Parasites & Infections…. When things go wrong

Parasites & Infections…. When things go wrong

Like humans, koi rely on a healthy immune system for protection from disease. Many of my articles focus on those aspects of koi-keeping that are necessary to keep their immune systems healthy, however, every now and then one needs to consider some of the health problems that can arise.

In spring and autumn when temperatures fluctuate widely it is imperative that you pay special attention to your koi. Make sure they are eating their food and swimming actively around the pond, if not you need to seek advice from your koi dealer. Koi are ectothermic, in other words they control their body temperature through their environment and their physiology is thus directly affected by the water temperature. Their immune systems work best between 18 and 28 degrees C and you will find that if your koi do get sick it will usually be during the cooler months.

Koi that are stressed over an extended period are more likely to suffer ill health because, when stressed, they release corticosteroids and catecholamines, hormones that ultimately suppress the immune system. If stress is chronic due to poor water quality their immune systems will be suppressed for longer than is safe and they will be especially vulnerable. Weekly water testing can save the lives of your koi (and reduce your own stress levels, because seeing one’s koi sick and struggling is extremely stressful).

What to look for

These are some of the most common problems that may affect your koi – if you familiarise yourself with the symptoms you will be able to recognise them more easily. If you notice a problem, immediately obtain assistance from a knowledgeable koi-keeper.

White spot

This parasite, Ichthyophthirius multifiliis, is visible to the naked eye as a series of white spots on the koi. The first few spots usually develop on the fins and infected fish will start to rub against objects in the water and may be restless. As the parasite spreads the fish lose their appetite and become lethargic. You need to pick up the infection early and begin treating immediately; if your fish get to the lethargic stage before you notice the problems then your chances of success are lessened and the infected koi may die.

Costia, chilodonella and trichodina

These three forms of parasite are all microscopic, so the first signs that your koi are affected will be a change in their behaviour. The koi scratch themselves as best as they can in an attempt to relieve the discomfort and to rid themselves of the parasites. The koi will scrape their bodies on the sides and bottom of the pond (called ‘flashing’) and they may also swim in a strange way, spiral or jump. The parasites also do further damage to the immune systems of the affected koi, and make them more susceptible to other health problems.

· Costia attaches itself to the gills, skin and fins of the koi. Affected fish become increasingly lethargic and may show signs of oxygen starvation.

· Chilodonella reaches high numbers when water quality is poor and koi that are exposed to bad water conditions become very susceptible to it. Chilodonella feeds off mucus, cell debris and bacteria.

· Trichodina also feeds off mucus, cell debris and bacteria. It attaches itself to the koi using a ring of small barbs.

Gill flukes and other flukes

Flukes are also only visible under a microscope; they look like small worms and attach to the skin and gills by means of hooks in their posterior body region. Flukes may attach themselves to any part of the koi, but gill flukes specifically congregate in groups at the openings of the gills and feed off the tissue, making it increasingly difficult for the koi to process oxygen from the water. Gill flukes also carry bacteria, which enters the gills through the holes drilled by the flukes and causes bacterial gill disease and ulcers.

As with other parasites, flukes irritate the koi so they scratch themselves against the bottom and sides of the pond. Gill flukes, because they specifically target the gills, affect the ability of the koi to take in oxygen and so gasping for air and head hanging occur along with scratching. Look out for these signs, along with the koi appearing lethargic and irritable.

Anchor worms and fish lice

Anchor worms and fish lice are both visible to the eye. They can cause tremendous irritation and create wounds, which are then subject to secondary bacterial infection. Anchor worms can be removed by hand if not too many are present.


The symptoms of dropsy are a swollen body and protruding scales; these symptoms come about due to an accumulation of body fluid within the body cavity of the fish. There are a number of causes, including bacterial infections. The prognosis is often not good, but use of antibiotics is the best option.


Ulcers develop on koi for a variety of reasons, including bad water management and insufficient filtration, bad nutrition and overstocking. They can also follow when a koi has bumped or injured itself, either on an object in the pond or while it is being netted, or during spawning.

Carp pox

Carp pox is a viral disease but affected koi do not act as if they are sick. They still look well and eat well, but develop whitish marks or tumours – most often on their heads and fins, and most often in the cooler months. Carp pox is difficult to treat, although there are treatments available. The marks sometimes disappear on their own over time, but this is not always the case. {

Article written by Angela Beckx of Koi @ Jungle. Contact Angela on 031 209 8781 or visit: Koi @ Jungle also stocks swimming pool products and equipment.

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