Schistosomiasis, also known as bilharzia or snail fever, is an acute and chronic disease caused by parasitic worms. It is transmitted when larvae, released by freshwater snails, penetrate human skin when in contact with infested water. Both people and the freshwater snails (carriers of schistosome worms) become infected in water.
The adult worms are not particularly harmful inside the body. The eggs they produce and release are spread via blood throughout the body and can get trapped in organ tissues, such as the liver, the bladder, the lungs, and even the brain, leading to inflammation and damage. This causes the chronic and more severe aspects of the disease such as organ damage, bladder cancer, liver fibrosis, and genital lesions.
A common characteristic of communities plagued by this disease is the lack of access to safe, clean water as well as the absence of a proper sanitation and hygiene infrastructure. These communities turn to open, natural water sources to wash, clean, drink and bathe, increasing the risk of exposure.
There are 23 species of schistosome parasitic worms: six infect people, others infect domestic animals. Urogenital schistosomiasis is caused by Schistosoma haematobium and intestinal schistosomiasis by any of the organisms S. guineensis, S. intercalatum, S. mansoni, S. japonicum, and S. mekongi.
Cause and transmission
Schistosomiasis is a disease caused by small, blood-dwelling parasitic worms, also known as blood fluke. A person can become infected after being in contact with freshwater snails. The parasite’s larvae leave the freshwater snail and penetrate a person’s skin, whilst (s)he is for example swimming or washing. Because the larvae are microscopic, the person is unaware when they get infected by a schistosome parasite.
The schistosome parasite enters the person’s blood where it lives and reproduces. It can release hundreds of eggs into the person’s blood stream. Some of these eggs are released into the environment when an infected person defecates or urinates. If the eggs reach freshwater, as they will in areas lacking safe water and sanitation facilities, they hatch into a larval form and infect a particular kind of freshwater snail. Here, the parasite grows and releases thousands of larvae into the water each day. The larvae will look for another person to infect. This is the life cycle of the schistosome parasite. Schistosomiasis transmission cannot occur from person to person without the freshwater snail as host and without water contact.
The risk of infection is linked to the frequency of contact people with infested freshwater.
When a person first comes in contact with a schistosome parasite, the parasite will pierce the skin and enter the blood. The penetration of the parasite cannot be seen with the naked eye and it is often asymptomatic. There can be, however, an initial, often itching rash that spreads around the area of infection.
After a couple of weeks, an infected person may experience a more severe skin rash, fever, headache, gland enlargement, diarrhea, abdominal pain, fatigue and malaise.
Signs and symptoms
Rash ("swimmer's itch")
A first symptom that can appear is an itching rash (dermatitis) of round bumps. The rash starts within hours to days the larvae penetration of the skin. Acute reactions are more frequently seen in tourists and migrants, because people living in affected areas have often been repeatedly exposed to the parasite. However, in most people skin penetration is symptom-free.
Katayama fever (acute schistosomiasis)
Katayama fever may occur weeks to months after the initial infection as a systemic reaction against schistosomulae which are migrating through the bloodstream to the lungs and to the liver. Nonspecific symptoms appear, which can be difficult to recognize: fever, fatigue, muscle pain, cough and shortness of breath (bronchospasm), diarrhea, weight loss, itching (urticarial) rash, joint pain, liver and spleen enlargement. Similarly to swimmer's itch, Katayama fever is more common in people suffering from their first infection, like migrants and tourists. The symptoms usually get better on their own, but a small proportion of people have persistent complaints.
In long-established disease, the eggs from the adult worms can cause inflammatory reactions. The long-term manifestations, which can become chronic, depend on the species of schistosome, as adult worms of different species migrate to different body areas.
Symptoms are: diarrhea, bloody stools, anemia, stunted growth, esophageal varices, enlarged liver and spleen, severe damage to the liver leading to liver fibrosis, and portal hypertension.
Symptoms are: blood in urine (haematuria), painful urination (dysuria), anemia and stunted growth. It can lead to damage of the genitals, kidneys and bladder, bladder cancer, and it leads to an increased risk of sexually transmitted diseases like HIV/AIDS.
When the dispersed schistosome eggs lodge in the genital tract, they cause genital schistosomiasis. These present slightly differently in women and men.
- Female Genital Schistosomiasis (FGS) can have serious consequences in girls and women that drastically decreases quality of life. It leads to pelvic inflammation, genital lesions, infertility and can increase the likelihood of contracting other dangerous diseases such as HIV/AIDS. Is it often misdiagnosed as a sexually transmitted disease due to lack of awareness of health staff.
- Male Genital Schistosomiasis (MGS) can cause abdominal and pelvic pain, genital inflammation, blood in the urine and semen, painful erection and ejaculation. The impact of MGS is not yet fully known and is currently being researched.
Central nervous system schistosomiasis
Only a minority of patients will develop central nervous system (brain or spine) symptoms or complications or from the caused by schistosomiasis (S. japonicum, S. mansoni and S. haematobium).
Schistosomiasis is diagnosed through the detection of parasite eggs in stool or urine specimens. Antibodies and/or antigens detected in blood or urine samples are also indications of infection.
For urogenital schistosomiasis, a urine filtration technique is used. Children with S. haematobium almost always have microscopic blood in their urine, which can be detected by special urine dipsticks.
The eggs of intestinal schistosomiasis can be detected in fecal specimens. For S. mansoni, a point-of-care test that detects the schistosome parasite antigen in urine, the Circulating Cathodic Antigen (CCA) test, can be used for diagnosis..
For people living in non-endemic or low-transmission areas, serological and immunological tests may be useful in showing exposure to infection and the need for thorough examination, treatment and follow-up.
Schistosomiasis can be treated by a medicine called praziquantel (brand name Biltricide®, among others), which kills the adult worms. This is the recommended treatment against all forms of schistosomiasis. It is a one day treatment (1-4 dosages) which is effective, safe, and available at low-cost. Sadly, it cannot prevent reinfection after treatment, but regular treatment cycles throughout childhood prevents the development of the disease and can also reverse disease progression. Praziquantel dosages should be height and weight adjusted.
The World Health Organization (WHO) also recommends treating preschool-aged children. Unfortunately, the current praziquantel tablet is not suitable for children under the age of six and can only be given under careful observation after a positive test that confirmed infection. The WHO states that praziquantel is safe to use during pregnancy.
An important challenge is to ensure that all infected people and all people at risk of developing schistosomiasis have adequate access to treatment. Praziquantel has a short shelf life of two years. It is a challenge to ensure sufficient supply of praziquantel, especially in peripheral health services in more remote settings.
Praziquantel is on the WHO’s List of Essential Medicines.
Prevention, control and elimination
For sustainable schistosomiasis control and elimination, affected communities need access to regular treatment, safe water supply and sanitation and hygiene infrastructure. Health education should be given on safe behaviour that stops transmission of the parasite (e.g., seeking treatment when feeling ill, using safe sanitation facilities) and on how to avoid exposure to infection.
The main strategy for the control and elimination of schistosomiasis is based on large-scale treatment (mass drug administration) of at-risk communities and groups; through school deworming and community treatment programmes; and by bahaviour change interventions.
The risk of schistosomiasis can also be reduced by controlling the number and spread of freshwater snails in areas where people have contact with water, either through snail control technologies or environmental management.
Schistosomiasis affects almost 240 million people worldwide, and more than 700 million people live in endemic areas. The infection is prevalent in tropical and sub-tropical areas, especially in poor communities without access to safe drinking water and adequate sanitation. It is estimated that at least 90% of those requiring treatment for schistosomiasis live in Africa (www.who.int).
There are 23 species of schistosome parasitic worms: six infect people, others infect domestic animals. Urogenital schistosomiasis is caused by Schistosoma haematobium and intestinal schistosomiasis by any of the organisms S. guineensis, S. intercalatum, S. mansoni, S. japonicum, and S. mekongi. S. haematobium is found mainly in Africa. S. mansoni has a similar distribution but is also endemic in South America and Asia.
Several million people all over the world suffer from severe morbidity as a consequence of schistosomiasis. Chronic schistosomiasis may affect people’s ability to work and in some cases can result in death.
Which of the cross-cutting issues are relevant
Large-scale treatment of at-risk populations (mass drug administration) and WASH interventions are the main strategies for control and elimination of schistosomiasis. WASH interventions include improving access to safe water and sanitation as well as hygiene promotion. Similar strategies are used to reduce transmission and prevention in other NTDs (Onchocerciasis, Dracunculiasis, Lymphatic Filariasis, Soil-transmitted Helminths and Trachoma). This provides opportunities for integrated NTD programmes that address multiple diseases by combining interventions related to healthy behaviour, water and sanitation, education, and other sectors in a coordinated way.
Relevant tools / interventions
The Pediatric Praziquantel Consortium is working on developing a paediatric formulation for children under the age of six , which is currently studied in clinical trials.
Great strides have been made with collaborative work on WASH and NTDs, and there are exciting collaborative projects ongoing in Ethiopia, Uganda, Kenya where schistosomiasis programmes are working closely with the WASH sector and behaviour change initiatives.
WHO and UNAIDS have called for the integration of schistosomiasis treatment and FGS prevention and diagnosis in women’s health services. Research in this topics is ongoing, visit the website of the Global Schistosomiasis Alliance for more info.