It is known as the velvet disease a condition characterized by an aspect of the skin of the fish which seems to be covered with a yellowish dust.

The causative agent is a protozoan with a biological cycle similar to the causative agent of the white spot disease.

The agent of the velvet disease is the dinoflagellate Oodinium pillularis discovered by Schaperclaus in 1951. The Oodinium is round or oval in shape (pear-shaped).

Different species of the genus Oodinium are known: pillularis, cyprinodontum, limmeticum and ocellatum.

It attacks all types of ornamental fish (exept Oodinium ocellatum that is typical of marine fish) and is a major cause of mortality of the offsprings.

The disease manifests itself as a fine dust on the skin of the fish of gray-yellow color, hence this disease is also known as Rust Disease and Gold-Dust Disease.

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 the infection advances the skin of the sick fish becomes dusty and opaque due to the excess of mucus that generates the immune system of the fish as a mechanism of defense against parasites.

Since the parasite affects the gills, it is common that respiratory failure appears.

If the condition becomes severe, the fish may present the following symptoms:

• Opaque eyes
• Fins against the body
• Ulcers in the skin
• Skin detachment
• Exophthalmia

The Oodinium presents a life cycle very similar to the cycle of 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 of alive organic matter, through roots that it uses to attach to the skin.

In the free phase form a protective shell inside which reproduces by cellular divicion. It can produce more than three hundred spores.

This disease is typical of the aquariums in poor maintenance conditions. The following factors influence in its appearance:

• Abrupt changes in water temperature
• Introduction of new fish without quarantine
• Fish tired and lacking in defenses
• Old water (it's advisable to make frequent changes of water)
• Introduction of plants with cysts (disinfect prior to their introduction)

The protozoa begins the parasitosis in the gills. Complete the infecting phase feeding from the host fish, until it reaches maturity. During the biological cycle the parasite presents the following phases:

• Infecting phase immobile in the skin, during this stage it feeds and grows.
• Cyst phase in the substrate when the parasite exits the fish. Inside the cyst the multiplication by cellular division takes place.
• Dinospores phase. The dinospores are the product of the divisions of the previous phase. If dinospores not 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.

The used treatments are based on salts of copper. Since not all fish tolerate this type of medication, take cautions before applying.

As alternative to the treatment with salts of copper are the salts of quinine or the methylene blue.

Some species of the parasite also obtain energy through photosynthesis, then cover the aquarium can favor to the weakening of the parasite.

To increase the temperature of the water can help to the treatment, because the parasite's life cycle is accelerated.

To prevent, it is necessary to know how the disease can enter to the aquarium.

The parasite can be latent waiting for favorable conditions. When the fish are weakened, the disease attacks. When the fish are stressed, they weaken.

The stress can be due to poor conditions of maintenance of the water or abrupt changes of temperature when making the water replacements.

Cysts of the parasite can also enter through sick new fish or through infected objects. The quarantine and the hygiene are basic precautions to be taken into account.