Despite their appearance and common name, the anchor worm is not a worm at all. Lernaeids, better known as anchor worms, are highly modified copepod crustaceans, related more closely to shrimp and crabs than to parasitic worms. As crustaceans, lernaeids must periodically molt or shed their shell to grow and mature. Only a portion of the tubular parasite is visible on the fish's body surface; however, the name anchor worm comes from the branching holdfast structure the parasite embeds in the fish's musculature that resembles a ship's anchor.
Anchor worms are common parasites of ornamental fish, particularly koi and goldfish, but Lernaea cyprinacea, the most common species in North America, has little host specificity, which means it will infect many species. This parasite has been a problem in the bait minnow industry and has been seen in farm raised rainbow trout, channel catfish, and others. A single anchor worm can kill a small fish, although larger fish may tolerate several of the parasites. Infestation typically results in gradual loss of physical condition and weakening of the host to such an extent that it becomes susceptible to secondary bacterial infections, particularly at the site of attachment. Occasionally a fish's body cavity or skull is penetrated by the anchor, which leads to more rapid death.
Only the female copepod attaches to and burrows into a fish host. Mature females can be seen with the unaided eye, and are about one inch in length. Females have a slender, white, thread-like body, with paired egg sacs trailing from the posterior end. The bases of the fins are the preferred sites for attachment, which begin initially as a small hemorrhagic (bloody) spot. Over time, a raised nodule develops from which the body extends, usually lying against the fish's side. Less commonly, anchor worms will attach to the gills.
Outbreaks occur in summer months when water temperatures are above 57 degrees F. The speed of the life cycle is temperature dependent, occurring most rapidly above 77 degrees F, but short enough that several generations can elapse in most areas. Eggs released into the water by the female hatch into free-swimming nauplii in 2-3 days. After a brief period of development, the first copepodid stage is reached and the parasite must find a fish host on which to live within a few days or it will die. The parasite continues to metamorphose, proceeding through a series of molts on the fish's skin. When the fifth copepodid stage is reached, males and females mate and the males die. The female then begins to grow rapidly, burrows into the fish's body, develops its anchor, and emerges from the fish's skin as an adult copepod.
A diagnosis of anchor worm infestation is based upon identification of the parasite attached to a fish. Your veterinarian can manually remove small numbers of anchor worms on individual fish using light sedation. Heavy infestations in ponds or large tanks, however, are best treated chemically.
Multiple treatments may be needed to eliminate all stages of the parasite. Organophosphate insecticides (trichlorfon) have been used in the past, but these products have come under scrutiny by government regulatory agencies and may no longer be available. Resistance to the organophosphates by the parasite may also develop with repeated exposure.
Chitin inhibitors (diflubenzuron and lufenuron) prevent a new shell from developing after the crustacean molts and dies. Some of these agents are found in certain products used to treat fleas on dogs and can be prescribed by your veterinarian. Many pesticide chitin inhibitors require an EPA permit for use as they can be dangerous to invertebrate wildlife in streams and ponds. Antibiotic therapy may also be recommended to prevent bacterial infections at attachment sites.
Read and follow your veterinarian's instructions carefully and apply all medications with caution. Provide an environment that is as low stress as possible by maintaining good water quality and assure the fish is eating.
The keys to prevention are avoidance and quarantine. When purchasing new fish, avoid anchor worms by inspecting closely for the presence of the parasite, but remember that immature females embedded beneath the skin may take several weeks to become visible. For this reason, always quarantine new aquarium or pond fish for at least one month. This period of time will usually allow any signs of disease to manifest before they have a chance to spread to your established fish population.