenner, R.M. The Conscientious Marine Aquarist. TFH Publications. Neptune City, NJ; 2001.
Noga, E.J. Fish Disease Diagnosis and Treatment. Iowa State University Press; 2000.
Post, G. Textbook of Fish Health. TFH Publications. Neptune City, NJ; 1987.
Stoskopf, M.K. Fish Medicine. W.B. Saunders Co. Philadelphia, PA; 1993.
Tullock, J.H. Natural Reef Aquariums. TFH Publications. Neptune City, NJ; 2001.