Blood Trematodes: Schistosomiasis

Biology

The complicated life cycle alternates between parasitic forms in the snail intermediate host and human definitive host and the free-living forms in water, the cercariae and the miracidia ( Fig. 285.1 ). The cercariae are released from infected snails and penetrate intact human skin, where they transform into schistosomula that travel through the venous system to the lungs. From the lungs, schistosomula enter the arterial system and reach the liver, where they attain sexual maturity in 4–6 weeks. Adult worms then descend against the blood flow of the portal venous system to the venules of the intestine ( S. mansoni or S. japonicum ) or bladder ( S. haematobium ).

Unlike other trematodes, schistosomes are not hermaphroditic. Males and females mate, and the females begin to deposit eggs. Eggs must pass through tissue to reach the lumen of the intestine or bladder and then on to the environment in feces or urine. About one-half of the eggs are retained in intestinal or vesicular tissues, where they cause granulomatous inflammation and fibrosis. In the case of the intestinal Schistosoma species, eggs also can be swept by the portal blood system back to the liver, where they can cause similar pathologic changes.

Eggs that exit the body and reach freshwater hatch and release miracidia that infect certain genera of snails. S. mansoni infects snails of the Biomphalaria species, S. haematobium infects Bulinus species, and S. japonicum infects Oncomelania snails. In the receptive snail, a single miracidium forms sporocysts from which thousands of cercariae are released into the water over the lifetime of the infected snail, typically a period of 6–12 months.

Epidemiology

Schistosomiasis is transmitted in 78 countries in tropical and subtropical areas, but 85% of infected people reside in sub-Saharan Africa, where S. mansoni and S. haematobium occur and often are coendemic. S. mansoni was transplanted, likely with the slave trade, to South America (especially Brazil, Venezuela, and Suriname) and several islands in the Caribbean (Antigua, Dominican Republic, Guadeloupe, Martinique, Montserrat, Puerto Rico, and Saint Lucia). Several Caribbean islands may have successfully eliminated schistosomiasis, largely due to successful mass treatment campaigns, snail control, and improvements in water, sanitation, and hygiene. ^, S. haematobium also occurs in a few countries of the Middle East, and a recent focus of transmission has been reported in Corsica, France. S. japonicum is endemic in China, the Philippines, and Indonesia.

People with higher intensity infections are responsible for most environmental contamination with parasite eggs. Cross-sectional studies of S. haematobium and S. mansoni show that the highest prevalence of infection occurs among school-aged children 10–14 years of age and that rates decrease in adulthood. Infants have been infected from being bathed in irrigation canals or in lake or river water. ^, Males are infected more often than females, and certain daily activities and occupations (e.g., fishing, doing laundry, washing cars) are important risk factors. Wide variations in community prevalence patterns and disease focality point to complex interactions with the environment and practices of human waste disposal, water contact, and acquired immunity. Hydraulic dams and irrigation projects often increase snail habitats and the risk of transmission.

Long-term residents of endemic areas, including expatriates, missionaries, diplomats, and Peace Corps volunteers, are at particular risk. However, tourists also can become infected, particularly those who take part in adventure or ecotourism.

Pathogenesis

The host immune response to schistosome egg antigens results in the major clinical and pathologic manifestations of schistosomiasis. Cercarial penetration of skin can give rise to a local hypersensitivity reaction and papular dermatitis, usually within 72 hours of exposure. This reaction typically is milder than the allergic rash associated with skin penetration of animal schistosome cercariae (“swimmer’s itch”). Adult worm maturation and sudden exposure to antigens with the onset of egg laying by young female worms can result in an acute illness resembling serum sickness, which is more often reported in travelers than lifelong residents of endemic regions. ^{, ,}

Chronic organ damage and dysfunction are related to the number of eggs in tissues, which is related to the number of egg-laying female worms and the host immunological response. People with tissue infection within the highest category of number of worms and numbers of eggs are at the greatest risk of disease. The worm burden in humans grows only through repeated exposure to water containing cercariae. Some evidence suggests that humans acquire partial immunity to reinfection, but the mechanism of a protective immune response has not been defined.

The eggs secrete antigenic materials that induce a host granulomatous response by macrophages, lymphocytes, giant cells, and eosinophils. Over time, granulomas decrease in size and are replaced by fibrosis. Fibrosis along the portal vein triads of the liver (i.e., periportal fibrosis or pipestem fibrosis) in intestinal schistosomiasis can result in portal hypertension. Liver function remains intact unless there is concurrent insult, such as alcoholic cirrhosis or viral hepatitis. As portal hypertension advances and collateral blood flow channels increase, eggs can be shunted to the lungs, which can result in granulomatous pulmonary arteritis leading to pulmonary hypertension.

In urogenital schistosomiasis (caused by S. haematobium ), chronic fibrosis of the urinary tract can lead to obstructive uropathy and hydronephrosis. Urogenital schistosomiasis can be complicated by pyelonephritis, calculi, and squamous cell carcinoma. Female genital schistosomiasis is characterized by inflammatory lesions of the genital tract, especially cervix, fallopian tubes, and vagina, including characteristic sandy patches associated with inflammation of the vaginal mucosa.

Organs less commonly involved include skin, lungs, seminal vesicles, and the central nervous system. S haematobium is associated with hematospermia (associated with seminal vesicle involvement) in males. Genital schistosomiasis in men and women can enhance transmission of human immunodeficiency virus (HIV) infection. Schistosome infection also can promote progression of HIV by reducing the host immune response control of virus replication. S. haematobium also is associated with human papillomavirus (HPV) infection and infertility.

Granulomatous reaction around ectopic eggs in the brain most often results from S. japonicum infection. Ectopic eggs in the spinal cord can cause transverse myelitis. ^,

Clinical Features