Monogenean Parasites of Fish 1
Peggy Reed, Ruth Francis-Floyd, and RuthEllen Klinger2
Introduction
Monogeneans (flukes) are a group of parasites best described as flatworms.
Monogeneans are commonly found on the gills, skin or fins of fishes and lower
aquatic invertebrates. A few may invade the rectal cavity, ureter, body cavity
and even the blood vascular system. There are more than 100 families of
monogeneans found on fishes of the world, in fresh and salt water, and at a
variety of temperatures.
Most monogeneans are browsers, moving about the body surface and feeding on
dermal (skin) mucus and gill debris. Monogeneans have a series of hooks that
enable them to attach while feeding (
Figure 1 ). Most species are host- and site-specific, requiring only one
host to complete an entire life cycle. In fact, some adult monogeneans will
remain permanently attached to a single site on the host.
Morbidity and mortality epidemics in cultured fish caused by excessive
parasite loads are associated with crowding, inadequate sanitation and
deterioration of water quality. Although monogeneans are commonly found on
wild fish, they are rarely a direct cause of disease or death in free-ranging
populations.
Dactylogyrus is usually attached to the gills of freshwater fish. It
reproduces by laying eggs, which are often resistant to chemical treatment,
therefore weekly treatment over a period of 34 weeks is recommended.
Gyrodactylus is usually found on the skin and fins of freshwater
fish and produces live young, so one treatment may be adequate to control an
infestation.
Benedeniella is a large monogene that can cause chronic problems in
marine systems and is difficult to eliminate from a system once established.
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Figure 1. Diagrams of three common genera of monogeneans.
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Clinical Signs and Effects
Freshwater fish infested with skin-inhabiting flukes become lethargic, swim
near the surface, seek the sides of the pond and their appetite dwindles. They
may be seen rubbing the bottom or sides of the holding facility (flashing).
The skin, where the flukes are attached, shows areas of scale loss and may
ooze a pinkish serous fluid. Heavy gill infestations (
Figure 2 ) result in respiratory disease. Gills may be swollen and pale,
respiration rate may be increased, and fish will be less tolerant of low
oxygen conditions. "Piping," gulping air at the water surface, may be observed
in fish with severe respiratory distress. Large numbers of monogeneans on
either the skin or gills may result in significant damage and mortality.
Secondary infection by bacteria and fungus is common on tissue that has been
damaged by monogeneans.
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Figure 2. A heavy infestation of mongeneans on the gills of a discus
fish (100x magnification).
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In salt water fish, sharks, skates and rays, the monogenean,
Neobenedinia spp., may infest the skin and gills, resulting in extreme
irritation to the host. Sharks with heavy infestations swim erratically, and
exhibit behaviour such as flashing and rubbing on the bottom of the tank. Gray
patches and open wounds may appear on the skin. Ulcerated skin lesions are
susceptible to secondary bacterial infections, which may result in mortality.
Affected gills may become irritated, hemorrhaged and swollen. Sand grains may
stick to the gills as infested sharks suck in sand in an attempt to rub off
the parasites.
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 monogenes (i.e.,
Dactylogyridae) release eggs into the water column that hatch and mature prior
to seeking a new host. Viviparous monogenes (i.e., Gyrodactylidae) release
live larvae that are immediately able to attach to the same host as the parent
or be carried by the water to another host. The direct life cycle can
contribute to population explosions in aquaculture systems, resulting in
clinical disease.
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Figure 3. Adult monogeneans attach to fish tissue and reproduce by a
direct life cycle.
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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
viviparous (produce live young), and are generally found on the body and fins
of fish. Each individual parasite has both male and female reproductive
organs. Adult parasites carry fully developed embryos (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
are egg layers. The eggs can be resilient to chemical treatment, therefore
multiple treatments (1 dose per week) are appropriate to control this group of
organisms. When the free-swimming ciliated larvae emerge from the eggs they
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 water temperatures of
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 by following a quarantine protocol whenever
feasible. If quarantine is not possible, a simple way to minimize the
introduction of monogeneans, as well as other external parasites, is to dip
fish in fresh or salt water, depending on the fish 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 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 identified using biopsy techniques can then be
specifically treated and eliminated. If biopsies cannot be done, 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 150-250 mg/L for 30 minutes. Sick fish do not
tolerate formalin well and all fish should be carefully watched during
chemical administration (see UF/IFAS Extension Fact Sheet VM-77 Use of
Formalin to Control Fish Parasites). 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 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. For further information see UF/IFAS Extension Fact Sheets
FA-23 The Use of Potassium Permanganate in Fish Ponds and FA-37 Use
of Potassium Permanganate to Control External Infections of Ornamental Fish.
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 monogeneans are established in a system, complete
elimination is difficult. However, copper treatments applied at 0.2 mg/L
active copper ion for up to three weeks are helpful in controlling certain
flukes, such as Neobenedinia. Certain marine fish, particularly sharks
and rays, do not tolerate copper well, and it is lethal to invertebrates.
Summary
Monogeneans are found on fresh and salt water fishes throughout the world.
They have a direct life cycle and can reproduce in a wide range of
temperatures. Monogeneans have a series of hooks that attach to the fish
causing irritation, excessive mucus production, and which create an open
window for bacterial invasion. A few flukes on a healthy mature fish are not
usually significant, however, moderate numbers on a young fish can cause
significant mortalities. When fish are crowded or sanitation practices are
poor, the potential damage from monogeneans is greater. Prevention of
monogenean infestations by following appropriate quarantine practices is
preferable to treating the parasites after they have become established in a
system. Formalin and potassium permanganate baths are effective for
controlling monogenean infestations in freshwater fish, and copper is often
used to control infestations of marine fish.
Footnotes
1. This document is FA-28, one
of a series of the Department of Fisheries and Aquatic Sciences, Florida
Cooperative Extension Service, Institute of Food and Agricultural Sciences,
University of Florida. First Published June 1996. Revised December 1998;
Reviewed September 2003. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.
2. Peggy Reed, All
Florida Veterinary Laboratory, Alachua, FL; Ruth Francis-Floyd, Professor,
Department of Fisheries and Aquatic Sciences and Department of Large Animal
Clinical Sciences (College of Veterinary Medicine), and RuthEllen Klinger,
Biological Scientist, Department of Large Animal Clinical Sciences (College of
Veterinary Medicine), Florida Cooperative Extension Service, Institute of Food
and Agricultural Sciences, University of Florida, Gainesville, FL 32611.
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extension publications, contact your county Cooperative Extension Service
office.
Florida Cooperative Extension Service / Institute of Food and Agricultural
Sciences / University of Florida / Larry R. Arrington, Interim Dean
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