Fish Lice (Argulus): symptoms, treatment, prevention

Argulus spp. are disc-shaped crustacean parasites up to about 10 mm long, visible to the naked eye, that attach via suckers and pierce fish to feed on blood.

Overview

Fish Lice are caused by Argulus spp., disc-shaped crustacean parasites that reach about 10 mm and are visible to the naked eye. They attach with suckers and pierce the host with a stylet to feed on blood, leading to lesions and secondary infection. Most common in pond and cold-water aquaria.

Symptoms

Visible disc-shaped parasites moving on the body
Scratching against decor
Small hemorrhagic lesions
Lethargy
Secondary infections at bite sites
Stress behaviours

Causes

Introduced with new pond fish or plants from outdoor water. Eggs deposited on hard surfaces hatch over weeks, so a single missed treatment cycle can restart the outbreak.

Diagnosis

Adults can be seen with the naked eye as flat, almost translucent discs moving over the fish. Confirm by close inspection or magnification; differentiate from Lernaea (worm-like protrusion, not free-moving).

Treatment

Combine manual removal with a chitin-synthesis inhibitor cycle in the tank to kill juveniles hatching from egg banks.

Quarantine

For light infestations the fish can stay in the tank; for heavy infestations move fish to a hospital tank and clean the original tank thoroughly while the chitin-inhibitor course runs.

Medication

Diflubenzuron at 0.01 mg/L in the tank, once weekly for 4 weeks, to kill juvenile stages as eggs hatch; unsafe for invertebrates.
Manually remove visible parasites with forceps, then a salt bath at about 15 g per litre for 30 minutes; the bath does not kill eggs but reduces the population.

Recovery

Treat any open wounds with a fish-safe antiseptic, monitor for secondary bacterial infection and continue water changes throughout the chitin-inhibitor course.

Prevention

Quarantine pond fish
Inspect new arrivals visually
Diflubenzuron prophylaxis for koi imports
Remove egg masses from hard surfaces