IchthyophthiriasisClinical signs, differential diagnosis, treatment and prevention.

Overview

A common and potentially fatal parasitic disease of freshwater fish, causing visible white skin spots. Ichthyophthirius multifiliis, a ciliated protozoan parasite with a direct life cycle (no intermediate host).

Clinical signs

Characterized by white spots on the skin and gills ("velvet-like" appearance), lethargy, piping (gasping for oxygen at the water surface), anorexia, extreme weight loss (especially in fry and juveniles), and…

Predisposing factors

Poor water quality (especially organic-rich, anoxic environments), overcrowding, and temperature fluctuations.
The causative parasite, Ichthyophthirius multifiliis, has a direct life cycle but massive reproductive potential, with environmental cysts releasing hundreds of infective tomites.

Treatment

In tanks and raceways, administer Formalin at 50–100 µL/L (ppm) as an immersion bath for up to 4 hours daily.
In earthen ponds, use Formalin at 25 µL/L as a single treatment.
Repeat treatment in 5–10 days if necessary.
Do not apply when water is warmer than 80°F, during heavy phytoplankton blooms, or if dissolved oxygen is <5 mg/L.
Do not treat striped bass; always test a small number of fish first to check for sensitivity.

Prevention

1. Maintain dissolved oxygen above 5 mg/L (ideally near 100% saturation). 2. Quarantine all new arrivals. 3. Disinfect eggs with formalin at 1,000–2,000 ppm for 15 minutes.

Source

The Merck Veterinary Manual 11th Edition (Passages 319, 320, 321)

MSD Veterinary Manual — Ichthyophthiriasis in Fish (accessed 2026-08-11)

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