Trematode parasites (flukes or flatworms) come in two main groups: the monogeneans, which have a direct life cycle and live on a single host, and the digeneans, which have indirect and often very complex life cycles with one or more intermediate hosts. The digeneans include the yellow grub, black spot, white grub and eye fluke and are the topics of a separate article.

Monogenean flukes are primarily parasites of gills, skin and the oral cavity, although a few occur internally in nasal cavities, intestine, ovipositor and urinary tract. They are quite small, with the largest being up to four millimeters (1/10 of an inch) in length, but generally less than one to two millimeters. Therefore, they are most readily diagnosed with low-power magnification of skin scrapings and gill biopsies.

Most monogeneans are browsers, moving about the body surfaces, and feeding on dermal (skin) mucus and gill debris. Monogeneans have a series of hooks that enable them to attach while feeding. Most species are host- and site-specific, requiring only one host to complete an entire life cycle. In fact, some adults will remain permanently attached to a single site on the host.

What To Watch For

Monogenean flukes cause irritation to the skin and gills. This results in the following signs:

The flukes can create sufficient damage to allow secondary bacterial infections or may transmit some bacteria themselves. Heavy infestations of flukes often indicate poor water quality (e.g., overcrowding, high ammonia, nitrite, organic load). The flukes can reproduce rapidly, with doubling time of live-bearing flukes potentially as short as twenty-four hours under optimal conditions.


Clinical signs of cloudy skin, eroded fins, rapid respirations (gilling), flashing or red skin lesions may suggest fluke infestation, but definitive diagnosis requires microscopic examination of samples taken from an affected fish. Your veterinarian can safely obtain small samples from the skin and gills (skin scraping, gill biopsy) from a fish sedated for a brief period.

The presence of only one or two flukes in a sample from an otherwise healthy fish may be of minimal significance to that fish, but does indicate the presence of a potential problem if water conditions deteriorate, if the fish becomes debilitated for some other cause, or if the fish is intended for introduction into a previously fluke-free aquarium or pond.


Many treatments or combinations of treatments have been used for external monogenean trematodes, including: baths or dips with praziquantel (Droncit®), formalin, freshwater/saltwater, acetic acid, organophosphates, potassium permanganate, Chloramine-T and copper. Because of the many species involved, no single mode of treatment can be expected to work for all fluke infestations.

Organophosphates, including trichlorfon (available as the active ingredient of some cattle dips) and its breakdown product dichlorvos, are generally effective and economical to apply, but resistance commonly develops.

Praziquantel is effective for many fluke species but rather expensive for even moderate volumes of water.
In many instances, a reasonable goal is control, rather than eradication, of the flukes. Osmotic treatments (freshwater dips for saltwater fish, and vice versa) are more effective for small flukes than for larger flukes, and are not very effective at all for estuary species. Treatment of egg-laying flukes requires multiple or prolonged applications to kill larvae as they emerge from the resistant eggs. Weekly treatments for at least 3 weeks are recommended. Your veterinarian can help you design a treatment plan.

Home Care

Management practices and home care are just as important as the application of a chemical or drug treatment in the control of flukes. Partial water changes are an important part of therapy by improving water quality and reducing loads of free-swimming parasite larvae that can re-infect the fish. Aerate the treatment water well, as some treatments may interfere with normal efficiency of respiration.

Changing out and/or disinfecting the gravel and ornaments can reduce the egg burden in cases of egg-laying flukes. Another option is to remove the fish to a treatment tank and let the regular pond or aquarium lie fallow, as the adults cannot survive more than two weeks off the host. Reducing the density of fish in the pond or aquarium can reduce the parasite load disproportionately, by decreasing transmission efficiency. Work with your veterinarian to monitor effectiveness of treatment or re-infestation.

Handle the chemical or drug used for treatment with care and according to directions, as some (e.g., formalin, organophosphates) are potentially hazardous or irritating to humans. The chemicals used should be pre-mixed to a slightly more dilute form before addition to the treatment water to prevent exposing the fish to "hot spots" of high concentration.

Preventative Care

Maintain good water quality and nutrition, as debilitated fish are less able to resist the effects of fluke infestation than otherwise healthy fish.

Once flukes infest a pond or aquarium, complete elimination can be difficult, so preventing introduction in the first place is preferable. A quarantine of at least thirty days can avoid major headaches and expense later. It is tempting to introduce a new fish in with your other fish right away, but resist the temptation and observe it for a period for signs of developing illness.

During the quarantine you can have your veterinarian perform pre-entry screening to reduce the chances of introducing flukes or other parasites into your collection. Even if the fish comes up clean on the gill biopsy and skin scraping, it may be advisable to perform a five-minute precautionary dip with saltwater (or freshwater for a marine fish), possibly in combination with formalin, just prior to introducing the fish to its intended home. This can cause the parasites to fall off the fish even if they are not killed, reducing the odds of parasite transfer.