Anchor Worms in Freshwater Fish

anchor worm in fish

Dnatheist / Wikimedia Commons / CC by 2.0

Anchor worms are macroscopic parasites, meaning they can be seen by the naked eye. They are commonly found on koi and goldfish, but can be found on many freshwater fish species. The "worm" part extending out into the water is actually the female reproductive structures. Treating these parasites can be challenging depending on your aquarium setup and the parasite load.

What Are Anchor Worms?

Anchor worms (Lernaea spp.) are not actually worms, but a crustacean copepod parasite that embeds into the fish’s skin and has a worm-like tail that sticks out of the skin. These parasites have multiple non-parasitic stages that take place in the water. Once a male mates with a female, she attaches to a fish to mature into a reproductively-active adult. These parasites get the name "worm" from the extending female reproductive structure. Juvenile anchor worms will be free-swimming in your aquarium, but will not be a nuisance for your fish.

The Lernaea genus of copepods infect most freshwater fish. They are commonly found on goldfish and koi. There are other similar genera of copepods that infect other freshwater and marine fish.

Signs of Anchor Worms

Anchor worms are one of the macroscopic freshwater fish parasites that are visible to the naked eye. You will see the female reproductive structures that resemble short white worms sticking out from behind scales. The end of the white “worm” may have one or two egg sacs attached to it. Anchor worms can also be found in the oral cavity and give the mouth a "baleen" appearance, like the long teeth of baleen whales.

Worms that have fallen off may leave behind skin patches of hemorrhage or fibrosis. Bacteria can infect the site of the anchor worm attachment, causing red lesion on the fish’s skin.

Causes of Anchor Worms

Anchor worms occur when a new fish is added to an aquarium carrying juvenile anchor worms or a reproductively-active female in their skin. Skipping proper quarantine makes this parasite spread very rapidly. A single female anchor worm can produce hundreds of larvae every two weeks for up to 16 weeks in a 77F (25C) tank.

Anchor worm juveniles can also be spread with the introduction of live plants. Although they may not be on the plants themselves, free-swimming juveniles may be in the water surrounding the plants. Aquatic plants can bring many bacteria and parasites into your aquarium if not properly quarantined. Like your new fish, quarantining plants in a plant-only system will break the parasite life cycle, since there are no fish to host them. This only applies to plants kept with fish. If your new plants have never been kept with any fish, they will be free from fish diseases. Though they may bring over some invertebrate pests, such as snails.

Treatment of Anchor Worms

Once anchor worms are present on your fish, it is very tempting to simply pull them off, but resist the urge to do so. Anchor worms need to be removed correctly with a fish under sedation by your veterinarian. They will need to remove the entire parasite including the feeding end from under the fish’s skin. Depending on the level of infestation, sedation makes the process less stressful for the fish and allows the veterinarian to work more effectively without a squirming fish.

Once the mature females have been removed, you may still have a microscopic problem: the juvenile stages. Over-the-counter "anchor worm" treatments are usually fairly effective against the juvenile stages, but they will not kill the adults. Another way of killing the free-swimming juvenile stage in the aquarium can be accomplished by removing your substrate and decor and running your water through a UV light.

Treating anchor worms with organophosphates or diflubenzuron (dimilin) is effective, but needs to be undertaken with severe caution. Only use veterinary-approved products, keep them away from your other pets and children, and make sure to wear proper protection (i.e., gloves) when handling medications.

The sites of adult attachment may also develop secondary bacterial infections. Monitor these sites carefully after the adults have been removed. They may require treatment with antibiotics, depending on their site and severity. Antibiotics should not be purchased over-the-counter and should only be used when prescribed by a veterinarian. The best method of deterring these secondary bacterial infections is good water quality.

How to Prevent Anchor Worms

The best method of preventing anchor worms is properly quarantining any new additions to your aquarium. If you see any anchor worms present in a tank of fish you are looking to purchase, assume they are all contaminated. Remember, juvenile larvae are microscopic and may be present without you realizing it until it's too late. By properly quarantining your new fish, you will prevent the spread to the main aquarium.