Blood Parasites in Koi
Flukes, Fish Lice, Anchor Worm & Sleeping DiseaseSleeping Disease (Cryptobia)
Koi produced in soft water regions occasionally carry Cryptobia, a blood parasite that remains dormant until the fish’s immune system weakens — often after transport or a stressful pond transfer. Affected fish become profoundly lethargic, lying on their sides at the bottom of the pond as though asleep. Pale or white gills indicate a severe drop in red blood cell count.
Gill and Skin Flukes
Flukes are hookworms that attach to their host using posterior hooks. Gill flukes possess eye spots and reproduce by laying eggs, while skin flukes lack eye spots and give birth to live young. Symptoms include flashing and scraping against surfaces, gaping gill covers, excessive mucus production, and heavy breathing near the surface.
Fish Louse (Argulus) and Anchor Worm (Lernaea)
The fish louse is a visible, disc-shaped parasite that attaches using suction pads and hooked antennae to pierce the skin and feed on blood. It injects a toxic substance during feeding that can kill small fish outright. Anchor worm appears as a small green or white stick protruding from beneath scales or from inflamed spots. Both create wounds that become entry points for bacteria and fungus.