Schistosomiasis, also known as bilharzia, is an acute and chronic disease caused by parasitic worms.
Please find more information on this disease in the text below or watch expert interview:
What is the disease?
How dangerous is Schistosomiasis?
In children, Schistosomiasis can cause anaemia, stunting and a reduced ability to learn, although the effects are usually reversible with treatment.
Chronic Schistosomiasis may affect people’s ability to work and in some cases can result in death.
The number of deaths due to Schistosomiasis is difficult to estimate because of other involved pathologies such as liver and kidney failure and bladder cancer.
Who is at risk?
People living in or traveling to areas where Schistosomiasis occurs and their skin comes in contact with freshwater from canals, rivers, streams, ponds, or lakes, are at risk of getting Schistosomiasis.
How many people are affected by Schistosomiasis?
Seven hundred million people are at risk in 74 countries, and 240 million are already infected.
Schistosomiasis ranks second only to malaria as the most common parasitic disease, and is the most deadly NTD, killing an estimated 280,000 people each year in Africa alone.
Where is Schistosomiasis found?
Schistosomiasis is prevalent in tropical and subtropical areas, especially in poor communities without access to safe drinking water and adequate sanitation. It is estimated that at least 92 percent of those patients requiring treatment for Schistosomiasis live in Africa.
How do people get Schistosomiasis?
People become infected when larval forms of the parasite – released by freshwater snails – penetrate the skin during contact with infested water.
Human Schistosomiasis is not acquired by contact with saltwater like in oceans or seas.
What are the signs and symptoms of Schistosomiasis?
Symptoms of Schistosomiasis are caused by the body's reaction to the eggs produced by worms, not by the worms themselves.
Although most people have no symptoms at the early phase of infection, others may develop a rash or itchy skin within days after becoming infected. Fever, chills, cough, and muscle aches can begin within one to two months of infection.
Adult worms produce eggs that usually travel to the intestine, liver or bladder, causing inflammation or scarring.
Children who are repeatedly infected can develop anemia, malnutrition, and learning difficulties
How is the disease developing?
Intestinal Schistosomiasis can result in abdominal pain, diarrhea, and blood in the stool. Enlargement of liver and spleen is common in advanced cases.
The classic sign of urogenital Schistosomiasis is blood in the urine.
Fibrosis of the bladder and ureter, and kidney damage are sometimes diagnosed in advanced cases.
Bladder cancer is another possible complication in the later stages.
In women, urogenital Schistosomiasis may present with genital lesions, vaginal bleeding, pain during sexual intercourse, and nodules in the vulva.
In men, urogenital Schistosomiasis can induce pathology of the seminal vesicles, prostate, and other organs.
This disease may also have other long-term irreversible consequences, including infertility.
More in-depth information:
The incubation period is typically 14–84 days for acute Schistosomiasis, but chronic infection can remain asymptomatic for years.
Penetration of larvae (cercariae) can be associated with a rash that develops within hours or up to a week after contaminated water exposures. Acute Schistosomiasis is characterized by fever, headache, myalgia, diarrhea, and respiratory symptoms. Eosinophilia is often present, as well as painful hepatomegaly or splenomegaly.
The clinical manifestations of chronic Schistosomiasis are the result of host immune responses to schistosome eggs by the human body. Eggs, secreted by adult worm pairs living in the bloodstream, become lodged in the capillaries of organs and cause granulomatous reactions.
S. mansoni and S. japonicum eggs most commonly lodge in the blood vessels of the liver or intestine and can cause diarrhea, constipation, and blood in the stool. Chronic inflammation can lead to bowel wall ulceration, hyperplasia, and polyposis and, with heavy infections, to periportal liver fibrosis.
S. haematobium eggs typically lodge in the urinary tract and can cause dysuria and hematuria. Calcifications in the bladder may appear late in the disease. S. haematobium infection can also cause genital symptoms and has been associated with increased risk of bladder cancer. As with acute Schistosomiasis, eosinophilia may be present during chronic infection with any species.
Rarely, central nervous system manifestations of Schistosomiasis may develop; this is thought to result from aberrant migration of adult worms or eggs depositing in the spinal cord or brain. Signs and symptoms are related to ectopic granulomas in the central nervous system and can present as transverse myelitis.
How is Schistosomiasis diagnosed?
Stool or urine samples are examined for the parasite. A blood sample can also be tested for evidence of infection. For accurate results, patients must wait six to eight weeks after their last exposure to contaminated water before the blood sample is taken.
How is Schistosomiasis treated?
Praziquantel is the recommended treatment against all forms of Schistosomiasis. It is effective, safe, and low-cost. Even though reinfection may occur after treatment, the risk of developing severe disease is diminished and even reversed when treatment is initiated and repeated in childhood.
How can I prevent infection?
No vaccine is available.
No drugs for preventing infection are available.
- Avoid swimming or wading in freshwater. Swimming in the ocean and in chlorinated swimming pools is safe.
- Drink safe water. Schistosomiasis is not transmitted by swallowing contaminated water but contact of mouth or lips with water containing the parasites can result in infection.
Boil water for one minute or filter it before drinking. Iodine treatment alone will not guarantee that water is free of all parasites.
- Bath water should be heated to a rolling boil for at least one minute. Water held in a storage tank for at least one to two days should be safe for bathing. Don’t use towels that came into contact with potentially unsafe water.
How can Schistosomiasis be controlled?
In countries where Schistosomiasis causes significant disease, control efforts usually focus on:
- reducing the number of infections in people
- eliminating the snails that are required to maintain the parasite’s lifecycle
For all species that cause Schistosomiasis, improved sanitation could reduce or eliminate transmission of this disease.
More in-depth information:
The control of Schistosomiasis is based on large-scale treatment of at-risk population groups, access to safe water, improved sanitation, hygiene education, and snail control.
The WHO strategy for Schistosomiasis control focuses on reducing disease through periodic, targeted treatment with praziquantel through mass drug administration (MDA). It involves regular treatment of all at-risk groups. In a few countries, where there is low transmission, the interruption of the transmission of the disease should be aimed for.
Groups targeted for treatment are:
- school-aged children in endemic areas
- adults considered to be at risk in endemic areas, and people with occupations involving contact with infested water
- entire communities living in highly endemic areas
The frequency of treatment is determined by the prevalence of infection in school-age children. In high-transmission areas, treatment may have to be repeated every year for a number of years. Monitoring is essential to determine the impact of control interventions.
The aim is to reduce disease morbidity and transmission. Periodic treatment of at-risk populations will cure mild symptoms and prevent infected people from developing severe, late-stage chronic disease.
However, a major limitation to Schistosomiasis control has been the limited availability of praziquantel. Data for 2016 show that 34.4% of people requiring treatment were reached globally, with a proportion of 51.6% of school-aged children requiring preventive chemotherapy for Schistosomiasis being treated.
For certain species of the parasite, such as S.japonicum, animals such as cows or water buffalo can also be infected
Run-off from pastures if the cows are infected can contaminate freshwater sources.
How is Schistosomiasis transmitted?
Transmission occurs when people suffering from Schistosomiasis contaminate freshwater sources with their excreta containing parasite eggs. These eggs hatch in water and release miracidia, which swim and penetrate specific snail intermediate hosts. Their stages in the snail include two generations of sporocysts and the production of cercariae. Upon release from the snail, the infective cercariae swim, penetrate the skin of the human host , and shed their forked tail, becoming schistosomulae.
In the body, the larvae develop into adult schistosomes. Adult worms live in the blood vessels where the females release eggs. Some of the eggs are passed out of the body in the feces or urine to continue the parasite’s lifecycle. Others become trapped in body tissues, causing immune reactions and progressive damage to organs.
What exactly does the vector do?
Freshwater becomes contaminated by Schistosoma eggs when infected people urinate or defecate in the water.
The eggs hatch, and if certain types of freshwater snails are present in the water, the parasites develop and multiply inside the snails. The parasite leaves the snail and enters the water where it can survive for about 48 hours.
Schistosoma parasites can penetrate the skin of persons who are wading, swimming, bathing, or washing in contaminated water.
Within several weeks, parasite mature into adult worms, residing in the blood vessels of the body where the females produce eggs. Some of the eggs travel to the bladder or intestine and are passed into the urine or stool.
How can the vector be controlled?
At present, niclosamide (used in many Bayer vector control products) is the only WHO recommended molluscicide. It comes in different formulations, such as 70 percent wettable powder and 25 percent emulsifiable concentrate, as well as in granules and sand and gelatin forms, and has been used with some success. The need to repeat treatment makes this strategy time-consuming and less cost-effective, especially for large areas.
The most economic and effective approach to controlling the snail population would be to use focal and seasonal application of molluscicide, targeting implementation based on the transmission cycle. Aside from the cost, other concerns regarding the use of molluscicides is their toxic effect on macro-organisms and micro-organisms and environmental pollution.
Other means of controlling snails are by environmental methods, such as burying their habitats, flooding the snails with water up to several meters in depth, and digging ditches or water drainage tunnels.
What are the difficulties and challenges in combating Schistosomiasis?
- maintaining and consolidating advances made in disease control
- enhancing access to diagnosis and treatment for millions of infected people
- developing better diagnostics and/or a vaccine to prevent infection
- increasing awareness of the disease among both the vulnerable population and all decision-makers who have an impact on the elimination of the disease
- underfunding of vector control programs and decentralization of facilities and manpower
What programs exist against Schistosomiasis?
Many but not all countries endemic for Schistosomiasis have established control programs. Countries where development has led to widespread improvements in sanitation and water safety, as well as successful Schistosomiasis control programs, may have eliminated this disease. However, there are currently no international guidelines for certification of elimination.
WHO coordinates the strategy of preventive chemotherapy in consultation with collaborating centers and partners from academic and research institutions, the private sector, nongovernmental organizations, international development agencies, and other United Nations organizations. WHO develops technical guidelines and tools for use by national control programs.
Working with partners and the private sector, WHO has advocated for increased access to praziquantel and resources for implementation. A significant amount of praziquantel, to treat more than 100 million children of the school age per year, has been pledged by Merck KGaA and development partners.
Merck KGaA initiated the Praziquantel Donation Program in cooperation with WHO back in 2007. Since then, more than 500 million tablets have been donated and over 100 million patients treated, mainly school children. Merck has committed itself to maintaining its efforts in the fight against the tropical disease until Schistosomiasis has been eliminated. To this end, each year Merck is donating up to 250 million tablets to WHO. In addition, Merck is supporting awareness programs at schools in Africa in order to educate children about the causes of Schistosomiasis and ways to prevent it. Furthermore, as part of a public-private partnership, the company is researching a pediatric formulation of praziquantel that can also be administered to very young children. To date, the tablets are only suitable for children older than six. At the end of 2014, Merck founded the Global Schistosomiasis Alliance together with partners such as the Bill & Melinda Gates Foundation, the U.S. development agency USAID, the Schistosomiasis Control Initiative (SCI) and World Vision International.
What does Bayer contribute to the fight against Schistosomiasis
currently no activities
What is the societal burden of Schistosomiasis?
Children with long-term or repeat infections can suffer from anemia and malnutrition, which can contribute to lost days at school and serious learning disabilities.
How did the history of the disease proceed?
Schistosomiasis seems to have affected human health for at least 4,000 years; characteristic symptoms are described in early Egyptian papyri
- in 1851 German physician Theodor Bilharz, who first described the cause of urinary Schistosomiasis. That is why Schistosomiasis is also known as bilharzia or bilharziosis in many countries.
- in 1904 S. japonicum was discovered
- in 1907 S. mansoni was described and named in the honor of Sir Patrick Manson, the first scientist to speculate that the difference in egg morphology and manner of excretion, terminal versus lateral spine; fecal versus urinary, of African schistosomes was due to the existence of two separate species namely S. haematobium and S. mansoni.
- in 1908 the Brazilian parasitologist Pirajá da Silva was the first physician who described the entire disease cycle.
- 1913 Keinosuka Miyairi and Masatsuga Suzuki elucidated transmission and life cycle details for S. japonicum.
- in the 1950’s Mao recognized the problem of Schistosomiasis in China and decided to work towards eradication. He raised public awareness, and funded many public health projects.
- in 2014 the first known case of infection was discovered by archaeologists, who discovered a parasite egg near the pelvis of a child skeleton in northern Syria.
The child lived 6,200 years ago in a time when ancient societies first used irrigation systems to grow crops. Scientists suspect the new farming technique meant people were spending a lot of time wading in warm water. That may have triggered outbreaks of the disease.