Anchor worm (Lernaea)
Key facts: Anchor worm refers to infestation by Lernaea species, crustacean copepod parasites whose females embed an anchor-shaped head into the skin and muscle of koi, causing localized inflammation and secondary infection.[1][2]
Anchor worm is caused by Lernaea, a genus of copepod crustaceans related to crabs, lobsters, and shrimp, that parasitize freshwater fish including koi, common carp, and goldfish.[1] Infestations are most prevalent in summer and in stagnant or slow-moving water.[1] About 110 species of lernaeids have been described, with Lernaea cyprinacea being one of the most common and found worldwide.[1]The parasite is listed among crustacean parasites commonly seen in koi and goldfish, alongside fish lice (Argulus).[3] A commercial health checklist from the US koi dealer Kodama Koi Farm also lists anchor worm among diseases affecting koi, describing worm-like protrusions, red sores, and rubbing as symptoms.[4]Life cycle
Lernaea has a direct life cycle that does not require an intermediate host; it can spread from fish to fish and typically takes 18 to 25 days to complete.[1] After mating, the male dies and the female burrows into the host's tissue, eventually embedding a large anchor on her anterior end permanently into the skin and muscle.[1][2] Within 24 hours the female may begin releasing eggs from a pair of posterior sacs, with each egg hatching within 24–36 hours.[1] Females are prolific, producing batches of up to 250 juveniles (nauplii) every two weeks for up to 16 weeks at temperatures warmer than 25°C (77°F).[1]Newly hatched nauplii are free-living and pass through three naupliar stages in about four days before molting into the first parasitic copepodid stage, often attaching to the gills.[1] Over the next seven days the parasite develops through five copepodid stages; mating occurs during the last of these free-swimming stages.[2] Once the female reaches the final stage, the male detaches and dies within 24 hours, while the female remains parasitic, either on the current host or after moving to another fish.[1]The optimal temperature range for Lernaea is 26–28°C (79–82°F).[1] Below 20°C (68°F) juveniles cannot complete development, and at 14°C (57°F) females will not reproduce; however, adult females can overwinter on the host and resume egg production as the water warms in spring.[1]Symptoms and diagnosis
Common infection sites include the skin, fins, gills, and oral cavity.[1] Reported signs include frequent rubbing or flashing, localized redness and inflammation, visible thread-like worms in wounds, breathing difficulty, and general lethargy.[2] Large numbers of copepodid-stage parasites on the gills can impair respiration and kill small fish, while embedded females cause intense focal inflammation and hemorrhage that can appear red and ulcerated.[1]The adult female, the most commonly observed life stage, appears to the naked eye as a thin thread or hair approximately 25 mm long.[1] Under a microscope it shows a long tubular body with an anchor on the anterior end and paired egg sacs on the posterior end; the anchor is embedded in tissue while the egg-sac end extends into the water.[1] A separate pictorial parasite guide gives a size range of approximately 5–25 mm and notes the parasite is generally seen fixed to the fish by its anchor.[3] Because adult females can be mistaken for plant fibers, fungi, or other organisms, consultation with a fish health professional using microscopic diagnosis is recommended for confirmation.[1]Disease caused by Lernaea
Infection with small numbers of parasites is not necessarily fatal but is extremely irritating to the fish.[1] Lernaea causes intense inflammation that can lead to secondary bacterial infection, including Aeromonas hydrophila, and fungal infection, which can worsen and kill affected fish.[1] Large numbers of parasites on the gills can interfere with respiration and cause death, and chronic infection frequently results in poor growth and body condition even when fish survive.[1] Only females are parasitic, and infection can cause secondary bacterial infection from the wound created by the anchor.[3]An extension publication on koi and goldfish species profiles lists anchor worm alongside fish lice, Ich, Trichodina, Chilodonella, and monogenetic flukes as common parasites, noting that fish are more vulnerable when water quality is poor and that anchor worm and fish lice are visible to the naked eye and can be identified easily, unlike protozoan and monogenean parasites that require microscopic confirmation.[5]Treatment
- Individual parasites can be removed from affected fish using forceps, although the anchor portion sometimes remains embedded; this method is impractical on a large scale.[1] A similar manual-removal technique is described for the aquarium, holding the fish and using tweezers while dipping it back into water periodically so it can breathe, though deeply embedded parasites may cause more trauma if pulled out.[2]Treatment should address the entire system, since parasite stages exist off the host; adult females can survive 30 days on a fish and are hardier than younger stages, so treatment should continue for several weeks.[1]For food fish, the only legal treatment is salt; research found 4.8 g/L seawater for up to about 30 days effective at preventing development of younger life stages, though adult females are more tolerant and require additional measures.[1]For non-food and ornamental fish, prolonged immersion with an organophosphate such as trichlorfon is effective, a 30-minute bath with 25 mg/L potassium permanganate kills larval stages (though adults may survive), and diflubenzuron (Dimilin) interferes with growth and kills molting adult and larval stages at 0.066 mg/L.[1]A secondary source lists potassium permanganate as generally considered the best treatment, usable as a tank treatment or dip, with salt dips, formalin dips, and modern antiparasitics as other options, and notes that aquarium salt at 1–2 tablespoons may help prevent secondary infection.[2]Removing fish from the system for 7 days can break the parasite's life cycle in the tank, since larval stages cannot survive without a host for that period.[1]
Prevention
Incoming fish must be strictly quarantined and tested to avoid introducing Lernaea to a system, which is especially important for food fish because treatment options are limited.[1] Before introduction, high-risk species such as koi and goldfish should be visually screened for adult parasites and monitored regularly during quarantine; early intervention after detection produces the most successful outcome.[1] There is also evidence suggesting that successfully treated fish may become resistant to future Lernaea infections.[1]Related parasites
Lernaea is a crustacean copepod parasite, distinct from protozoan parasites such as Ich, Costia, and Trichodina, and from monogenetic flukes, all of which require microscopic confirmation.[5] It is often discussed alongside fish lice (Argulus), another visible crustacean parasite, though the two differ biologically: unlike the non-parasitic male anchorworm, all life stages and both sexes of Argulus are parasitic, and Argulus continues to molt periodically even after reaching maturity.[6] Secondary infection from Lernaea wounds can contribute to conditions discussed on the pages for Aeromonas and ulcers and Saprolegnia (fungus).[1]Common questions
What does anchor worm look like on koi?
The adult female appears as a thin, thread- or hair-like structure about 25 mm long, with one end (the anchor) embedded in the fish's tissue and the other end, bearing paired egg sacs, extending into the water.[1]
Is anchor worm contagious between fish?
Lernaea has a direct life cycle and can spread from fish to fish without an intermediate host, with juvenile stages becoming parasitic on gills before maturing and some moving between hosts.[1]
Can anchor worm be removed with tweezers?
Individual parasites can be removed with forceps, but the anchor portion sometimes remains embedded, and the method is impractical on a large scale, so treatment of the whole system is usually also needed.[1]
Does salt kill anchor worm?
Salt has variable effects: eggs failed to hatch at 8 g/L and younger stages were controlled at 4.8 g/L for 5–6 days, but adult females survived up to 22.4 g/L for up to 6 days, so salt alone may not eliminate adults.[1]
How is anchor worm diagnosed?
Because adult females can resemble plant fibers or fungi, microscopic examination of wet mounts from skin, gill, oral cavity, or fin samples by a fish health professional is recommended to confirm the diagnosis.[1]
Related koi topics
References
- ^ UF/IFAS EDIS FA185: Lernaea (Anchorworm) Infestations in Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA185. Retrieved November 2026.
- ^ Lernaea (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Lernaea. Retrieved November 2026.
- ^ UF/IFAS EDIS FA115: Common Freshwater Fish Parasites Pictorial Guide: Crustaceans. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA115. Retrieved November 2026.
- ^ HELP, Is My Koi Sick?!? Diagnose Symptoms & Fish Diseases - FREE Health Checklist. Kodama Koi Farm (koi dealer). kodamakoifarm.com/koi-sick-health-diagnosis-symptoms-diseases/. Retrieved November 2026.
- ^ SRAC Publication No. 7201: Species Profile: Koi and Goldfish. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/7201%20Species%20Profile-%20Koi…. Retrieved November 2026.
- ^ UF/IFAS EDIS FA184: Argulus (Fish Louse) Infections in Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA184. Retrieved November 2026.