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White spot disease

What is it?

White spot is a contagious parasitic disease of fish. Caused by Ichyophthirius multifilis, the parasite infects the fish after moving from the bottom of the pond. The parasite attaches itself to the fish, moving under the skin where it feeds on cells and body fluids. Later in the parasite's life cycle, it moves out of the fish and attaches itself to pond material where it repeatedly divides itself thousands of times. These then hatch and the process begins again.

The parasite causes damage to fish tissue which can lead to bacterial and fungal infections

Where and when might it occur?

The disease can affect most fish species but is most common in carps.

The parasites are often introduced through infected fish or pond equipment.

Changes in environmental conditions such as the introduction of lower temperature water to that of the pond or tank can also cause the release.

Diagnosing white spot disease

The illness is first detected through behavioural changes in the fish. Fish will often be seen to rub themselves against stones as the parasite enters the skin.

Disorientated swimming close to the surface or staying close to the bottom and tightly folded fins are also observed.

In the later stages, small white cysts will start to appear over the fish, including the gills. These are roughly the size of a grain of salt.

Control and treatment

To stop the parasites from being released, zinc-free malachite is used in the pond (0.1ppm). Instructions on use must be followed carefully to prevent toxicity.

To prevent the disease it is advised to quarantine new fish for two weeks before introduction.

References
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Boden. E, (2001) Black's Veterinary Dictionary. A&C Black, London, 20th Edition