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		Monogenean Trematodes
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		TRANSMISSION Transmission of monogenean flukes from fish to fish is primarily by direct contact. Monogeneans tend to have direct life cycles, which means that no intermediate host is required for the parasite to reproduce ( Figure 3 ). Adults are hermaphroditic which means that each organism has both male and female reproductive structures, oviparous monogeneans release eggs into the water which hatch and mature into adult forms. Viviparous monogeneans release live larvae which may attach to the same host as the parent or be carried by the water to another. The direct life cycle can contribute to population explosions in aquaculture systems, resulting in clinical disease. Figure 3: Oviparous monogenes (i.e., Dactylogyridae) release eggs into the water column which hatch and mature prior to seeking a new host. Viviparous monogenes (i.e., Gyrodactylidae) release live larvae which are immediately able to attach to host tissue. See Figure 3 . There are two common genera of freshwater flukes, Gyrodactylus and Dactylogyrus, which differ markedly in their reproductive strategies as well as their preferred attachment sites on host fish. Gyrodactylus are generally found on the body and fins of fish. They are viviparous which means that they produce live young. Each individual parasite has both male and female reproductive organs. Adults parasites carry a fully developed embryo (identical to the adult) which in turn, carry young of the next generation. Therefore, each individual parasite may represent several generations. This reproductive strategy allows populations of Gyrodactylus to multiply very quickly, particularly in a closed system where water exchange is minimal. Members of the genus Dactylogyrus prefer to attach to gills of host fish. Unlike Gyrodactylus, parasites in the genus Dactylogyrus attach to gills and are egg layers. The eggs can be resilient to chemical treatment which make the use of multiple treatments appropriate to control this group of organisms. When the eggs hatch, free-swimming larvae emerge which are ciliated and which are carried to a new host by water currents as well as by their own movement. The time required for maturation of Dactylogyrus from eggs to adult is temperature dependent. At water temperatures of 72--75°F only a few days are required for completion of the life cycle, whereas at cooler temperatures (i.e., 34--36°F), generation time is extended to five or six months. MANAGEMENT The best way to manage monogeneans is to avoid introducing parasites to a new system. This can be done by dipping fish prior to placing them into the system, and following a quarantine protocol whenever feasible. If quarantine is not possible, a simple way to minimize introduction of monogeneans, as well as other external parasites, is to dip fish in fresh or salt water, depending on species. Salt water fish can be dipped in freshwater which will eliminate many single-celled external parasites, and freshwater fish can be dipped in sea water to accomplish the same goal. This practice will not completely eliminate the risk of introducing parasites to an established tank or system, but guarantee that no parasites will be introduced with new fish, but will help minimize the numbers brought in. Ideally, fish should be quarantined for at least three weeks prior to being placed into a new system. While in quarantine, gill and skin biopsies can be performed to determine whether monogeneans, or other gill or skin pathogens are present. Any parasites which are identified using biopsy techniques can then be specifically treated and eliminated. If biopsies cannot be done, then prophylactic treatment with a broad spectrum parasiticide, such as formalin or potassium permanganate, should be carried out. A quarantine system should be very simple so that fish are readily accessible for observation and handling, water can be easily changed, and treatments readily administered. Treatment of monogeneans is usually not satisfactory unless the primary cause of increased fluke populations is found and alleviated. The treatment of choice for freshwater fishes is formalin, administered as a prolonged bath at 25 mg/L or a short term bath at 175--250 mg/L for 30--60 minutes. Fish which are very sick do not tolerate formalin well and all fish should be carefully watched during chemical administration (see IFAS Extension Fact Sheet VM-77 ). If adverse reaction is observed, fish should be removed from the treatment tank at once and placed in clean water. Potassium permanganate is also effective against monogeneans, and is the treatment of choice if columnaris bacteria or fungi are invading damaged tissues. Potassium permanganate can be administered as a prolonged bath at a concentration of 2 mg/L or as a short-term bath (30--60 minutes) at a concentration of 10 mg/L. Again, fish must be observed carefully while they are in contact with the chemical and they should be removed at once if adverse effects are noticed (see IFAS Extension Fact Sheet FA-23 ). Freshwater dips should be given to saltwater fish prior to placing them into a new system. This is very helpful in preventing the initial introduction of monogeneans. Once established, however, copper treatments applied at 0.2 mg/L active copper ion for up to three weeks are helpful in controlling Neobenedinia , however complete elimination of the parasites from the system is difficult. Certain marine fish, particularly sharks and rays, do not tolerate copper well, and it is lethal to invertebrates. more ...

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