Digeneans are flatworms (Phylum Platyhelminthes, Class Trematoda, Subclass Digenea) that develop by molluscan first intermediate hosts, which in most cases are snails or clams. After a complicated development in the first intermediate host, the cercariae either infect the final host directly or find a second intermediate host, such as a snail, a crustacean or a fish, in which they become metacercariae, ready to infect a final host, such as a piscivorous fish, bird or mammal.
Although some adult-stage trematodes infecting fish are regarded as serious pathogens (e.g.
blood flukes), the most well-known trematode diseases of fish are caused by their metacercariae.
15.1 Sanguinicolosis of common carp
This disease is caused by a blood fluke, Sanguinicola inermis, which inhabits the blood vessels of common carp. This, about 1 mm long worm produces characteristic triangular eggs, which are carried by the blood stream to different organs. During the first larval stage, the miracidia hatch from eggs that are already in the blood, but they can only exit the fish through superficial organs, like the gills (Figure 62).
Figure 62. Developmental cycle of the blood fluke Sanguinicola inermis in common carp.
a) Mature fluke lives in the heart and gill arteries of common carp, b) Eggs are carried to capillaries by the blood stream, c) Miracidia break through the capillaries and enter the water, d) Ciliated miracidia swim in water to find a snail, e) In the body of the snail, sporocyst, redial and cercarial stages develop, f) Cercaria leaving the snail infect fish.
Redrawn after Kocylowski and Myaczynski, 1960.
Other larvae found in the capillaries of parenchymatous organs die and became small nodules surrounded by a capsule of connective tissue. Entering the water, larvae bore themselves into snails of the genus Limnaea, and within some weeks cercariae develop by multiplying in this mollusc. Cercariae swimming in the water actively invade common carp and develop to adults in the blood vessels. Miracidia obstruct capillaries, and cause stasis and localized degeneration.
In intensive infections, the gills are pale or marbled and the gill filaments break off. Malpighi bodies in the kidney die, and sera accumulate in the abdominal cavity, under the scales and in the eyes.
Clinical signs: The gills show clinical signs resembling gill rot caused by fungi or dactylogyrosis. In cases involving changes to the kidney, sanguinicolosis must be differentiated from spring viraemia of carp (SVC). With microscopic investigation, the characteristic triangular eggs of S. inermis can be easily recognized.
Prevention and treatment: Prevention is based on reducing the number of intermediate-host snails in the pond.
15.2 Diplostomosis of cyprinids
This is a commonly occurring disease, almost all cyprinids cultured in ponds having some metacercariae of Diplostomum spathaceum in the lens of their eyes (Figure 63). Adult worms live in the gut of water birds, the eggs entering the water with their faeces.
Miracidia hatched from the eggs infect snails of the genus Limnaea in which cercarial stages develop. Cercariae swimming in water actively attack fish, boring into the skin and migrating towards the eyes. If the number of invasive cercariae is high, heavy infection develops and the migrating larvae damage tissues and cause bleeding.
In heavily infected fry and fingerlings, haemorrhage develops in the muscles, primarily at the dorsal part of the head, causing death of the fish. In less severe infections, cercariae sporadically migrate to the lens, where they accumulate without encystation, being located in the vitreous humor. In this chronic form of infection, the vigorously moving cercariae cause the lens to become opaque, and this change can be easily seen with the naked eye. In silver and grass carp stocks, extremely intensive infections may develop, but fingerlings of common carp are less frequently affected. In severe cases, fish become blind and may lose one of their eyes.
Although blind fish can survive, their condition is poorer than that of less-infected specimens. Within some months, infected fish can recuperate.
Figure 63. Developmental cycle of Diplostomum spathaceum
a) Fluke eggs in faeces are voided by birds, b) On the bottom, miracidia develop from eggs and swim, c) Further stages develop in snails, d) Cercaria swim in the water, e) Cercaria infecting fish develop to metacercariae in the lens of the eye. Birds become infected when infected fish are consumed.
Redrawn after Bauer, Musselius and Strelkov, 1969
Prevention and treatment: For killing metacercariae, Mebendazol is effective if mixed in the food or administered in bath solution. Prevention relies on killing the snails in the pond by desiccation, wintering or disinfecting the bottom. Chasing off water birds is also useful.
15.3 Blackspot disease
This disease is caused by metacercariae of trematodes of the genera Posthodiplostomum and Apophallus (Figures 64 and 65A). Both types of fluke infect fish with their encysted metacercariae. Mature flukes live in the gut of water birds, and cercariae develop in snails, but when infecting fish they encyst and cause a host reaction, the formation of a black pigment layer around them.
Metacercariae of Posthodiplostomum cuticola, (also reported under the synonym Neascus cuticola), form relatively large cysts
(about 1.5 mm in diameter) in the muscle of fish (Figure 64). However, the large (approximately 0.2–0.3 mm) metacercarial cysts of Apophallus muehlingi (Figure 65A) infect mainly the fin rays, and less frequently, the skin. In small fish, they can cause serious deformation of the body. Fish may survive heavy infections but because they become unsightly, they are not marketable.
Prevention and treatment: Prevention is similar to that suggested for diplostomosis. No treatment is known.
15.4 Tetracotylosis
A great number of strigeid-type flukes (Apharyngostrigea, Cotylurus, Apathemon, etc.) form encysted metacercariae, called Tetracotyle in the abdominal cavity, inner organs or the muscle of fish. In heavy infections, these transparent metacercariae, which are surrounded only by a thin wall, can cause dropsy that resembles infectious bacterial dropsy. Hundreds of cysts attached to the pericardium hamper the movement of the heart and can cause the death of the host. The presence of cysts in the serous membranes makes diagnosis easy.
Figure 64. Heavy black spot infection in the skin of freshwater bream caused by
Posthodiplostomum cuticola metacercariae
Fresh-mount preparation
Figure 65. Encysted metacercariae
A) Apophallus muehlingi, B) Metagonimus yokogawai, C) Opistorchis felineus
Prevention and treatment: Prevention is similar to that suggested for diplostomosis. No treatment is known.
15.5 Other metacercarial infections
In Siberia and some Far Eastern countries, metacercariae of Metagonimus yokogawai (Figure 65B) are common the muscles of cyprinid fish, while Opisthorchis felineus often occurs on the scales (Figure 65C). Both species are of zoonotic importance, causing human infections in these regions. Their intermediate host snails are not common in ponds; therefore, their occurrence in pond farms is less probable.