Costia Ichtyobodo

Duncan Griffiths




Costia Ichtyobodo is classed as a flagellate parasite and one of the deadliest, smallest parasites that affect koi and gold fish, it can also be found on other freshwater and saltwater species of fish.

Flagellate parasite can infest fish both externally and internally and are characterised by one or more Flagella, (hairs).

The flagella cause a flagellate parasite to move in fashion that is very erratic and jerk like, Costia Ichtyobodo, is no exception.

Although a magnification of between x200 to x400 is needed to identify this parasite, its jerk like and tumbling movements, can clearly be seen as low as x100 magnification, as it gambols and jerks in and out of the field of view under the microscope, should you witness this spectacle, it should be your first alert to the possible presence of Costia.
Even though it may still be to small at this magnification to be seen clearly, no other parasite behaves in this manner.

The two most common species of Costia are, Ichtyobodo necatrix and Ichtyobodo pyriformis, with necatrix being by far the most common.

Both are extremely small, necatrix being about 20 micron and pyriformis about 15 micron.

Costia has two pairs of flagella, not one. One of the two pairs is used for propulsion and attaching itself to the host, the other pair is used in feeding, Costia also has a macronucleus.

Costia is pyriform in shape, (pear shaped), although, it only takes on this shape when physically attached to the host, at all other times it is the shape of a comma or bean.

The parasite itself is concave on one side and convex on the other and attaches itself to the host with a sucker at one end.

Costia reproduces by longitudinal fission, and cannot live for more than a couple of hours without a host, however when conditions are unfavourable to the parasite they are believed capable of forming into a cyst. Further, in this state they are also believed to be resilient to being dried out, and will remain in this state until conditions improve in their favour.

Costia is also temperature dependant and proliferates between the temperatures of about 10 to 25 deg c and cannot survive at temperatures 28 deg C and above.
Below 10 deg C the parasite will from into a cyst for protection.

The parasite is transmitted by direct contact and also is capable of swimming from host to host in its free swimming form. More importantly it is capable of being transferred from pond to pond on nets and koi socks.

In very small numbers costia presents little problem, feeding on sloughed off dead cells from the host, but in overwhelming numbers it can be responsible for mass mortalities in koi, by attacking living cells. Costia usually inhabits the gill and skin and once attached to the host actually destroys tissue at that site, let me repeat, it destroys tissue! Which is why when found in the gill, Costia is so devastating, when present in great numbers Costia will destroy gill tissue rendering the koi weak and debilitated and unable to mount an immune defence,
Costia is a parasite that is very much an opportunist on weakened fish, in so much as, when the koi is unwell and cannot mount an immune response. This allows more of each generation of Costia to survive what is considered “normal” mortalities of the parasite. Result Costia proliferates and will steadily multiply, and the end is usually not far away.

young koi succumb to costia a lot more readily than older mature koi.


A koi that is displaying laboured and heavy breathing,off its food, is also lethargic and covered in excess mucus is showing classic signs of Costia, however a microscopic examination should be done before commencement of any and all treatments, also koi suffering from heavy infestation of Costia may not make it through the treatment regime, as is the case with many parasitic infections. Every opportunity must taken advantage of to help a debilitated koi whilst treatments are underway, E.I. good pond hygiene and lots of extra O2, by means of air stones.



Costia can often be found with other parasites that are opportunistic, and may result in treatment modification, e.g. On lots of occasions, Costia is found nestling along side Chilodonella, in such a case the choice of treatment would be malachite green and formalin, you would have to make that decision based on the evidence you have presented at that time.


Treatments are many but the main working treatments are as follows:


Potassium Permanganate,  @ 1.5 gm per 220 U.K. Gallons or 265 U.S. Gallons one treatment only, for three to four hours.


Malachite Green and Formalin@

Malachite green 2% 10ml per 176 U.K.gallons

Formalin  30% 10 ml per 150 U.K. gallons

Used both together for one treatment only.


Chloramines T @ 2 gm per 100 U.K gallons

For three treatments only, days one / two and three. Switching off the U.V. light, making sure the ph is above 7.5  as UV light and ph, greatly increase the speed of release into the water of chloramines T and thus alter the toxicity.