Fish Flukes: How to Recognize and Treat This Fish Disease

Dactylogyrus and Gyrodactylus are monogenean butterflies that live on the skin, fins, and sometimes gills of freshwater and marine fish. Unlike Lernaea and leeches (other worm-like parasites of fish that infest the skin), Gyrodactylus and Dactylogyrus are microscopic. Marine monogenetic trematodes are large enough to be seen with the naked eye. They are attached to the fish via a rostral hook attachment organ. This attachment to the skin causes small puncture wounds in the skin each time the hooks are attached. It is the wandering action of the mouth that causes damage. Gyrodactylus and Dactylogyrus can be distinguished according to their breeding methods. Some fish species, such as Discus, are particularly susceptible to gill parasite infections; where >3 butterflies on 5-10 gill filament biopsies are considered a heavy infestation.

The main difference between the two different types of chance is their mode of reproduction and therefore the way we manage and treat disease. Gyrodactylus are live-bearing parasites (that is, they produce live offspring) and so a single treatment will suffice. Dactylogyrus, on the other hand, are egg layers, and treatments never kill their eggs. Multiple treatments will be required to treat dactylogyrosis. Therefore, it is important to know who you are dealing with.

clinical signs

These parasites cause extensive skin and fin damage due to their attachment to hooks and feeding activities. Affected fish, present with increased mucus production, are often restless and irritated and may “shine” or splash out of the water. Chronic infections cause loss of appetite, which leads to weight loss. The skin often shows epithelial damage, which can be complicated by secondary bacterial (usually Aeromonas hydrophila) and fungal infections. Fins are often frayed. Gyrodactylus is occasionally found on the gills, but of particular importance here is the Dactylogyrus parasite. Known as ‘gill parasite’, they cause localized to extensive gill damage and heavy infestations can lead to respiratory failure.


Diagnosis can be made by microscopic observation of these monogenean butterflies obtained from skin scrapings or gill biopsies. Even fish that were dead a few hours ago can be examined for remnants of chance haptors (in the case of Gyrodactylus). The importance of distinguishing the two main butterfly species in freshwater fish is to determine if multiple treatments are required (egg-laying Dactylogyrus will require multiple treatments as the egg stage is resistant to treatment).

Sometimes dactylogyrosis is difficult to detect in wet preparation as the distal gill filament clips may miss organisms – gill butterflies tend to live in the proximal part of the gills. Sacrificing the fish for histology examination will help give you a more precise answer.


Some commonly used drugs effective against monogenetic trematodes include formalin, organophosphates (trichlorfon, fenthion), anthelmintics (praziquantel, fenbendazole), peroxide, and osmotic control.

This article is from Loh & Landos (2011) Fish Vetting Essentials, Richmond Loh Publishing, Perth.

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