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,
