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Related FAQs:  Freshwater Worms 1 Planaria, FW Worm Identification, FW Worm Behavior, FW Worm Compatibility/Control, FW Worm Selection, FW Worm Systems, FW Worm Feeding, FW Worm Disease, FW Worm Reproduction & FAQs on: Worm Caused Diseases,  Platyhelminths/Flatworms: ( Flukes, Planaria, Tapeworms and Leeches), Acanthocephalans, Nematodes/Roundworms (e.g. Camallanus),... Anchor "Worms": See FW Crustacean Parasitic Disease, & Worms as Foods,

Nematodes, Flatworms, Anchor Worms and Other Worm Parasites of Freshwater Fish

© Neale Monks 2018

The vast majority of worms are not parasitic


Worms of various kinds are very common parasites of wild fish, and several sorts can be seen among farmed aquarium fish as well. The genus Camallanus is probably the best known because it is so widespread and quite easy to identify. Most of the other worms are difficult to identify without the use of a microscope, making diagnosis and treatment rather difficult. For this reason many aquarists adopt a precautionary approach to worm parasites, quarantining and treating new livestock with anti-worm (also known as antihelminthic) medications.


A quick (but important) note on taxonomy

Over the years, the word ‘worm’ has been used to describe pretty much any invertebrate animal without legs. This includes several large groups (or phyla) that are not parasitic at all, including the predatory arrow worms (phylum Chaetognatha) and the deposit-feeding peanut worms (phylum Sipuncula).

Most of the fish parasites come from three distinct phyla, the flatworms (Platyhelminthes), roundworms (Nematoda), and the segmented worms (Annelida) but it should be noted that occasionally the worm name gets applied to parasites that might look wormy but actually belong to phyla that wouldn’t normally be considered ‘worms’ by even the most disinterested observer. Anchor worms, Lernaea spp., are actually highly specialised crustaceans, and therefore more closely related to shrimps, crabs and barnacles than they are to, say, earthworms. Similarly, acanthocephalans, or spiny-headed worms, are merely worm-like in shape, and physiologically quite different. This is important because antihelminthic medications will have little to no effect on these non-worm ‘worms’, and instead the aquarist will need to use something particularly designed to treat these sorts of parasites.

Returning to the true worms, the flatworms include those parasites commonly referred to as flukes, and are characterised by their small, flat, often leaf-like body shape. The nematodes or roundworms tend to look more like hairs, pointed at each end, but otherwise featureless. While most nematodes are colourless, Camallanus worms are distinctive because of their red colouration. Finally, the annelids are distinctly segmented, and under the microscope the head often bears distinctive hooks and other structures used to hold onto their host. Among the annelids, it is flatworms that are the most familiar, the ones infecting fish not looking very different to the flatworms that sometimes infect humans and their pets.



Low-level infestations of worms that infect the digestive system are probably quite common, whether we’re talking about wild-caught fish or farmed fish. Assuming the fish is otherwise healthy and properly looked after, such low-level infestations may never cause any apparent problems, so without examining the faeces of the fish, you might never know your fish are infected.

On the other hand, heavy infestations can manifest themselves in several ways. The classic situation is where a fish becomes swollen around the abdomen while the rest of the fish becomes emaciated. Such fish may also become lethargic, and in some cases developmental abnormalities, such as crooked spines, will become apparent.

Only very rarely are intestinal worms themselves visible to the aquarist. Camallanus worms extend a part of themselves outside the anus of the fish as part of their reproductive cycle. When this happens the worms are visible as pink, red or reddish-brown threads.

External worms such as skin and eye flukes can be so small that they are difficult to see without a microscope, though persistent ‘flashing’ behaviour as the fish tries to rub them off can be a useful clue. But anchor worms are usually big enough they can be seen as Y-shaped flecks attached to the host, often the fins, but not necessarily so. Because these parasites are damaging the surface of the fish, they do make it more prone to bacterial or fungal infections, which might be useful when trying to make your diagnosis.



Although anchor worms and Camallanus worms might be diagnosed relatively easily, most of the other flukes and intestinal worms are impossible to identify to species level without a microscope and/or a degree in parasitology. However, while anchor worms require specific treatment (usually an organophosphate-based medication) most of the other types of worms can be treated using general purpose antihelminthic medications.


Types of worms and worm-like parasites


Camallanus and other nematodes

These worms are very common among farmed aquarium fish, in particular livebearers and cichlids. As mentioned above, these worms reveal themselves as pink, red or reddish-brown threads emerging from the anus of the infected fish. The worms themselves can be quite big, comparable in size to the bloodworms we use to feed fish, but much thinner and without the segments.

Some species of Camallanus require an intermediate host to complete their life cycle, typically small crustaceans such as copepods. But at least one species can complete its life cycle without such hosts, meaning that an infected fish have the potential to infect other fish in the aquarium. It is believe that this Camallanus species releases eggs that end up in the faeces of their host, and when other fish accidentally consume these eggs (perhaps while foraging on the bottom of the tank) they becoming infected.

Flubendazole and fenbendazole work well against Camallanus, though more than one cycle may be necessary. Other anthelminthics such as praziquantel may work, but less reliably so.



There are many types of tapeworm infecting a wide variety of animals, but they all belong to a group of annelid worms known as the Cestoda, or cestodes. Most, perhaps all, require two different hosts to complete their life cycle. The ones infecting fish typically have to pass through a small invertebrate first, commonly a specific type of crustacean or snail, which is eaten by the fish, and in this way infects the fish. Once the tapeworm matures it starts producing eggs that end up in the faeces of the fish, and these are, in turn, consumed by whatever small invertebrates are its intermediate host, ready to infect the next fish.

Whereas Camallanus usually reveal themselves once the reddish thread-like worms emerge from the infected fish’s anus, tapeworms are much more cryptic. As mentioned earlier, the usual situation is while the fish generally becomes emaciated, its abdomen swells up. But if a vet or parasitologist examines the faeces of infected fish under a microscope, they can find the reproductive segments or eggs that will help to identify the specific type of tapeworm present in the host.

Prevention is better than cure, when it comes to tapeworms. Because of their complex life cycle, they are unlikely to get inside healthy aquarium fish unless those fish consume infected snails or crustaceans. Snails taken from ponds containing fish should not be used as food for those species, such as puffers and loaches, that consume small snails. It’s potentially possible for snails to be transported from one pond to another by floods or even on the feet of waterfowl, so the wise aquarist would probably avoid snails entirely unless cultured at home in a demonstrably safe environment, such as another aquarium set up somewhere indoors.

Crustaceans are the main risk though, being among the most common hosts for tapeworms as well as providing a much more useful range of live foods. Brine shrimp are the exception, being completely safe because of the hypersaline conditions used to rear them; it’s Daphnia and Cyclops species that may have been exposed to tapeworm larvae at some point, and therefore pose the main risk. While both can be cultured safely at home, the safety of those bags of live Daphnia sold in pet shops cannot be ascertained so easily, so are best avoided.

Praziquantel is the most widely used medication for tapeworms, though it isn’t always the most effective. It works by causing the digestive tract to spasm, which will hopefully cause the worms to ‘let go’ and be passed out in the faeces. Because of the way it works, you may need to perform more than one course to deal with heavy infections. The usual approach is to complete the course as recommended by the vet or manufacturer, and then after a 10-14 day break, repeat the course once more.



Also known as spiny-headed worms, these worm-like parasites are actually more closely related to rotifers. They have complex life cycles involving two or three hosts, typically a small crustacean of some sort for the first part of the life cycle, which is in turn consumed by a larger animal, such as fish or bird, where the parasite develops into its final, reproductive stage.

Diagnosing spiny-headed worms is extremely difficult, the worms themselves being small, even microscopic, and at low levels of infection the host may manifest no outward symptoms of any kind. It’s quite likely a lot of wild-caught fish are infected with spiny-headed worms, and provided the fish are well looked after, the spiny-headed worms don’t actually cause any real problems. Heavier infections cause lethargy and emaciation, but only a trained expert will be able to identify spiny-headed worms, whether by examination of the infected fish’s faeces or through dissection of a fish that has died under suspicious circumstances.

Prevention is the only way to deal with spiny-headed worms. Crustaceans are by far their most common initial host, freshwater shrimps and Cyclops being two known sources of infection On the other hand, their complex life cycle does mean that an infected fish is unlikely to infect its companions within the aquarium. Furthermore, brine shrimp should be completely safe, given the way that they are cultured involves hypersaline conditions that would quickly kill off spiny-headed worm eggs and larvae.

Because spiny-headed worms are not really worms, antihelminthic medications do not work against them.


Skin and gill flukes

These are small flatworms (platyhelminths) that live attached to the outside of their host. They are most often seen on wild-caught fish, perhaps most notoriously bichirs, and in the wild do seem to be quite ‘choosy’ about their hosts, each fluke species preferring to infect just a small number of host fish species, perhaps even just a single fish species. But under aquarium conditions flukes are sometimes more opportunistic, so it is a good idea to treat flukes as they appear to prevent them infected other types of fish in the tank.

The most commonly seen skin flukes are probably Gyrodactylus species, which are known to infect a wide range of hosts from guppies to goldfish.   Dactylogyrus is another genus of skin fluke widely seen on aquarium fish. Macrogyrodactylus flukes specific to bichirs have acquired a certain notoriety in recent years, being very common in the hobby, and easily spotted as small black flecks on the head and body of infected bichirs.

Most flukes are just about visible to the naked eye, at about 0.5 mm in length, more or less rectangular in shape, and when observed over a period of several days they will often be seen to have moved from place to place as they graze on mucous and skin cells. Infected fish will betray the presence of flukes by ‘flashing’, darting against rocks or plants in an attempt to scratch off the irritating flukes. Heavy infections may cause an abundance of mucous to be produced, resulting in the skin looking more cloudy than usual. If the gills are infected, the fish may ventilate heavily, flash, and generally show signs of respiratory distress, such as lethargy and subdued colouration. Heavy infections will also make the fish more vulnerable to secondary infections because of the damage done to their skin.

Positively identifying flukes requires examination of mucous or skin tissue under the microscope, which probably exceeds the abilities of most aquarists. Treatment is via antihelminthic medications, including praziquantel, mebendazole and flubendazole. Salt-tolerant fish species (such as mollies) can also be moved into strongly brackish or fully marine conditions for a week or two, which should kill these external parasites reliably.


Anchor worms

Anchor worms (Lernaea spp.) are primarily an issue for pondkeepers, though they are sometimes seen on aquarium fish as well. As noted earlier on, anchor worms are actually crustaceans. Their common name comes from their distinctive anchor-shaped body, the two ‘hooks’ of the anchor being egg sacs, while the central ‘shank’ is the main body of the animal. Once attached to their host, the anchor worms burrow into the skin, feeding on blood and skin tissues. These are quite big by parasite standards, getting to several mm in length, and easily visible to the naked eye. Interestingly, only the females are parasitic: the males are planktonic and die after mating.

Infected fish will often flash, and once the aquarist examines the fish more closely, the anchor worms should be apparent. The areas where the anchor worms have attached themselves often exhibit dead (white) or inflamed (orange-pink) tissue, and such areas can be prone to bacterial and fungal infections.

Anchor worms have a simple life cycle, the females producing larvae capable of infecting other fish directly without the need for an intermediate host. This means they can quickly spread throughout a population of fish, so prompt treatment is important.

It is possible to remove the anchor worms from large fish (such as koi) by hand, though the damage caused by doing this will make the fish more prone to secondary infections. Vets will usually apply some sort of topical antiseptic (such as iodine) to the wound afterwards, before returning the fish to the pond or aquarium, and aquarists should do likewise.

Physical removal of anchor worms is stressful for the fish and difficult to do, and cannot be safely done to fish smaller than koi or goldfish. So for most aquarists treating the pond or aquarium will be necessary. Commercial anchor worm treatments are available, most based on some sort of organophosphate chemical. These need to be used very carefully to be effective. Dosage is important, and because organophosphates break down quite quickly, they usually need to be used several times across a period of several days.

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