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Anchor worms in fish: how to identify, remove and treat Lernaea

Act today

Anchor worms are not really worms: they're parasitic crustaceans called Lernaea that bury their heads in a fish's skin. You can pull visible ones out with tweezers, but you also need to treat the whole tank or pond for several weeks, because young stages you can't see are still in the water.

They're most common on goldfish, koi and other pond fish, and in summer. The wounds they leave often become infected, so good water quality during recovery is essential.

How to identify it

  • Thin threads sticking out of the skin, fins, gills or mouth, about 5 to 25 mm (up to 1 inch) long. White, greenish or brownish.
  • A Y-shaped tail end on adult females, often with two small egg sacs.
  • A red, swollen, sometimes ulcerated spot where the parasite enters the body. The head is anchored under the skin, which gives the parasite its name.
  • Other signs: rubbing against objects (flashing), clamped fins, poor appetite, weight loss, and red sores.

Easily confused with: a bit of plant or a fibre stuck to the fish, fuzzy fungus (soft and cotton-like, see columnaris), or worms hanging from the vent (camallanus). Anchor worms are firmly attached and don't wipe off. A fish vet can confirm with a microscope.

When it's a problem

  • Normal: anchor worms are never normal. Even one means there are probably more on the way.
  • Early: one or two parasites on a single fish, with little redness.
  • Serious: several fish affected, red ulcers, or parasites on the gills or in the mouth.
  • Emergency: small fish with many parasites, fish breathing hard (young stages can attack the gills), or ulcers turning white or fuzzy. Act now.

What to do right now

  1. Check every fish in the tank or pond in good light.
  2. Test the water and do a water change if ammonia or nitrite is above zero. Wounds heal far better in clean water.
  3. Remove visible parasites: wet your hands, hold the fish gently in a wet net or cloth, grip the parasite with fine tweezers as close to the skin as you can, and pull out slowly and steadily. Keep the fish out of water for as short a time as possible (seconds, not minutes). For large or valuable fish, ask a vet, who may sedate the fish first.
  4. Watch the wound. Part of the anchor may stay behind; it usually works its way out. Some vets dab the wound with a fish-safe antiseptic.
  5. Isolate badly wounded fish in a clean hospital tank, if you have one.
  6. Start treating the whole tank or pond. Pulling parasites alone won't clear the outbreak.

What to do next

Understand the life cycle. Females on the fish release batches of tiny larvae every couple of weeks. The larvae swim free for a few days, then attach to fish (often the gills) and grow. At 25 to 30°C (77 to 86°F) the whole cycle takes about 18 to 25 days. Below about 20°C (68°F), young stages can't finish developing, but adult females can survive winter on a fish and start again in spring. Adult females can live on a host for about 30 days. That's why treatment has to last several weeks.

Treatment options by active ingredient. Product names and legal rules vary between countries. Check the label, follow it exactly, and ask a vet if unsure.

  • Diflubenzuron (and related "chitin inhibitors" such as lufenuron): stop parasites growing their outer skeleton, so they die when they moult. Veterinary sources describe diflubenzuron as the most effective option. It kills all crustaceans, including shrimp, and may be restricted-use in some places.
  • Organophosphates (such as trichlorfon): historically used for ornamental fish, but legal restrictions limit their use and some products are no longer made. Toxic to people and many animals; handle with great care.
  • Potassium permanganate: a strong oxidiser that kills larvae but may not kill adults. Easy to overdose, and harsh on gills. Best used under vet guidance.
  • Salt: the Merck Veterinary Manual describes a short salt dip followed by a lower level of salt in the tank for about three weeks, which kills young stages as they hatch. Only for salt-tolerant fish such as goldfish and koi. Not for scaleless fish, many catfish and loaches, most plants, shrimp or snails.
  • Emamectin (an in-feed drug): shown in experiments to work against fish lice, a related crustacean parasite, in goldfish and koi. Generally only available through a vet, if at all.

Breaking the cycle without drugs. Free-swimming larvae can't survive long without a fish. Moving all fish out of a tank for about a week, while treating the fish themselves, can help clear the tank. This works best for small setups.

General cautions. Remove activated carbon from the filter while medicating. Keep shrimp and snails out of treated water. Take care with scaleless fish, plecos and other catfish, and loaches. Medicine can harm plants and the filter bacteria, so test ammonia and nitrite often. Finish the full course, even if you can't see any more parasites.

Recovery. Watch wounds closely for 2 to 3 weeks. Red sores that turn white, fuzzy or spread suggest bacterial infection (see fin rot and columnaris). Antibiotics should be chosen with a vet; many countries require a vet prescription.

Ponds and goldfish. Ponds are harder to treat because of their size and wildlife. Calculate the volume carefully, treat all connected water, and remember that pond treatments may harm frogs, insects and other invertebrates. Anchor worms often arrive with new koi or goldfish in summer.

Products that help

  • Fine-tipped tweezers or forceps: for removing visible parasites.
  • Anti-parasite treatment for crustacean parasites: products containing a chitin inhibitor or other approved active ingredient. Choose one sold for anchor worms and fish lice.
  • Aquarium or pond salt (pure sodium chloride): supportive treatment for salt-tolerant fish only.
  • Hospital tank: keeps wounded fish in clean water and makes treatment easier.
  • Water test kit: to keep ammonia and nitrite at zero during treatment.
  • Keep on hand: a soft net, a dechlorinator, and a quarantine tank for every new fish.

How to prevent it

  • Quarantine all new fish, especially goldfish, koi and pond fish, for at least four weeks, and inspect them closely.
  • Look for threads and red spots on fish before you buy.
  • Don't add water from the shop bag to your tank or pond.
  • Be careful with plants and live food from ponds, which can carry larvae.

Why it happened

Anchor worms almost always come in on a new fish, or occasionally in pond water or live food. In warm water they breed quickly, and every female can release hundreds of young over a few months.

When to get help

See a fish vet if parasites are on the gills or in the mouth, if many fish are affected, if wounds become infected, or if you're treating valuable koi or a large pond. A vet can confirm the parasite with a microscope and advise which treatments are legal and safe where you live.

In the game

Multiple Tank Syndrome doesn't have an anchor worm card yet. The "Quarantine tank" card is the best defence against hitchhiking parasites.

Sources

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