eMedicine Specialties > Pediatrics: General Medicine > Parasitology

Schistosomiasis

Author: Vinod K Dhawan, MD, FACP, FRCP(C), Professor, Department of Clinical Medicine, University of California at Los Angeles; Professor of Medicine, Charles R Drew University of Medicine and Science; Chief, Division of Infectious Diseases, MLK-Harbor Hospital
Coauthor(s): Michael D Nissen, MBBS, BMedSc, FRACP, FRCPA, Associate Professor in Biomolecular, Biomedical Science & Health, Griffith University; Director of Infectious Diseases and Unit Head of Queensland Paediatric Infectious Laboratory, Sir Albert Sakzewski Viral Research Centre, Royal Children's Hospital; John Charles Walker, MSc, PhD, Head, Department of Parasitology, Center for Infectious Diseases and Microbiology, Westmead Hospital, Westmead, Australia; Senior Lecturer, Department of Medicine, University of Sydney, Australia
Contributor Information and Disclosures

Updated: May 15, 2008

Introduction

Background

Human schistosomiasis is principally caused by one of the following 6 species of parasitic worms: Schistosoma haematobium, Schistosoma intercalatum, Schistosoma japonicum, Schistosoma malayensis, Schistosoma mansoni, and Schistosoma mekongi. Other species of animal schistosomes cause human infection, including schistosomes of birds and small mammals that cannot mature in the human host but die in the skin where they cause dermatitis. 

Schistosomes are blood flukes and belong to the class Trematoda. Unlike other trematodes, schistosomes are elongated but become round as they adapt to residing in blood vessels of the genitourinary or GI tract. They require a vertebrate and an intermediate water-dwelling snail host to complete their life cycle. Geographic distribution and maintenance of human infection by schistosomes depends on and is limited by the presence of a suitable snail host.

Clinical features of human schistosomiasis depend on the species, developmental stage, and site of infection in the body. This can be summarized into 3 major syndromes: (1) cercarial dermatitis, (2) acute schistosomiasis or Katayama fever, and (3) chronic fibro-obstructive disease.

Pathophysiology

Schistosomes are digenetic, which means that mature adult worms reproduce sexually in the definitive human host with asexual reproduction in the larval forms (common to all trematodes or flukes). They are also diecious, which means that adults are sexually distinct and that male and female genitals do not occur within the same individual (a characteristic that separates them from other flukes).

The schistosome life cycle differs from that of other trematodes in that humans become infected by the penetration of cercariae through the skin rather than through oral ingestion. Human schistosomes can infect other vertebrates and provide an animal reservoir of infection, although this is of epidemiologic significance only for S japonicum and, possibly, S mekongi. Snails of the genera Biomphalaria, Bulinus, Neotricula, and Oncomelania are the principle intermediate hosts for S mansoni, S haematobium, S mekongi, and S japonicum, respectively.

Frequency

United States

Frequency of infection has been estimated to exceed 400,000 persons and is principally because of immigrants from endemic areas. Transmission of the disease cannot occur in the United States because of lack of suitable snail hosts. Epidemics of acute schistosomiasis have been reported in Americans traveling in or returning from endemic areas. Cercarial dermatitis due to avian schistosomes has been reported in the Great Lakes region of the United States.

International

Human schistosomes currently infect more than 200 million people in 74 countries worldwide in the endemic areas of Africa, the Caribbean, Central America, South America, East Asia, and the Middle East.^1,2 Prevalence is thought to be increasing. The most severely affected countries in Africa include Angola, Chad, Congo, Egypt, Ghana, Kenya, Madagascar, Malawi, Mali, Mozambique, Nigeria, Senegal, Sudan, Tanzania, Uganda, Zambia, and Zimbabwe. Yemen has the most infected people in the Middle East. Brazil is the most affected country in the Americas, with 25 million people living in endemic areas and an estimated 3 million infected.

S mansoni is found in 54 countries, including the Arabian peninsula (the most heavily infested population is in Yemen), Egypt, Libya, Mauritania, Somalia, Sudan, sub-Saharan Africa, Brazil, the Caribbean (except Antigua, Guadeloupe, Martinique, Montserrat, and St. Lucia), Suriname, and Venezuela.

S haematobium is endemic in 53 countries in the Middle East and most of the African continent, including the islands of Madagascar and Mauritius. The infection is unlikely to be of public health significance in Lebanon, Mauritius, Oman, Syria, Tunisia, and Turkey because transmission is low or nonexistent. A disputed and ill-defined focus is noted in India and requires further confirmation.

S japonicum is endemic in China, the Sulawesi province of Indonesia, and the Philippines.³ China is the most affected country, with an estimated 900,000 people infected. The parasite has been eradicated from Japan since 1982.

S intercalatum has been reported in 10 countries in central and western Africa. S mekongi is confined to Cambodia and Laos, where the borders run along the Mekong River. Neotricula snails have been reported in southern China, but S mekongi has not been reported in these areas. S malayensis has been reported among aboriginal people in a small jungle focus in Malaysia.

Mortality/Morbidity

Mortality from human schistosomiasis is related to complications of fibro-occlusive disease secondary to the immune stimulus of schistosome eggs and end organ damage.

Morbidity of schistosomiasis correlates well with the worm burden as calculated by fecal and urinary egg counts. The disease significantly disrupts the nutritional status and growth from middle childhood to adolescence. Prevalence of infection in endemic communities demonstrates a negative binomial distribution, with most infected individuals having low worm burdens and only a small percentage with heavy infestations. A possible genetic link has been observed between human leukocyte antigen (HLA) antigens and the occurrence of end-stage liver disease in S japonicum and S mansoni infections.

Sex

Both sexes are equally susceptible. Because of different local and cultural work and social practices, either sex may be more exposed to infection.

Age

People of all ages are susceptible. Immune responses are heightened and more intense with secondary and subsequent exposures to schistosome cercariae. Surveys in endemic areas have demonstrated that severity and prevalence of infection takes several years to peak in children and that both decrease with age. Most manifestations of disease occur in the second decade of life with a relation to peak egg output. Evidence points to a partial and acquired immunity to schistosomes that is determined by specific antibodies and eosinophils, and the immunity targets immature adult worms.

Clinical

History

Symptoms of schistosomiasis depend on the species of the schistosome and the stage of disease. Most infected individuals are asymptomatic or have only mild nonspecific symptoms. Only 5-10% of infected individuals develop severe clinical symptoms, which are usually associated with heavy infestations. Such symptoms are as follows:

• Edema and pruritus are symptoms of cercarial dermatitis and are rarely observed in primary exposure (see Media file 1).
• High fever or Katayama fever is described in heavy primary infections.
• Nausea is observed in acute schistosomiasis and at the onset of fevers.
• Vomiting may be experienced in acute schistosomiasis.
• Generalized lymphadenopathy is described as part of the Katayama fever syndrome.
• Profuse diarrhea or dysentery is associated with Katayama fever.
• Hematemesis or melena is the usual presenting symptom of chronic schistosomiasis due to S japonicum and S mansoni.
• Abdominal pain may be observed with acute and chronic schistosomiasis secondary to liver and splenic enlargement (see Media file 2).
• Urticaria may occur with cercarial dermatitis or may be more extensive, affecting large body areas in acute schistosomiasis.
• Malaise, generalized fatigue, and weakness are sometimes observed with acute schistosomiasis.
• Focal epilepsy is a recognized complication of chronic S japonicum infection in East Asia (see Media file 3).
• Spinal cord lesions due to S mansoni or S haematobium can cause a myelopathy.
• A dry cough (with or without hemoptysis) secondary to inflammation in the lungs can be experienced in acute schistosomiasis and is associated with migration of schistosomula through the lungs. Cough is also a symptom of cor pulmonale resulting from chronic pulmonary schistosomiasis. The subsequent pulmonary hypertension may lead to hemoptysis.
• Palpitations may be felt as a complication of cor pulmonale in chronic pulmonary schistosomiasis.
• Shortness of breath may be observed in both acute and chronic pulmonary schistosomiasis as a result of cor pulmonale in chronic cases. Acute pulmonary symptoms of cough and dyspnea may occur 3-6 weeks after exposure and occur without fever.
• Weight loss may be experienced as a consequence of vomiting and profuse diarrhea.
• Urinary frequency is regularly associated with acute S haematobium infection.
• Dysuria is a common feature of acute S haematobium infection.
• Terminal hematuria is also a regular presenting symptom for S haematobium infection or bilharzia.
• Referred suprapubic or perineal pain may be associated with S haematobium infection.

Physical

Physical findings vary with the species and the stage of the disease.

• Urticaria: Urticaria may be observed in cases with cercarial dermatitis or acute schistosomiasis with Katayama fever.
• Papular dermatitis: Papular lesions may develop from cercarial dermatitis and remain for 5-7 days and up to 10 days after water exposure. However, this is more often observed with avian schistosomiasis.
• Generalized lymphadenopathy: General enlargement of lymph nodes may be detected in acute schistosomiasis with Katayama fever.
• Abnormal pulmonary sounds: Areas of moist crackles may be heard over both lung fields in acute schistosomiasis.
• Hepatosplenomegaly: Hepatosplenomegaly is often detected in acute and chronic schistosomiasis with S japonicum and S mansoni. Jaundice is a rare clinical finding.
• Purpura: Purpura resulting from thrombocytopenia is observed with hypersplenism.
• Hematuria: Hematuria with S haematobium typically progresses from microscopic to frank bloody urine as the bladder mucosa ulcerates.
• Myelitislike syndrome: A transverse myelitislike syndrome may be observed with S mansoni or S haematobium disease.
• Focal neurologic deficits: Focal neurologic deficits may be found due to cerebral granulomas in cases with seizures and chronic S japonicum infection.
• Right ventricular hypertrophy: Clinical signs of right ventricular hypertrophy are observed in cases with cor pulmonale and chronic schistosomiasis. This includes palpable sternal heave and increased and delayed pulmonary second heart sound.
• Female genital tract disease: Female genital tract disease (schistosomiasis) is defined as the presence of schistosome eggs and/or worms in the upper and/or lower genital tracts. Obvious vulval, vaginal, and/or cervical schistosomal disease has been reported from both endemic and nonendemic areas. Possible consequences include hypogonadism, retarded puberty, infertility (primary and secondary), ectopic and tubal pregnancy, tubal abortion, hemoperitoneum, anemia due to chronic blood loss, metaplasia, miscarriage and preterm delivery, carcinoma, increased risk for sexually transmitted diseases, destruction of the hymen and/or clitoris, and vesicovaginal fistula.

Causes

The diagnosis of schistosomiasis should be considered in any individuals who have resided in endemic areas with significant freshwater contact.

• Life cycle of human schistosomes
  • Once cercariae have entered the body, they are called schistosomulae, which migrate through the tissues to invade blood vessels. The organisms are transported to the lungs and then to the liver, where they mature into an adult worm within 6 weeks before descending to their final positions in the venous circulation.
  • Adults of S haematobium are mostly found in the venous plexi of the bladder, prostate, and uterus, whereas S intercalatum, S japonicum, S mansoni, and S mekongi are observed in the portal, inferior, and superior mesenteric veins. Maturity of female worms depends on the presence of a mature male because they form pairs, with the female lying enclosed within a groove formed by the male.
  • Adult worms have a mean life span of 5-10 years, with females releasing 300-3000 eggs per day. Eggs are deposited in the terminal venules of the bladder (S haematobium), intestine, and rectum (S intercalatum, S japonicum, S mansoni, S mekongi), where they mature to contain a miracidium over the next 10 days. The miracidium releases proteolytic enzymes, which facilitate larval movement through the tissues into either the genitourinary tract or the gastrointestinal tract.
  • Schistosome eggs contain spines in positions characteristic of each species (see Media file 4). Eggs are passed in human urine and excrement, and motile miracidia are released upon contact with fresh water. Miracidia then actively seek out snail hosts in which they develop first into mother and daughter sporocysts by asexual division and then into cercariae over 4-6 weeks. Cercariae then leave the snail and penetrate the human skin on contact with the assistance of their glandular secretions.
• Granuloma formation
  • Not all schistosome eggs are excreted from the body, and as many as 50% can embolize to other body areas, leading to a host immune reaction and granuloma formation. Granulomas begin to form with maturation of the miracidium at 6 days and are focal within 2 weeks.
  • The most common sites are the liver for S intercalatum, S japonicum, and S mansoni and the bladder for S haematobium. Other areas less commonly affected include the lungs, CNS, and kidneys.
• Immune response
  • The immune response to schistosomiasis is highly regulated and of the delayed hypersensitivity type, including TH1 and TH2 responses with local cytokine production. Granulomata are notable for the presence of eosinophils and lead to the development of widespread collagen deposition and scar tissue. This causes major lesions of chronic schistosomiasis with blood flow obstruction in affected tissues.
  • Human skin appears to be a critical site in which the initial events of the host and parasite interaction occur and where the immune response is commenced. Induction and modulation of granuloma formation is under the control of clones of CD4 and CD8 T cells. Cytokines produced in response to the parasite, such as interleukin 7 in the skin and interferon-gamma in the liver, also seem to influence the development of schistosomal immunity.
• Specific schistosomal parasites
  • S mekongi is thought to be the principle human schistosome pathogen of Cambodia and Laos. It has smaller eggs and a different intermediate snail host (Neotricula aperta) than S japonicum. Hepatosplenomegaly, portal hypertension, or both are the main clinical findings in infected individuals.
  • S intercalatum (localized to West and Central Africa) is of less epidemiologic importance in these regions than S haematobium and S mansoni. Observed symptoms include abdominal pain and bloody diarrhea.
• Cercarial dermatitis: Cercarial dermatitis is more commonly observed in human infection with avian schistosomes, cercarial demise, and immediate hypersensitivity reactions occurring at skin invasion sites.
• Katayama fever: This occurs with the onset of egg production. The syndrome includes lymphadenopathy and diarrhea, with symptoms and signs similar to a serum sickness reaction. It is commonly associated with S japonicum infection and is probably due to the larger number of eggs released by this species. It is less common with S mansoni infestation and is rare with S haematobium. Onset of symptoms is 20-40 days after exposure, and temperatures may reach 105°F. Fevers spontaneously subside 2-10 weeks after onset.
• S haematobium infection: During infection by S haematobium, eggs are deposited in the mucosa and submucosa of the bladder and lower ureters. Granulomas are very cellular and form intraluminal polyploidal lesions that can lead to hydronephrosis. Lesions tend to necrotize, ulcerate, and bleed. With age, the lesions become acellular, fibrose, and calcify and are termed sandy patches. Calcification may lead to bladder deformation, ureteric obstruction, secondary infections, hydronephrosis, chronic pyelonephritis, and renal failure. Carcinoma of the bladder is a long-term sequela of chronic infection.
• Liver and spleen disease: In acute schistosomiasis, the enlarged liver and spleen are initially soft from passive venous congestion and granuloma cellular proliferation. In chronic disease, the liver and spleen remain large, nodular, and nontender. Stigmata of liver disease and cirrhosis such as ascites, spider nevi, peripheral edema, testicular atrophy, and feminization are usually absent. However, a small number of patients decompensate and show the above signs of chronic liver disease.
• Focal epilepsy: This is secondary to a localized brain granuloma or generalized encephalopathy. It constitutes 3% of the complications of chronic disease.
• Transverse myelitis: In transverse myelitis, spinal cord lesions are the result of the retrograde flow of eggs and granuloma formation in either S mansoni or S haematobium infection.
• Pulmonary symptoms: Pulmonary involvement (ie, cor pulmonale) is most commonly observed with S haematobium infection because ectopic egg deposition can occur more readily from the vesical plexus to the lungs via the inferior vena cava, bypassing the liver. Pulmonary complications with S japonicum and S mansoni are usually only observed after the development of a collateral circulation secondary to severe hepatosplenic disease.

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