Gill Flukes

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Some of the material on this page is the same as on the pages for Gyrodactylus, the skin flukes. Information found there may be applicable to gill flukes as well.

This is typical appearance for a gill fluke such as Dactylogyrus vastator at about 0.2 mm length. Note the four pigment spots on the four-lobed anterior. Hooks are not especially prominent on this specimen.

Dactylogyrus et al.

Phylum: Platyhelminthes
Class: Monogenea

Previously these were listed in the class Trematoda and are often still referred to as monogenetic trematodes. Commonly we refer to monogenetic and digenetic flukes (the Digenea still considered in the class Trematoda), and divide the monogeneans into skin flukes and gill flukes. Gill flukes are sometimes called “ Dactyls” for short. While Dactylogyrus vastator is the most well-known of the gill flukes there are several other genera and many species involved. Here I will refer to them simply as Gill Flukes. To add a bit to the confusion there are some flukes of the Digenea that sometimes are found encysted in the gills of fish. I will refer to those separately.
Large view of Dactylogyrus from a Silver Dollar (Metynnis sp.).

Gill flukes parasitize freshwater, marine, and brackishwater fish. They cause great numbers of mortalities in aquaculture and in the ornamental fish industry, being found on fish from the wild as well as farmed fish. These parasites attach to the gills of the fish and feed on mucus, epithelial cells, and blood from the host, much of this material being produced by the fish in response to the physical injury incurred from the parasites. Damage is physical and inflammatory, with secondary bacterial infections including septicemia. Small fish may die from the stress of acute infections. Chronic infections may result in ematiation and poor growth.
Wild-caught species that are common carriers of gill flukes: Plecostomus, Otocinclus and other Loricariids, Characins, Corydoras, and Elephant Nose (Gnathonemus petersi).
Captive raised fish that are common carriers: Koi, Discus, livebearers (Poeciliidae), some tetras (Characins) and Barbs (Cyprinidae).
Imported Clown Knife Fish (Notopterus) are usually infected with gill flukes as are many tropical brackish water fish such as Scatophagus. Morphological differences between types of gill flukes found on various fishes are noteworthy for comparing effectiveness of medications and the host range of the species of parasite.

Gill flukes in a gill sample from a Black Skirt Tetra, Gymncorymbus ternetzi. The fish measured 15 mm total length.

Diagnosis:
The first step in diagnosis is to ask if you are dealing with a species of fish in which gill flukes are a common problem. Distention of the operculum is sometimes noticed due to gill swelling. Chronically infected individuals may appear ematiated. In small fish mortalities may arise early in the infection and the parasites observed microscopically in the gills. If gills of dead fish have decayed too badly the parasites will have moved on, but frozen samples of gill may show evidence of infection such as hook sets and cell debris consistent with having been a parasite in proximity of the hooks. Occassionally it is suggested that gill samples be removed from fish for examination, but this should be done on large fish specimens if at all. Alternatively a short bath in a solution of Acriflavin or other appropriate medication will often facilitate the release of some parasites from the host and these or their hooks can later be seen in samples of settled sediment from the treatment container. Examination for gill flukes is best done on samples of gill from recently deceased fish. In necropsy they may be hard to spot despite their relatively large size. It helps to be on the look out for their movement when they contract or expand. The parasites are best seen when gill filaments have been separated from the gill arch and flattened somewhat under the cover slip. Bear in mind that such preparation can distort the shape of the organism slightly. Identifying characteristics are hooks, if present, and the pigment spots (usually four in Dactylogyrids).
The Dactylogyrids and other gill flukes reproduce via eggs and these eggs can sometimes be found in the gills of fish. Care should be taken not to mistake other organisms (such as Oodinium) for gill fluke eggs or vice versa. The attachment stem is characteristic of fluke eggs.
The presence of only a few gill flukes in the gills of a moribund fish may be enough to establish cause of death if there is associated trauma, gill damage, mucous production, or septicemia. Other causes should not be dismissed out of hand, of course. In aquaria any detection of gill flukes is considered cause for therapeutic treatment. In pond fish the numbers of organisms is to be considered as well as their apparent impact and environmental factors such as organic load, temperature, and size of fish population. In some cases it may be advisable to remove and treat affected individuals in order to merely reduce the parasite load. The egg-laying aspect of the fluke life cycle makes erradication difficult in many such situations.

Egg of a gill fluke with attachment base evident.

Gill fluke, about 0.4 mm, from a Koi.

Page two shows some variety in gill flukes commonly seen.

A small young gill fluke from a Discus.

Treatment:
Most frequently flukes are treated with organophosphates: Trichlorphon, Dylox, Fluke Tabs®, and Lifebearer®. These are usually effective in treating gill flukes on wild-caught fish, but gill flukes on captive raised fish may have developed resistance to organophosphates.
Addition of salt to the water is only moderately effective when treating organisms on freshwater fishes. Salt dips (strong salt solution for a period of one minute or less) are usually effective in removing the parasites.
Formalin baths are effective if done accurately. Long-term formalin baths of two drops per gallon standard formalin may or may not be effective. Acriflavin has been found useful in causing the parasites to detach from the gills when used in sufficient strength and this should be considered as a treatment or a prophylactic when dealing with species of fish which are sensitve to other treatments. Metynnis species, for instance are sensitive to the organophosphates, and formalin should never be used on the Elephant Nose or other Mormyrids.
Dactylogyrus and their eggs are known to be killed by drying and this can be helpful in controlling the spread of the disease. Ponds are often dried out to rid them of the parasites, and nets and other equipment can be air dried to help keep transmission low.

The most immediate and perhaps most effective treatment is the use of a potassium permanganate dip (KMnO4) which I will not describe here except to recommend this site: http://edis.ifas.ufl.edu/FA032 . The use of 2ppm potassium permanganate as an extended bath is sometimes recommended and its effectiveness depends partly on water quality and substrate conditions as well as filtration. Fish species differ in their sensitivity to potassium permanganate, goldfish and koi handling it quite well. Hydrogen peroxide is handy for neutralizing potassium permanganate if necessary.
In marine aquaria copper compounds are generally used to control gill flukes.
There are a number of new anthelminthic drugs which may be useful in treating these parasites. Flubendazol (5% Flubenol) reportedly gives excellent results. Praziquantel (Droncit) is also said to work well.

Other gill fluke genera: Angyrocephalus, Cichlidogyrus, Cleidodiscus, Monocoelium, Neodactylogyrus, Urocleidus

Page two: More gill flukes.

Pathogens List

Gyrodactylus, skin flukes.