Freshwater Lice

Freshwater Lice

Argulus species

Freshwater lice are crustaceans and are the largest external parasite which can infest your fish. They can be readily seen with the naked eye and are very distinctive, although they can hide on fish!

There are three main species of freshwater lice in the UK. They are Argulus foliaceous, A. coregoni and A. japonicus.

They are found throughout the world, and can cause problems in a wide range of freshwater fish species. The severity of the problems that they can cause is related to the size of the fish and the number of parasites attached to the fish. However, in addition to the mechanical damage inflicted by these parasites and the consequent stress to the host fish, they also carry the risk of transmitting bacterial and viral diseases to the fish.

Life cycle

The female lays her eggs on hard surfaces, not on the host fish. The eggs are in rows, attached to the surface by a gelatinous material, and a single female can lay up to 2000 eggs at any one time. This is the first problem in the control of this parasite, since once the eggs have been laid there is no easy way of getting to them to kill them.

The rate of hatching depends on water temperature, and the eggs do not hatch at temperatures below 8 – 10°C. However, eggs are able to overwinter, which can also give problems in the control of this parasite since the eggs will not be killed by low temperatures but simply lie dormant until the following spring when once the water temperatures rise above 10°C the whole life cycle will begin again. Above 15°C eggs can hatch within 25 days. The next stage of development after hatching is the metanauplius stage, and these larvae must find a host fish within 6 days after hatching or they will not survive. After attaching to a host fish, they hatch into the next, juvenile, stage which commences feeding on the host fish. There can be up to ten juvenile stages, depending on the species of the parasite. After a couple of weeks these juvenile stages develop into the adult mature parasite. Depending on water temperature, the whole life cycle can be completed in 4 – 6 weeks. The parasites spend quite a considerable time swimming in the pond or tank water, and mating takes place while they are free-swimming.

These parasites develop through the summer and reach maximum numbers in late autumn.

Fish LiceSymptoms

Argulus have a sucker-like mouth and hooks on their legs, both of which they use to attach to fish and feed. They first of all insert a long sting-like structure called a stylet which injects enzymes into the skin and flesh of the fish to break down these tissues to make them easier for the parasite to consume. They can repeatedly puncture the skin, and also their mouthparts are serrated all of which can cause considerable damage to the fish on which they are feeding. Also the fact that they are penetrating the skin of the fish means that they can easily transmit bacteria or viruses from one fish to another.

This aggressive attachment and feeding behaviour can cause great irritation to the fish which will rub against objects and flick or flash to try to get rid of the parasites. Fish can also tend to congregate around areas of water movement, all in an attempt to relieve the irritation of the parasites. Severely affected fish can stop feeding and high numbers of Argulus on an individual fish can cause it serious problems since heavy infestations can be very stressful and some small fish can even die as a result of these parasites.

The route by which fish become infested with Argulus is usually subsequent to the introduction of infected fish. Once a pond is infected, the only way the infestation can be completely cleared is by a complete draining and clear out since the eggs are laid on hard structures within the pond and can resist low temperatures, even overwintering. However, this is not usually an option with hobbyists ponds, so the next best solution is to routinely treat the fish with Lice-Solve™ on an annual basis to ensure that there is no build-up of parasites on the fish.

Freshwater ParasitesErgasilus Species

These belong to the group of parasitic copepods and are widespread in fish populations. The most common Ergasilus species is Ergasilus sieboldi, although other Ergasilus species such as E. briani are found to cause problems in fish in the UK.

Ergasilus Sieboldi

This parasite, otherwise known as the gill maggot, can cause considerable problems to the fish it infests. It feeds on blood and mucus and can have a serious effect on gill function in heavy infestations. Also the parasites can act as disease vectors carrying both bacteria and viruses.

Freshwater ParasitesLife Cycle

The Ergasilus sieboldi life cycle comprises six nauplius and five copepod stages. These stages are free living, and it is only the adult females who are parasitic. The adult male parasites are free-swimming and after mating with the females the males die

Unlike the Argulus species, Ergasilus females lay eggs while still attached to the gills of the host fish. The rate of hatching is temperature-dependent, and up to three generations can be produced every season. Overwintered females start to lay eggs as the temperatures rise in March, and continue throughout the summer and early autumn, with the highest levels of infestation in fish occurring in late summer/early autumn. Up to three generations can be produced every season.

Parasites can be transferred to clean systems either on fish themselves or in infected water or dirty utensils – observe good hygiene when dealing with an outbreak.


Since the parasites tend to live in the gills, some of the main symptoms relate to the site of attachment –

  • Congregating near water inlets
  • Swimming near the surface
  • Gasping or showing other signs of respiratory distress
  • White spots on the gills (see image below)
  • Scratching or rubbing
  • General loss of condition

Fish DiseasesTreatment

Both Argulus and Ergasilus can be treated with Lice-Solve™ which is a unique formulation of emamectin specially designed for administration directly either to the pond or tank water. One  treatment will kill both Argulus and Ergasilus parasites, and it is filter-safe. Keep the water well-oxygenated during treatment.


Thanks to Bernice Brewster for the photographs