FLUKES – Class Trematoda

The second class of flukes is known as class Trematoda which is further broken down into two subclasses; Digenetic flukes and Aspidogastrean flukes.

However, for Aspidogastream flukes, there is no known relation to human infections. Therefore, the term ‘flukes’ when associated with human infection, it is most likely referring to digenetic flukes.

Trematodes are mostly endoparasitic (infects hosts internally) and most of the adults live on vertebrates. The digenea flukes live in guts, bloodstream or the host tissues. Similar to class Monogenea flukes, their body are also covered with tegument whereby cilia are absence in adults.

Attachment

Unlike Monogenea flukes, trematodes do not have prohaptor and opisthaptor. Instead, they are characterized by two hookless suckers on the anterior ends which are used for adhesion to hosts. The two suckers known as oral suckers,which surrounds the mouth, and acetabulum which is ventrally located (circled red in the diagram on the right).

These suckers are supplied with adhesive gland cells that facilitate attachment and operate on the suction produced by the muscles of trematodes.

Reproduction

Trematodes are also generally hermaphroditic, just like the Turbellarians. For the male system, the number of testes differs in monogenetic flukes (many testes) and digenetic flukes (only two). The sperms produced in testes and later stored in seminal vesicle, will eventually move via the sperm duct into the penis, where the sperm and semen are ejaculated into the female orifice.

LIFE CYCLE

The life cycle of the Trematodas involves two or three hosts, such as human hosts. The life cycle is complex as the larvae act as parasites to different species that the adults will be attached to.

At the first stage of the life cycle, the adult trematode living inside a mammal species (eg. human host, sheep host, cow host) will release immature or unembroynated eggs through the faeces. Then, the eggs will develop and hatched in water and seek its host, snails. In the snails, the egg will undergo several stages of development. It will then develop into sporocysts, rediae and developed into cercariae in the snail tissues. The pelagic cercariae will then emerge from the snail and enclosed in a cyst on water plants. When mammals (humans, cows etc) eat this infected vegetation with the larvae, the larvae will be transferred to the secondary hosts, the mammals. The larvae will then travel to the intestines and to the specific organs where the species survive and begins to attach using the suckers. The Trematodes will then grow and the whole cycle continues.

A typical example of a Trematode that undergo similar life cycle as the above is the blood flukes also called Schistosoma. The disease is called schistomiasis. The eggs have sharp spines on its surface and it is produced in the capillaries (blood stream) of the bladder wall or intestines. The eggs block the flow of the blood vessels and the sharp protruding spines on the surface tear the surrounding tissues, thus causing hemorrhages. It is a pandemic disease particularly in countries such as Africa. Among the symptoms for human infected with trematodes include having diarrhea, ulcers and inflammation.

Watch the video here (Lifecycle of Schistosoma) to understand the life cycle of this particular Trematode or the animation (Animation of the lifecycle of Schistosoma) here. If you are interested, you can watch a clip of this Crime Scene Investigation (CSI) movie about blood flukes. CSI CLIP