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Chapter 17 : Blood Trematodes: Schistosomes
Schistosomes belong to the phylum Platyhelminthes, family Schistosomatidae, and are a group of digenetic, dioecious trematodes requiring definitive and intermediate hosts to complete their life cycles. Four species are important agents of human disease: Schistosoma mansoni, Schistosoma japonicum, Schistosoma mekongi, and Schistosoma haematobium. Schistosoma intercalatum is of less epidemiologic importance. The earliest known instance of schistosomiasis was found in Egyptian mummies of the predynastic period, using enzyme-linked immunosorbent assay (ELISA) to detect circulating anodic antigen. Schistosomes are somewhat different from other human trematodes since they (i) have two sexes, (ii) live in the blood vessels, (iii) have nonoperculated eggs, and (iv) have no encysted metacercarial stage in the life cycle. Cercarial dermatitis follows skin penetration by cercariae, and the reaction may be partly due to previous host sensitization. Few clinical manifestations are associated with primary exposure, but both humoral and cellular immune responses are elicited on subsequent exposure. After infection and the transformation of cercariae to schistosomula, the schistosomulum expresses antigens on its surface that evoke a host immune response that provides some degree of resistance to reinfection. Symptoms of acute schistosomiasis include fever, diarrhea, abdominal pain, weight loss, and eosinophilia. Symptoms of chronic disease include chronic diarrhea, abdominal pain, and hepatomegaly or hepatosplenomegaly. Patients who have been treated for schistosomiasis should be monitored posttherapy to determine the effectiveness of the treatment.
Life cycle of schistosomes.
Schistosome cercariae. (Top, illustration by Sharon Belkin.)
Scanning electron micrograph of male and female schistosome in copula.
Schistosoma mansoni male and female in copula; note the female within the male worm.
Schistosoma mansoni egg with a mature miracidium. (Illustration by Nobuko Kitamura; adapted from Figure 27.15A in P. C. Beaver, R. C. Jung, and E. W. Cupp, Clinical Parasitology, 9th ed., Lea & Febiger, Philadelphia, Pa., 1984.)
Schistosoma mansoni eggs. Note the large lateral spine.
Schistosoma miracidium. (Armed Forces Institute of Pathology photograph.)
Hepatic fibrosis with multiple schistosome egg granulomas. (Armed Forces Institute of Pathology photograph.)
Schistosome eggs in tissue. (Left) S. haematobium, (right) S. mansoni.
Pipe-stem fibrosis in the liver as a result of Schistosoma mansoni. (Original photograph by L. Millman; from A Pictorial Presentation of Parasites: A cooperative collection prepared and/or edited by H. Zaiman.)
Schistosomiasis. Note the engorgement of collateral circulation of the abdomen. (From A Pictorial Presentation of Parasites: A cooperative collection prepared and/or edited by H. Zaiman.)
Schistosoma mansoni infection showing liver and splenic enlargement; 800 worms were removed from this patient. (From A Pictorial Presentation of Parasites: A cooperative collection prepared and/or edited by H. Zaiman. Photograph courtesy of R. Goldsmith and B. H. Kean.)
Schistosoma mansoni eggs in crushed rectal biopsy tissue. (Armed Forces Institute of Pathology photograph.)
Schistosoma mansoni (top), S. haematobium (middle), and S. japonicum (bottom) eggs. Note how much smaller the S. japonicum egg normally is than those of the other two species, and note the small size of the lateral spine. (Illustration by Nobuko Kitamura.)
Schistosoma japonicum eggs. Note the small lateral spine.
Schistosoma japonicum paired adult male and female in a mesenteric vein. (From A Pictorial Presentation of Parasites: A cooperative collection prepared and/or edited by H. Zaiman.)
Schistosoma mekongi eggs. Note the subspherical shape and smaller size than S. japonicum eggs.
Schistosoma haematobium eggs. Note the terminal spine and slight size variations.
Schistosoma haematobium. (Upper) Egg in the appendix. Note the terminal spine. (Lower) Eggs in a rectal biopsy specimen. (From A Pictorial Presentation of Parasites: A cooperative collection prepared and/or edited by H. Zaiman.)
Calcified Schistosoma haematobium eggs in a crushed sliver of fixed tissue from the bladder of an Egyptian patient. Unstained, ×239. (Armed Forces Institute of Pathology photograph.)
Schistosoma haematobium: (Left) X ray of the pelvis in a 24-year-old Egyptian with schistosomal obstructive uropathy. Linear calcification is seen in the base of the bladder and along the lower ureters, which are dilated (arrows). (Armed Forces Institute of Pathology photograph. Courtesy of Marcia Young.) (Right) Urinary bladder. An exophytic squamous cell carcinoma is present in the central portion of the photograph, and infiltrating squamous cell cancer extends to the left. Magnification, ×4. (Armed Forces Institute of Pathology photograph.)
Schistosoma intercalatum egg. The miracidium tends to be somewhat narrower in the middle, giving it an hourglass shape. (From A Pictorial Presentation of Parasites: A cooperative collection prepared and/or edited by H. Zaiman.)
Characteristics of blood trematodes
Improved immunologic testing for the diagnosis of S. mansoni infection
Management and therapy of schistosomiasis a
Pathogenesis, pathology, and symptoms seen with S. japonicum infection
Distribution of infection and morbidity due to Schistosoma mekongi in northern Cambodia
Disease manifestations seen with Schistosoma haematobium infection
Mechanisms of bladder carcinogenesis due to Schistosoma haematobium
Schistosomiasis control measures