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Parasitology and Epidemiology Lab Report
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Lab report describing the parasites found in freshwater fish in Dorset UK, and the potential relationship between host and parasite body size

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  • August 14, 2021
  • 9
  • 2021/2022
  • Essay
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Parasitology and Epidemiology Lab Report
Section 1: Life history traits of the blood fluke ( Schistosoma sp.)
1.1: Introduction
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
(Sokolow et al. 2016). 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
(Zarowiecki et al. 2007).



1.2: 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 (Theron et al. 2014). 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
(Rinaldi et al. 2009). Miracidia are sensitive to chemical odours released by the snail and are
able to locate hosts using chemical gradients (Hertel et al. 2006). 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 (Koprivinikar et al. 2010).
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
(Moraes et al. 2012, Wang et al. 2005). 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 (Crosby et al. 2011), 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 (Beltran and Boissier 2009, Steinauer 2009). 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 (Hove et al. 2008). 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 (Gobert et al. 2005, Skìrnisson and Kolářová 2008).



1.3: 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 (Coutinho et al. 2007). 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 (Fitzpatrick et al. 2007).
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 (Prüss-Ustün et al. 2014). 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 (Ibrahim 2007, Rodgers et al. 2005).

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