What are Anchor Worms?

The female anchor worm (Lernaea), a parasitic crustacean, burrows into the flesh of freshwater fish until it reaches the muscle. The parasite may spend three or more months attached to the muscle tissue. Upon departure, the anchor worm leaves serious wounds on the fish. The wounds may make the fish susceptible to diseases or infection. Anchor worms frequently attack koi and goldfish, but they also infest other species, such as the rainbow trout.

  1. Infestations

    • A small fish can die from the effects of just one anchor worm. A heavy infestation of anchor worms on a large fish will seriously weaken it. Most anchor worms attach themselves at the base of the fins, but some may penetrate the fish's gills. The parasite will use its wedge-shaped head to burrow beneath protective scales. The infected fish will often suffer a swift death if the worm penetrates a vulnerable area.

    Identification and Symptoms

    • The anchor worm is approximately 1 inch long, so it can be seen by the human eye. Its slender, whitish-green body trails outward from the point of penetration. Two egg sacs often extend from the posterior of the worm. Each sac contains approximately 250 eggs. The fish owner may notice the fish rubbing itself against the sides of the tank, pond, or rocks in a vain attempt to rid itself of the anchor worm. Visible bloody circular wounds may also be visible on the fish.

    Life Cycle

    • Anchor worms hibernate in the winter. They break dormancy when the water temperature tops 57 degrees Fahrenheit, and multiply rapidly when the water temperature tops 77 degrees Fahrenheit. The eggs hatch approximately two or three days after the female releases them in water. Female parasites have about three days to find a host fish. As the female grows, the male attaches itself to her hindquarters to briefly breed before he dies.

    Control and Treatment

    • To avoid introducing anchor worms to a pond, quarantine a new fish for at least one month prior to placing it into a community tank or pond. For a single infected fish, fill a tub with 10 mg of potassium permanganate per liter of water. Soak the fish in this water for up to 30 minutes. To treat a tank full of infected fish, add 2 mg of potassium permanganate to the tank for each liter of water. For a large pond with a widespread anchor worm problem, add 8 oz. of dimilin for every 1,000 gallons of water. After this treatment, fish with seriously ulcerated wounds may still require antibiotics to prevent secondary infections.