Anchor Worm (Lernaea)

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by Karen Paul

Lernaea (anchor worm) is not actually a worm, it is a copepod crustacean. The adult anchor worm is visible without magnification, measuring 1-1.5cm in length. Only the female Lernaea is parasitic, the egg sacks giving her a Y-shaped appearance. Male Lernaea are relatively short lived and die soon after mating.

Lernaea with egg sacks

Life Cycle

The female anchor worm becomes an adult on her host after fertilization by the male. The typical copepod body then changes to become unsegmented and worm-like. The head is thrust into the skin of the host, often through the body wall and into the internal organs. The head develops cephalic horns (anchors) to hold it in place. Two egg sacks develop near the posterior end of the body. The egg sacks are shed when the eggs mature.

Egg sacks

Once the eggs hatch there are several juvenile stages. The first stage juvenile (nauplii) is about 140µ long and molts within about 50 hours, depending on water temperature. The second stage (metanauplii) is about 180µ and molts within 20 to 40 hours to become the first copepodid form measuring about 230µ in length. This juvenile stage has two pairs of swimming legs and must find a host within a few days to survive. Once on its host the juvenile goes through more metamorphoses. Mating occurs during the sixth copepodid stage. The male is about 780µ and the female 1,040µ in length. The male dies after mating. The whole cycle takes about 25 days at 20°C. Lernaea can over winter as an adult female on a host or as eggs.

Fifth copepodid (larval) stage

Details of egg sacks and posterior end (left)
and the anterior end site of attachment to host (right)

Symptoms

The attached adult Lernaea can be identified without magnification. A raised ulcer usually develops at the attachment site and secondary fungal and bacterial infections are common. The cephalic horns (anchors) may be embedded into vital organs such as the liver, heart or brain and can cause serious damage, even death.

The larval stages of Lernaea can cause itching and excess mucus secretion. Heavy infestation can cause gill damage. The fish may be lethargic and show increased gill movements.

Treatment

Any fish with attached adult anchor worms should be moved to a quarantine tank to prevent eggs being released into the pond. The attached adult Lernaea can be carefully removed with tweezers, the fish may need to be anaesthetised for this. Care must be taken to remove all parts of the parasite. Dipping the tweezers in a strong Potassium Permanganate solution first will cause the anchor worm to release its grip and make it easier to remove completely. Once removed the wound should be treated with an antiseptic spray or gel to reduce the risk of secondary bacterial infections or fungus. Alternatively, Malachite Green can be applied with a cotton bud.

The most effective chemical treatments against Lernaea larvae were organophosphates, Dimilin, Masoten and Malathion, but they are now banned in the UK for use in fish treatments.

Lernaea with egg sacks anchored on a host

Lernaea with ciliates attached to the posterior end

Lernaea is often introduced to the pond on new fish. Proper quarantine procedures should help to prevent its introduction into an established pond.

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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