Knowing Parasite Worms in Fish

As anyone who has tried to maintain a fresh or saltwater aquarium is certainly aware. Fish in captivity are subject to a myriad of health problems. In addition to the often discussed external pathogens, such as protozoa, bacteria, trematodes, etc. Fish may also host internal infestation of parasites. One of a lot type parasites that may cause problems in fish is the worms. Such as the roundworms (nematodes), tapeworms (cestodes), thorny-headed worms (acanthocephalans), and flukes (digeneans). Most of these worms do not pose a serious health risk to the fish because they often have complicated life cycles, in which the fish may serve as only one of possibly several intermediate hosts. But, there also parasite worms in fish that may cause problems. So take your time and read carefully.

A fish with internal infestation of parasites worms may showing completely normal, healthy, exhibiting no symptoms of infestation. Nematodes inside the body to musculature for instance. Do not impair a fish’s maneuverability when they are present in small numbers. (They may, however, prove unsightly and unappetizing to the fish market patron!). Another apparently benign association involves the flashlight fish. Most flashlight fish mortality, upon necropsy (autopsy) are found to harbor a pair of degenetic trematodes in the gall bladder. This relationship has not been investigated, but the trematodes do not seem to cause the host any great harm.

Other worms can threaten a fish’s well being and do warrant treatment by the home aquarist. The challenge is diagnosis, not always a simple task. Juvenile and adult worms may be observed in the course of a necropsy. But ideally the aquarist would like to identify the problem before mortality occur. Usual spot of internal infestation include nematodes coiled in the mesenteries or musculature. Cestodes in the gut or inside other organs, acanthocephalans in the gut. Or boring through the intestinal wall, and trematodes in the gut, organs, blood, gills, etc. After a problem has been identified through necropsy. It’s important to ensure if the mortality was the only fish harboring worms or if all fish in the tank are infected.

Considering the worms complicated life cycles, most infested fish must come in contact with the worms before they get into the pet trade. Since the necessary intermediate hosts are not usually found in the tank along with the fish, transmission of worms between fish within a hobbyist’s tank does not readily occur. If several fish of the same species were obtained as a group, it is usually assumed that the survivors are also carriers and they are treated accordingly.

The next most direct method of diagnosis, if observation of worms during a necropsy is not possible, is finding adult worms, larvae, or eggs in the feces. Microscopic examination of feces is not a routine procedure nor is it a very easy one. The aquarist may try this procedure if experience, through necropsies, has shown that a particular species typically has intestinal worms of if the individual fish exhibits external symptoms of infestation (discussed below).

In order to secure a fecal sample, the fish is best kept in a scrupulously clean, bare-bottomed tank in order to see the dropped feces. Also, the fish must be eating and thus producing feces, and the aquarist must be very lucky to be looking in the right place at the right time. Not all fish produce pelleted feces and the waste material can break down fairly rapidly, so this technique may not always be possible.

A third method of diagnosing a problem with worms is indirect and, at best, just a guess. When a fish is eating well yet is still not putting on weight, and intestinal infestation may be suspected. This is particularly when a fish eats regularly yet actually looses weight, metabolizing body musculature to stay alive. This is usually seen as thinning along the back on either side of the dorsal fin. In an extreme case this may result in a well-fed fish starving to death.

Treatment of worms has not been well studied and medications and dosages must often be extrapolated from veterinarian practices. This can result in some rather broad generalizations and assumptions that are difficult to substantiate. The list of treatments discussed here is by no means exhaustive but merely some of the things tried at the John G. Shedd Aquarium in Chicago.

As with treatment of fish diseases, medications for treating parasite worms in fish can be administered in one of three forms: as a bath, in treated food, or by injection. Powdered medications or capsules can be dissolved and added directly to the tank. Dosages are often listed with parts per million (ppm) as the units. This is the same as milligrams per liter (mg/l), although this still may not be a convenient measurement for the hobbyist.

A stock solution of medication can be prepared by dissolving 75 grams (g) active ingredient in one liter (L) of water. One drop of this solution in a gallon will yield one ppm. As an example, to treat a 50 gallon tank at 25 ppm (with a solution that produces one ppm at one drop per gallon) requires 50 gallons x 25 ppm= 1.250 drops. Don’t panic! it is not necessary to count drops all day long. Twenty drops equals one milliliter (ml), so 1,250 drops/20 =62.5 milliliter = approximately 12.5 teaspoons (tsp.).

If the fish to be treated is eating well, it is often easier to medicate the food. For large fish this may simply require slipping a tablet or a gelatin capsule containing the medication into a piece of cut food , such as smelt tail. Deworming medications often do not well in water. But a suspension can sometimes be injected with a hypodermic needle into a piece of cut food. For dosing one ml. of the stock solution mentioned above contains 75 mg of active ingredient.

Medications can also be applied to pellets or ground food. The proper amount of medication can be mixed with the water and the pellets soaded in the solution. This dosing is not precise, but at least some medication makes its way into the animals. With practice, the amount of water used to prepare the suspension can be reduced to just enough to be completely absorbed by the pellets. A common rule of thumb is that a healthy fish eats 3% of its body weight per day. So that quantity of food will have to contain the prescribed dose for that fish. Dosages are often listed as milligram per kilogram (mg/kg) body weight.

One way to estimate the weight of a fish is to consider how its size compares with that of a quarter pound hamburger (precooked weight). A quarter pound is about 100 g (= 0.10 kg). Mixing the medication with pellets or ground food allows for easier treatment of many fish at a time. That way individual capsules do not have to be weighed for each patient. This technique of soaking the food in medication does not work well with flaked food.

As a last resort, at the Shedd Aquarium. We may force feeding a fish that refused to eat, for whatever reason, and is in danger of starvation. This requires anaesthetizing the fish. Putting a tube down its throat and pumping in a slurry of food and medication with a syringe and catheter tube. The material and expertise required for force feeding generally precludes its use by the home aquarist.


Several medications have been tried with varying results. Praziquantel (for example, Droncit) is routinely used orally as a canine and feline dewormer. It has also been used as a bath against mono genetic trematodes in fish. The dose is 250 mg/L added once to the tank and left in for three days. It has been suggested that praziquantel in water may make its way into the bloodstream across the gills. But this has not been proven and does not appear to be a very efficient way of treating internal problems.

Oral dewormers include praziquantel, piperazine (for example, Pipfuge) and fenbendazole (for example, Panacur). All of them are commonly used in veterinarian work. The dosage rate for praziquantel in canines is one half of a 34 mg tablet for dogs under 10 kg and one 34 mg tablet for dogs up to 20 kg. Such a wide re a range suggests this treatment is best used on large fish (one kg and larger) to avoid overdosing. In establishing the dosage, researchers maybe did not consider treating 110 gr fish or smaller animals, (a more realistic weight for aquarium fish!). Praziquantel is not a hard medicine so it easy to dose and is fairly easy to administer in large fish. We have had good success with embedding half of a tablet in a piece of cut smelt and offering it to the infested fish.

Fenbendazole is used in horses against roundworms. A typical recommendation is one 5 gr packet/500 lb. A difficult question to answer is how many angelfish make up one horse-equivalent. This borders on the ridiculous when trying to weigh out medication for one fish. This treatment would be much more practically administered mixed in a large batch of treated food distributed to several fish.

Praziquantel is available as an injectable medication. But we at the Shedd Aquarium have had no experience using this form against worms. The treatment of parasite worms in fish is not an exact science. Most of the medications were not developed specifically for fish nor has their application to fish hosts been well studied. However, when even circumstantial evidence indicates a worm infestation, the hobbyist does have some treatment options. The protocols mentioned here may be worth experimenting with using the above mentioned medications. Or with other drugs as they u become available.

By: James A. Anderson
Assistant Curator of Fishes
John G. Shedd Aquarium, Chicago

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