Anchor Worms in Freshwater Fish

anchor worm in fish

Dnatheist / Wikimedia Commons / CC by 2.0

Anchor worms (Lernaea spp.) 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 setup and parasite load.

What Are Anchor Worms?

Anchor worms (Lernaea spp.) are external copepod parasites that attach to your fish under scales. 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. Juveniles 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 most commonly found on goldfish and koi. There are other similar genuses of copepods that infect other freshwater species 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. Anchor worms can also be found in the oral cavity and give the mouth a "baleen" appearance.

Worms that have fallen off may leave behind patches of hemorrhage or fibrosis.

Causes of Anchor Worms

Anchor worms are caused by a new fish being 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 fish tank 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 disease. This doesn't mean they may bring over some invertebrate pests!

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 from the feeding end. 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 touch the adults. This can also 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 make sure to wear proper protection (i.e. gloves).

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

How to Prevent Anchor Worms

The best method of preventing anchor worms is properly quarantining any new additions. 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 tank.