Koi Disease Identification Guide: Ich, Ulcers, KHV, Dropsy, and Anchor Worm

Early disease identification is the most important skill in koi keeping. Most koi diseases are treatable when caught early. The same disease, treated a week later, often results in fish loss. Regular observation of behavior and physical condition during feeding is your primary diagnostic tool.

White Spot (Ich)

Ichthyophthirius multifiliis, known as ich or white spot, presents as small white dots roughly 1mm in size scattered across the body and fins. In heavy infestations, fish flash (rub against surfaces), show clamped fins, and group near aeration sources. Ich is caused by a protozoan parasite with a temperature-dependent life cycle. At 77 degrees F (25 C), the cycle completes in about 4 days. At 60 degrees F (15 C), it takes three weeks.

Treatment involves salt at 0.3 percent concentration, raising temperature to accelerate the parasite life cycle, and antiparasitic medications containing malachite green or formalin for severe cases. Treat all fish in the system simultaneously as the parasite spreads through water.

Bacterial Ulcers

Ulcers are wounds in the skin and underlying muscle caused by opportunistic bacteria, most commonly Aeromonas hydrophila and Pseudomonas species. They appear as red, circular lesions ranging from a small red area to large open wounds exposing muscle tissue. Ulcers develop secondary to immune suppression from poor water quality, parasites, or handling stress.

Treatment requires addressing the root cause (water quality, parasites) alongside topical wound care and sometimes injectable or oral antibiotics under veterinary guidance. Isolate severely affected fish to reduce stress and facilitate treatment.

Koi Herpesvirus (KHV)

KHV is a notifiable disease in many countries, meaning suspected cases must be reported to fish health authorities. It causes mass mortality events in koi and common carp, with death rates of 80 to 100 percent in susceptible populations. Active viral replication occurs between 60 and 77 degrees F (16 to 26 C). Symptoms include sunken eyes, gill pallor and necrosis, skin lesions, and erratic behavior before death.

There is no cure. Surviving fish become lifelong carriers. New fish should be tested for KHV carrier status before entering a valuable collection. Any unexplained mass mortality event in koi should be investigated for KHV involvement.

Dropsy

Dropsy is not a disease itself but a syndrome characterized by fluid accumulation in body cavities. Visually it presents as a pinecone-like appearance caused by raised scales that stand out from the body. The underlying cause is typically bacterial (often Aeromonas), though kidney failure, viral disease, or advanced parasitism can also cause dropsy. By the time scales are raised, internal organ damage is usually extensive. Prognosis is poor. Isolate affected fish and consult a fish health specialist for treatment options.

Anchor Worm

Lernaea cyprinacea, the anchor worm, is a parasitic copepod that burrows its anchor-shaped head into the fish's muscle tissue while the body and egg sacs protrude visibly from the skin. Infestations cause localized inflammation, secondary bacterial infection at attachment sites, and stress in heavily parasitized fish. Remove visible anchor worms with forceps, treat the wound with antiseptic, and use Dimilin (diflubenzuron) to eliminate larvae in the water column before they mature and reinfest.