Life History of the Blood Fluke (Schistosoma sp.)
Blood flukes (Schistosoma sp.) are parasitic flatworms which belong to the Trematoda class of platyhelminths, a group which also includes the lung fluke Paragonimus westermani and the liver fluke Fasciola hepatica. Schistosoma is the causative agent of schistosomiasis, an intravascular infection which affects an estimated 250 million people worldwide, but is most prevalent in developing countries including Indonesia, Brazil and sub-Saharan Africa. Symptoms include diarrhoea, muscle aches, fatigue, and fever. If flukes are successful in reproducing in the host, organ inflammation can occur in response to the eggs. Over twenty species of Schistosoma are known, with five main species infecting humans: S. haematobium, S. intercalatum, S. japonicum, S. mansoni, and S. mekongi .
Life Cycle and Transmission
Schistosoma has an indirect life cycle, with two hosts involved: freshwater snails, such as Biomphalaria glabrata, a South American species which acts as an intermediate host for Schistosoma mansoni, and humans, which are the definitive host. The eggs of Schistosoma can be shed in either the urine or faeces of an infected human, depending on the species, and providing conditions are favourable, the eggs hatch, releasing miracidia. The miracidium is motile, possessing many somatic cilia and is able to locate particular species of freshwater snail, and upon doing so penetrates the tissue of the foot. Miracidia are sensitive to chemical odours released by the snail and are able to locate hosts using chemical gradients. In the snail, the miracidia develop further to two successive generations of sporocysts, and eventually into free swimming cercariae which are identified by a fork-shaped tail.
Once the infective cercariae are released from the snail, they swim towards a human host, and penetrate the skin. During penetration, the cercariae shed their tails and become schistosomulae, which migrate through the skin and blood (Schistosomulae can stay within the skin for up to 72 hours) and travel through several layers of tissue, maturing as they go. The adult worms reside in the mesenteric veins of the bowels or rectum (venous plexus of the bladder in S. haematobium) at several locations, which can be specific to the species - for example S. mansoni is often found in the superior mesenteric vein draining the large intestine whereas S. japonicum is more commonly found in the superior mesenteric vein draining the small intestine, however both species are able to inhabit both locations.
Schistosoma is sexually dimorphic, with males being larger than females. Once a female comes into contact with a male, she resides in a large groove of the male known as the gynecophoral canal, where she will mate with him and deposit eggs in the venules of the perivesical and portal systems. The eggs move progressively towards the lumen of the intestines (in S. mansoni and S. japonicum) and the bladder (in S. haematobium) of the human host, and are expelled via faeces and urine, respectively. The spine orientation and shape of Schistosoma eggs are characteristic for each species, and studying the morphology of eggs in the stool (or urine) has proven an efficient method of diagnosis of Schistosoma infection; for example the egg of S. haematobium is elongated with a lateral spine, compared to that of S. mansoni which has a lateral spine, and S. japonicum which is more rounded in shape and has a much less prominent spine.
Virulence and Control
The adult worms of Schistosoma can live in the blood vessels of the intestine (or bladder) for over 10 years, and during this time will produce continue to produce eggs. Any eggs which are not expelled can be lodged in the body, which results in hepatic fibrosis (liver scarring) due to the host’s immune response to the eggs. Schistosomiasis is particularly prevalent in developing regions including sub-Saharan Africa, the Middle East and north-east Brazil, with approximately 250 million cases of schistosomiasis reported globally per year, with around 300, 000 deaths annually associated with schistosome infection.
Since schistosomiasis is a water-borne disease transmitted through the faecal-oral route, an efficient method of control is maintaining hygiene and adequate clean drinking water in order to mitigate risk factors of infection, reducing the likelihood of encountering cercariae and the host snails. A method of biological control by the oligochaete worm Chaetogaster limnaei limnaei which is a commensal organism of Biomphalaria glabrata has been proposed, suggesting that the oligochaete worm acts as a control of trematode larvae.
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