Parasitism by sea lice (Lepeophtheirus and Caligus)
The sea lice is a small external parasitic crustacean that lives on the skin, fins and sometimes the gills of salmon. Two genera are mainly involved: Lepeophtheirus salmonis, specific to salmonids, and various species of Caligus, less speci...
What is it?
The sea lice is a small external parasitic crustacean that lives on the skin, fins and sometimes the gills of salmon. Two genera are mainly involved: Lepeophtheirus salmonis, specific to salmonids, and various species of Caligus, less specific.
These parasites feed on the mucus, skin and sometimes blood of the fish. They are visible to the naked eye in adults, in the form of small moving dots on the body.
This is a major problem in sea cage salmon farming, with an economic and animal welfare impact recognized worldwide.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
The high density of caged fish promotes rapid transmission between individuals.
The proximity of several farming sites along wild salmon migration routes increases the risk of cross-contamination.
Water temperature influences the rate at which the parasite develops, with warmer waters speeding up its cycle.
A weakening of the fish linked to other diseases or handling stress increases its sensitivity.
Transmission
Transmission occurs through the free-living larval stages of the parasite which swim in the water column and attach to a new host fish. It can occur between fish from the same farm but also between neighboring farms or with migratory wild fish.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis is made by direct visual observation of parasites on the skin, fins and gills, with counting the number of lice per fish during regular sampling.
The veterinarian or farm health manager distinguishes between the different stages of the parasite (larvae, young, adult, egg-bearing female) to assess the extent of the infestation and the need for treatment.
Treatment
Treatment is based on antiparasitics administered either orally mixed with food, or by medicated bath in tarpaulins or temporarily closed cages.
Mechanical and thermal methods also exist, such as rinsing with fresh water or lukewarm water which is poorly tolerated by the parasite but tolerated by the salmon.
The use of organic fish cleaners often complements chemical treatments.
Rotation between different families of antiparasitics is recommended to limit the development of resistance, a major issue for this disease in the global salmon farming industry.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Management is based on regular monitoring of the number of lice per fish on farms, with defined intervention thresholds.
Cleaner fish, such as certain species of wrasse or lumpfish, are used in cages to eat lice directly from salmon.
Physical barriers such as anti-lice nets or skirts limit the entry of floating larvae into the enclosures.
The sanitary rest of the sites, the rotation of breeding areas and the synchronization of production cycles between neighboring farms reduce the overall parasitic pressure.
Preventive treatments coordinated across an entire area limit the appearance of resistance.
Possible complications
Open skin wounds expose the fish to secondary bacterial or fungal infections.
Severe infestation can cause anemia, osmotic imbalance through loss of skin integrity, chronic stress and increased mortality, especially in young fish or already weakened individuals.
On a regional scale, the infestation of farms can also impact populations of wild salmon migrating near production sites.
When to see a veterinarian
Visible presence of numerous small mobile parasites on the skin or fins of the fish.
Areas of eroded, reddened skin or open sores on the heads or backs of fish.
Abnormal behavior of repeated rubbing against enclosure structures.
Decreased appetite, weight loss or unusual mortality in a batch of salmon.
Prognosis
The prognosis depends on the level of infestation and the speed of treatment.
A mild to moderate infestation, treated in time, has a good prognosis with healing of skin lesions.
A massive infestation, especially in young fish or already weakened individuals, can lead to significant mortality if it is not brought under control quickly.
Frequently asked questions
Are salmon lice dangerous for humans?
No, these are not parasites transmissible to humans. They are specific to fish and pose no risk to people who handle or consume salmon.
Can you see sea lice with the naked eye?
Yes, adult stages are visible as small moving dots on the skin and fins, especially towards the head and back of the fish.
Can farmed salmon sold in fish stores be infested?
Fish intended for consumption are monitored and treated before marketing. A visible infestation does not affect the healthiness of the flesh but reflects an upstream animal welfare problem.
How do farms limit this parasite without chemical pest control?
They notably use cleaner fish which eat the lice directly from the salmon, as well as physical barriers and coordinated management of breeding cycles between neighboring farms.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
Content reviewed by the TunisieVet editorial team · Updated 25 September 2026
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