How do I deal with aquarium flukes?
Please could you advise me on the best treatment for flukes in a tropical aquarium?
GARY VINCENT
PETER REPLIES: Flukes are multicelled parasites comprising many hundreds of species. Some parasitise mammals; others specifically affect birds, reptiles or amphibians, and numerous species affect fish. The two groups of flukes that are of concern to fishkeepers are the monogenean flukes and the digenean flukes.
Monogenean flukes: These include the surface-dwelling skin- and gill-flukes, such as Gyrodactylus and
Dactylogyrus species. Key general features of monogeneans are their small size, with adult flukes ranging between 0.2-2mm depending on the species, hence mostly invisible to the naked eye, and their ‘direct’ life cycle, meaning they spread directly from fish to fish. Most monogeneans are egg-layers, but the skin-dwelling gryodactylids give birth to (usually pregnant) live young. Monogeneans can affect both coldwater and tropical freshwater fish. Low numbers may cause no obvious signs, but heavy infestations can cause severe damage to the skin and/or gills, resulting in death. Badly affected fish exhibit signs of skin and/or gill irritation, manifesting as body-rubbing or fast gill beats.
Digenean flukes: Key features of these flukes are their larger size and their internal parasitic lifestyle. Unlike the surface-dwelling monogeneans, digenean flukes are found inside the bodies of fish, such as within the skin, gut, muscles, liver, swimbladder, eyes or blood, depending on the species of digenean involved. Some digeneans occur as the adult flukes within the fish—such as the blood flukes—others as larvae, with the latter typically encysted within the fish’s tissues. Digeneans have an ‘indirect’ life cycle, meaning they cannot spread directly from fish to fish but instead need to pass through two or three different types of hosts, such as birds and aquatic snails—notably lymnaeid and planorbid snails—to complete each generation.
Another feature of digenean flukes is they are less likely to cause significant harm or death to their host (although eye flukes can cause visual impairment and even blindness). So, in most cases
it isn’t necessary to treat fish with digenetic flukes, and, if your aquarium happens to be snail-free, they cannot spread from fish to fish. Even if a chemical treatment is administered, bear in mind that larval flukes are trapped within the fish, risking complications and possibly even a bacterial infection as the dead and dying flukes decompose within the fish’s body. In rare circumstances, however, it may be appropriate to use a treatment such as praziquantel for fish that are harbouring adult digenean flukes in their blood or gut.
To eradicate skin- and gill-parasitic monogenean flukes, the best treatments are certain ‘wormer’ drugs known as anthelmintics. These include praziquantel and flubendazole, which cause intense muscular spasms in flukes and alter their calcium balance, so the flukes lose their grip on the fish and eventually die. Anthelmintics can also be used to eradicate tapeworms and other gut worms of fish.
A number of commercial anthelminticbased remedies are available from your fish store, such as NT Labs’ Anti-Fluke & Wormer which uses flubendazole, and Waterlife’s Fluke Free which uses praziquantel. Some of these drugs may harm aquatic invertebrates, so it may be necessary to remove any snails or shrimps from the tank before treatment. You may have to re-dose the tank one or more times, particularly in the case of the egg-laying monogeneans, as the egg-stage, which can last several days, is resilient to chemical treatments.
Salt (sodium chloride) has also been advocated as a treatment for monogeans, but some species of fluke are more salt-tolerant than others. One study showed that even a relatively high salt level of 7g per litre caused a reduction – but not total elimination – of
Gyrodactylus bullatarudis on guppies. Many freshwater tropicals won’t easily tolerate such high salt levels.
You may also see formalin and potassium permanganate advocated for killing flukes, but these are harsh chemicals when used at therapeutic levels, so they wouldn’t be my first choice of action.