Fish Louse

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Fish Louse (Argulus sp.) attached to the skin of a fish (Wikimedia Commons)

The fish louse (Argulus spp.) is a metazoan crustacean skin parasites of freshwater fish. They pose a major threat to the health of commercially produced fish.

Life Cycle

The parasite's life cycle is direct. The lice mate on the fish and the female then lays her eggs on surfaces or plants within the habitat. These then progress through a series of metamorphic stages, each of which moults its exoskeleton in order to grow.

After 4 days the hatched adult crustacean will seek out a host fish to feed on.

The total life cycle takes 30-100 days and is temperature dependent. Eggs can undergo latency and delayed hatching if water temperatures are not suitable over Winter.

Pathology and Clinical Signs

The louse has a sharp stylet along the ventral midline that it uses to pierce the skin of the fish. It then injects digestive enzymes into the surrounding tissues and feeds on the digested bodily fluids by sucking them out with its proboscis.

The damage caused by the fish louse is two-fold; firstly localised inflammation and irritation of the skin caused directly by the parasite and secondarily the risk of developing opportunistic infections, both bacterial and viral via the damaged skin. Pseudomonas spp. and Aeromonas spp. are often of particular concern, as they can potentially invade the gills.

Fish may show rubbing and flashing behaviour due to the intense irritation caused by the infection.


The louse can be seen with the naked eye, growing up to 7-10 mm in length. They appear as flat, oval, darkly coloured spots on the skin of the fish and are easier to see on fins or when moving. Red lesions may be visible from previous feeding areas.


It is vital that any treatment plan is effective against both adults and juvenile stages of the parasite.

Infestations can be treated with an increase in water salinity. If this is ineffective, ectoparasiticides such as organophosphates or chitin inhibitors can be used. Three treatments of organophosphates are usually required to eliminate all life stages. It should be noted that organophosphate treatment is not permitted in fish in the UK and should be confirmed in any other region before use.

Fish Louse Learning Resources
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Test your knowledge using flashcard type questions
Ornamental Fish Q&A 10


Fishdoc (2009) Argulus/Fish Louse at, accessed online 22/11/2011

Lewbart, G. A. (1998) Self-Assessment Colour Review of Ornamental Fish Manson

Maclean, B (2006) Common dermatoses of ornamental fish and amphibians In Practice 2006 28: 604-613

Southgate, P (1994) Laboratory diagnosis of fish disease In Practice 1994;16:252-255

Stuart, N (1988) Common skin diseases of farmed and pet fish In Practice 1988 10: 47-53

Wildgoose, W (1998) Skin disease in ornamental fish: identifying common problems In Practice 1998 20: 226-243

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