There are several diseases that Koi fish can suffer from and many of them are related directly to fluctuations in temperatures and extreme weather changes. Koi fish are very susceptible to the cold and tend to fall very ill if exposed to the cold for too long. Broadly speaking Koi Carp disease causes can be classified into five major categories – bacterial, fungal, viral, internal parasites and external parasites. Apart from this there are several environmental factors that can disturb a Koi fish’s lifestyle and health like too much exposure to the sun and sub zero temperatures. Koi fish are generally very strong fish and it does not take much effort to keep these fish as pets. Some diseases among Koi fish take care of themselves but others may cause a lot of issues for the fish and can even be fatal.
A disease known as Ichthyophthirius multifilis causes white spot like formations on the body of the fish. This disease is not visible to the naked eye but can be seen clearly under a microscope. This disease usually takes over when the immunity of the fish is weak and the surrounding pond temperature is low. This disease can be fatal but only to smaller fish as the adult fish can fight it off. As far as treatment in concerned, the best way to do away with this disease is to raise the temperature in the pond.
The velvet disease though not very common in Koi can be a source of great discomfort for the fish. It is caused by an external parasite called as Oodinium which basically attaches itself to the outside of the fish ad begins to spread its tentacles until it covers a major part of the skin of the fish. After that it begins to kill off the cells it is nearest to eating away the nutrients from inside. Once that is done, it leaves the fish and begins to multiply, releasing more parasites into the water which have to seek out their own hosts. An entire fish pond can get sick with this type of disease in no time. The most apparent symptom of this kind of disease is a gold dust like formation on the skin of the fish. In many cases this disease causes very tiny puss filled pockets on the skin of the fish that can only be seen through a microscope. These puss pockets cause itchiness and therefore the fish are seen rubbing themselves against objects in the pond.
Koi fish are attacked by worms as well and a very commonly found one is the Lernea elegans which can be a very big problem for the Koi. These worms attach themselves to the belly of the fish and stay there for about a fortnight causing a lot of damage. The worst thing about these worms is that they reproduce rapidly and also leave the fish with bad gashed in the areas they have infected. Fish that are attacked by these worms have to then cope with the wounds on their bodies getting infected by other bacteria and fungi. Once these worms are spotted, immediate action is necessary otherwise the fish will develop multiple problems at one time.
Another type of external parasite are the Argulus lice. They are much easier to spot because of their green color and therefore can be nipped in the bud. The problem with these lice is that they attach themselves to the fins and the sides of the fish and begin sucking out their nutritional needs. While they are attached, the skin of the fish can undergo quite a bit of physical trauma leading to ulceration and infection.