Parasitic Crustacea: Ergasilus, Lernaea, Argulus, and Leeches

Parasitic crustacea are increasingly serious problems in cultured fish and can impact wild populations. Most parasitic crustacea of freshwater fish can be seen with the naked eye as they attach to the gills, body and fins of the host.

Three major genera are discussed below.

1. Ergasilus

Ergasilus (Figure 24) are often incidental findings on wild or pond- raised fish and probably cause few problems in small numbers.

However, their feeding activity causes severe focal damage and heavy infestations can be debilitating. Most affect the gills of freshwater fish, commonly seen in warm weather.

A 3% salt dip, followed by 0.2 %-prolonged bath for three weeks, may be effective in eliminating this parasite.

2. Lernaea

Lernaea, also known as anchor worm (Figure 25), is a common parasite of goldfish and koi, especially during the summer months. The copepod attaches to the fish, mates, and the male dies.

The female then penetrates under the skin of the fish and differentiates into an adult. Heavy infections lead to debilitation and secondary bacterial or fungal infections.

Removal of the parasite by hand with forceps may control lernaeid infestations with careful monitoring of the wound. A 3% salt dip followed by 0.2%-prolonged immersion has been used to effectively control Lernaea in goldfish and koi ponds

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3. Argulus

Argulus or fish louse is a large parasite (Figure 26) that attaches to the external surface of the host and can be easily seen with the unaided eye.

Argulus is uncommon in freshwater aquarium fish but may occur if wild or pond-raised fish are introduced into the tank. It is especially common on goldfish and koi.

Individual parasites can be removed from fish with forceps, but this does not eliminate parasites in the environment. A prolonged immersion of 0.02 – 0.2% salt may control re-infection to the fish host.

4. Leeches

Although not a common problem, occasionally, fish will be observed infected with either leeches or copepods. Leeches have long, slender flexible bodies and actively swim for an attack on their prey.

Skin and underlying soft tissues are damaged and allow blood to flow into the leeches digestive tract. Leeches are not host-specific, and the damage to the skin and gills is dependent upon the number of leeches present at any time.

Small fishes can be seriously injured or die due to excessive leech infestation. Leeches are occasionally seen in wild and pond-raised fish. They have a direct life cycle with immature and mature worms being parasitic on host’s blood.

Pathogenesis varies with number and size of worms and duration of feeding. Heavily infested fish often have chronic anemia. Fish may develop secondary bacterial and fungal infections at the attachment site.

Leeches resemble trematodes but are much larger and have anterior and posterior suckers (Figure 27). Dips in 3% saltwater are effective in controlling leeches.

Ponds with heavy leech infestation require drainage, treatment with chlorinated lime, followed by several weeks of drying. This will destroy the adults and their cocoons containing eggs.

In summary, most fish health problems occur because of environmental problems: poor water quality, crowding, dietary deficiencies, or “stress”.

The best cure for any fish health problem is prevention. Good water quality management and proper fish husbandry techniques will eliminate most parasites described here.

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