SCHISTOSOMIASIS

Schistosomiasis (blood fluke infection) is a tropical disease that mainly affects the urinary and intestinal tracts of people living in poor rural communities of both tropical and subtropical countries including Africa, the Middle East, Asia, and the Americas. It is among the leading cause of morbidity and mortality in Africa, Asia and South America. The disease rarely occurs in developed countries where there has been significant improvement in public health, sanitation and sewage management; and infections in these areas are usually attributed to travelers or immigrants who may harbour the parasite and who are from schistosomiasis-endemic regions. However, Schistosoma species could also be acquired by individuals in schistosomiasis-free countries (China, Japan, and USA) from somewhere else.

Schistosomiasis has significantly changed in its global distribution over the past 5 decades in most part of the world (where the disease used to be common) including China, Japan, Brazil, USA, and some parts of the Middle East, and this has been largely attributed to significant improvements in public health policies and other environmental and sanitation practices that brought the blood-fluke infection under control. The disease affects both the old and young but morbidity is highest among children in endemic regions, and over 200 million people are either at risk of infection or are already infected with the parasite that causes the disease. Schistosomiasis occurs in both humans and animals, and the disease is strongly associated with poor personal and public hygiene among the heavily populated people.

Schistosomiasis is caused by a group of parasites (flukes) which are generally known as Schistosoma species. Schistosoma species are found in the Phylum Platyhelminthes, Class Trematoda, and Genus Schistosoma. Five Schistosoma species are known to infect humans, and they include: Schistosoma haematobium (causes urinary schistosomiasis or Bilharziasis), S. mansoni (causes intestinal schistosomiasis), S. japonicum (causes oriental schistosomiasis), S. intercalatum, and S. mekongi. S. malayensis and S. mattheei (which causes animal schistosomiasis) are other forms of the parasite. Only S. haematobium is known to cause urinary schistosomiasis. S. mansoni, S. japonicum, S. intercalatum, and S. mekongi cause intestinal schistosomiasis and other forms of the blood-fluke disease. Schistosoma species are not hermaphrodites like some trematodes, and the adult Schistosomes live as male and female parasites in their hosts. Schistosoma species are known to live in the vascular channels of their human hosts, i.e. in the portal vascular (venous) systems where they penetrate and invade the veins of their hosts.

Type and morphology of Schistosoma

Schistosoma species exists in three morphological forms which are eggs, miracidia (singular: miracidium) and cercaria (singular: cercariae). Miracidia are the infective stage of the parasite in snail while cercaria is the infective stage of Schistosoma species in humans. The eggs and adult worms of the various species of Schistosoma species vary in their morphology. Adult worms reproduce sexually in humans to produce eggs which are released in the urine and faeces of infected human hosts into the environment. The eggs hatch in freshwater to release free-swimming, short-lived and motile miracidia which are sexually separate (i.e. they occur as males and females). Schistosomes are cylindrical and elongated in their morphology, and the adult worm measures about 1-2 cm in length.

Vector, reservoir and habitat of Schistosoma

Schistosoma species do not have any insect hosts like other parasites. Amphibious snails (snails that live on land and in water) are the most important intermediate hosts while human beings are the most important definitive host of Schistosoma species. Water bodies including brackish and freshwater are the most important natural habitats for snails that harbour human Schistosoma species. These freshwater snails which can also be found on vegetations are the source of miracidia which reproduce asexually to release numerous sporocysts and cercaria that eventually swim into water from where humans become infected. Species of snail hosts that harbour human Schistosomes include: Bulinus species (which harbour Schistosoma haematobium and S. intercalatum), Biomphalaria species (which harbour Schistosoma mansoni) and Oncomelania species which transmits Schistosoma japonicum. An infection with Schistosomes in humans normally occurs via contact with water containing infective cercariae of the parasite.

Dogs, goats, rodents, horse, pigs, and cats are examples of animals that can serve as reservoirs for Schistosoma species. Though these animals can be infected experimentally with Schistosomes, they do not however, serve as reservoir in human transmission of the parasite. Dogs are usually the main animal reservoir for S. mekongi. Constant human contacts with rivers, streams, muddy areas, lakes and even vegetations that inhabit snail hosts of the parasite in the environment encourages the onward transmission of Schistosomes to humans. The miracidia of Schistosomes hatch fast when eggs are deposited by definitive hosts in freshwater, and the production of cercaria are accelerated when the miracidia locates and invades a suitable species of snail specific for it.

         Clinical signs and symptoms of schistosomiasis

Majority of people infected with Schistosoma species are asymptomatic and rarely show any significant clinical presentation of the disease because the parasite creates a balanced parasite-host relationship. Also, Schistosoma species have mechanisms that allow the parasite to evade immune destruction by their host’s immune system. The clinical presentation of Schistosoma species varies depending on the type of infecting parasite. Developing parasites and adult worms of Schistosoma species can survive for many years within the vascular system of their human host undetected. However, some specific clinical manifestations of schistosomiasis includes: transient rash (from systemic and local hypersensitivity reactions), protracted feverish episodes, intestinal inflammation, abdominal pain, bloody diarrhea, haematuria (blood in urine), dysuria, and anaemia (due to repeated haemorrhages or bleeding of the affected organs).

Pathogenesis of schistosomiasis

Human infections with Schistosoma species results from the penetration of the skin by Schistosoma cercariae, and this occurs following mans contact with freshwater (standing or slow-moving water bodies). Enzyme secretion by Schistosomes facilitates the penetration of the parasite into the skin of their human hosts. Only the head of the cercariae penetrates the skin. The tail is lost during penetration. Cercariae can also be ingested by humans via drinking water contaminated with infected human feaces, and they penetrate the mucosal wall of the intestinal tract when they gain entry through this route (Figure 1). Some human activities such as washing, fishing and bathing in feacally-contaminated waters containing the intermediate host of the parasite can also serve as route via which Schistosomes infect humans. Rashes (known as Schistosome dermatitis) that increases in severity appears on the skin following the parasites penetration but the rash begins to vanish after few days of infection, and as soon as they invade the liver of their human hosts. Maturation of worms in the liver, their migration to small veins and the production of eggs sparks up a febrile reaction (known as Katayama syndrome or fever) in human hosts. Vomiting, headache, fever, abdominal pain, nauseas, diarrhea and loss of weight are some of the clinical manifestations that mark this acute stage of schistosomiasis in humans.

Figure 1: Life cycle Schistosoma. 1. Eggs of Schistosoma species are eliminated with the feaces or urine of a human host. 2. Under optimal conditions the eggs hatch and release miracidia. 3. Miracidia then swims and penetrates specific snail intermediate hosts. 4. The stages of the miracidia in the snail include 2 generations of sporocysts (i.e. the successive generations). 5. Infective cercariae (which are free-swimming) are later produced and released by the snail into water. 6. Upon release from the snail, the infective cercariae swim; and then penetrate the skin of the human host. 7. Infective cercariae shed their forked tail to become schistosomulae. 8,9. The schistosomulae migrate through several tissues and stages to their residence in the veins. Adult worms in humans reside in the mesenteric venules in various locations, which at times seem to be specific for each species. Inside the human host, S. japonicum is more frequently found in the superior mesenteric veins draining the small intestine (A) while S. mansoni occurs more often in the superior mesenteric veins draining the large intestine (B). It is noteworthy that S. japonicum and S. mansoni can occupy either of these locations (small intestine and large intestine), and they are also capable of moving between any of these sites, so it is not possible to state clearly that one species only occurs in one particular location.  S. haematobium most often occurs in the venous plexus of the bladder (C), but it can also be found in the rectal venules. After deposition of eggs by the female worm in the small venules of the portal and perivesical systems, the eggs are moved progressively toward the lumen of the intestine (for S. mansoni and S. japonicum) and of the bladder and ureter (for S. haematobium), and are eliminated with feces or urine, respectively. CDC

Katayama fever is a serum sickness that occurs in schistosomiasis patients in which the antibody level (including IgG, IgM, and IgE immunoglobulins) is elevated from substantial antigen production following the deposition of fresh eggs of Schistosoma species. The chronic stage of schistosomiasis sets in following the oviposition, deposition and retention of the parasites eggs in the lumen of the bladder, bowel and other body tissues. Released eggs are extended through the blood to other vital organs of the body where they inhabit. The most significant pathological changes of schistosomiasis is characterized by the retention of eggs of the parasite in the tissues of its human host. Thousands of Schistosoma eggs are usually retained and trapped in the wall of the urinary bladder and ureter, and this condition (which is quite common in children at an early age) can result to kidney failure if the disease is left untreated. Obstruction and bleeding of the bladder can result in Schistosoma haematobium infection (urinary schistosomiasis) while ulceration and congestion of the intestinal mucosa can occur in other Schistosoma infections.

As earlier stated, S. japonicum, S. mansoni, S. intercalatum and S. mekongi are the main causative agents of intestinal schistosomiasis. The eggs of Schistosoma species including those of S. mansoni, (Figure 2), S. haematobium (Figure 3), S. japonicum (Figure 4), S. intercalatum (Figure 5), and S. mekongi (Figure 6) are usually morphologically distinct; and they are sought for in the stool and urine samples of patients suffering from the Schistosoma infection. These parasites cause ulceration, diarrhea, and bleeding of the intestinal tract. Neurological anomaly including epilepsy can occur when the eggs of Schistosoma species are occasionally deposited in the CNS. Adult worms of Schistosoma species are known to live in the lumen of veins (including those of the urinary and intestinal tracts) where they cause inflammation, ulceration and bleeding in these areas and its associated organs.

Figure 2: S. mansoni egg (ovoid-shaped) in an unstained wet mount of stool specimen. The characteristic lateral spine of S. mansoni is shown (arrow). CDC

Figure 3: Eggs of S. haematobium (ovoid-shaped) in a wet mount of urine specimen. Note the characteristic terminal spine of S. haematobium (arrow). CDC

Figure 4: Egg of S. japonicum (round-shaped) in an unstained wet mount of stool specimen. S. japonicum usually has an unnoticeable small spine that is laterally placed (arrow). CDC

Figure 5: Eggs of S. intercalatum (ovoid-shaped) in a wet mount of urine specimen. Spines of S. intercalatum are usually conspicuous, small and terminally placed (arrow). CDC

Figure 6: Egg of S. mekongi (round-shaped) in an unstained wet mount of stool specimen. The spine of S. mekongi is usually inconspicuous, small and laterally placed (arrowhead). CDC

         Laboratory diagnosis of schistosomiasis

Schistosomiasis is basically an asymptomatic infection in humans. The diagnosis of schistosomiasis is mainly based on the detection of parasites egg in feaces, urine or biopsy of infected individuals. Detection of blood in urine (i.e. haematuria) of infected individuals in endemic regions indicates urinary schistosomiasis. The eggs of all Schistosoma species that infect or parasitize human beings are all found in urine and stool specimen of infected individuals. Eggs of Schistosomes are morphologically distinct, and the number of eggs present in clinical samples also varies depending on the type of infecting Schistosoma species.

Stool specimens are subjected to various concentration and sedimentation techniques (excluding floatation concentration technique) to detect the parasites eggs while urine samples are usually filtered or centrifuged to detect eggs of Schistosomes. Floatation concentration technique is not recommended for the laboratory diagnosis of schistosomiasis because this method cannot allow Schistosoma eggs (which are usually heavier than the eggs of other parasites) to float on the solvent used. Biopsies from the rectum, intestines and bladder can also be examined be examined for Schistosoma eggs. Serodiagnosis which employs monoclonal antibodies (MAbs) facilitates the detection of specific antibodies and/or antigens of Schistosoma species in blood serum samples of infected individuals.

Though antigen-antibody based serological tests for the diagnosis of schistosomiasis has their disadvantages including that they cannot different between acute infections and previous infectious, they still remain a veritable tool for diagnosing Schistosome infections in endemic regions, and they also eliminate the cumbersomeness and logistical difficulties associated with the processing and collection of stool and urine samples from infected individuals.

Treatment of schistosomiasis

Praziquantel, a schistosomicide (i.e. Schistosome-killing agent) is the best antihelminthic drug amongst others for the treatment of schistosomiasis in humans. Proper usage of praziquantel is very effective against all species of Schistosomes that infect human hosts because the drug is rapidly taken up and concentrated by adult worms, and egg excretion is significantly reduced as well. Other antihelminthics usually considered for the treatment of Schistosoma infections include oxamniquine (active against S. mansoni infection) and metrifonate (active against S. haematobium infection). However, praziquantel is still universally used for the effective treatment of schistosomiasis since the production of both oxamniquine and metrifonate may have been discontinued.

          Control and prevention of schistosomiasis

Improvements in sanitation and health have significantly helped to reduce and even eliminate schistosomiasis in some parts of the world where the disease used to be endemic including China. Increase in the building of dams and irrigation systems for agricultural purposes (which both serve as natural habitats for snail species harbouring Schistosomes) has contributed significantly in increasing the prevalence of schistosomiasis in Africa and other parts of the world where these practice dominates. Schistosomiasis can also be prevented and controlled by avoiding human contacts with water bodies that may contain infective cercariae of the parasite.

The provision of portable water supply for communities living in endemic areas, construction of bridges and crossroads to minimize water contact and preventing human-feacal contamination of waters will go a long way in minimizing the disease. Provision of sanitary facilities and proper health education for the populace in endemic regions coupled with effective treatment of active infections with praziquantel is very effective in managing the disease. Mass deworming of children and other target groups in endemic regions in addition to improvement in their nutrition, decontamination of water through boiling or chlorination before use is also very useful for controlling schistosomiasis. Effective vaccine for the disease is still under research and development.

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