Flukes (Trematoda)

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Flukes (Trematoda)

All flukes are internal parasites and they are common in fish and fowl as well as mammals. A mature fluke is flat, oval-shaped, and can grow up to a few mm in length. They have two suckers, the frontal one surrounding the mouth of the animal. The organism is usually hermaphrodite, laying eggs that must be transmitted to water. The life cycle is of differing complexity, usually contains one intermediate host at the least, but sometimes they are two or more. Fish can be intermediate hosts or final hosts, depending on the species of fluke. Infection by flukes is usually not lethal to the final host. Transmission of the infection is aided by the fact that intermediate hosts are weakened by the infection, becoming an easy prey to the final host, which speeds up the life cycle of the trematode.

Typical life cycle:

An egg of a sexually mature fluke is carried into water, a ciliate larva is hatched from it, and seeks out its first intermediate host. These are some kinds of molluscs, usually snails. In the mollusc, asexual reproduction takes place and a lot of tailed (cercaria) larvae are formed, and seek out their next host, which may be either a final or a second intermediate host (a fish or a crustacean). In the second intermediate host usually just slight changes in the morphology of the infecting agent take place. The flukes carried by fish in the second intermediate stage are often covered by a mantle or cyst but this is not always the case. Most often warm blooded vertebrates are the final hosts for these flukes.

When fish is the final host, it is most common that the second intermediate host is a crustacean or insect. The mature stage of the fluke tends to stay in the fish's intestines or organs, such as gut, bile duct, stomach and kidneys.

Black spot disease caused by Cryptocotyle lingua

This species of fluke has fish as the second or intermediate host and the final host is often a fisheating species, such as birds or mammals. The fluke causes black spot disease in fish reared in seawater. Tailed larvae from molluscs seek the fish and penetrate its skin where they develop and become dormant. The fish surrounds them with dark defense cells. The larvae can also be found in musculature, heart, peritoneum and eyes.

Symptom:

Dark spots on fish skin, these can be seen with bare eyes. Possible eye damage resulting in bad sight or blind fish. Mortality loss is mainly in young fish and mortalities in seawater stocked charr have reached 100%. This parasite usually does not cause disease in the on-growing phase.

Treatment:

None

Prophylactics:

Interrupt the lifecycle by

Decrease salinity, below 18 promill.
Choose cage sites where there are no intermediate hosts or decrease their numbers.
Try to prevent the final host, such as birds, from excreting faeces around the cage site.

Eye fluke (Diplostomum spp.)

These have fish as intermediate hosts, but the final hosts are either birds or mammals feeding on fish. This infection is common in wild arctic charr in Iceland. Eggs are released from a final host in feaces. The eggs hatch into ciliate larvae, that look for water snails. There they multiply in number and leave the snail as cercariate larvae. These have to find a fish host within two days to survive. When found, the larvae penetrate in through the skin or the gills. In the fish further development takes place (metacercaria) with the fluke moving to the eyes of the fish. In this stage the larva looks like the fully mature fluke, except it is without reproductive organs.

Symptom:

The parasite damages the eyes and sight of the fish and a white cloud covers the eyeball. Finally the fish becomes blind and is an easy prey to birds, thus closes the life cycle. A blind fish usually darkens in colour.

Treatment:

None is known.

Prophylactics:

Infection in enclosed land based fish farms is unlikely, unless the water resource is heavily populated by birds. In earthen ponds and in lake cages the risk is greater. Water resources should therefore be protected as possible and kept free of birds.

Kidney fluke (Phyllodistomum conostomum (umblae)

This fluke has fish as final hosts. It lives in the renal collecting ducts in the kidneys and is common in arctic charr and brown trout in Iceland. The first intermediate host is a small shellfish (bivalve or snail), the second is a larvae of an insect transmitted to the fish when the insect larvae is eaten by the fish.