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Anchor Worms: a Common Springtime Pest in Koi and Goldfish Ponds

Hello, Frank Indiviglio here.

Anchor worms are crustaceans (specifically copepods) and as such are more closely related to shrimp than to worms.  They often make their presence known in outdoor ponds as winter turns to spring.  Lernaea elegans, the most commonly encountered species, remains dormant during the winter and becomes active as the water reaches 55 F or so. 


Anchor worms bury below their host’s scales, but betray their presence by trailing ¼ to ½ inch-long portions of their black, thread-like bodies from infected fishes.  They usually attach about a fish’s gills, eyes or fins, but can occur most anywhere.  Other signs include inflamed or raised scales and efforts by fishes to dislodge the parasites (leaping, rubbing).


Infestations most commonly occur during the spring, when the parasites are searching for new hosts after their winter dormancy.  Conveniently for the anchor worms, the immune systems of pond fishes are at their weakest at this time, having been stressed by cool temperatures and the long winter fast. 

Secondary Bacterial Infections

Anchor worms rarely cause fatalities, but the wounds they inflict frequently become infected by opportunistic Pseudomonas and Aeromonas bacteria.  Ever present in the pond, these pathogens can easily kill fishes, especially those with depressed immune systems.

Avoiding Anchor Worms

Channel CatfishBe especially careful to check for anchor worms when purchasing koi or goldfishes in the late winter or spring…those that have been wintered outdoors may be infected.  They also parasitize weather fishes, channel catfishes, hi-fin loaches and other species commonly kept in outdoor ponds.

Anchor worms of various species can also be introduced to your collection via tropical fishes which have been raised outdoors.

Treating Parasitized Fishes

Fortunately, Jungle Lab’s Anchors Away is an effective treatment for infestations of anchor worms and certain other parasites.  Be aware that this medication will kill crayfishes, snails and other invertebrates, and that carbon should be removed from your filter while treatment is ongoing.

It is also useful to add an ultraviolet sterilizer to your pond’s filtration system.  UV sterilizers will kill anchor worms in the free swimming larval stage (they are not effective against adults), thereby preventing re-infestation.

Further Reading

A detailed article (Food and Agriculture Association of the United Nations) on the life cycle of anchor worms and related parasitic copepods is posted at.

Please write in with your questions and comments.  Thanks, until next time, Frank Indiviglio.

Image referenced from Wikipedia and originally posted by Brian Gratwicke


  1. avatar

    I got some free koi from a breeder that were infested with anchor worms and I can’t seem to get rid of them. Treatment keeps them down but as soon as I stop they’re back. Could they be resistant to traditional treatments.? Help

  2. avatar

    What have you treated with and for what duration? You may need to use a stronger medication like anti-fluke, or it may be that you need to treat them for a little longer after they seem at bay. You may want to consider dosing with pond salt and investing in a UV Sterilizer if you have not already as another/additional measure once they are under control. It may also be a good idea to have some melafix on hand once the worms are dead to help heal the holes the worms have left so that no infections and ulcers develop. I doubt that they are resistant to the treatments, but the weather may play a role in how effective some medications are. The water temp should be >60 F for many meds to work reliably.

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  4. avatar

    I don’t think you have anything to worry about. Fish lice and Anchor worms are pretty specialized in the host and conditions they need, and fortunately humans are not part of their host repertiore, and should not be a concern. I have found no information on human anchorworm infections, and due to their need for aquatic hosts and conditions there is virtually no likelihood that that parasites could infect a human. If you are still concerned you may want to consult a physician.

  5. avatar

    Please help
    I had an anchor worm problem this year with some of my
    1-2″ goldfish that I had in a brand new 600 gal pond that I dug up this spring. I only had about 10 small goldfish in the pond, but the pond was drying up fast from the lack of rain this year 2012, so I moved the fish back into there 25 gal tank inside the house.
    After about a month fish started to die, I had no idea what the problem was till there was only 3 fish left, then I started to look real close and that’s when I noticed the anchor worms, new to this problem I treated the tank with PraziPro, and took out the carbon out of the filter, but it was to late for the fish. 4 days later after I treated the tank I took the dead fish out and looked at the worms with a loop and I can see that some were still alive. My question is how long does it take for this product to kill the worm?
    Also how does this parasite effect frogs and tadpoles in a pond?
    And the most important question I have is
    If children or adults are playing in a small pond like mine catching frogs and tadpoles that is infested with parasites/anchor worms and eggs and someone gets these parasites or parasite eggs in an open wound/mouth/eyes will they become a problem and how will it effect the human body?. Thanks Frank

  6. avatar

    Prazipro won’t be effictive on anchorworms…you’ll need a product like Clout or microbe lift lice and anchorworm treatment Anchorworms should not cause a problem for frogs or humans.

  7. avatar

    Anchor worms are a real problem in the fish business. It takes some strong chemicals and a long time to get rid of them. Tadpoles & frogs also can be infected with them and hop from pond to pond to spread them to other ponds. Humans as far as I know do not get infected. Max

  8. avatar

    It’s going to be end of mine day, except before ending I am reading this impressive paragraph to improve my experience.

  9. avatar

    any thoughts where the initial anchor worm or lice come from? is it from poor fish tank/pond water quality?

  10. avatar

    Hi Han, They are naturally occurring in most areas in natural ponds and streams. They may be introduced into artificial ponds through fish and plants harvested from natural systems.

  11. avatar

    How long will a anchor worm larvae live with out a host, can they be in my pressure filter and how long can they live in it if I disconnect it, also will they be in my Lilly plants if I take the Lillie’s out and transfer them to another holding tank

  12. avatar

    Hi Jason, Anchorworms will usually reach the parasitic phases in about a week. I wouldn’t expect them to live in your filter or in a plant without a host; they will only host on animals like fish (or occasionally amphibians like tadpoles).

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About Frank Indiviglio

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Being born with a deep interest in animals might seem unfortunate for a native Bronxite , but my family encouraged my interest and the menagerie that sprung from it. Jobs with pet stores and importers had me caring for a fantastic assortment of reptiles and amphibians. After a detour as a lawyer, I was hired as a Bronx Zoo animal keeper and was soon caring for gharials, goliath frogs, king cobras and everything in-between. Research has taken me in pursuit of anacondas, Orinoco crocodiles and other animals in locales ranging from Venezuela’s llanos to Tortuguero’s beaches. Now, after 20+ years with the Bronx Zoo, I am a consultant for several zoos and museums. I have spent time in Japan, and often exchange ideas with zoologists there. I have written books on salamanders, geckos and other “herps”, discussed reptile-keeping on television and presented papers at conferences. A Master’s Degree in biology has led to teaching opportunities. My work puts me in contact with thousands of hobbyists keeping an array of pets. Without fail, I have learned much from them and hope, dear readers, that you will be generous in sharing your thoughts on this blog and web site. For a complete biography of my experience click here.