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Chilodonella

By the koi.wiki editorial team · Updated

Key facts: Chilodonella is a free-moving, oval to kidney-shaped ciliate protozoan parasite that infests the skin, fins, and gills of koi, often causing excessive mucus production[1][2].
Chilodonella is a genus of ciliated protozoan parasites; several species are described in the scientific literature, though only two are considered pathogenic to fish[2]. It is commonly listed among the protozoan parasites affecting koi and goldfish alongside Ich, Trichodina, and Costia[5].According to the Southern Regional Aquaculture Center, Chilodonella is an oval, flat protozoan with parallel rows of cilia and a notched anterior end that swims erratically, similar to Costia (Ichthyobodo), but is much larger[2]. The University of Florida IFAS Extension describes it as ovoid or kidney-shaped, dorsoventrally flattened, and translucent, with bands of cilia along its long axis, measuring approximately 30–80 μm by 20–60 μm[1].

Appearance and biology

Chilodonella targets the skin, fins, and gills of fish[1]. It is free-moving and glides over the gill and skin surfaces of the host[1][2]. The parasite's erratic swimming pattern resembles that of Ichthyobodo (Costia), another common koi ectoparasite, though Chilodonella is notably larger[2].A characteristic effect noted by the UF/IFAS pictorial guide is that Chilodonella infections often cause fish to excrete excessive mucus[1].

Signs and symptoms

According to the US koi dealer Kodama Koi Farm, Chilodonella infection in koi is associated with gray patches on the body, clamped fins, and gasping[3]. The same source notes that when the skin loses its natural slipperiness and becomes dry or sandpapery to the touch, this can indicate Chilodonella, Gyrodactylus, or Dactylogyrus infestation[3].Swelling of the gills is often observed under microscopic wet mount examination in fish infected with Chilodonella, and heavy concentrations of the parasite can cause mortalities[2].Koi and goldfish are described as more vulnerable to protozoan parasites, including Chilodonella, when water quality is poor[5]. See signs of illness for broader context on recognizing disease in koi.

Diagnosis

Because Chilodonella infections do not necessarily produce distinctive external lesions beyond mucus and gill swelling, diagnosis relies on microscopic examination of skin, fin, and gill scrapes[2][4]. UF/IFAS Extension recommends that fish be checked for microscopic parasites by analyzing biopsied tissue from gill, skin, and fins, especially where there are external signs such as excess mucus or abnormal behavior, and advises consulting a fish health specialist[4]. The presence of protozoans such as Chilodonella must be confirmed by microscopic examination rather than visual inspection alone[5]. For general diagnostic methods, see diagnosis: scrapes, microscopes and fish vets.

Treatment

Formalin, copper sulfate (CuSO4), and potassium permanganate (KMnO4) have been used successfully to treat Chilodonella in fish[2]. Potassium permanganate has been used experimentally at a concentration of 2 ppm to control external parasites including Costia, Trichodina, Ambiphrya, Chilodonella, and gill flukes, according to the Southern Regional Aquaculture Center; concentrations above 2 ppm can be toxic to fish if the organic content of the water is low[6].Among FDA-approved formalin products, Formacide-B, Parasite-S, and Formalin-F are listed as controlling external protozoa including Chilodonella, Ichthyobodo, Epistylis, Trichodina, and Ichthyophthirius on finfish[4]. The Southern Regional Aquaculture Center notes that very few chemicals are approved for use in aquaculture overall, and advises consulting a qualified fish health professional for current treatment rates and legal status of specific chemicals[2][4]. See treatments overview for further general information.

Prevention and risk factors

Protozoan parasites such as Chilodonella can be present in low numbers without causing disease; overall parasite load combined with environmental factors determines whether disease results[2]. Koi and goldfish become more vulnerable to parasitic infection, including Chilodonella, when water quality is poor[5]. Maintaining good water quality is therefore relevant to prevention; see water quality and quarantine for related practices.Examination of new fish for microscopic parasites, including biopsy of gill, skin, and fin tissue, is recommended before or upon arrival at a holding facility, particularly where signs such as excess mucus or abnormal behavior are present[4].

Common questions

What does Chilodonella look like under a microscope?
It appears as an oval or kidney-shaped, dorsoventrally flattened, translucent protozoan with bands of cilia along its long axis, measuring roughly 30–80 μm by 20–60 μm[1].
What are the signs of Chilodonella in koi?
Reported signs include gray patches on the body, clamped fins, gasping, excessive mucus, and skin that feels dry or sandpapery rather than slippery[3][1].
How is Chilodonella diagnosed?
Diagnosis is made through microscopic examination of skin, fin, and gill scrapes, since the infection does not produce easily identifiable external lesions on its own[2][4].
How is Chilodonella treated in koi?
Formalin, copper sulfate, and potassium permanganate have been used to treat Chilodonella, with potassium permanganate commonly applied at around 2 ppm; a qualified fish health professional should be consulted for current treatment rates and legal status of chemicals[2][6].
Is Chilodonella related to poor water quality?
Koi and goldfish are described as more vulnerable to protozoan parasites, including Chilodonella, when water quality is poor, linking environmental conditions to infection risk[5].

References

  1. ^ UF/IFAS EDIS FA108: Common Freshwater Fish Parasites Pictorial Guide: Motile Ciliates. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA108. Retrieved November 2026.
  2. ^ SRAC Publication No. 4701: Protozoan Parasites. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4701%20Protozoan%20Parasites.pdf. Retrieved November 2026.
  3. ^ HELP, Is My Koi Sick?!? Diagnose Symptoms & Fish Diseases - FREE Health Checklist. Kodama Koi Farm (koi dealer). kodamakoifarm.com/koi-sick-health-diagnosis-symptoms-diseases/. Retrieved November 2026.
  4. ^ UF/IFAS EDIS FA120: Preparation of Ornamental Fish for Shipping. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA120. Retrieved November 2026.
  5. ^ SRAC Publication No. 7201: Species Profile: Koi and Goldfish. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/7201%20Species%20Profile-%20Koi…. Retrieved November 2026.
  6. ^ SRAC Publication No. 410: Calculating Treatments for Ponds and Tanks. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/410%20Calculating%20Treatments%…. Retrieved November 2026.
  7. ^ NVN – Nishikigoi Vereniging Nederland. Nishikigoi Vereniging Nederland. nvn-koi.nl/. Retrieved November 2026.