The infectious disease schistosomiasis is caused by parasitic worms (helminths) that inhabit freshwater rivers and other sources of fresh water. It is spread to humans by an intermediate host, namely, freshwater snails. When the disease advances, it causes diarrhea and bloody stool and, when left untreated, it can in the long run damage the liver. Schistosomiasis might also induce cancer in internal organs such as urinary bladder, which can be fatal.
According to data from the Centers for Disease Control and Prevention (CDC) in 2011, 240 million people worldwide were infected with this disease.
Causes of Infection
Schistosomiasis is caused by parasitic worms (helminthes) that inhabit several visceral organs. It is also infectious and is transmitted by freshwater snails (e.g., Oncomelania hupensis) as the intermediate host. Most human infections are caused by Schistosoma mansoni, S. haematobium, or S. japonicum. Additionally, with much less frequency than these three, S. mekongi and S. intercalatum, both harmful to people, can also cause this disease.
Cercariae of schistosomes that are egested from freshwater snails can penetrate the skin of people swimming, washing, bathing, or excreting in contaminated water. After entering the body of the host, these helminths will then mature to the adult stage in the space of several weeks. After maturation, they migrate in pairs to blood vessels, where the female will release eggs that also travel to the bladder and/or intestines and are passed into the urine or stool, thereby starting a new cycle of infection. Furthermore, some of the eggs can become trapped in body tissue and in doing so cause progressive damage to internal organs.
Schistosomiasis symptoms are thus caused by the reaction of the host’s body to the eggs produced by the worms.
Disease Agent：Schistosoma, parasitic worms
Intermediate Host：Freshwater snails
Usually, the total incubation period for patients with acute schistosomiasis is 14–74 days, although many patients are asymptomatic immediately after infection. It is some time after egg laying that symptoms emerge, mainly in intestines and urogenital organs.
In cases of acute infection, the patient may develop a rash or itchy skin within the space of a few days from the onset of the initial infection date, while fever, chills, coughing and muscle ache can also present themselves from within one to two months. Often eosinophilia is present with hepato- and/or splenomegaly.
If adult worms are also present, the eggs will be produced in the body that will subsequently migrate to the intestines, liver or bladder and thereby, cause inflammation, scarring, diarrhea, constipation, and bloody stool/urine. For pediatric patients, repeated infection can also result in anemia, malnutrition and learning difficulties.
Left untreated for a few years after infection, host immune responses to schistosome eggs result in damage to the liver, intestines, and bladder. Chronic inflammation can lead to such serious problems as bowel wall ulceration, hyperplasia, polyposis, liver fibrosis and bladder cancer. In addition, it occasionally causes damage to genital organs (the fallopian tubes and cervix in females and the seminal vesicles and prostate in males). Rarely, eggs are found in the brain or spinal cord and these can cause seizures, paralysis, or spinal cord inflammation.
Diagnosis and Treatment
The primary method for diagnosing schistosoma is by examining stool and/or urine for worm eggs.
At the adult stage, S. mansoni, S. japonicum, S. mekongi and S. intercalatum inhabit mesenteric venous plexuses in infected patients, with eggs being shed in the feces. In the case of adult S. haematobium, the venous plexuses of the lower urinary tract are preferred and eggs are thus shed in urine.
There are regions where S. mansoni and S. haematobium are found together in sub-Saharan Africa. In these regions, examination of both stool and urine is conducted. For accurate results, there is a waiting period before the blood sample is taken: until the antibody is formed within those infected (six to eight weeks after the last exposure to contaminated water).
Regardless of the species, praziquantel is an effective treatment against all major types of schistomes, although timing is of crucial importance as it is adult worms that are the most susceptible to the drug. Furthermore, a mature antibody response to the parasite is needed for praziquantel to have the desired effect and thus the treatment must begin at least six to eight weeks from the time the patient was last exposed to a contaminated freshwater source. Usually a single course of treatment is necessary, and this is verified with a follow-up examination one to two months after treatment. Mild cases in patients with weakened immune systems may also require repeat treatment two to four weeks following initial treatment to increase treatment effectiveness.
No vaccine is available for this disease. At-risk communities in countries endemic to schistosomiasis should not swim or otherwise enter freshwater sources and should also either boil (for at least one minute) or filter drinking water before consumption. Similarly, bathwater should be heated to a rolling boil for at least one minute before use. If the water is from a storage tank, it should sit from one to two days at least before being considered safe for bathing.
Regions at High Risk of Infection
About 90% of infections occur in Africa. In particular, freshwater in southern and sub-Saharan Africa is at risk for schistosomiasis transmission. Transmission also occurs in the Maghreb region of North Africa and the Nile River valleys in Egypt and the Sudan.
Aside from Africa, regions with infection risks include Brazil, Suriname, and Venezuela in Latin America; Dominica, Guadeloupe, Martinique, and Saint Lucia in the Caribbean; Iran, Iraq, Saudi Arabia, and Yemen in the Middle East; southern China; and some parts of Southeast Asia, including the Philippines and Lao PDR.
Estimated Number of Infected People
There are more than 258 million people in 78 countries infected with this disease. It was also reported that over 61.6 million of those infected received medical treatment in 2014.
Estimated Number of Deaths
CDC data in 2011 show that more than 280,000 people die of this disease every year in Africa alone, making it the NTD that has the largest number of deaths. Of these casualties, 200,000 are in sub-Saharan Africa.
WHO- Neglected Tropical Diseases, accessed March 19, 2014,
CDC- Neglected Tropical Diseases, accessed March 19, 2014,