Ichthyophthiriasis (White Spot Disease in Fish)

Clinical signs

• Pinhead-sized white spots (0.5–1 mm) scattered across skin, fins and gills
• Flashing — fish rubbing against pond walls, nets or bottom
• Rapid gill movement and gasping at the surface as gill parasites destroy respiratory tissue
• Excess mucus production giving fish a faded, slimy appearance
• Lethargy, loss of appetite, schooling at water inlets where DO is highest
• Mortality 20–100% in untreated outbreaks, reaching peak 7–14 days after first spots

Transmission

• Direct life cycle — no intermediate host required; one infected fish reseeds an entire pond in 5–7 days at 25°C
• Free-swimming theronts (infective stage) released from cysts on the pond bottom; theronts survive 24–48 h searching for a host
• Introduction via infected fish, untreated live feed, contaminated water from another pond, or shared equipment
• Stress triggers: handling, transport, sudden temperature drops, overcrowding and low DO precede most outbreaks
• Lifecycle speed is temperature-dependent: 4 days at 25°C, 10 days at 15°C — colder water gives more treatment windows but more cumulative damage

Diagnosis

• Field: visible 0.5–1 mm white spots + flashing behaviour is virtually diagnostic in freshwater fish
• Microscopy of a skin or gill scrape — the trophont stage is large (up to 1 mm), round, slow-rotating with a horseshoe-shaped macronucleus
• Differentiate from columnaris (irregular grey-white patches, not raised spots) and lymphocystis (larger, cauliflower-like growths)
• Count parasites per gill filament — >10 trophonts/filament indicates severe infection

Treatment & emergency response

• Only the free-swimming theront stage is treatment-sensitive — trophonts under the skin are protected, so treatment must run long enough to catch all newly hatched theronts
• Salt bath at 0.3–0.5% (3–5 g/L) sustained for 7–14 days — most cost-effective for tilapia, carp and most pond fish (not for Corydoras or salt-sensitive species)
• Raise water temperature to 28–30°C where the species tolerates it — accelerates lifecycle and shortens treatment window
• Formalin at 25 mg/L every 48 hours for 3–5 treatments (use only with strict PPE and consult local regulations)
• Daily water changes (10–20%) during treatment physically remove free-swimming theronts
• Increase aeration — damaged gills mean fish need more dissolved oxygen, not less
• Stop feeding for the first 48 hours of treatment to reduce stress and water fouling

Prevention

• Quarantine all new fish for 21 days at 25–28°C before introducing to the main system
• Source fingerlings only from hatcheries with documented Ich-free history
• Disinfect nets, buckets and grading equipment between ponds (KMnO₄ 10 mg/L or chlorine 200 ppm)
• Maintain stocking density within species norms — overcrowded fish stress easily and reseed Ich faster
• Avoid abrupt temperature drops >3°C; cover ponds during cold snaps in northern India and Bangladesh
• Keep DO above 5 mg/L; hypoxia is the most common trigger for sub-clinical Ich to flare
• Use UV sterilisation or sand filtration on recirculating systems to kill free-swimming theronts in source water

Biosecurity checklist

• Quarantine pond with separate water supply for all new fish — 21 days minimum
• Daily mortality + feeding-response log per pond; flashing behaviour triggers same-day skin scrape
• Disinfect handling equipment between every pond, not just at end of day
• Dry and sun-treat empty ponds for 7–14 days between batches — kills cysts on the pond bottom
• Do not transfer water from a treatment pond to a healthy pond without 24 h chlorination
• Train workers to recognise the difference between Ich (raised white spots) and saprolegniasis (cotton-wool tufts)

Frequently asked questions