Gill Rot (Branchiomycosis): causes, symptoms and treatment

Gill Rot (Branchiomycosis) (Branchiomyces sanguinis / B. demigrans) — etiology, symptoms, diagnosis, active-substance medication, recovery and prevention; mortality without treatment: very high.

Overview

True fungal infection of gill blood vessels and tissue causing necrosis and asphyxiation. Outbreaks in warm, eutrophic, poorly oxygenated water. Causative agent: Branchiomyces sanguinis / B. demigrans. Transmission: water. Incubation: 3-14 days. Reported mortality without treatment: very high.

Symptoms

• mottled red and pale gill patches
• rapid breathing
• gasping at surface
• lethargy
• loss of appetite
• high mortality in days

Causes

Outbreaks are typically triggered by chronic stress, poor water quality, temperature swings, overcrowding, or the introduction of unquarantined fish. The pathogen spreads via free-swimming or waterborne stages in shared water.

Diagnosis

Diagnosis is based on clinical signs (cottony or fuzzy growths on skin, fins, or gills) and microscopy of a fresh scrape, which reveals hyphae of Branchiomyces sanguinis / B. demigrans. Most cases are secondary to injury or pre-existing damage, so search for the underlying trigger.

Treatment

Effective treatment requires isolating affected fish in a quarantine tank, identifying the pathogen, administering the appropriate active substance at the correct dose and duration, and supporting recovery with stable water parameters and nutrition.

Step 1: Quarantine

Set up a bare-bottom quarantine tank with a mature sponge filter, heater, and aeration. Match temperature and pH to the display tank, and acclimate fish slowly. A bare bottom simplifies daily siphoning and prevents medication from being absorbed by substrate.

Step 2: Medication

1. Improve water + formalin. Massive water change, increase aeration, lower temperature, formalin 25 ppm bath 60 min; copper sulfate 0.2 mg/L as alternative. (duration: single bath, repeat in 5 days)

Step 3: Recovery

After medication, perform a 30-50% water change and run fresh activated carbon for 24-48 hours to remove residues. Continue feeding a high-quality, varied diet with vitamins and immunostimulants. Reintroduce fish to the display tank only after at least one week without recurrence of symptoms.

Prevention

• avoid summer temperature peaks
• strong aeration
• low organic load
• avoid overfeeding in warm water