Branchiomycosis Gill Rot in Koi: Recognition and Emergency Treatment

By KoiQuanta Editorial Team|Updated Mar 2026

Branchiomycosis can cause 50-100% pond-after-pond-treatment) mortality within 2-5 days of first symptoms appearing during warm, high-organic-load summer conditions. This is not an exaggeration for effect - it's a documented outcome for Branchiomycosis outbreaks in unprepared facilities. The disease progresses faster than most koi keepers expect, and by the time multiple fish are visibly affected, many more are likely in the early stages.

KoiQuanta's Branchiomycosis emergency response protocol guides immediate pond disinfection, fish isolation, and treatment steps for this rapidly fatal condition.

TL;DR

Perform a 30-40% water change to reduce the nutrient base supporting fungal growth.
Fish with more than 50% of gill tissue involved typically do not recover.
Fish with early-stage disease and less than 50% gill involvement may recover.
Allow the pond to dry completely for at least 1 week.
Disinfect all equipment, nets, and containers that contacted the pond water with 1% bleach solution.

What Branchiomycosis Is

Branchiomycosis (gill rot) is caused by two related fungal pathogens: Branchiomyces sanguinis and Branchiomyces demigrans. These water molds infect the gill tissue, growing within the blood vessels of the gill filaments and gill lamellae. As the fungal hyphae proliferate, they block blood circulation within the gill, causing ischemic necrosis - the tissue dies from lack of blood supply.

The necrotic areas appear as pale, mottled, or marbled patches on the gill tissue - the gill that should be uniformly red-pink becomes patchy gray-white.

Why it's warm-weather specific: Branchiomyces thrives at temperatures above 18°C and grows fastest above 25°C. High organic load provides the nutrient base the fungus needs for rapid proliferation. This combination - summer heat plus elevated organic load - is the perfect Branchiomycosis culture medium.

Recognizing Branchiomycosis

Speed of recognition is everything with Branchiomycosis.

Fish behavior:

Increased respiratory rate and gill cover movement
Surface activity and hanging near aeration sources - the classic oxygen-deprivation behavior
Rapid progression from apparently normal to severe respiratory distress within hours
Fish may be seen "gasping" at the surface where oxygen is highest
Lethargy as the disease progresses

Physical examination:

Gill covers may be held open or moved asymmetrically
Gently lifting the gill cover reveals mottled, marbled, or patchy gill tissue - gray-white areas where tissue is necrotic alternating with remaining pink areas
Affected areas have irregular, geographic boundaries
Severe cases: gill tissue may be visibly fragmented or crumbling

Other signs:

Fish rubbing head against surfaces (indicating gill discomfort)
Sudden death without obvious external lesions - these fish often have completely destroyed gill tissue

Distinguishing from other gill diseases:

Bacterial gill disease: diffuse gill swelling and excess mucus
Gill flukes: patchy damage but organisms visible on scrape
Chlorine damage: history of chemical exposure, sudden onset
Branchiomycosis: geographic mottling specific to fungal vascular occlusion

Emergency Response Protocol

When Branchiomycosis is suspected, response must be immediate.

Step 1: Emergency isolation.

Remove visibly affected fish to a hospital tank immediately. Do not wait for laboratory confirmation - the delay cost is measured in fish lives. The hospital tank should have maximized aeration and matched water temperature.

Step 2: Pond disinfection (display pond).

KoiQuanta's emergency protocol includes pond disinfection as a critical step. Options include:

Potassium permanganate at 2-5 mg/L for 12-24 hours (with careful DO monitoring)
Copper sulfate at 0.5 ppm for 24-48 hours
Strong salt treatment at 0.5-0.7% for several days

These treatments target the free-floating fungal spores in the water and on surfaces, reducing the inoculum that remaining fish are exposed to.

Step 3: Maximize aeration.

Gill-damaged fish have severely compromised respiratory function. Maximum aeration is not just supportive - it may be the difference between survival and death for fish with moderate gill damage.

Step 4: Reduce organic load immediately.

Stop feeding. Perform a 30-40% water change to reduce the nutrient base supporting fungal growth. Clean any obvious organic debris. These measures directly reduce the conditions favoring fungal proliferation.

Treatment Options

There is no specific, highly effective pharmacological treatment for Branchiomycosis in koi. Management is primarily supportive and environmental:

Malachite green at low concentrations has antifungal activity and can help slow Branchiomycosis progression when applied as a bath treatment. Usage regulations vary by jurisdiction.

Formalin similarly has some antifungal activity and has been used in combination with malachite green.

Potassium permanganate baths have both oxidative antifungal effect and kill secondary bacterial infections that commonly follow gill necrosis.

Copper sulfate at therapeutic doses has antifungal properties.

The prognosis for fish with severe Branchiomycosis is poor regardless of treatment. Fish with more than 50% of gill tissue involved typically do not recover. Fish with early-stage disease caught before extensive necrosis may survive with appropriate care.

Post-Outbreak Pond Disinfection

After a Branchiomycosis outbreak, decontamination of the pond environment is important:

Complete drain-down and scrubbing of all pond surfaces if possible
Disinfection of all equipment with bleach or Virkon
Extended lime treatment of drained pond walls and bottom
Refill with fresh water after appropriate drying period

Without environmental decontamination, surviving Branchiomyces spores can seed future outbreaks when conditions return to favorable.

Your gill disease management guide covers the broader range of gill conditions. The koi disease identification guide supports differential diagnosis of the respiratory distress presentation.

Frequently Asked Questions

How do I recognize Branchiomycosis in koi?

Branchiomycosis presents as rapid respiratory distress - fish gathering at surface near aeration, fast gill movement, lethargy - combined with visible gill tissue mottling when gill covers are examined. The mottled appearance (gray-white necrotic patches alternating with remaining pink areas, with geographic irregular boundaries) is characteristic. The disease typically appears during warm summer conditions with elevated organic load. Speed of progression distinguishes it from most other gill diseases - fish that appeared normal in the morning may be in severe respiratory distress by evening. Any sudden respiratory distress event in summer warrants immediate gill examination.

Is Branchiomycosis treatable in koi?

The prognosis is guarded to poor depending on the extent of gill involvement by the time treatment is initiated. There is no highly specific antifungal treatment available for koi. Management combines isolation of affected fish, maximal aeration support, pond disinfection with potassium permanganate or copper sulfate, organic load reduction through water changes and feeding cessation, and supportive treatments like malachite green with some antifungal activity. Fish with early-stage disease and less than 50% gill involvement may recover. Fish with advanced necrosis covering the majority of gill tissue rarely survive. Early recognition is the primary determinant of outcome.

How do I disinfect my koi pond after a Branchiomycosis outbreak?

Drain the pond completely where possible. Scrub all surfaces to remove organic matter and biofilm. Apply a lime treatment (calcium hydroxide) to drained pond surfaces - the high pH kills Branchiomyces spores. Allow the pond to dry completely for at least 1 week. Disinfect all equipment, nets, and containers that contacted the pond water with 1% bleach solution. Refill with fresh water, establish biological filtration, and maintain very low stocking density and organic load during the recovery period. Ensure water quality is excellent before restocking to prevent recurrence.

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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