Koi Disease Reference Manual: Complete Pathogen Guide

By KoiQuanta Editorial Team|Updated Mar 2026

Koi are susceptible to over 30 identified pathogens requiring different treatment approaches. This isn't a number to memorize -- it's a reminder that "my koi is sick" is the beginning of a diagnosis, not the end. The correct treatment for koi herpesvirus (no curative treatment exists) is completely different from the correct treatment for a Gyrodactylus fluke infestation (Praziquantel), which is different again from the correct treatment for a bacterial ulcer (antibiotics). Getting the diagnosis right before treating saves fish.

This reference covers all major koi disease categories with identification keys, standard treatment approaches, and prevention guidance.

TL;DR

Mass mortality can occur rapidly -- dozens of fish dying within 24-72 hours.
Supportive care (excellent koi pond water quality tracker, maximum oxygenation) and temperature manipulation (holding above 30°C to suppress viral replication) are the primary management options.
Pond mortality rates in unmanaged KHV outbreaks range from 70-100% of susceptible fish.
Warm water management (maintaining temperature above 18°C when possible) suppresses lesions.
Spreads more rapidly in warm water (above 20°C) than most other bacterial koi infections.
At 25°C, treatments should be repeated every 3-4 days; at 15°C, every 10-14 days.
Saucer-shaped, circular protozoa with distinctive ring of denticles (tooth-like structures) visible at 100-400x magnification.

Viral Diseases

Koi Herpesvirus (KHV)

Causative agent: Cyprinid herpesvirus 3 (CyHV-3)

Temperature dependency: Active and lethal at 18-28°C. Slows below 13°C and above 30°C.

Identification: Sunken, milky, necrotic gill tissue is the hallmark presentation. Fish show respiratory distress, congregate at water surface or near aeration, and display sunken eyes and skin lesions. Behavioral changes (lethargy, loss of balance) precede gill changes. Mass mortality can occur rapidly -- dozens of fish dying within 24-72 hours.

Diagnosis confirmation: PCR testing of gill tissue is the definitive diagnostic method. Clinical signs alone are strongly suggestive but not confirmatory.

Treatment: No curative treatment. Supportive care (excellent water quality, maximum oxygenation) and temperature manipulation (holding above 30°C to suppress viral replication) are the primary management options. KHV-exposed fish that survive become lifelong carriers.

Prevention: Quarantine all new fish before introduction, especially during the 18-28°C active window. KHV is a notifiable disease in most US states -- confirm your state's reporting requirements.

Prognosis: Poor for affected fish. Pond mortality rates in unmanaged KHV outbreaks range from 70-100% of susceptible fish.

Spring Viremia of Carp (SVC)

Causative agent: Rhabdovirus carpio

Temperature dependency: Most pathogenic at 10-17°C. Rare above 22°C.

Identification: Hemorrhagic signs including petechiae (small red spots) on the skin and fins, abdominal swelling with fluid accumulation, and pale necrotic areas on skin. Often confused with bacterial hemorrhagic septicemia. SVC attacks primarily in spring when water temperatures are in the active range.

Diagnosis confirmation: PCR testing or virus isolation from tissue. Clinical signs are suggestive but not diagnostic.

Treatment: No curative treatment. SVC is a USDA-listed notifiable disease -- you are legally required to report suspected SVC to your state veterinarian.

Prevention: Import only from certified disease-free sources. Strict koi quarantine program before introduction.

Carp Pox (Herpesvirus cyprini)

Causative agent: Cyprinid herpesvirus 1 (CyHV-1)

Temperature dependency: Lesions typically regress when water temperatures rise above 18°C and recur in cooler water.

Identification: Smooth, waxy, "candle-wax" appearing raised lesions on skin and fins. Lesions are gray-white to pink in color. Unlike bacterial or fungal lesions, carp pox lesions have a smooth, almost plastic appearance. Usually not life-threatening but disfiguring.

Treatment: No curative treatment. Warm water management (maintaining temperature above 18°C when possible) suppresses lesions.

Prognosis: Excellent for survival; fish are lifelong carriers but lesions usually self-resolve in warm water.

Bacterial Diseases

Aeromonas Ulcer Disease (Motile Aeromonas Septicemia)

Causative agents: Aeromonas hydrophila, A. salmonicida, A. veronii and related species

Identification: Open skin ulcers, often starting as raised scales or redness before progressing to open erosions. Ulcers have a characteristic appearance: red inflamed margins with white or yellowish necrotic tissue at the center. Hemorrhagic streaking in fins may accompany ulcers.

Predisposing factors: Stress, water quality problems, physical injury, cold-water transition periods, and the spring temperature rise window.

Treatment: Antibiotic therapy (oxytetracycline, enrofloxacin, or other appropriate antibiotics based on sensitivity testing). Topical wound treatment with antiseptics and wound sealant. Address underlying water quality problems simultaneously.

Prevention: Maintain optimal water quality, minimize physical injury, quarantine new fish, and address spring transition vulnerabilities proactively.

Columnaris (Saddleback Disease)

Causative agent: Flavobacterium columnare

Identification: Pale gray or white lesion across the dorsal area, often with a saddle-like shape giving the disease its common name. Lesions can also appear at the fins and gills. Often confused with fungal infection (saprolegnia) -- columnaris lesions appear dry and have a characteristic bacterial "musty" odor. Spreads more rapidly in warm water (above 20°C) than most other bacterial koi infections.

Treatment: Broad-spectrum antibiotics. Potassium permanganate is effective for mild cases and as a surface treatment. The disease progresses rapidly -- early treatment is critical.

Bacterial Gill Disease

Causative agents: Flavobacterium and Cytophaga species primarily

Identification: Pale, discolored, swollen, or eroded gill tissue. Fish show respiratory distress, rapid gill movement, and surface gasping. Often secondary to poor water quality or concurrent parasitic gill infection.

Treatment: Potassium permanganate gill treatment, salt, and improved water quality. Antibiotics if systemic bacterial infection is suspected.

Furunculosis

Causative agent: Aeromonas salmonicida

Identification: Deep boil-like swellings (furuncles) that rupture and form ulcers. Unlike standard Aeromonas ulcers, furunculosis lesions tend to be deeper and may show extensive tissue liquefaction beneath the surface.

Treatment: Antibiotics (quinolones or trimethoprim-sulfonamide combinations are often effective). Furunculosis can be difficult to treat and prone to recurrence.

Parasitic Diseases

Monogenean Flukes (Skin Flukes and Gill Flukes)

Causative agents: Gyrodactylus (skin flukes, viviparous) and Dactylogyrus (gill flukes, oviparous)

Identification: Often invisible without skin scrape and microscopy. Fish show flashing (rubbing against surfaces), excess mucus production, rapid gill movement (for gill flukes), clamped fins, and lethargy. Heavy infestations cause erosion of skin and gill tissue. Gyrodactylus is frequently found during skin scrapes; Dactylogyrus requires gill biopsy for reliable detection.

Treatment: Praziquantel (2-4 mg/L, two treatments 5-7 days apart). Formalin and potassium permanganate are also effective.

White Spot (Ich, Ichthyophthirius multifiliis)

Causative agent: Ichthyophthirius multifiliis

Identification: White cyst-like spots on skin and fins, resembling salt grains. Fish flash and rub against surfaces. Advanced cases show extensive white cyst coverage with respiratory distress.

Treatment: Formalin, malachite green, or combination products. Multiple treatments required, timed to the temperature-dependent lifecycle. At 25°C, treatments should be repeated every 3-4 days; at 15°C, every 10-14 days.

Anchor Worm (Lernaea)

Causative agent: Lernaea species (crustacean)

Identification: Visible thread-like parasites embedded in the skin, typically 1-3cm long with forked "anchor" structures buried in muscle. Usually accompanied by a small ulcer at the attachment site.

Treatment: Manual removal of adult worms with forceps (sedate fish first), followed by topical antiseptic at the wound site. Diflubenzuron or other organophosphate treatments to interrupt the larval lifecycle and prevent reinfestation.

Fish Lice (Argulus)

Causative agent: Argulus species (crustacean)

Identification: Flat, disc-shaped transparent to greenish parasites, 5-10mm in diameter, visible to the naked eye on the fish surface. Fish scratch and flash; heavy infestations cause extensive irritation and secondary bacterial infection.

Treatment: Manual removal of adults. Diflubenzuron or similar treatment to interrupt larval stages. Potassium permanganate baths are also effective.

Trichodina

Causative agent: Trichodina species (ciliated protozoa)

Identification: Visible only under microscope on skin scrape. Saucer-shaped, circular protozoa with distinctive ring of denticles (tooth-like structures) visible at 100-400x magnification. Fish show excess mucus, flashing, and lethargy. Trichodina thrives in water with organic enrichment and poor water quality.

Treatment: Formalin or potassium permanganate. Salt at 0.3-0.5%. Improving water quality is essential to prevent recurrence.

Costia (Ichthyobodo)

Causative agent: Ichthyobodo necatrix

Identification: Very small (7-15 microns), pear-shaped flagellated protozoa visible on skin scrape at 400x magnification. Bluish-gray "sheen" on skin sometimes visible. Fish show flashing and excess mucus. Costia is particularly dangerous at lower temperatures (below 10°C) and can kill fish quickly.

Treatment: Formalin, potassium permanganate, or salt. At cold temperatures, treatment timing is critical -- costia can be lethal before other disease signs become obvious.

Velvet (Oodinium)

Causative agent: Oodinium pillularis

Identification: Fine gold or rust-colored dust-like coating on skin and fins, most visible in raking light. Unlike ich, which appears as distinct white spots, velvet has a continuous dusty appearance. Fish show respiratory distress and lethargy.

Treatment: Malachite green, copper sulfate, or formalin. Salt at treatment concentrations. Multiple treatments required.

Fungal Diseases

Saprolegnia (Cotton Mold)

Causative agent: Saprolegnia species (water mold, technically oomycete)

Identification: White to gray-brown cotton-like growth on skin, fins, or gills. Almost always secondary to an underlying wound, bacterial infection, or immune depression. "Primary" saprolegnia without an underlying cause is rare.

Treatment: Salt at 0.3-0.5%, potassium permanganate baths, or proprietary antifungal products. Malachite green (restricted in some jurisdictions) is effective. Critical: find and treat the underlying cause or the fungal infection will recur.

Prevention: Maintain skin integrity, treat all wounds promptly, and don't handle koi unnecessarily.

Parasitic Worms (Endoparasites)

Intestinal Worms

Causative agents: Various nematodes and cestodes

Identification: Rarely visible externally unless worms are passed in feces. Weight loss, poor body condition, and reduced growth despite adequate feeding may indicate endoparasite load.

Treatment: Praziquantel for cestodes (tapeworms). Fenbendazole or levamisole for nematodes. Administered in medicated food.

Using This Reference with KoiQuanta

KoiQuanta's disease reference links each condition to its built-in treatment protocol. When you identify a likely pathogen from the reference, you can load the corresponding treatment protocol directly -- which includes dosing calculations, treatment schedules, and monitoring checkpoints. For the treatment protocols in more detail, see the koi herpesvirus management guide and the koi parasite treatment guide.

Frequently Asked Questions

What is the most common koi disease?

Monogenean flukes (Gyrodactylus skin flukes and Dactylogyrus gill flukes) are arguably the most common disease organisms found in koi worldwide, particularly in imported fish from Asia. Aeromonas ulcer disease is the most common serious bacterial disease. Trichodina is the most common ciliated protozoan found on skin scrapes. The "most common" depends on what level you're looking at -- parasites are more prevalent than bacteria, and bacteria are more prevalent than viruses.

How do I tell if my koi has a viral vs bacterial infection?

Viral infections (KHV, SVC) typically present with more systemic signs -- gill necrosis, fluid accumulation, and mass mortality -- rather than localized lesions. Bacterial infections more often present as distinct ulcers, fin rot, or hemorrhagic streaking in specific areas. Temperature context matters: KHV is active at 18-28°C, SVC at 10-17°C. Definitive diagnosis of viral disease requires PCR testing; a skin scrape and clinical assessment can often distinguish bacterial from parasitic causes without lab testing. When in doubt, consult a vet or fish pathologist -- misdiagnosis is costly and delays effective treatment.

What is the treatment success rate for KHV?

Very low for active outbreak. Ponds experiencing KHV outbreaks with the right temperature conditions (18-28°C) typically see 70-100% mortality among susceptible fish. Survivors become lifelong carriers that can infect future introductions. Temperature manipulation (keeping water above 30°C to suppress viral replication) improves survival outcomes during outbreaks but is difficult to achieve in outdoor pond systems and stressful for fish. Prevention through rigorous quarantine is far more effective than treatment -- there is no curative antiviral medication for KHV.

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Sources

Associated Koi Clubs of America (AKCA)
Koi Organisation International (KOI)
University of Florida IFAS Extension Aquaculture Program
Fish Vet Group
Water Quality Association