Monogeneans are parasitic worms in the phylum Platyhelminthes. Previously they were considered a subclass of Trematoda, but now are considered a class of their own. There are several orders of monogeneans, with numerous families and genera. Estimates for number of species are in the tens of thousands. Why so many? Because there are tens of thousands of fish species, and monogeneans love fish and have been evolving with them for millions of years.

Monogeneans get their name by the fact that they have a direct life cycle. That is, they only require one host. They are hermaphroditic and can be viviparous or oviparous, and most are ectoparasites that have specific host preferences. Many also have preferences for specific host tissues, and some are even picky about microhabitats within a tissue. For example, Gyrodactylus salaris prefers skin over gills, and specifically prefers fins over skin on the body. Monogeneans have different dietary preferences too. Monopisthocotylea eat mucus and epithelial cells, while Polyopisthocotylea prefer a diet of blood.

In addition to having a direct life cycle and parasitizing fish, monogeneans are characterized by having opisthaptors and prohaptors. The opisthaptor is the main adhesive organ that monogeneans use to stay attached to their host. It is located on the posterior end and can be made of hooks, suckers, or clamps. In monopisthocotylids, the opisthaptor is a single unit composed of hooks, while in polyopisthocotylids, the opisthaptor consists of a series or clamps that pinch the host’s gill lamellae. Prohaptors are on the anterior end and are also used for attachment, but are not as impressive morphologically as opisthaptors. Often the prohaptor is simply glands that secrete sticky materials that help the monogenean attach to its host.

Polyopisthocotyleans, the blood feeding monogeneans, can cause anemia in their hosts. These fish tend to be lethargic, anorexic, and have pale gills, livers, and kidneys. Polyopisthocotylean infections can significantly lower hematocrit and serum protein levels, and leave the fish very susceptible to secondary infections. Monopisthocotyleans, who graze on epithelium and mucus, are pathogenic by destroying epithelium and inducing hyperplastic reactions. Hyperplasia is especially detrimental in the gills, where it results in less surface area for gas exchange and waste excretion. Like their blood-feeding cousins, monopisthocotyleans leave their hosts vulnerable to secondary infections. Furthermore, monogeneans may actually act as vectors for bacteria and viruses by introducing infectious microbes when they pierce epithelial cells. In general, however, monogeneans on their native hosts do not threaten their health unless the parasite load is very heavy or the fish is stressed by poor water quality, crowding, or handling.

Monogeneans that find suitable homes on non-native hosts can be very detrimental. In the Aral Sea, the monogenean Nitzschia sturionis was introduced with the stellate sturgeon from the Caspian Sea. N. sturionis found a suitable host in the native spiny sturgeon and cased mass mortalities of the population. Similarly, Gyrodactylus salaris was introduced into Norwegian rivers with the Baltic salmon. As a result, fry of the native Atlantic salmon were decimated.

Aquaculture and display aquaria can also favor conditions for monogenean survival and proliferation, which can result in high morbidity and mortality of the fish. In a crowded, stressful environment, such as an aquaculture facility, even fish that are naturally resistant to a given monogenean can succumb. In the aquarium trade, the stress of shipping, distribution to retailers, and conditions in pet stores can give monogeneans an edge over their hosts. Monogeneans of the genera Dactylogyrus and Gyrodactylus appear to be the most prevalent parasites of ornamental fish originating from Asia, especially in Cyprinids.

Clinically, monogeneans are easy to recognize at 40 x combined magnification from a skin or gill scrape. Dactylogyrus can be distinguished from Gyrodactylus by the presence of four eye spots on the anterior end of the worm. Treatment of monogeneans requires an understanding of the lifecycle of the genus in question. As mentioned above, Dactylogyrus and Gyrodactylus are common parasites of ornamental fish, and both are susceptible to similar dewormers. However, these genera have different reproductive strategies with Dactylogyrus laying eggs and Gyrodactylus being viviparous. Being viviparous, it is possible to eradicate Gyrodactylus infestation with a single drug treatment. In the case of Dactylogyrus, however, multiple treatments are required as the eggs are not as susceptible to dewormers.

Monogenean treatments are further complicated by incredible diversity of species, even within just the genera Dactylogyrus and Gyrodactylus alone. These different species can have different susceptibilities to medications. Therefore, efficacy of treatments should be confirmed by follow up skin and/or gill scrapes. Organophosphates, such as Trichlorfon, can be very effective against several monogeneans. However, the development of parasite resistance to organophosphates has been seen on farms after regular use. Also, organophosphates are neurotoxic to both fish and humans. Formalin can be effective against monogeneans, but can also deplete dissolved oxygen in the water and some fish are especially sensitive to it. Other chemicals used to control monogenean infections are copper sulfate, hydrogen peroxide, and potassium permanganate.

The antihelminth praziquantel has been shown to be toxic to Gyrodactylus and Dactylogyrus and safe for most fish. Praziquantel can negatively effect fish at high concentrations that will rapidly kill monogeneans, however, it can be very effective as long term baths at low concentrations. Praziquantel has been shown to cause vacuolisation in the tegument and contractile paralysis of Dactylogyrus and Gyrodactylus. In one study, no Gyrodactylus were found on previously infected sticklebacks after 16 hours in 10 mg/L praziquantel. For ornamental fish, current recommendations are to dose 2-3 mg/L weekly for Dactylogyrus. One dose left in the aquarium water may be sufficient for Gyrodactylus.

Mebendazole is another antihelminthic that can be effective against monogeneans at low concentrations for long term baths. However, the toxicity of mebendazole to different fish species varies considerably, even in closely related species. Mebendazole is well tolerated by koi carp, Cyprinus carpio, but is toxic to goldfish, Carassus auratus. Regardless of what treatment is used, it is critical to maintain excellent water quality and good nutrition in order for the fish to heal from lesions and prevent re-infection.

Fluke pictures taken by Jagger (Graham)

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Animated gifs put together by Jagger (Graham)

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