Saprolegniasis (Fungal Cotton Wool)

Clinical signs

• True cotton-wool tufts on skin, fins, gills or eggs — visible white to grey-brown fluffy growths that can be lifted with a needle
• Lesions concentrate around handling injuries, descaled patches, fin damage and the mouth
• On eggs: tufts spread radially from a single dead egg and engulf surrounding viable eggs within 24–48 hours
• Lethargy, loss of equilibrium and refusal of feed in heavily infected fish
• Secondary bacterial infection (Aeromonas, Pseudomonas) is common — wounds turn red and ulcerative
• Mortality 10–60% over 7–14 days in fingerlings; up to 100% egg loss in untreated hatchery trays

Transmission

• Waterborne — Saprolegnia zoospores are ubiquitous in fresh and brackish water year-round
• Opportunistic — disease requires a portal of entry (handling injury, parasite wound, dead tissue) and stress (temperature drop, low DO, poor water quality)
• Spreads from dead eggs to live eggs in hatchery trays via radial hyphal growth
• Cold-water trigger — outbreaks cluster below 20°C and accelerate at 8–15°C; rare above 25°C
• Stress triggers — grading, transport, sudden temperature drops, low DO and skin parasites precede most outbreaks

Diagnosis

• Field: true cotton-wool tufts that can be lifted with a needle — distinguishes from columnaris (flat grey patches) and Ich (raised round white spots)
• Microscopy of a skin scrape — branching, non-septate hyphae with terminal zoosporangia
• Culture on Sabouraud dextrose agar with antibiotics — fluffy white cotton-like growth in 48–72 hours
• PCR or ITS sequencing for species-level identification in research or aquaculture genetics work
• Always inspect for an underlying cause — parasite, handling injury, or temperature drop — before treating

Treatment & emergency response

• Treat the underlying stressor first — temperature stability, DO above 6 mg/L, gentle handling — or the infection recurs
• Salt bath at 0.3–0.5% (3–5 g/L) sustained for 7–14 days — most cost-effective treatment in tilapia, carp and most pond species
• Formalin at 25 mg/L every 48 hours for 3–5 treatments (where legal and with strict PPE) — effective on both fish and eggs
• Hydrogen peroxide at 250–500 mg/L for 15 minutes (fish) or 1,000 mg/L for 15 minutes (eggs) — fastest acting, broadest legal status
• Sodium chloride 2.5% dip for 5 minutes per egg tray, repeated daily until visible mould clears
• Remove dead fish and dead eggs at least twice daily — every dead unit reseeds zoospores into the system
• Antibiotics only if secondary bacterial ulceration develops, with vet prescription and withdrawal compliance

Prevention

• Minimise handling — grade only in cool early-morning hours, never during midday or under thermal stress
• Pre-treat all incoming fish with a 3 g/L salt dip for 15 minutes before stocking
• Disinfect grading equipment between batches (KMnO₄ 10 mg/L or chlorine 200 ppm)
• Cover ponds during cold snaps in northern India and Bangladesh — prevent water temperature drops below 18°C
• Hatcheries: pick out dead eggs at least twice daily and dip remaining trays in 1,000 mg/L hydrogen peroxide for 15 minutes
• Control external parasites (Ich, Trichodina, gyrodactylids) — every parasite wound is a Saprolegnia entry point
• Maintain DO above 6 mg/L; hypoxia compounds mucus loss and opens the skin to zoospore attachment

Biosecurity checklist

• Daily mortality + handling-injury log per pond and hatchery tray
• Twice-daily removal of dead fish and dead eggs — never leave them overnight
• Foot baths and equipment disinfection at every pond and hatchery room entry
• Dedicated nets per pond; colour-code to prevent cross-use
• No grading or transport when forecast water temperature will drop below 18°C in the next 48 hours
• Train workers to distinguish saprolegniasis (cotton-wool tufts, liftable with a needle) from columnaris (flat grey patches)
• Quarantine and salt-dip every incoming fish batch before introducing to the main system

Frequently asked questions