Understanding Branchiomycosis (Gill Rot) 🐠 Vet Guide 2025

By Dr Duncan Houston BVSc, Ask A Vet Blog Founder

1. What Is Branchiomycosis?

Branchiomycosis, often called “gill rot,” is an acute fungal disease primarily affecting the gill tissues of freshwater fish. It’s caused by Branchiomyces sanguinis and B. demigrans, water molds in the Oomycota group—not true fungi, but with a similarly destructive impact. This disease damages gill filaments, causing necrosis, congestion, and compromised respiration, often leading to high mortality rates in poorly maintained systems or warm water ponds :contentReference[oaicite:3]{index=3}.

2. Causative Agents & How Infection Develops

• B. sanguinis: Invades blood vessels within gill arches and lamellae.
• B. demigrans: Grows in gill tissue outside vessels, causing localized necrosis :contentReference[oaicite:4]{index=4}.

These organisms release spores that lodge on gills and germinate, forming non‑septate branched hyphae. Optimal growth occurs between 25–32 °C, but infections begin around 20 °C :contentReference[oaicite:5]{index=5}. The incubation period can be as fast as 2–4 days in warm, stressful aquatic environments :contentReference[oaicite:6]{index=6}.

3. Risk Factors & Epidemiology

Branchiomycosis outbreaks occur globally—in Europe, North America, Asia, Australia—particularly in fish farms, ponds, and aquaria exposed to warm temperatures and poor water quality :contentReference[oaicite:7]{index=7}.

• High water temperatures (>20 °C)
• Low dissolved oxygen
• Reduced water flow or stagnation
• Overcrowding and heavy organic load
• Nutrient-rich or eutrophic waters—often from overfeeding or decaying plant matter :contentReference[oaicite:8]{index=8}

4. Clinical Signs & Gross Lesions

Fish infected with this disease display:

• Respiratory distress—gasping, lethargy, clustered near water inlets :contentReference[oaicite:9]{index=9}
• Pale, necrotic, or marbled gill filaments—red patches may occur from congestion :contentReference[oaicite:10]{index=10}
• Weak or sluggish swimming; loss of appetite :contentReference[oaicite:11]{index=11}
• Mortality rates can exceed 50% within 48 hours in severe cases :contentReference[oaicite:12]{index=12}

5. Diagnostic Approach

5.1 Presumptive Diagnosis

Observation of classic gill rot lesions—necrosis, congestion, marbling—can lead to preliminary diagnosis :contentReference[oaicite:13]{index=13}.

5.2 Confirmatory Diagnosis

• Wet mounts: Look for non‑septate branching hyphae and spores in blood vessels or tissues :contentReference[oaicite:14]{index=14}.
• Histology: Gill sections with PAS or silver stains detect hyphae and inflammatory responses :contentReference[oaicite:15]{index=15}.
• Culture: Branchiomyces can be cultured on selective media, though histology remains more reliable :contentReference[oaicite:16]{index=16}.

6. Veterinary Care & Treatment Options

There are currently NO effective antifungal baths (e.g., copper, formalin, peroxide) that reach the systemic hyphae without harming fish :contentReference[oaicite:17]{index=17}. Recommended measures include:

• Improve water quality: Increase flow, aeration, reduce temperature, perform frequent partial water changes.
• Reduce stocking density: Remove weakened or dead fish and reduce overcrowding.
• Quarantine affected systems: Dry and disinfect ponds or tanks between batches, discard sediment.
• Supportive care: Maintain strong immune health with nutrition, stress reduction, and oxygenation.
• Preventive sanitation: Clean equipment thoroughly, sterilize nets and utensils, manage algae blooms.

7. Prevention Strategies

• Monitor and maintain temperature below 20 °C when possible.
• Ensure good aeration and consistent water flow.
• Perform regular water testing and scheduled maintenance.
• Remove organic debris and promptly cull dead fish.
• Quarantine new or wild-caught stock.
• Deploy live plants to help uptake nitrates and stabilize water quality.

8. Case Study Highlight

In a commercial pond outbreak, average mortality exceeded 60% within days. Intervention with immediate water exchange, removal of carcasses, aeration increase, and reduction in feeding halted the outbreak—with 80% survival over the following week.

9. Practical Pet Owner Tips (2025)

• Avoid outdoor tanks below shade covers in warm months.
• Install aerators or filters with surface agitation.
• Avoid overcrowding and overfeeding.
• Have Ask A Vet aquatic telehealth plan for rapid support.
• Use Purrz to set reminders for testing and tank cleanups during high-risk periods.

10. FAQ for Branchiomycosis

Can I medicate affected fish?

No reliable medications exist—focus on environmental fixes and supportive care.

Is branchiomycosis contagious?

Yes, spores spread quickly in shared water; quarantine and sanitization are essential.

Can my fish recover?

Recovery is rare once clinical signs appear. Prevention and early environmental intervention are critical.

11. Summary Checklist

🩺 If you suspect branchiomycosis, prompt veterinary consultation is vital. Visit AskAVet.com or download the Ask A Vet app for specialized aquatic telehealth, water‑quality monitoring, and tailored advice. With correct environment and expert guidance, you can protect your aquatic companions in 2025 and beyond. 🌟