Information on white spot
This document is FA28, one
of a series of the Fisheries and Aquatic Sciences Department, Florida
Cooperative Extension Service, Institute of Food and Agricultural Sciences,
University of Florida. Original publication date June, 1996. Reviewed July,
2002. Visit the EDIS Web Site at http://edis.ifas.ufl.edu.
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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opinions or affiliations. For information on obtaining other extension
publications, contact your county Cooperative Extension Service office. Florida
Cooperative Extension Service / Institute of Food and Agricultural
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University of Florida / Larry R. Arrington, Interim Dean
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.
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.
Figure 1. Diagrams of three
common genera of monogeneans. 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. Figure 2.
A heavy infestation of
mongeneans on the gills of a discus fish (100x magnification). 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 ortality. 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. Figure 3. Adult monogeneans attach to fish tissue
and
reproduce by a direct life cycle.
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.
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 reventing 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.
