Amyloodinium Ocellatum
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Magnification of Amloodinium Ocellatum |
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| One of the most frequently encountered marine fish diseases, velvet, or coral
fish disease has historically and notoriously been responsible for a tremendous number of
premature fish mortalities. Amyloodinium ocellatum is an algal protozoan belonging to a
group of single-celled organisms known as the dinoflagellates. Related to the
dinoflagellate Gypmnodinium brevis, it is a cousin of the organism which causes the red
tides to occur along all major coastal waters. Initially, symptoms may be subtle and
diagnosis is often difficult. Rapid gill motion may occur, as the gills are typically the
first site of infestation. After three to six days, "velvety" looking patches
will become apparent on greater body areas. Fins and eyes may become cloudy. Eventually,
the velvety appearance will spread over the entire body. Advanced cases display small,
white spots, fin necrosis, and an overall appearance of the fish having rolled in powdered
sugar. Fish have probably already begun to die by this point. If left untreated, this
disease is capable of killing all of the fish in an exposed system. I have seen a tanks
full of fish killed in as little as 48 hours many times, although the gestation period is
usually somewhat longer.
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| Reprint from Marine Aquarium Handbook-Beginner to Breeder by Martin Moe, Jr. – Green Turtle Publications |
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Although careful observation may reveal the disease organism (sighting down the length of the fish's side), diagnosis is best made microscopically. By preparing a wet mount on a slide with a small amount of body slime, one can easily identify these non-motile parasites at 100x magnification. At 200x magnification, the unicellular dinoflagellates take on the appearance of what looks like bunches of grapes. Some of these "grape-like organisms" have a teardrop shape, with the pointed end acting as the attachment point to fish tissue, while the majority of others appear as round cells, varying in size between 30 to 60 microns. First aid for treating this disease is to lower the specific gravity (reduce salinity) of the system down below 1.015, with marine fish tolerating a specific gravity as low as 1.010 for periods exceeding 60 days. Since these parasites are non-osmoregulatory, exposure to a radically lowered s.g. causes them to take in water across their semi-permeable membrane in an effort to equalize osmotic pressures. Soon, after taking in a volume of water greater than the cell wall is capable of holding, the parasite bursts, similar to what happens when overfilling a water balloon. Therefore, once diagnosis is positive for Amyloodinium ocellatum, one should radically drop the s.g. down toward 1.010., (it is not recommended to go lower than this level). Fish tolerate this radical drop quite well, and most species have done well for periods exceeding 90 days. Conversely, great care must be exercised when raising the specific gravity. The s.g. should not be increased more than .002 in a 24-hour period, as osmotic shock may occur. Typically for most salt mixes, one cup per 50 gallons will yield a rise in s.g. of approximately .002. Along with a lowered specific gravity, copper sulfate is considered the drug of choice when treating this disease, at concentrations ranging from .18 mg/lt. up to 0.25 mg/l. of non-chelated, ionic copper. Therapeutic levels of copper must be maintained for at least 21 to 30 days to be sure of eradicating all stages of the life cycle. It is worth mentioning here that since gill hyperplasia can occur if fish are exposed to copper at concentrations above .18 mg/l., close observation and great care should be taken during the duration of treatment. Since calcareous objects (such as coral and coral sand) tend to plate out copper from solution by binding free copper to Ca CO3, testing and treating should be done every day until the levels stabilize. Additionally, copper is toxic to live rock and invertebrates, and must never be used in reef aquariums. In conclusion, it is important to identify disease organisms quickly and accurately, as well as begin treatment as soon as possible. Lowering the specific gravity and adding copper sulfate to the system is the best way to control this parasite. Remember to remove all chemical filter media as it will remove the medication. Also, turn off ozonizers to prevent the possible formation of toxic, free radical compounds. The practice of providing a two to three week quarantine period when your fish first arrive will also dramatically diminish the likelihood of parasitic disease outbreak. Likewise, good water quality in your holding facilities, good and proper nutrition, and the assembly of compatible animals within the holding tanks) will also aid in reducing stress and disease outbreaks considerably. For more information and additional reading, please refer to the following references: Blasiola, George. 1991 Bassleer, Gerald. 1996 Gratzek, Dr. John B. 1992 Herwig, Nelson. 1979 Moe, Martin Jr. 1989 Post, Dr. George. 1983, 1987 Untergasser, Dieter. 1989. Veterinary Clinics. 1988. |
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The life cycle of
this parasite is usually completed in 6-12 days. During this time, Amyloodinium ocellatum
matures through three stages. Beginning as free-swimming dinospores, they attach
themselves to tissue of fish on contact. Once attached, the dinospores encyst to become
trophonts. These trophonts send out rhizoids, the filaments which allow the host's
nutrients to be taken up. After drawing nutrients from the host for several days, these
filaments are withdrawn and the parasite matures to become a tomont. While encysted, cell
division occurs, ultimately releasing upwards of 250 dinospores to attack a new host.
Normally, the tomont falls off the fish, once maturity is achieved. Life cycle
developments is completed in the substrate or other quiet areas of the environment. The
time span of maturation is dependant on temperature (the warmer the water, the quicker the
development).