Schistosomiasis is one of the many parasitic and bacterial neglected tropical diseases that cause global substantial illness in more than one billion people affecting especially the world's poorest people. Schistosomiasis, also known as bilharzia, is a major parasitic disease caused by trematode worms of the genus Schistosoma. Every year, there are over 230 million people worldwide requiring treatment for the infection with the number of people treated for bilharzia rose from 12.4 million in 2006 to 33.5 million in 2010 according to World Health Organization (WHO).1,2 These infections are prevalent in rural and outlying city areas of 74 countries in Africa, Asia, and Latin America. And it is the second largest disease burden after malaria or tuberculosis where almost 300,000 people die annually in Africa alone.3 Two important species are infecting humans plus one less epidemiological importance in Africa targeting particularly children who may acquire the disease by contact with the infected water such as swimming. Praziquantel is the chosen treatment to reduce the number of parasite in humans and at the same time strategies to minimise the spread of infection has been implemented.
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The geographically distribution of Schistosoma species type are dependent on the snail host available in the area. There are three human associated infections found in Africa. S. mansoni is the most widely spread species which is associated with intestinal and liver disease similar to that of S. intercalatum but S. intercalatum is found the least in geographic location in Africa. On the other hand, S. haematobium is the major agent of schistosomiasis that leads to renal disease and increased risks for bladder cancer in adults. It is seriously threatens the health and productive life of rural families. A study showed that schistosomiasis cases of 333 infected return travellers from an endemic areas in Africa reported in Europe were imported resulting 92 infections of S. haematobium, 130 cases of S. mansoni and 4 S. intercalatum cases.4
Species distinction is made further by the species of freshwater snails found to be supporting transmission of the parasite in different areas of the world (Table 1). The larvae of the parasite are harboured by snails, which serve as intermediate hosts, and infect humans who have come in contact with infested water.
Table 1: Schistosomiasis and snail genus according to the geographical locations.
Type of schistosome
Africa, Middle East, Brazil
Asia (previously in Japan, now mainly China)
Asia (only around the Mekong River)
Africa, Middle East
The worms discussed below are important Schistosomiasis in Africa as highlighted in Table 1 above:
S. mansoni is found in western and central Africa as shown in Figure 1. The parasite was first abundant in tropical Africa and entered the New World as a result of the African slave trade.5 S. mansoni infects most about 83.31 million people worldwide causing intestinal disease.6 The prevalence is greatest between 10-24 year age group associated with high water contact activities.6 The infections are acquired by humans transmitted by an aquatic snail called Biomphalaria species. Reservoir hosts are not available for this species.
Figure 1: Map showing distribution of S. mansoni in western and central Africa (Cameroon, Equitorial Guinea, Nigeria, Sao Tome, Gabon and Democratic Republic of Congo).7
S. haematobium occurs in many parts of Africa throughout the Nile Valley (Figure 2), where reservoir hosts include monkeys, chimpanzees and baboons. This species cause urogenital diseases where humans are the only significant reservoir acquired from the snail intermediate host Bulinus species.
Figure 2: Map showing the distribution of S. haematobium in large parts of Africa, parts of the Middle East, Iran, Mauritius and Madagascar.7
S. intercalatum infections is found focally in 10 countries around central West Africa as shown in Figure 3. The infection is transmitted by an aquatic snail, Bulinas species in humans and cause intestinal form of infection similar to S. mansoni. Disease is spread by seasonal migration of nomad populations and migration of African labourers where it is 5 to 25% of the population is prevalence in the endemic region.4
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Figure 3: Map showing the
distribution of S. intercalatum
in western and central Africa.7
Life cycle of Schistosoma spp.
Figure 3: Life-cycle of schistosomiosis.8
Schistosomes are different from other human trematodes since they have two sexes that live in the blood vessels. The life cycle of schistosomes is complex as it requires the use of two hosts to complete its life cycle. It involves a warm-blooded vertebrate definitive host in which the parasite undergoes sexual reproduction and an intermediate host, an aquatic snail, in which it undergoes asexual reproduction. Illustration of the life-cycle of the different schistosomes is in Figure 3.
Eggs are eliminated from a human host either in the faeces (S. mansoni and S. intercalatum) or urine (S. haematobium) of an infected human depending on the parasite species.
(2 & 3) Eggs can survive up to a week in dry land and larvae called miracidia, hatch if the faeces end up in water. The miracidia will swim to penetrate specific snail hosts under optimal conditions.
(4 & 5) Two generations of sporocysts and produce thousands of infectious cercariae within the snail takes a few months.
(6 & 7) The free-swimming cercariae are released from snail into the freshwater and penetrate the skin of the human host and other mammals. Cercariae, at this stage, lose their tails where the body is now known as schistosomulae.
(8 & 9) The schistosomulae migrate through several tissues and stages to reside in the veins. Each schistosomulae stays a few days in the skin and then enters the bloodstream through dermal lymphatic vessels or blood venules.
(10) Worms of schiostosomiasis reside in the mesenteric venules in various locations in human body that is specific for each species.
(B) S. mansoni and S. intercalatum occur more often in the superior mesenteric veins draining the large intestine. About half of the eggs laid by the female pass to the lower intestine/rectum while the other half is carried by the blood to other organs in the body, specifically in the liver and lungs. Those that enter the intestine are passed out in the faeces.
(C) S. haematobium often occurs in the venous plexur of bladder, but can also be found in the rectal venules. About half of the eggs laid by the female pass into the bladder, ureters, vagina and cervix. Those eggs that enter the bladder are passed out with the urine while others are deposited in other organs.
The cycle will start again once the eggs get out of the body into the freshwater.
Schistosome infections differ between the early or acute stages of infection and towards the chronic infection in the later stage.9 The initial entry of the cercarial forms into the skin causes a localized dermatitis and may progresses to subsequent migration of the developing schistosomulae through the lung and hepatic circulation during its development into mature schistosome parasite. It is primarily results from the host's immune response to the eggs.
â€¢ Schistosoma mansoni disease:
The pathology related to S. mansoni infection is divided into two main stages, acute and chronic schistosomiasis.
Acute Schistosomiasis - 'Katayama' fever is associated with heavy primary infection and the onset of the female parasite laying eggs and granuloma formation around eggs deposited in the liver and intestinal wall. This takes approximately 5 weeks after infection. It resembles 'serum sickness' with hepatosplenomegaly, and leucocytosis with eosinophilia. This phase of the infection is often asymptomatic, but when symptoms do occur they include fever, nausea, headache, irritating cough and, in extreme cases bloody diarrhoea with mucus and necrotic material. These symptoms last from a few weeks to several months if present.
Chronic Intestinal Schistosomiasis - This stage manifest after many years of infection due to the production of eggs by the adult worms. (Figure 4) The eggs become trapped in the tissues results in cellular pathogenic reaction and granulomatous inflammation around the eggs with subsequent fibrosis. Infection in both the small and large intestine may also be involved, but the large intestine will show more severe lesions that lead to symptoms including fever, abdominal pain, malaise and liver tenderness. The patients of S. mansoni infection may have diarrhoea or dysentery with blood and mucus present in stool samples.
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Figure 4: Schistosoma mansoni eggs.10
Chronic Hepatosplenic Schistosomiasis - This stage is also seen after a few years of chronic infection. It is similar to that seen in the intestine where granulomatous inflammation in the liver where eggs are trapped leading to fibrosis and hepatosplenic disease. Hepatosplenomegaly is where congested spleen increases greatly in size with large quantities of antigenic material present in the spleen.
â€¢ Schistosoma haematobium disease:
This infection is similar to S. mansoni at the early stages of infection with symptoms like dermatitis, malaise, fever and allergic reactions. But the difference is that, it produces dysuria, haematuria and frequent urination as the early symptoms due to cystitis from eggs deposition. The organ frequently affected is the urinary bladder and can be chronic with bloody urine. Eggs (Figure 5) got calcified and trap in the tissues. It is one of the leading causes of bladder cancer in parts of Africa where squamous cell carcinoma of the bladder frequently demonstrate.13
Figure 5: Eggs of Schistosoma haematobium.13
â€¢ Schistosoma intercalatum disease:
S. intercalatum infections are often asymptomatic with bloody diarrhoea similar to those of S. mansoni. This is diagnosed by the presence of terminally spined eggs in the faeces. (Figure 6) Symptoms include pain in the left iliac fossa, nausea, diarrhoea, anorexia and abdominal pain. There is an association with Salmonella infection where if schistosomiasis is left untreated, Salmonella infection cannot be cured and result in complications such as severe rectal or genital lesions.4
Figure 6: Eggs of Schistosoma intercalatum.13
The safest and most effective drug for treating all forms of schistosomiasis is praziquantel. It effectively kills larval and adult worm, but is less effective in killing the 2-4 weeks old parasites.11 Oxamniquine has been effective in treating infections caused by S. mansoni in some areas in which praziquantel is less effective. Metrifonate is the drug to treat S. haematobium infection. This involves regular treatment of all people in high-risk groups:
Irrigation workers and fishermen - those with occupations expose to infected freshwater.
School age children - age between 6 and 15 years old that have the potential to swim and play for long hours in nearby infected lakes and irrigation channels.
Pregnant and non-pregnant women - those who carry out household work by collecting water to wash clothes and cooking utensils and therefore more likely to become infected and it is of particularly important in pregnant women to avoid potential risks of infection associated with anaemia in pregnancy.
Treatment should also be complemented with health education on the water safety and sanitation method in the community. The risk of developing severe disease in childhood is minimised and even reversed with this treatment whenever re-infection may occur.12 WHO has implemented a continuous support in providing the drug to the infected areas as the cure rates have been equal to or greater than 85%.11
In order to allow the world's poorest populations to live healthier, develop fully, learn effectively, raise families, and be productive members of their communities; the strategies in controlling the infection of bilharzia are taken into practice. The following control strategies are of different ways to prevent transmission of infection or to reduce the likelihood of heavy infection:
Personal protection - residence, farmers, fishermen and others who live around the infected areas should wear rubber boots to protect their skin from penetration by the swimming forms of the schistosoma parasites. Swimming or bathing should also be prohibited in contaminated area.
Parasite control measures - treating water for washing with chlorine or iodine to kill the eggs and immature schistosoma organisms in order to reduce the number of parasites.
Rapid case detection and referral - to seek medical advice for early diagnosis of schistosomiasis at the nearest health centre for any suspected cases and effective treatment including return travellers.
Education in the community - teach the community the causes and modes of transmission of schistosomiasis especially to avoid urinating or defecating in water to reduce contamination of schistosoma eggs.
An important control measure that has been successfully used with many other infectious diseases is the use of vaccine. There are a few candidate vaccines being tested out where one approach has been established to target and eliminate eggs excretion by the female schistosome with successful reports in mice and large animal reservoir hosts in S. japonicum.13
To completely eradicate schistosomiasis transmission in Africa, the problem of poverty, due to poor sanitation and health care must be overcome. The control of the infection is also based on drug treatment, snail control and health education in the affected community. The World Health Organisation is making the drugs accessible to countries that need that drugs the most.
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