For educational purposes only. This page does not provide medical or veterinary advice, diagnosis, or treatment. Always consult a qualified healthcare or veterinary professional before making any health-related decisions.
Sea Lice
Lepeophtheirus salmonis / Caligus spp.
Size: Visible to naked eye — adults 5–12 mm; egg strings up to 22 mm
Major global problem in salmon aquaculture; found in wild salmon populations in the North Atlantic and North Pacific; causes hundreds of millions of dollars in annual losses to fish farming
Sea lice are parasitic copepods (tiny crustaceans) that attach to the skin, scales, and mucus of marine fish, particularly salmon. They are one of the most economically damaging parasites in the global aquaculture industry. Sea lice feed on skin, mucus, and blood, causing lesions, stress, and secondary infections. Heavy infestations can be fatal. Salmon farms create dense fish populations that allow sea lice to multiply rapidly and spread to wild salmon populations nearby — a major environmental controversy.
Lifecycle
Sea lice have a complex lifecycle with multiple free-swimming larval stages (nauplii and copepodid) before attaching to a host. Once attached, they progress through chalimus stages (anchored to the fish) and then mobile adult stages. Adult females produce egg strings continuously. The entire lifecycle takes 2–8 weeks depending on water temperature.
Sea Lice — Lepeophtheirus salmonis / Caligus spp.
Symptoms
Pharmaceutical treatments
Evidence-based medications used by healthcare and veterinary professionals.
Reference only. Medication names, types, and notes are provided for general awareness. Dosage, suitability, and prescribing decisions must be made by a licensed physician or veterinarian.
Emamectin benzoate (SLICE)
Veterinary prescriptionAdministered in feed. Highly effective against mobile sea lice stages. Resistance has developed in some populations with heavy use.
Azamethiphos
Veterinary prescriptionBath treatment administered in enclosed net pens. Effective against adult and chalimus stages. Requires careful environmental management.
Hydrogen peroxide bath
Veterinary treatmentNon-chemical bath treatment used in aquaculture. Effective against mobile stages. Requires precise dosing to avoid fish stress.
Holistic & natural approaches
Traditional, herbal, and integrative approaches for general awareness.
Not a substitute for medical treatment. Natural remedies listed here have not been evaluated by the FDA or equivalent regulatory bodies. Do not use these in place of prescribed treatment. Always consult a qualified healthcare or veterinary professional first.
Cleaner fish (wrasse and lumpfish)
Biological control using cleaner fish that eat sea lice off salmon. Widely adopted in Norwegian and Scottish salmon farming as a chemical-free alternative. Wrasse and lumpfish are the most commonly used species.
Laser delousing systems
Automated laser systems (such as Stingray) detect and target individual sea lice with precision laser pulses, killing them without chemicals or stress to the fish. Increasingly adopted in modern aquaculture.
Freshwater treatment
Brief immersion in freshwater kills sea lice (which cannot tolerate low salinity) without harming the fish. Used as a non-chemical treatment in some aquaculture operations.
Prevention
Zoonotic risk — low or none
Sea lice do not infect humans. They are exclusively parasites of marine fish. Salmon and other fish with sea lice are safe to eat — the parasites do not survive outside of saltwater and pose no human health risk.
Related parasites
Anisakis
Anisakis simplex
Anisakis is a parasitic nematode (roundworm) found in the flesh of many marine fish and squid. Humans become accidental hosts by eating raw or undercooked seafood — sushi, sashimi, ceviche, and lightly cured fish are common sources. The larvae cannot complete their lifecycle in humans but burrow into the stomach or intestinal wall, causing a painful condition called anisakiasis. Marine mammals such as dolphins and seals are the definitive hosts.
Tongue-Eating Louse
Cymothoa exigua
Cymothoa exigua is one of the most remarkable parasites known to science — the only parasite known to functionally replace a host organ. This marine isopod (a crustacean related to pill bugs) enters a fish through the gills as a juvenile male, attaches to the tongue, severs its blood supply, and causes the tongue to atrophy. The parasite then takes the tongue's place, functioning as a replacement tongue while feeding on the fish's blood and mucus. The fish can still eat with the parasite in place. It poses no known risk to humans.
Gnathia
Gnathia spp.
Gnathia are tiny parasitic isopods (crustaceans) that are among the most prevalent blood-feeding parasites on coral reefs. Only the juvenile stage (zuphea/praniza) is parasitic — they attach to fish, feed on blood and tissue fluid, then drop off to molt and mature. Adults are free-living and do not feed. Despite their small size (1–7 mm), gnathia can cause significant blood loss, stress, and disease transmission in heavily infested fish. They are a primary food source for cleaner wrasses and cleaner shrimp, making them a key part of reef ecosystem dynamics.