Trichodina are a genus of ciliated protozoans that primarily parasitize fish and occasionally amphibians. Ciliates are creatures that possess cilia, which are tiny hair-like protuberances that enable the creatures to “swim” about fairly efficiently. In parasites, this is key for being able to travel from host to host.
Trichodina infestations are typically rare, as they prefer water bodies with stagnant, unhealthy waters over clean water. Goldfish are most often affected by Trichodina, but koi are susceptible as well. If you know or suspect that your pond has Trichodina, it’s imperative that you confirm and treat as quickly as possible to prevent your fish from becoming ill or, in extreme cases, dying.
Trichodina is a genus of protozoan parasites found in the family Trichodinidae. All members of this family are obligate ectoparasites, meaning that they only live outside of the host’s body and must have access to a host to fully complete their life cycle.
There are over 150 species in the Trichodina genus, with the majority of them not actually being true parasites to their hosts. Rather, most of them simply use fish as an anchor point to attach to while they feed on bacteria and other microscopic organic material that floats by.
However, even these can cause issues in fish as they may damage the sensitive slime coating and epidermal skin layer, resulting in an increased risk of the fish developing infections and secondary illnesses. The collective name for diseases caused by trichonids is trichodinosis.
A handful of species may feed on dead tissues and settled organic material that is found on the surface of the fish’s skin, but otherwise do not directly parasitize the fish. It is not Trichodina that kills fish; rather, it’s the secondary ailments that host fish become much more susceptible to upon colonization.
Most often, Trichodina establish in an ecosystem due to poor water quality and overcrowding. If found in a relatively healthy pond, they were likely introduced via a new fish that was raised in less than ideal conditions (such as an overstocked fish farm), and then not properly quarantined by the new owner prior to being placed in the pond with other fish.
They can also pioneer via new plants that were kept in poor conditions. For this reason, any new plants should be thoroughly rinsed off and checked over, if possible, with a microscope to ensure that parasites are not present on their roots. If potted in soil or substrate upon purchase, remove and dispose of the soil/substrate just in case Trichodina are present, and again thoroughly rinse off the plant’s roots before planting in or near your pond.
They are most often found on the gills and fins of fish, but can exist anywhere on the skin. Some species prefer the urogenital ducts, where a great deal of bacteria can be found for them to feed on.
Trichonids are quite small, approximately 18 to 100 micrometers in diameter depending on the species, with a spherical shape and a basal (located on the underside of the body) disc that enables them to attach to hosts.
These basal discs have dozens of denticles, which are incredibly small, sharp, tooth-like structures that enable secure affixion to their host organism, which could be a fish, frog, salamander, or other such aquatic or semi-aquatic organism. Due to their size, trichonids are most easy to identify under a microscope by taking water and/or small tissue samples.
Trichodina species have a pretty simple life cycle, relying on only one host and only needing to find a new one if they somehow become detached from the initial one.
Trichonids attach to a host, reaching maturity within a span of only a few days. Once maturity is reached, they don’t need a mate to reproduce – rather, because they are single-celled organisms, they reproduce via binary fission, literally splitting themselves in half.
The daughter cell either attaches to the current host, or utilizes its cilia to swim to a new one. They must find a host within 24 hours or most trichonids will die. Once a host is secured, the process repeats itself.
Koi infected with Trichodina, or any parasite, may display erratic behaviors such as flashing, jumping about, and rubbing against rocks and walls to try to rid themselves of the uncomfortable parasite.
The longer that Trichodina are present, the more damage they are likely to do to skin as they sort of spin in place continuously while attached to the fish. Over time, skin becomes damaged enough that it sloughs off, which can then be ingested by the trichonids.
Koi that have trichonids attached to them may produce excess epidermal mucous to try to fend off the parasite. This can cause fish to appear to have a whitish-grey, sometimes grey-blue, muted hue.
The skin around trichonid attachment sites may be pink or red, depending on the severity of irritation and whether or not a secondary infection is present.
Lesions and ulcers can develop in more serious cases as parasite attachment sites become more agitated and are invaded by bacteria, fungi, and any viruses present. This is of particular concern if fish are rubbing against things and thus scrape their sides raw, inviting in even more bacteria and secondary infections.
When trichonids attach to gills, the gills can become damaged and somewhat ratty looking over time as skin dies and falls off. They can also become a pinkish or deep red color. Damaged gills can result in impaired swimming over time.
As skin dies around Trichodina attachment sites, your fish will display obvious, small white splotches.
[su_note note_color="#ffffff" text_color="#000000" radius="0"]If you suspect that a fish may be ill, or notice that it’s already infected with trichodina, quarantine it from the rest of the pond immediately to hopefully prevent the spread. Treat as needed in a quarantine tank, and keep a close eye on the rest of the pond.[/su_note]
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Luckily Trichodina are not incredibly hardy, making them easy to treat. In most cases, a simple salt bath should do the trick. You can simply quarantine and treat only infected fish, or all fish to ensure that there are no trichonid survivors.
While your fish are quarantined in tanks, you may drain your pond to ensure any unattached trichonids die, though this shouldn’t be necessary as most die within 24 hours of not having a host.
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Potassium permanganate also effectively treats Trichodina, and is gentle enough that you can also use this to treat the roots of any new plants you acquire to ensure no trichonids are transferred to your pond from them. It’s incredibly oxidative, and works by breaking down the cell walls of parasites as well as bacteria, algae, and other organic matter.
Larger organisms like fish and plants are unharmed so long as proper dosage is followed, as per the product label. Absolutely do not us in conjunction with salt baths, as salt causes potassium permanganate to become much more toxic.
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For more serious cases, formalin can be used. However, this chemical can be more dangerous to fish and must be dosed very carefully. Use it only if other means haven’t worked, as trichodina are easily treatable with cheaper salt baths or potassium permanganate (never both together!).
If your fish has a particularly bad symptoms, including secondary infections, formalin is a good choice for rapid removal of the parasites.
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As always, maintain healthy water quality! A clean, well-oxygenated, well-filtered pond with properly balanced temperature and pH to suit koi will help deter parasites from establishing themselves.
Include a quality filter or two in your pond, an aerator to provide both water movement and oxygen, and add a variety of marginal and submerged plants to aid in filtering water, providing shade to regulate water temperature, and naturally increasing dissolved oxygen.
Check your water quality at least once daily, at the same time each day to minimize the risk of data bias. Do not overstock your pond, as this will greatly decrease water quality and stress your fish, thus making them more susceptible to parasites and illness.
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Providing a high quality, balanced diet is also key. A healthy koi is one that is better able to fend off parasites and secondary infections with minimal harm! Feed koi a well-balanced pellet feed, and supplement with occasional fruits and veggies like shelled peas, spinach, oranges, and watermelon.
Alternatively, you can make your own food so that you know exactly what you’re koi are eating!
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