Costia in Koi

An Aggressive Parasite That Demands Immediate Attention

Costia (Ichthyobodo necator) is one of the most destructive parasites a koi keeper can encounter.

Identifying Costia

This microscopic parasite appears bean-shaped when free-swimming and pear-shaped when attached to a host. It anchors itself to the skin via its narrow end, progressively destroying the protective mucus layer and underlying tissue. Costia cannot survive without a living host and dies at water temperatures above 30°C.

Symptoms to Watch For

Infected koi typically display clamped fins and a marked change in behaviour — often described as “parking,” where fish become listless and cluster in corners or along the pond floor. As the condition worsens, sunken eyes develop. Distinctive “spiderweb” wounds, excessive mucus production, and dry patches on the skin are hallmark indicators of Costia.

Getting Professional Help

Costia thrives when a fish’s immune defences are compromised. A microscopic examination is essential for accurate identification, as symptoms can overlap with other parasitic infections. Reach out to a koi health professional at the first sign of trouble.

Suspect This Disease in Your Koi?

Early diagnosis saves lives. Contact KoiDoc for an on-site microscopic assessment.

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Frequently Asked Questions

Can Costia survive in my pond without fish?

No. Costia requires a living fish host and cannot survive independently in the water column.

Is Costia more common at certain times of year?

Costia is often more prevalent in cooler months when koi immune systems are naturally suppressed.

My koi has sunken eyes — is it too late?

Sunken eyes suggest a prolonged infection, but recovery is still possible with aggressive, professionally guided treatment.