Fish Louse (Argulus)

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by Karen Paul

Argulus (fish louse) is a common fish parasite. It is a flattened disc-shaped, jelly-like crustacean of the class Branchiura. All Branchiurans are fish parasites. The adult Argulus is visible without magnification, measuring up to 10mm in diameter.

Argulus attaches itself to the skin or fins and feeds on blood by inserting it's stylet feeding apparatus into the flesh.The various hooks, barbs and suckers that Argulus uses to attach itself to the fish can cause further damage. Reddened lesions develop where the Argulus has been feeding and may become infected with bacteria or fungus. In addition to the physical damage, affected fish are subject to stress which often leads to secondary parasitic infestations such as white spot and Costia.

Life Cycle

The female leaves the fish to lay eggs on plants and other submerged objects. The water temperature needs to be above 16°C for egg production. The eggs take 3-4 weeks to hatch at summer temperatures. They can over-winter and hatch in spring as water temperatures increase.

After hatching the juvenile passes through several metamorphic stages as it develops into an adult. The juvenile must find a host within 4 days of hatching to survive but the adult can survive without a host for several days.

The whole cycle takes between 30 - 100 days depending on temperature.

Symptoms

The first sign of Argulus infestation is usually the fish flashing or rubbing themselves against the sides and bottom of the pond to try to dislodge the parasites. Serious infestation can cause fish to become lethargic. Skin, fins and gills secrete excess mucus.

Treatment

The most effective treatments against Argulus were organophosphates but they are now banned in the UK for use in fish treatments. A salt bath can be effective for individual fish but there is no simple solution to ridding a seriously infested pond of these parasites.

As with all parasites and disease, prevention is better than cure. Poor water quality, overcrowding and poor nutrition will cause stress and weaken the immune system. Healthy fish will be able to fight off most diseases and parasites.

Before using any chemical treatments it is advisable to test the water quality. Some symptoms, such as flashing, may be caused by high nitrite levels. Adding chemicals in these circumstances will do more harm than good.

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