Schistosomiasis, also known as snail fever and bilharzia, is a disease caused by parasitic flatworms called schistosomes. The urinary tract or the intestines may be infected. Symptoms include abdominal pain, diarrhea, bloody stool, or blood in the urine. Those who have been infected for a long time may experience liver damage, kidney failure, infertility, or bladder cancer. In children, it may cause poor growth and learning difficulty.
The disease is spread by contact with fresh water contaminated with the parasites. These parasites are released from infected freshwater snails. The disease is especially common among children in developing countries, as they are more likely to play in contaminated water. Other high-risk groups include farmers, fishermen, and people using unclean water during daily living. It belongs to the group of helminth infections. Diagnosis is by finding eggs of the parasite in a person's urine or stool. It can also be confirmed by finding antibodies against the disease in the blood.
Methods to prevent the disease include improving access to clean water and reducing the number of snails. In areas where the disease is common, the medication praziquantel may be given once a year to the entire group. This is done to decrease the number of people infected, and consequently, the spread of the disease. Praziquantel is also the treatment recommended by the World Health Organization for those who are known to be infected.
Schistosomiasis affected about 252 million people worldwide in 2015. An estimated 4,400 to 200,000 people die from it each year. The disease is most commonly found in Africa, Asia, and South America. Around 700 million people, in more than 70 countries, live in areas where the disease is common. In tropical countries, schistosomiasis is second only to malaria among parasitic diseases with the greatest economic impact. Schistosomiasis is listed as a neglected tropical disease.
|Other names||Bilharzia, snail fever, Katayama fever|
|11-year-old boy with abdominal fluid and portal hypertension due to schistosomiasis (Agusan del Sur, Philippines)|
|Symptoms||Abdominal pain, diarrhea, bloody stool, blood in the urine|
|Complications||Liver damage, kidney failure, infertility, bladder cancer|
|Causes||Schistosomes from freshwater snails|
|Diagnostic method||Finding eggs of the parasite in urine or stool, antibodies in blood|
|Prevention||Access to clean water|
|Frequency||252 million (2015)|
Many individuals do not experience symptoms. If symptoms do appear, they usually take 4-6 weeks from the time of infection. The first symptom of the disease may be a general feeling of illness. Within 12 hours of infection, an individual may complain of a tingling sensation or light rash, commonly referred to as "swimmer's itch", due to irritation at the point of entrance. The rash that may develop can mimic scabies and other types of rashes. Other symptoms can occur 2-10 weeks later and can include fever, aching, a cough, diarrhea, chills, or gland enlargement. These symptoms can also be related to avian schistosomiasis, which does not cause any further symptoms in humans.
The manifestations of schistosomal infection vary over time as the cercariae, and later adult worms and their eggs, migrate through the body. If eggs migrate to the brain or spinal cord, seizures, paralysis, or spinal-cord inflammation are possible.
In intestinal schistosomiasis, eggs become lodged in the intestinal wall and cause an immune system reaction called a granulomatous reaction. This immune response can lead to obstruction of the colon and blood loss. The infected individual may have what appears to be a potbelly. Eggs can also become lodged in the liver, leading to high blood pressure through the liver, enlarged spleen, the buildup of fluid in the abdomen, and potentially life-threatening dilations or swollen areas in the esophagus or gastrointestinal tract that can tear and bleed profusely (esophageal varices). In rare instances, the central nervous system is affected. Individuals with chronic active schistosomiasis may not complain of typical symptoms.
The first potential reaction is an itchy, papular rash:432 that results from cercariae penetrating the skin, often in a person's first infection. The round bumps are usually one to three centimeters across. Because people living in affected areas have often been repeatedly exposed, acute reactions are more common in tourists and migrants. The rash can occur between the first few hours and a week after exposure and lasts for several days. A similar, more severe reaction called "swimmer's itch" reaction can also be caused by cercariae from animal trematodes that often infect birds.
Another primary condition, called Katayama fever, may also develop from infection with these worms, and it can be very difficult to recognize. Symptoms include fever, lethargy, the eruption of pale temporary bumps associated with severe itching (urticarial) rash, liver and spleen enlargement, and bronchospasm.
Acute schistosomiasis (Katayama fever) may occur weeks or months after the initial infection as a systemic reaction against migrating schistosomulae as they pass through the bloodstream through the lungs to the liver. Similarly to swimmer's itch, Katayama fever is more commonly seen in people with their first infection such as migrants and tourists. It is seen, however, in native residents of China infected with S. japonicum. Symptoms include:
The symptoms usually get better on their own, but a small proportion of people have persistent weight loss, diarrhea, diffuse abdominal pain, and rash.
In long-established disease, adult worms lay eggs that can cause inflammatory reactions. The eggs secrete proteolytic enzymes that help them migrate to the bladder and intestines to be shed. The enzymes also cause an eosinophilic inflammatory reaction when eggs get trapped in tissues or embolize to the liver, spleen, lungs, or brain. The long-term manifestations are dependent on the species of schistosome, as the adult worms of different species migrate to different areas. Many infections are mildly symptomatic, with anemia and malnutrition being common in endemic areas.
The worms of S. haematobium migrate to the veins around the bladder and ureters. This can lead to blood in the urine 10 to 12 weeks after infection. Over time, fibrosis can lead to obstruction of the urinary tract, hydronephrosis, and kidney failure. Bladder cancer diagnosis and mortality are generally elevated in affected areas; efforts to control schistosomiasis in Egypt have led to decreases in the bladder cancer rate. The risk of bladder cancer appears to be especially high in male smokers, perhaps due to chronic irritation of the bladder lining allowing it to be exposed to carcinogens from smoking.
The worms of S. mansoni and S. japonicum migrate to the veins of the gastrointestinal tract and liver. Eggs in the gut wall can lead to pain, blood in the stool, and diarrhea (especially in children). Severe disease can lead to narrowing of the colon or rectum. Eggs also migrate to the liver leading to fibrosis in 4 to 8% of people with chronic infection, mainly those with long-term heavy infection.
Central nervous system lesions occur occasionally. Cerebral granulomatous disease may be caused by S. japonicum eggs in the brain. Communities in China affected by S. japonicum have rates of seizures eight times higher than baseline. Similarly, granulomatous lesions from S. mansoni and S. haematobium eggs in the spinal cord can lead to transverse myelitis with flaccid paraplegia. Eggs are thought to travel to the central nervous system via embolization.
Infected individuals release Schistosoma eggs into water via their fecal material or urine. After larvae hatch from these eggs, the larvae infect a very specific type of freshwater snail. For example, in S. haematobium and S. intercalatum it is snails of the genus Bulinus, in S. mansoni it is Biomphalaria, and in S. japonicum it is Oncomelania. The Schistosoma larvae undergo the next phase of their lifecycles in these snails, spending their time reproducing and developing. Once this step has been completed, the parasite leaves the snail and enters the water column. The parasite can live in the water for only 48 hours without a human host. Once a host has been found, the worm enters its blood vessels. For several weeks, the worm remains in the vessels, continuing its development into its adult phase. When maturity is reached, mating occurs and eggs are produced. Eggs enter the bladder/intestine and are excreted through urine and feces and the process repeats. If the eggs do not get excreted, they can become engrained in the body tissues and cause a variety of problems such as immune reactions and organ damage.
Diagnosis of infection is confirmed by the identification of eggs in stools. Eggs of S. mansoni are about 140 by 60 µm in size and have a lateral spine. The diagnosis is improved through the use of the Kato technique, a semiquantitative stool examination technique. Other methods that can be used are enzyme-linked immunosorbent assay, circumoval precipitation test, and alkaline phosphatase immunoassay.
Microscopic identification of eggs in stool or urine is the most practical method for diagnosis. Stool examination should be performed when infection with S. mansoni or S. japonicum is suspected, and urine examination should be performed if S. haematobium is suspected. Eggs can be present in the stool in infections with all Schistosoma species. The examination can be performed on a simple smear (1 to 2 mg of fecal material). Because eggs may be passed intermittently or in small numbers, their detection is enhanced by repeated examinations or concentration procedures, or both. In addition, for field surveys and investigational purposes, the egg output can be quantified by using the Kato-Katz technique (20 to 50 mg of fecal material) or the Ritchie technique. Eggs can be found in the urine in infections with S. haematobium (recommended time for collection: between noon and 3 PM) and with S. japonicum. Quantification is possible by using filtration through a nucleopore filter membrane of a standard volume of urine followed by egg counts on the membrane. Tissue biopsy (rectal biopsy for all species and biopsy of the bladder for S. haematobium) may demonstrate eggs when stool or urine examinations are negative.
Antibody detection can be useful to indicate schistosome infection in people who have traveled to areas where schistosomiasis is common and in whom eggs cannot be demonstrated in fecal or urine specimens. Test sensitivity and specificity vary widely among the many tests reported for the serologic diagnosis of schistosomiasis and are dependent on both the type of antigen preparations used (crude, purified, adult worm, egg, cercarial) and the test procedure.
At the U.S. Centers for Disease Control and Prevention, a combination of tests with purified adult worm antigens is used for antibody detection. All serum specimens are tested by FAST-ELISA using S. mansoni adult microsomal antigen. A positive reaction (greater than 9 units/µl serum) indicates infection with Schistosoma species. Sensitivity for S. mansoni infection is 99%, 95% for S. haematobium infection, and less than 50% for S. japonicum infection. Specificity of this assay for detecting schistosome infection is 99%. Because test sensitivity with the FAST-ELISA is reduced for species other than S. mansoni, immunoblots of the species appropriate to the patient's travel history are also tested to ensure detection of S. haematobium and S. japonicum infections. Immunoblots with adult worm microsomal antigens are species-specific, so a positive reaction indicates the infecting species. The presence of antibody is indicative only of schistosome infection at some time and cannot be correlated with clinical status, worm burden, egg production, or prognosis. Where a person has traveled can help determine which Schistosoma species to test for by immunoblot.
In 2005, a field evaluation of a novel handheld microscope was undertaken in Uganda for the diagnosis of intestinal schistosomiasis by a team led by Russell Stothard from the Natural History Museum of London, working with the Schistosomiasis Control Initiative, London.
Many countries are working towards eradicating the disease. The World Health Organization is promoting these efforts. In some cases, urbanization, pollution, and the consequent destruction of snail habitat have reduced exposure, with a subsequent decrease in new infections. The drug praziquantel is used for prevention in high-risk populations living in areas where the disease is common.The Centers for Disease Control and Prevention advises avoiding drinking or coming into contact with contaminated water in areas where schistosomiasis is common.
For many years from the 1950s onwards, vast dams and irrigation schemes were constructed, causing a massive rise in water-borne infections from schistosomiasis. The detailed specifications laid out in various United Nations documents since the 1950s could have minimized this problem. Irrigation schemes can be designed to make it hard for the snails to colonize the water and to reduce the contact with the local population. Even though guidelines on how to design these schemes to minimise the spread of the disease had been published years before, the designers were unaware of them. The dams appear to have reduced the population of the large migratory prawn Macrobrachium. After the construction of fourteen large dams, greater increases in schistosomiasis occurred in the historical habitats of native prawns than in other areas. Further, at the 1986 Diama Dam on the Senegal River, restoring prawns upstream of the dam reduced both snail density and the human schistosomiasis reinfection rate.
In China, the national strategy for schistosomiasis japonica control has shifted three times since it was first initiated: transmission control strategy (from mid-1950s to early 1980s), morbidity control strategy (from mid-1980s to 2003), and the "new integrated strategy" (2004 to present). The morbidity control strategy focused on synchronous chemotherapy for humans and bovines and the new strategy developed in 2004 intervenes in the transmission pathway of schistosomiasis, mainly including replacement of bovines with machines, prohibition of grazing cattle in the grasslands, improving sanitation, installation of fecal-matter containers on boats, praziquantel drug therapy, snail control, and health education. A 2018 review found that the "new integrated strategy" was highly effective to reducing the rate of S. japonicum infection in both humans and the intermediate host snails and reduced the infection risk by 3–4 times relative to the conventional strategy.
There are two drugs available, praziquantel and oxamniquine, for the treatment of schistosomiasis. They are considered equivalent in relation to efficacy against S. mansoni and safety. Because of praziquantel's lower cost per treatment, and oxaminiquine's lack of efficacy against the urogenital form of the disease caused by S. haematobium, in general praziquantel is considered the first option for treatment. The treatment objective is to cure the disease and to prevent the evolution of the acute to the chronic form of the disease. All cases of suspected schistosomiasis should be treated regardless of presentation because the adult parasite can live in the host for years.
Schistosomiasis is treatable by taking by mouth a single dose of the drug praziquantel annually.
The WHO has developed guidelines for community treatment based on the impact the disease has on children in villages in which it is common:
Other possible treatments include a combination of praziquantel with metrifonate, artesunate, or mefloquine. A Cochrane review found tentative evidence that when used alone, metrifonate was as effective as praziquantel.
Another agent, mefloquine, which has previously been used to treat and prevent malaria, was recognised in 2008–2009 to be effective against Schistosoma.
The disease is found in tropical countries in Africa, the Caribbean, eastern South America, Southeast Asia, and the Middle East. S. mansoni is found in parts of South America and the Caribbean, Africa, and the Middle East; S. haematobium in Africa and the Middle East; and S. japonicum in the Far East. S. mekongi and S. intercalatum are found locally in Southeast Asia and central West Africa, respectively.
In 2010, approximately 238 million people were infected with schistosomiasis, 85 percent of whom live in Africa. An earlier estimate from 2006 had put the figure at 200 million people infected. In many of the affected areas, schistosomiasis infects a large proportion of children under 14 years of age. An estimated 600 to 700 million people worldwide are at risk from the disease because they live in countries where the organism is common. In 2012, 249 million people were in need of treatment to prevent the disease. This likely makes it the most common parasitic infection with malaria second and causing about 207 million cases in 2013.
S. haematobium, the infectious agent responsible for urogenital schistosomiasis, infects over 112 million people annually in Sub-Saharan Africa alone. It is responsible for 32 million cases of dysuria, 10 million cases of hydronephrosis, and 150,000 deaths from renal failure annually, making S. haematobium the world’s deadliest schistosome.
Estimates regarding the number of deaths vary. Worldwide, the Global Burden of Disease Study issued in 2010 estimated 12,000 direct deaths while the WHO in 2014 estimated more than 200,000 annual deaths related to schistosomiasis. Another 20 million have severe consequences from the disease. It is the most deadly of the neglected tropical diseases.
The first physician who described the entire disease cycle was the Brazilian parasitologist Pirajá da Silva in 1908. The earliest known case of infection was discovered in 2014, belonging to a child who lived 6,200 years ago.
In 2016 more than 200 million people needed treatment but only 88 million people were actually treated for schistosomiasis.
Schistosomiasis is named for the genus of parasitic flatworm Schistosoma, whose name means 'split body'. The name Bilharzia comes from Theodor Bilharz, a German pathologist working in Egypt in 1851 who first discovered these worms.
Schistosomiasis is endemic in Egypt, exacerbated by the country's dam and irrigation projects along the Nile. From the late 1950s through the early 1980s, infected villagers were treated with repeated injections of tartar emetic. Epidemiological evidence suggests that this campaign unintentionally contributed to the spread of hepatitis C via unclean needles. Egypt has the world's highest hepatitis C infection rate, and the infection rates in various regions of the country closely track the timing and intensity of the anti-schistosomiasis campaign. From ancient times to the early 20th century, schistosomiasis' symptom of blood in the urine was seen as a male version of menstruation in Egypt and was thus viewed as a rite of passage for boys.
Among human parasitic diseases, schistosomiasis ranks second behind malaria in terms of socio-economic and public health importance in tropical and subtropical areas.
As of September 2014, a vaccine called "Bilhvax" against S. haematobium infection is being developed. As of September 2016, no results from the Phase III clinical trials completed in 2012 have been reported.
Bilharzia caused by poor civil engineering design due to ignorance of cause and prevention
citing public domain text, published by the CDC
Theodor Bilhharz (who discovered schistosomiasis in Egypt), and Pirajá da Silva (who established its life cycle)
By the early twentieth century, the Egyptian population was well aware of the widespread occurrence of haematuria to the point where the passing of blood by boys was considered as a normal and even necessary part of growing up, a form of male menstruation linked with male fertility (Girges 1934, 103).
The ancient Egyptians also wrote of boys becoming men when blood was seen in their urine, as this was likened to the young female's first menstruation (Despommier et al. 1995). Also, archaeological evidence such as wall reliefs, hieroglyphs, and papyri all confirm that their lifestyle encompassed activities such as bathing, fishing and playing in the Nile, and this combined with bad sanitation habits, would make almost everyone susceptible to this infection.
Anthelmintics or antihelminthics are a group of antiparasitic drugs that expel parasitic worms (helminths) and other internal parasites from the body by either stunning or killing them and without causing significant damage to the host. They may also be called vermifuges (those that stun) or vermicides (those that kill). Anthelmintics are used to treat people who are infected by helminths, a condition called helminthiasis. These drugs are also used to treat infected animals.
Pills containing anthelmintics are used in mass deworming campaigns of school-aged children in many developing countries. For example, the treatment of choice for soil-transmitted helminths is mebendazole and albendazole and praziquantel for schistosomiasis.Biomphalaria tenagophila
Biomphalaria tenagophila is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk in the family Planorbidae, the ram's horn snails.
This species is medically important pest, because of transferring the disease intestinal schistosomiasis. (Intestinal schistosomiasis is the most widespread of all types of schistosomiasis).
The parasite Schistosoma mansoni, which Biomphalaria snails carry, infects about 83.31 million people worldwide.The shell of this species, like all planorbids is sinistral in coiling, but is carried upside down and thus appears to be dextral.Health in Tunisia
In 2016, life expectancy in Tunisia was 74 years for males and 78 years for females. By comparison, in the 1960s it was only 47.1 years. Infant mortality in 2017 was 12.1 per 1,000 live births.Measles, tetanus, and polio have been largely eliminated by a major immunization program. Schistosomiasis and malaria are rare, though rabies, stings, and leishmaniasis are still an issue. Non-communicable diseases associated with an unhealthy lifestyle are now the leading causes of death.Helminthiasis
Helminthiasis, also known as worm infection, is any macroparasitic disease of humans and other animals in which a part of the body is infected with parasitic worms, known as helminths. There are numerous species of these parasites, which are broadly classified into tapeworms, flukes, and roundworms. They often live in the gastrointestinal tract of their hosts, but they may also burrow into other organs, where they induce physiological damage.
Soil-transmitted helminthiasis and schistosomiasis are the most important helminthiases, and are among the neglected tropical diseases. This group of helmianthiases have been targeted under the joint action of the world's leading pharmaceutical companies and non-governmental organizations through a project launched in 2012 called the London Declaration on Neglected Tropical Diseases, which aims to control or eradicate certain neglected tropical diseases by 2020.Helminthiasis has been found to result in poor birth outcome, poor cognitive development, poor school and work performance, poor socioeconomic development, and poverty. Chronic illness, malnutrition, and anemia are further examples of secondary effects.Soil-transmitted helminthiases are responsible for parasitic infections in as much as a quarter of the human population worldwide. One well-known example of soil-transmitted helminthiases is ascariasis.Jiang Shaoji
Jiang Shaoji (simplified Chinese: 江绍基; traditional Chinese: 江紹基; pinyin: Jiāng Shàojī; 1919–1995) was a Chinese internist and gastroenterologist. He was a professor in Shanghai Jiao Tong University, School of Medicine and elected an Academician of the Chinese Academy of Engineering in 1994.Jiang was a native of Wuxi, Jiangsu Province. He graduated from Medical school of Saint John's University, Shanghai and received M.D. in 1945. After that, he became an internist in Shanghai Hongren Hospital and promoted to the vice president of that hospital in 1954. In 1957, he was appointed as the vice president of Renji Hospital and also worked as the vice chief of internal medicine department. After 1978, he successively held the posts of vice president of medical science department in Shanghai Second Medical University (SSMU), vice president and honorary president of Shanghai Institute of immunology, vice president of school administration committee in SSMU. He also held posts of vice president, president of Chinese Society of Gastroenterology, which is a specialty society affiliated to Chinese Medical Association. He was one of the founders and the first president of Shanghai Institute of Digestive Disease, which was found in 1984.
In 1950s', Jiang joined the campaign of schistosomiasis control. He manifested that dwarfism caused by schistosomiasis could come back to growing state after treatment. In China, he was the first to use colonoscopy observing pathological change in the colon of those suffered from schistosomiasis. He also do researches about chronic hepatitis, gastritis and gastric cancer. His team established the gastric cancer model of wolf dog, observed the relationship of vitamins and gastric cancer. They applied folic acid, selenium, and tretinoin to induce differentiation of gastric precancerous lesions.List of drugs used to treat schistosomiasis
A schistosomicide is a drug used to combat schistosomiasis.Examples listed in MeSH include:
oxamniquineAnother example is praziquantel.Metrifonate
Metrifonate (INN) or trichlorfon (USAN) is an irreversible organophosphate acetylcholinesterase inhibitor. It is a prodrug which is activated non-enzymatically into the active agent dichlorvos.
It is used as an insecticide.
It can be used to treat schistosomiasis caused by Schistoma haematobium, but is no longer commercially available.It has been proposed for use in treatment of Alzheimer's disease, but use for that purpose is not currently recommended.Neglected tropical diseases
Neglected tropical diseases (NTDs) are a diverse group of tropical infections which are common in low-income populations in developing regions of Africa, Asia, and the Americas. They are caused by a variety of pathogens such as viruses, bacteria, protozoa and helminths. These diseases are contrasted with the big three infectious diseases (HIV/AIDS, tuberculosis, and malaria), which generally receive greater treatment and research funding. In sub-Saharan Africa, the effect of these diseases as a group is comparable to malaria and tuberculosis. NTD co-infection can also make HIV/AIDS and tuberculosis more deadly.In some cases, the treatments are relatively inexpensive. For example, the treatment for schistosomiasis is US$0.20 per child per year. Nevertheless, in 2010 it was estimated that control of neglected diseases would require funding of between US$2 billion and US$3 billion over the subsequent five to seven years. Some pharmaceutical companies have committed to donating all the drug therapies required, and mass drug administration (for example, mass deworming) has been successfully accomplished in several countries. However, preventive measures are often more accessible in the developed world, but not universally available in poorer areas.Within developed countries, neglected tropical diseases affect the very poorest in society. In the United States, there are up to 1.46 million families including 2.8 million children living on less than two dollars a day. In countries such as these, the burdens of neglected tropical diseases are often overshadowed by other public health issues. However, many of the same issues put populations at risk in developed as developing nations. For example, other problems can stem from poverty which expose individuals to the vectors of these disease, such as lack of adequate housing.Twenty neglected tropical diseases are prioritized by the World Health Organization (WHO), though other organizations define NTDs differently. Chromoblastomycosis and other deep mycoses, scabies and other ectoparasites and snakebite envenoming were added to the list in 2017. These diseases are common in 149 countries, affecting more than 1.4 billion people (including more than 500 million children) and costing developing economies billions of dollars every year. They resulted in 142,000 deaths in 2013—down from 204,000 deaths in 1990. Of these 20, two were targeted for eradication (dracunculiasis (guinea-worm disease) by 2015 and yaws by 2020), and four for elimination (blinding trachoma, human African trypanosomiasis, leprosy and lymphatic filariasis by 2020).Oxamniquine
Oxamniquine, sold under the brand name Vansil among others, is a medication used to treat schistosomiasis due to Schistosoma mansoni. Praziquantel, however, is often the preferred treatment. It is given by mouth and used as a single dose.Common side effects include sleepiness, headache, nausea, diarrhea, and reddish urine. It is typically not recommended during pregnancy, if possible. Seizures may occur and therefore caution is recommended in people with epilepsy. It works by causing paralysis of the parasitic worms. It is in the anthelmintic family of medications.Oxamniquine was first used medically in 1972. It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system. It is not commercially available in the United States. It is more expensive than praziquantel.Praziquantel
Praziquantel (PZQ), sold under the brandname Biltricide among others, is a medication used to treat a number of types of parasitic worm infections. Specifically it is used for schistosomiasis, clonorchiasis, opisthorchiasis, tapeworm infections, cysticercosis, hydatid disease, and other fluke infections. It should not be used for worm infections of the eye. It is taken by mouth.Side effects may include poor coordination, abdominal pain, vomiting, headache, and allergic reactions. While it may be used during pregnancy, it is not recommended for use during breastfeeding. Praziquantel is in the anthelmintic class of medications. It works partly by affecting the function of the worm's sucker.Praziquantel was approved for medical use in the United States in 1982. It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system. The wholesale cost in the developing world is about US$0.4–18.15 per day. In the United States a typical course of treatment costs $100–200.Schistosoma
Schistosoma is a genus of trematodes, commonly known as blood flukes. They are parasitic flatworms responsible for a highly significant group of infections in humans termed schistosomiasis, which is considered by the World Health Organization as the second-most socioeconomically devastating parasitic disease (after malaria), with hundreds of millions infected worldwide.Adult flatworms parasitize blood capillaries of either the mesenteries or plexus of the bladder, depending on the infecting species. They are unique among trematodes and any other flatworms in that they are dioecious with distinct sexual dimorphism between male and female. Thousands of eggs are released and reach either the bladder or the intestine (according to the infecting species), and these are then excreted in urine or feces to fresh water. Larvae must then pass through an intermediate snail host, before the next larval stage of the parasite emerges that can infect a new mammalian host by directly penetrating the skin.Schistosoma haematobium
Schistosoma haematobium (urinary blood fluke) is species of digenetic trematode, belonging to a group (genus) of blood flukes (Schistosoma). It is found in Africa and the Middle East. It is the major agent of schistosomiasis, the most prevalent parasitic infection in humans. It is the only blood fluke that infects the urinary tract, causing urinary schistosomiasis, and is the leading cause of bladder cancer (only next to tobacco smoking). The diseases are caused by the eggs.
Adults are found in the venous plexuses around the urinary bladder and the released eggs travels to the wall of the urine bladder causing haematuria and fibrosis of the bladder. The bladder becomes calcified, and there is increased pressure on ureters and kidneys otherwise known as hydronephrosis. Inflammation of the genitals due to S. haematobium may contribute to the propagation of HIV.S. haematobium was the first blood fluke discovered. Theodor Bilharz, a German surgeon working in Cairo, identified the parasite as a causative agent of urinary infection in 1851. After the discoverer, the infection (generally including all schistosome infections) was called bilharzia or bilharziasis. Along with other helminth parasites Clonorchis sinensis and Opisthorchis viverrini, S. haematobium was declared as Group 1 (extensively proven) carcinogens by the WHO International Agency for Research on Cancer (IARC) Working Group on the Evaluation of Carcinogenic Risks to Humans in 2009.Schistosoma intercalatum
Schistosoma intercalatum is a parasitic worm found in parts of western and central Africa. There are two strains: the Lower Guinea strain and the Zaire strain. S. intercalatum is one of the major agents of the rectal form of schistosomiasis, also called bilharzia. It is a trematode, and being part of the genus Schistosoma, it is commonly referred to as a blood-fluke since the adult resides in blood vessels.
Humans are the definitive host and two species of freshwater snail make up the intermediate host, Bulinus forskalii for the Lower Guinea strain and Bulinus africanus for the Zaire strain.Schistosoma japonicum
Schistosoma japonicum is an important parasite and one of the major infectious agents of schistosomiasis.This parasite has a very wide host range, infecting at least 31 species of wild mammals, including 9 carnivores, 16 rodents, one primate (Human), two insectivores and three artiodactyls and therefore it can be considered a true zoonosis.Schistosoma mansoni
Schistosoma mansoni is a water-borne parasite of humans, and belongs to the group of blood flukes (Schistosoma). The adult lives in the blood vessels (mesenteric veins) near the human intestine. It causes intestinal schistosomiasis (similar to S. japonicum, S. mekongi, S. guineensis, and S. intercalatum). Clinical symptoms are caused by the eggs. As the leading cause of schistosomiasis in the world, it is the most prevalent parasite in humans. It is classified as a neglected tropical disease. As of 2016, 206.5 million people have schistosomiasis and S. mansoni is the major parasite. It is found in Africa, the Middle East, the Caribbean, Brazil, Venezuela and Suriname.Unlike other flukes (trematodes) in which sexes are not separate (monoecious), schistosomes are unique in that adults are divided into males and females, thus, (dioecious). However, the two adults live in permanent partnership, a condition called in copula; for this, they are considered as hermaphrodites. The life cycle of schistosomes includes two hosts: humans as definitive hosts, where the parasite undergoes sexual reproduction, and snails as intermediate hosts, where a series of asexual reproductive takes place. S. mansoni is transmitted through water, where freshwater snails of the genus Biomphalaria act as intermediate hosts. The larvae are able to live in water and infect the hosts by directly penetrating the skin. Prevention of infection is done by improved sanitation and killing the snails. Infection is treated with praziquantel.
S. mansoni was first noted by Theodor Maximillian Bilharz in Egypt in 1851, while discovering S. haematobium. Sir Patrick Manson identified it as unique species in 1902. Louis Westenra Sambon gave the name Schistosomum mansoni in 1907 in honour of Manson.Schistosoma mekongi
Schistosoma mekongi is a species of trematodes, also known as flukes. It is one of the five major schistosomes that account for all human infections, the other four being S. haematobium, S. mansoni, S. japonicum, and S. intercalatum. This trematode causes schistosomiasis in humans.
Freshwater snail Neotricula aperta serves as an intermediate host for Schistosoma mekongi.Schistosomiasis Control Initiative
The Schistosomiasis Control Initiative (SCI) at Imperial College London, is a non-profit initiative that works with governments in sub-Saharan African countries to develop sustainable programmes against parasitic worm infections (schistosomiasis and soil-transmitted helminthiasis).
These diseases affect over 1 billion people in some of the poorest and most marginalised communities in the world, and are part of a group called Neglected Tropical Diseases.
They can impair child development, reduce school attendance and productivity, increase the risk of HIV in women and lead to infertility and internal organ damage. If left untreated, the damage can be permanent.
However, treatment is safe and effective. Early and regular treatment has the potential to reduce the damaging effects of infection, increase school attendance by 25% and increase future earnings by 40%.
Treatment is also very cost-effective. Up to three people can be treated for as little as £1 with SCI-supported programmes. In fact, the international charity-evaluator, GiveWell, recognises the SCI as one of the most cost-effective non-profits in the world.
Also, the impact of SCI-supported programmes can be seen very quickly. The number of people infected by these parasites can be reduced by up to 60% after just one round of treatment.
SCI continually conducts research to assess programme performance and investigate novel approaches to disease management. This ensures that control strategies are optimised and will ultimately lead to disease elimination.Schistosomiasis vaccine
A Schistosomiasis vaccine is a vaccine against Schistosomiasis (also known as bilharzia, bilharziosis or snail fever), a parasitic disease caused by several species of fluke of the genus Schistosoma. No effective vaccine for the disease exists yet. Schistosomiasis affects over 200 million people worldwide, mainly in rural agricultural and peri-urban areas of developing countries, and approximately 10% suffer severe health complications from the infection. While chemotherepeutic drugs, such as praziquantel, oxamniquine and metrifonate both no longer on the market, are currently considered safe and effective for the treatment of schistosomiasis, reinfection occurs frequently following drug treatment, thus a vaccine is sought to provide long-term treatment. Additionally, experimental vaccination efforts have been successful in animal models of schistosomiasis.Paramyosin has been proposed as a vaccine candidate.At present Sm-p80 (calpain) is the sole schistosome vaccine candidate that has been tested for its prophylactic and antifecundity efficacy in different vaccine formulations and approaches (e.g., DNA alone, recombinant protein and prime boost) in two very different experimental animal models (mouse and baboon) of infection and disease. Sm-p80-based vaccine formulation(s) have four effects: Reduction in adult worm numbers; Reduction in egg production (complete elimination of egg induced pathology both in baboons and mice); Protection against acute schistosomiasis; Therapeutic effect on adult worms. This vaccine is now ready for human clinical trials.Another target is Sm14.Stibophen
Stibophen is an anthelmintic classified as antimony compound and used as treatment of schistosomiasis by intramuscular injection.
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