Bacterial, viral and fungal diseases

Bacterial diseases
Common bacterial diseases in Koi include fin-rot and mouth-rot, which cause the affected tissues to erode, often accompanied by localised whitening or reddening. Other bacteria cause ulcers, manifesting as crater-like sores on the skin. A bloating condition known as ‘dropsy’, in which the Koi’s belly region swells and the scales stick out, can also have a bacterial origin.

Mild bacterial infections can be treated using a commercial remedy (such as fin-rot remedy or ulcer remedy), added to the water. Serious or persistent cases (including dropsy) may require antibiotics from a vet (your Koi dealer may be able to recommend local vets who deal with fish). Many bacterial problems are triggered or aggravated by unhygienic water conditions or overcrowding, so good Koi husbandry is the best prevention.

Viral diseases
A relatively common viral disease of Koi is ‘carp pox’, which causes whitish wax-like growths on the skin and/or fins. Fortunately, this virus is more disfiguring than harmful and eventually the growths should disappear of their own accord. There are no treatments for viral diseases of Koi. The provision of clean water conditions and optimal nutrition will help the Koi’s immune system to overpower many viral problems.

Fungal diseases
Fungal infections manifest as one or more white-grey fluffy growths that stick out from the Koi’s body surface. These fungal masses may resemble tufts of cotton wool. They are not highly contagious and generally only a single Koi is affected. Use a commercial fungus remedy or salt treatment. If you have a spare pond or other suitable temporary housing system, you can treat the affected Koi in isolation.

Generally, it is only stressed or injured Koi that succumb to fungus attack, so always investigate for any underlying causes (such as damage to the Koi’s skin) if a fungus problem occurs.

Written by Dr Peter Burgess

Photo courtesy of The Environment Agency