Schistosomiasis

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Schistosomiasis

Author: Palaniandy Kogulan, MBBS, MD, Assistant Director of Internal Medicine, Synergy Medical Education Alliance; Assistant Professor of Medicine, Michigan State University College of Human Medicine
Coauthor(s): Daniel R Lucey, MD, MPH, Chief, Fellowship Program Director, Department of Internal Medicine, Division of Infectious Diseases, Washington Hospital Center; Professor, Department of Internal Medicine, Uniformed Services University of the Health Sciences
Contributor Information and Disclosures

Updated: Feb 26, 2010

Background

After malaria, schistosomiasis is the second most prevalent tropical disease in the world. In some parts of the world, it also is known as bilharzia in honor of Theodore Bilharz, who first identified the etiological agent for Schistosoma hematobium in Egypt in 1851.

The pathophysiology of schistosomiasis involves the immune response against the schistosome eggs. The clinical manifestations depend on the species of parasite, intensity of worm burden, and immunity of the person to the parasite. Recent World Health Organization (WHO) reports estimate that 500-600 million people in 74 tropical and subtropical countries are at risk for schistosomiasis. More than 200 million people in these countries are infected. Of these, 120 million are symptomatic, with 20 million having severe clinical disease.

Egg of Schistosoma hematobium, with its typical terminal spine.

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Egg of Schistosoma hematobium, with its typical terminal spine.

Persons at risk include those who live or travel in areas where schistosomiasis occurs and who come into contact with fresh water that contains the appropriate type of snail intermediate host. The main forms of human schistosomiasis are caused by 5 species of flatworm in the genus Schistosoma, within the class Trematode. The 5 species are as follows: S hematobium, Schistosoma mansoni, Schistosoma japonicum, Schistosoma intercalatum, and Schistosoma mekongi. The worms also are called blood flukes because they live in the vascular system of humans and other vertebrates.

The life cycle of the flatworms that cause human schistosomiasis involves a sexual stage in the human and an asexual stage in the freshwater snail host. The adult worms are small, 12-26 mm long and 0.3-0.6 mm wide, and vary with the different species. S hematobium lives in the venous plexus near the urinary bladder and ureters, S mansoni lives in the inferior mesenteric vein, and S japonicum lives in the superior mesenteric vein of both the large and small intestines.

Adult worms mate and lay eggs. The eggs are nonoperculate, possess a spine, and contain a miracidium. The microscopic appearance of the egg allows diagnostic differentiation of the 5 species. An adult S hematobium produces 20-200 round, terminally spined eggs per day; S mansoni produces 100-300 ovoid, laterally spined eggs per day; and S japonicum produces 500-3500 round, small, laterally spined eggs per day. The eggs of S intercalatum have prominent terminal spines, and S mekongi have small lateral spines.

When the ova reach the fresh water, the miracidia are released and penetrate the snail. Within 3-5 weeks, they asexually multiply into hundreds of fork-tailed cercariae. The cercariae leave the snail and swim to a human or nonhuman animal, where they penetrate the skin. Once inside, cercariae travel to the heart, to the lungs, and through the systemic circulation to reach the portal veins, where they develop into adult worms. The time between cercariae penetration and the first ova production is 4-6 weeks.

Humans are estimated to excrete approximately 50% of the eggs. The rest are trapped in various parts of the body. Occasionally, the worm can be in ectopic positions, such as in the spinal cord, where it produces unusual clinical manifestations.

Pathophysiology

The pathophysiology of infection correlates with the life cycle of the parasite, as follows:

• Cercariae: Skin penetration of cercariae produces an allergic dermatitis at the site of entry. With prior sensitization, a pruritic papular rash develops. This also is observed with nonhuman avian schistosomes.
• Schistosomula: These are tailless cercariae that are transported through blood or lymphatics to the right side of the heart and lungs. Heavy infection can cause symptoms such as cough and fever. Eosinophilia may be observed.
• Adult worm: Adult worms do not multiply inside the human body. In the venous blood, adult male and female worms mate, and the female lays eggs 4-6 weeks after cercarial penetration. Adult worms are rarely pathogenic. The female adult worm lives for approximately 3-8 years and lays eggs throughout her life span.
• Eggs: They cause Katayama fever and schistosomiasis.
• Katayama fever: The exact pathophysiology is not known. It occurs 4-6 weeks after infection, at the time of the initial egg release. It is reported most commonly with S japonicum but also has been reported with S mansoni. Katayama fever is believed to be due to the high worm and egg antigen stimuli that result from immune complex formation and lead to a serum sickness–like illness. This syndrome is not due to granuloma formation.
• Schistosomiasis: It is due to immunological reactions to Schistosoma eggs trapped in tissues. Antigens released from the egg stimulate a granulomatous reaction comprised of T cells, macrophages, and eosinophils that results in clinical disease. Symptoms and signs depend on the number and location of eggs trapped in the tissues. Initially, the inflammatory reaction is readily reversible. In the latter stages of the disease, the pathology is associated with collagen deposition and fibrosis, resulting in organ damage that may be only partially reversible.

Granuloma in the liver due to Schistosoma mansoni. The S mansoni egg is at the center of the granuloma.

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Granuloma in the liver due to Schistosoma mansoni. The S mansoni egg is at the center of the granuloma.

Frequency

United States

Frequency is estimated at more than 400,000 infected persons. The appropriate snail intermediate host for endemic schistosomiasis is not endemic to the United States. All cases are imported, except for the rare laboratory accident. The diagnosis should be considered in a person from an endemic area or a traveler who had contact with fresh water in an endemic area.

International

Intestinal schistosomiasis caused by S mansoni occurs in 52 countries, including Caribbean countries (ie, Saint Lucia, Antigua, Montserrat, Martinique, Guadeloupe, Dominican Republic, Puerto Rico), eastern Mediterranean countries, South American countries (ie, Brazil, Venezuela, Surinam), and most countries in Africa. Other Schistosoma species that can cause intestinal symptoms and diseases include S intercalatum, S japonicum, and S mekongi. S intercalatum is found in 10 countries within the rain forests of central Africa. S japonicum is endemic in 4 countries in the western Pacific region (ie, China, Philippines, Indonesia, Thailand). S mekongi infection occurs in the Mekong River area of Southeast Asia (ie, Kampuchea, Laos, Thailand). Urinary schistosomiasis caused by S hematobium affects 54 countries in Africa and the eastern Mediterranean.

According to the WHO, the global distribution of schistosomiasis has changed in recent years. It has been eradicated from Japan and the Lesser Antilles islands; transmission has been stopped in Tunisia; and transmission is very low in Morocco, Saudi Arabia, Venezuela, and Puerto Rico.

Mortality/Morbidity

• Acute schistosomiasis: Although most clinical manifestations are benign, some are severe and may require hospitalization. If acute schistosomiasis is not suspected clinically and treated appropriately, it can result in severe morbidity or death.
• Chronic schistosomiasis: Most patients are asymptomatic or mildly symptomatic and do not require medical attention. Only a small proportion of the endemic population harbors a heavy worm burden that later leads to clinical complications.
• Gastrointestinal schistosomiasis: The most common complication is periportal fibrosis, also termed Symmers clay pipestem fibrosis. This leads to portal hypertension and gastrointestinal hemorrhage. Liver failure is uncommon except for in persons with concomitant chronic hepatitis or cirrhosis. Recently, people co-infected with either hepatitis B or C and S mansoni have been shown to have rapid progression of liver disease.
• Urinary tract schistosomiasis: This can lead to renal failure due to obstructive uropathy, pyelonephritis, or bladder carcinoma (occurring usually 10-20 y after the initial infection). In addition, immune complexes that contain worm antigens may deposit in the glomeruli, leading to glomerulonephritis and amyloidosis.
• Female genital schistosomiasis (FGS): S haematobium causes lesions in the female lower genital tract (ie, cervix, valva, vagina). FGS has been identified as a major social and medical problem that may facilitate the spread of some sexually transmitted diseases such as HIV and human papillomavirus (HPV).¹
• Schistosomal cor pulmonale: This is an important complication that develops in about 5% of patients with hepatosplenic S mansoni.
• CNS schistosomiasis: Most cases of cerebral schistosomiasis are observed with S japonicum, constituting 2-4% of all S japonicum infections. However, CNS schistosomiasis can also occur with other species. Spinal schistosomiasis usually presents as transverse myelitis and is primarily due to S mansoni infection.
• Schistosomiasis in pregnancy: This has been associated with anemia and low birth weight.²

Race

No racial predilection exists.

Sex

Schistosomiasis is more common in males, most likely because of increased exposure to infected water via bathing, swimming, and agricultural activities.

Age

• Exposure to infection can start shortly after birth.
• People aged 10-14 years are at the maximum risk of exposure.
• The lower prevalence in adults is possibly due to partial immunity and decreased exposure to infected water.

Clinical

History

• Acute manifestations
• Cercarial dermatitis: Individuals who have been exposed to fresh or salt water may develop a pruritic rash due to cercarial dermatitis (also called swimmer's itch). This has been described in North America due to nonhuman avian schistosomes.
• Katayama syndrome: Fever, lethargy, and myalgia are the most common symptoms. Less common symptoms include cough, headache, anorexia, and rash. These symptoms mimic any acute viral, bacterial, or malarial illness. Consequently, acute illness is often missed unless schistosomiasis is suspected. A travel history that includes exposure to fresh water in an endemic area is an important part of the medical history.
• Chronic manifestations
• Symptoms depend on the species of schistosome causing infection, the duration and severity of infestation, and the immune response to the eggs.
• Typically, onset is insidious.
• S mansoni, S mekongi, S intercalatum, and S japonicum cause intestinal tract and liver disease.
• S hematobium only rarely causes intestinal or liver disease but characteristically causes urinary tract disease.
• Hepatic schistosomiasis: In the early stage, dyspepsia, flatulence, and pain are present in the left hypochondrium due to spleen enlargement. Anemia or cor pulmonale may cause generalized pain, weakness, and shortness of breath. In the later stages, abdominal distention, lower limb edema, hematemesis, and melena can occur. Symptoms of liver failure are rare unless other infectious, toxic, or malignant causes of hepatitis are present.
• Intestinal schistosomiasis: Fatigue, abdominal pain, diarrhea, and dysentery occur.
• Urinary schistosomiasis: Dysuria, urinary frequency, and terminal hematuria occur.
• Cardiopulmonary schistosomiasis may cause larval pneumonitis with a cough, mild wheezing, and a low-grade fever.
• In schistosomal cor pulmonale, easy fatigability, palpitations, dyspnea on exertion, and hemoptysis are present.
• CNS schistosomiasis involves focal and generalized seizures; headache; and myeloradiculopathy with lower limb and back pain, bladder dysfunction, paresthesia, and lower limb weakness.
• Female genital schistosomiasis (FGS): The history includes postcoital bleeding, genital ulceration, irregular menstruation, and pelvic pain.

Physical

• Acute schistosomiasis
• Generalized lymphadenopathy
• Hepatosplenomegaly
• Rash
• Fever
• Chronic schistosomiasis
• Portal hypertension with abdominal distention, hepatosplenomegaly, pedal edema, pallor, distended abdominal veins, and ascites
• Intestinal polyposis with heme-positive stool, pallor, and signs of malnutrition
• CNS symptoms, including focal neurological findings, seizures, and spinal cord lesions
• Renal failure with anemia and hypertension
• Cor pulmonale with signs of right heart failure
• Genital lesions including ulcer or nodular lesions of the cervix, valva, or vagina or vesicovaginal fistula