Monogenean (skin or gill fluke) infestation occurs in both freshwater and marine fish. Monogeneans are ectoparasites that
live on skin, gills, and fins. They contain a haptor (attachment organ) and have a direct life cycle. Dactylogyrus species has four points at the anterior end, an anterior sucker, four eyespots, and a haptor with two large hooks surrounded
by several small hooklets. Gyrodactylus species has two points at the anterior end, an anterior sucker, no eyespots, and a haptor with two large hooks surrounded
by several small hooklets. This monogenean is viviparous with internal embryos containing hooks. Clinical signs associated
with monogenean infestation include flashing and skin disorders because of the injury to skin caused by the parasite's attachment
and feeding behavior. The hooklets on the haptor penetrate epithelial cells.
Figure 12A: A gill biopsy from a Corydoras species exhibiting nodules or pseudocysts on the gill filaments (100X).
Monogeneans can be treated by using formalin (0.125 to 0.250 ml 37% formaldehyde/L) as a one-to 60-minute bath; praziquantel
(2 mg/L for freshwater fish or 20 mg/L for marine fish) as a one-to three-hour bath; trichlorfon (dimethyl phosphonate; 0.25
to 1 mg/L) as a one-hour bath; or a salt bath (30 to 35 g/L) in freshwater fish.1,2
Digenetic trematodes are endoparasites with an indirect life cycle. The indirect life cycle of digenetic trematodes involves
a definitive host that is a fish-eating bird, a first intermediate host that is a snail, and a second intermediate host that
is a fish. Adults live in the gastrointestinal tract and have two suckers and a Y-shaped gut. The metacercariae encyst throughout
the body of the fish intermediate host. Most fish with encysted metacercariae exhibit no signs of illness; however, those
with heavily infested organs may have organ dysfunction. Encysted digeneans are difficult to treat, but praziquantel may reduce
Figure 12B: Spores of Henneguya species from a ruptured pseudocyst on the gill filaments in Figure 12A. This myxozoan, frequently found in wild caught species of Corydoras species, usually causes a self-limiting disease (1,000X).2
Parasitic crustaceans include the Branchiura, which are ectoparasites with a dorsoventrally flattened body and prehensile
suckers that attach to the bodies of fish.11 An example is Argulus species (fish louse), which has a direct life cycle and causes cutaneous lesions and respiratory distress. It is large enough
to be seen grossly, and clients may observe the parasite's movements (Figure 13).