Velvet disease is a condition characterized by a a yellowish dust aspect on the skin of the fish. The disease manifests itself as a fine dust on the skin of the fish. The dust is typically gray-yellow in color, which explains why this disease is also also known as rust disease or gold-dust disease.
- The causative agent is a protozoan with a biological cycle similar to the causative agent of the white spot disease.
- The most common type of fish prone to this disease is the dinoflagellate Oodinium pillularis species, discovered by Schaperclaus in 1951. The oodinium is round or oval in shape (pear-shaped).
- Different species of the genus Oodinium include: pillularis, cyprinodontum, limmeticum and ocellatum.
- It attacks most types of ornamental fish (except Oodinium ocellatum, a type of marine fish) and is a major cause of mortality of the offsprings.
The symptoms depend mainly on the severity of the infection.
- During the beginning of the infection, the affected fish will show some behavioral changes, like fin twitching or body rubbing.
- As it advances, the skin of the sick fish becomes dusty and opaque due to the excess of mucus that their immune system generates as a defense mechanism.
Since the parasite affects the gills, it is common that respiratory failure appears.
If the condition becomes severe, the fish may present the following:
- Opaque eyes
- Fins against the body
- Ulcers in the skin
- Skin detachment
Oodinium has a life cycle very similar to the Ichthyophthirius (white spot disease), however these organisms are not related—Oodinium is a flagellate, whereas Ichthyophthirius is a ciliate.
- During the infecting phase, the parasite feeds on live organic matter through roots that it uses to attach to the skin.
- In the free phase, a protective shell forms on the inside, which then reproduces by cellular division. It can produce more than three hundred spores.
Velvet disease is a common occurrence in aquariums that undergo poor maintenance conditions. The following factors influence this:
- Abrupt changes in water temperature
- Introduction of new fish without quarantine
- Tired fired that are lacking in defenses
- Old water—you should be frequently changing your water.
- Introduction of plants with cysts (if bringing in new plans, make sure to disinfect prior to their introduction)
Biological Cycle of the Parasite
The protozoa begins the parasitosis in the gills, then it complete the infecting phase when it begins feeding from the host fish, until it reaches maturity.
During the biological cycle, the parasite presents the following phases:
- Infecting phase, which is immobile in the skin (during this stage, it feeds and grows).
- Cyst phase in the substrate, when the parasite exits the fish. Multiplication by cellular division takes place inside the cyst in this phase.
- Dinospores phase. The dinospores are the product of the divisions of the previous phase. If dinospores don't find a new host, they die within 24 to 36 hours.
The life cycle is completed in ten to fourteen days at a temperature of 23 to 25 degrees.
Treatments are commonly comprised of copper salts. Since not all fish tolerate this type of medication, take caution before applying. Here are some other alternatives:
- Quinine salts or methylene blue.
- Some species of the parasite also obtain energy through photosynthesis. Try to cover the aquarium, which can lend to the weakening of the parasite.
- Increasing the temperature of the water can also help with treatment because the parasite's life cycle is accelerated through the heat.
Successful prevention requires first knowing how the disease entered the aquarium.
- The parasite can be latent, waiting for a favorable condition. When the fish are weakened, the disease then attacks. When the fish are stressed, they weaken.
- Stress can also be due to poor maintenance conditions of the water or abrupt changes of temperature when making the water replacements.
- Parasite cysts can also enter through sick new fish or through infected objects. To further prevent this, make sure you quarantine any new fish or objects and ensure hygiene before introducing them to the tank.