CLASSIFICATION OF DIGENETIC TREMATODES ACCORDING TO THEIR HABITAT

   The digeneans are a group of specialised endoparasitic platyhelminthes. A common feature is that all have complex lifecycles, involving one or more intermediate hosts, the first of which is always a mollusc, which is usually aquatic. As adults they are found in most vertebrates groups, including fish, amphibians, reptiles, birds, and mammals, acting as definitive hosts, where they may be highly pathogenic.

  They may be located in most of the internal organs of these definitive hosts, including the lungs, bladder and blood stream, although the majority are found in the gastrointestinal tract, or closely associated organs such as the bile
duct and liver. They exhibit a flattened leaf-like body, structurally similar to many of the free living turbellarians. The digeneans are classified below based on their locations in the definitive hosts;

• Blood flukes - Schistosoma haematobium, S. mansoni, S. japonicum, S. mekongi,
and S. intercalatum

• Liver flukes - Fasciola hepatica, F. gigantica, C sinensis, Opisthorchis felineus,
O viverrini, Dicrocoelium dendriticum, and D. hospes Pancreatic flukes - Eurytrema pacreaticum, E. coelomaticum, and E. ovis

• Lung flukes -Paragonimus westermani, P. mexicana, and P. skrjabini

• Intestinal flukes – Fasciolopsis buski, Metagonimus yokogawai, Echinostoma
ilocanum, Watsonius watsoni, Heterophyes heterophyes, and Gastrodiscoides
hominis

Blood flukes (Schistosoma species)

  Schistosomiasis, or bilharzia, is a tropical parasitic disease caused by blooddwelling fluke worms of the genus Schistosoma. Over 200 million people are infected in at least 75 countries with 600 million or more people at risk of infection. The main schistosomes that infect human beings include S haematobium (transmitted by Bulinus snails and causing urinary schistosomiasis in Africa and
the Arabian peninsula), S mansoni (transmitted by Biomphalaria snails and causing intestinal and hepatic schistosomiasis in Africa, the Arabian peninsula, and South America), and S japonicum (transmitted by the amphibious snail Oncomelania and causing intestinal and hepatosplenic schistosomiasis in China, the Philippines, and Indonesia).

  S. intercalatum and S. mekongi are only of local importance. S. japonicum is a zoonotic parasite that infects a wide range of animals, including cattle, dogs, pigs, and rodents. S. mansoni also infects rodents and primates, but human beings are
the main host. A dozen other schistosome species are animal parasites, some of which occasionally infect humans.

  Unlike other trematodes, schistosomes have separate sexes, but males and females are found together. The male is short and stout and holds the relatively long female worm in its gynaecophoric canal, a groove like structure. With S. haematobium, both male and female live together in the veins that drain the urinary bladder, pelvis, and ureter, whereas S. japonicum and S. mansoni live in the inferior and superior mesenteric veins, respectively.

  Hence, these flukes are known as blood flukes. These species are distinguished from the other Schistosoma species based on the morphology of their eggs and their adult and cercarial forms. S. haematobium eggs have a terminal spine, whereas S. mansoni
and S. japonicum eggs have lateral spines and central spines, respectively

Morphology

  The adult males measure up to 15 millimetres in length and females up to 10 mm. The schistosomes remain in copula throughout their life span, the uxorious male surrounding the female with his gynaecophoric canal. The male is actually flat but the sides roll up forming the groove. The cuticle of the male is covered with minute papillae.

The female only possess these at the anterior and posterior end as the middle section being covered by the male body. Oral and ventral suckers are present, with the ventral one being lager serving to hold the worms in place, preventing them from being carried away by the circulatory current.

The ova of S. mansoni are 114-175 µm long by 45-68 µm wide. They are light yellowish brown, elongate and possess a lateral spine. The shell is acid fast when stained with modified Ziehl-Neelsen Stain.