Schistosomiasis

Clinical Description

Identification

A blood fluke (trematode) infection with adult male and female worms living within mesenteric or vesical veins of the host over a life span of many years. Eggs produce minute granulomata and scars in organs where they lodge or are deposited. Symptoms are related to the number and location of the eggs in the human host: Schistosoma mansoni and S. japonicum give rise primarily to hepatic and intestinal pathology, and early signs and symptoms include diarrhea, abdominal pain and hepatosplenomegaly. S. japonicum can also cause CNS disease, with Jacksonian seizures. S. haematobium gives rise to urinary manifestations, and early signs and symptoms include dysuria, urinary frequency and hematuria at the end of urination; CNS disease has, rarely, been reported.

The WHO-recommended case definitions in endemic areas are a) For urinary schistosomiasis: visible hematuria or positive reagent strip for hematuria, or with eggs of S. haematobium in urine (confirmed case); b) For intestinal schistosomiasis: non-specific abdominal symptoms, blood in stool, hepato(spleno)megaly (suspected case), or presence of eggs in stools (confirmed case).

The most important effects of schistosomiasis are the late complications that arise from chronic infection: liver fibrosis, portal hypertension and its sequelae, and possibly colorectal malignancy in the intestinal forms; and obstructive uropathy, superimposed bacterial infection, infertility and bladder cancer in the urinary form. Eggs can be deposited at ectopic sites, including the brain, spinal cord, skin, pelvis and vulvovaginal areas.

The larvae of certain schistosomes of birds and mammals may penetrate the human skin and cause a dermatitis, sometimes known as “swimmer's itch”; these schistosomes do not mature in humans. Such infections may be prevalent among bathers in lakes in many parts of the world. However, the clinical entity of “seabather's eruption”, a pruritic dermatitis that appears principally where the bathing suit has been worn, has been shown to be caused by the larval stage of some jellyfish species, and not by a schistosome.

Infectious Agents

Schistosoma mansoni, S. haematobium and S. japonicum are the major species causing human disease. S. mekongi, S. malayensis, S. intercalatum and S. mattheei are of importance only in limited areas.


Diagnosis

Definitive diagnosis of schistosomiasis depends on demonstration of eggs in biopsy specimens, in the stool by direct smear or on a Kato thick smear, or in urine by the examination of a urine sediment or Nuclepore® filtration. Urine filtration is especially useful for S. haematobium infections. Useful immunological tests include immunoblot analysis, the circumoval precipitin test, IFA and ELISA with egg or adult worm antigen, and RIA with purified egg or adult antigens; positive results on serological antibody detection tests could be indicative of prior infection and are not proof of current infection. More recently, various assays developed to detect schistosome antigens directly in serum or urine have proved useful in detecting current infection, and in assessing cure after treatment.


Epidemiology

Mode of Transmission

Infection is acquired from freshwater containing free-swimming larval forms (cercariae) that have developed in snails. The eggs of S. haematobium leave the mammalian body mainly in the urine, those of the other species in the feces. The eggs hatch in water and the liberated larvae (miracidia) penetrate into suitable freshwater snail intermediate hosts. After several weeks, the cercariae emerge from the snail and penetrate human skin, usually while the person is in contact with infected water—e.g. working, swimming or wading. Cercariae then enter the bloodstream, are carried to blood vessels of the lungs, migrate to the liver, develop to maturity, then migrate to veins of the abdominal or pelvic cavity.

Adult forms of S. mansoni, S. japonicum, S. mekongi, S. mattheei and S. intercalatum usually remain in mesenteric veins; those of S. haematobium usually migrate through anastomoses into the vesical plexus of the urinary bladder. Eggs are deposited in venules and escape into the lumen of the bowel or urinary bladder, or end up lodging in other organs, including the liver and the lungs.

Incubation Period

Acute systemic manifestations (Katayama fever) may occur in primary infections 2–6 weeks after exposure, immediately preceding and during initial egg deposition. Acute systemic manifestations are uncommon, but can occur with S. haematobium infections.

Period of Communicability

Not communicable from person to person; persons with schistosomiasis may spread the infection by discharging eggs in urine and/or feces into bodies of water for as long as they excrete eggs. It is common for human infections with S. mansoni and S. haematobium to last in excess of 10 years. Infected snails will release cercariae for as long as they live, a period that may last from several weeks to about 3 months.

Reservoir

Humans are the principal reservoir of S. haematobium, S. intercalatum and S. mansoni, although the latter has been reported to occur in rodents. Humans, dogs, cats, pigs, cattle, water buffalo and wild rodents are potential hosts of S. japonicum; their relative epidemiological importance varies in different regions. S. malayensis appears to be a rodent parasite that occasionally infects humans. Epidemiological persistence of the parasite depends on the presence of an appropriate snail as intermediate host—i.e. species of the genera Biomphalaria for S. mansoni; Bulinus for S. haematobium, S. intercalatum and S. mattheei; Oncomelania for S. japonicum; Neotricula for S. mekongi; and Robertsiella for S. malayensis.

Susceptibility

Susceptibility is universal; any immunity developing as a result of infection is variable and not yet fully investigated.

Occurrence

S. mansoni is found in Africa (including Madagascar); the Arabian Peninsula; Brazil, Suriname and Venezuela in South America; and in some Caribbean islands. S. haematobium is found in Africa (including Madagascar) and the Middle East. S. japonicum is found in China, the Philippines and Sulawesi (Celebes) in Indonesia; no new cases have been found in Japan since 1978 after an intensive control program. S. mekongi is found in the Mekong River area of Cambodia and the Lao People's Democratic Republic. S. intercalatum occurs in parts of western Africa, including Cameroon, Chad, the Democratic Republic of Congo, Gabon, and Sao Tome. S. malayensis is known only from peninsular Malaysia. Human infection with the bovine parasite S. mattheei has been reported from southern Africa.


Prevention and Control

1.    Preventive measures:

a.    Treat patients in endemic areas with praziquantel to relieve suffering and prevent disease progression. Regularly treat high-risk groups—such as school-age children, women of childbearing age, or special occupational groups in endemic areas—with presumptive curative doses. A height-measuring pole (http://whqlibdoc.who.int/trs/WHO_TRS_912.pdf) has been tested in Africa and facilitates praziquantel dosage.

b.    Educate the public in endemic areas to seek treatment early and regularly, and to protect themselves.

c.    Dispose of feces and urine so that viable eggs will not reach bodies of freshwater containing intermediate snail hosts. Though difficult, control of animals infected with S. japonicum is desirable.

d.    Improve irrigation and agriculture practices; reduce snail habitats by removing vegetation, by draining and filling, or by lining canals with concrete.

e.    Treat snail-breeding sites with molluskicides. Cost may limit the use of these agents.

f.    Individual protection: prevent exposure to contaminated water (e.g. by wearing rubber boots). To minimize cercarial penetration after brief or accidental water exposure, vigorously and completely towel dry skin surfaces that are wet with suspected water. Apply 70% alcohol immediately to the skin to kill surface cercariae.

g.    Provide water for drinking, bathing and washing clothes from sources free of cercariae or treated to kill them. Effective measures for inactivating cercariae include water treatment with iodine or chlorine. Allowing water to stand 48–72 hours before use is also effective.

h.    Travelers visiting endemic areas should be advised of the risks and informed about preventive measures.

2.    Control of patient, contacts and the immediate environment:

a.    Report to local health authority: in selected endemic areas; in many countries, not a reportable disease, Class 3.

b.    Isolation: Not applicable.

c.    Concurrent disinfection: Sanitary disposal of feces and urine.

d.    Quarantine: Not applicable.

e.    Immunization of contacts: Not applicable.

f.    Investigation of contacts and source of infection: Examine contacts for infection from a common source.

g.    Specific treatment: Praziquantel is the drug of choice against all species. Alternative drugs are oxamniquine for S. mansoni, and metrifonate for S. haematobium.

3.    Epidemic measures:

Examine for schistosomiasis and treat all who are infected, but especially those with disease and/or moderate to heavy intensity of infection; pay particular attention to children. Provide clean water, warn people against contact with water potentially containing cercariae, and prohibit contamination of water. Treat areas that have high snail densities with molluskicides.

4.    Disaster implications:

None.

5.    International measures:

WHO Collaborating Centres provide support as required. More information can be found at: http://www.who.int/collaboratingcentres/database/en/

6.    Further information may be found at: http://www.who.int/tdr/diseases/schisto/default.htm

Source: Heymann (Ed.). (2008). Control of Communicable Diseases Manual, 19th edition. Washington, DC: American Public Health Association.

Common Disease Taxonomy: