Fungal
🔴 Fatal
Saprolegniasis (Water Mould)
🦠 Saprolegnia parasitica, S. diclina · Saprolegniaceae (Oomycota)
📋 Overview
Saprolegniasis is the most important non-bacterial fish disease in freshwater aquaculture. Saprolegnia spp. are oomycetes (related to algae, not true fungi), but are traditionally grouped under fungal diseases. They colonise injured skin, stressed fish, or dead eggs, and can spread to adjacent living tissue. Malachite green (the most effective treatment) was banned in food fish production in the EU and US. Control remains challenging.
📊 Key Facts
Causative Agent
Saprolegnia parasitica, S. diclina
Agent Family
Saprolegniaceae (Oomycota)
Geographic Range
Worldwide — freshwater environments
Transmission Routes
Water-borne zoospores to wounds, compromised skin, or stressed fish
Primary Host
Salmonids, cyprinids, many freshwater species; also fish eggs
Vaccine
✗ None / Experimental
🩺 Clinical Signs
White/grey cotton-wool tufts (hyphae) on skin, fins, gills or eggs; underlying tissues reddened and haemorrhagic; rapid spread in water.
🔬 Symptoms
🔬
White cotton-wool tufts on skin
Integumentary Acute
🔬
Underlying haemorrhagic tissue
Integumentary Subacute
🔬
Egg mortality
Systemic Acute
🧪 Diagnosis
Wet mount of hyphae — aseptate filaments with zoosporangia; culture.
💊 Treatment
ℹ️ Treatment options are limited. Consult a qualified fish health professional.
Salt bath (20 g/L), potassium permanganate, hydrogen peroxide (licensed in some countries), bronopol.
🛡️ Prevention
ℹ️ Prevention is the most effective strategy for managing this disease in aquaculture and aquarium settings.
Prevent injuries, remove dead eggs (egg picking), maintain good water quality, UV treatment.