Schistosomiasis and Other Trematode Infections: Introduction

Trematodes, or flatworms, are a group of morphologically and biologically heterogeneous organisms that belong to the phylum Platyhelminthes. Human infection with trematodes occurs in many geographic areas and can cause considerable morbidity and mortality. For clinical purposes, significant trematode infections of humans may be divided according to tissues invaded by adult flukes: blood, biliary tree, intestines, and lungs (Table-1). Table-1 Major Human Trematode Infections Trematode Transmission Endemic Area(s) Blood Flukes  Schistosoma mansoni  Skin penetration by cercariae released from snails Africa, South America, Middle East S. japonicum  Skin penetration by cercariae released from snails China, Philippines, Indonesia S. intercalatum  Skin penetration by cercariae released from snails West Africa S. mekongi  Skin penetration by cercariae released from snails Southeast Asia S. haematobium  Skin penetration by cercariae released from snails Africa, Middle East Biliary (Hepatic) Flukes  Clonorchis sinensis  Ingestion of metacercariae in freshwater fish Far East Opisthorchis viverrini  Ingestion of metacercariae in freshwater fish Far East, Thailand O. felineus  Ingestion of metacercariae in freshwater fish Far East, Europe Fasciola hepatica  Ingestion of metacercariae on aquatic plants or in water Worldwide F. gigantica  Ingestion of metacercariae on aquatic plants or in water Sporadic, Africa Intestinal Flukes  Fasciolopsis buski  Ingestion of metacercariae on aquatic plants Southeast Asia Heterophyes heterophyes  Ingestion of metacercariae in freshwater or brackish-water fish Far East, North Africa Lung Flukes  Paragonimus westermani  Ingestion of metacercariae in crayfish or crabs Global except North America and Europe Trematodes share some common morphologic features, including macroscopic size (from 1 cm to several cm); dorsoventral, flattened, bilaterally symmetric bodies (adult worms); and the prominence of two suckers. Except for schistosomes, all human parasitic trematodes are hermaphroditic. Their life cycle involves a definitive host (mammalian/human), in which adult worms initiate sexual reproduction, and an intermediate host (snails), in which asexual multiplication of larvae occurs. More than one intermediate host may be necessary for some species of trematodes. Human infection is initiated either by direct penetration of intact skin or by ingestion. Upon maturation within humans, adult flukes initiate sexual reproduction and egg production. Helminth ova leave the definitive host in excreta or sputum and, upon reaching suitable environmental conditions, they hatch, releasing free-living miracidia that seek specific snail intermediate hosts. After asexual reproduction, cercariae are released from infected snails. In certain species, these organisms infect humans; in others, they find a second intermediate host to allow encystment into metacercariae—the infective stage. The host-parasite relationship in trematode infections is a product of certain biologic features of these organisms: they are multicellular, undergo several developmental changes within the host, and usually result in chronic infections. In general, the distribution of worm infections in human populations is overdispersed; i.e., it follows a negative binomial mathematical relationship in which most infected individuals harbor low worm burdens while a small percentage are heavily infected. It is the heavily infected minority who are particularly prone to disease sequelae and who constitute an epidemiologically significant reservoir of infection in endemic areas. Equally important is an appreciation that worms do not multiply within the definitive host and that they have a relatively long life span, ranging from a few months to a few years. Morbidity and death due to trematode infections reflect a multifactorial process that results from the tipping of a delicate balance between intensity of infection and host reactions, which initiate and modulate immunologic and pathologic outcome. Furthermore, the genetics of the parasite and of the human host contribute to the outcome of infection and disease. Infections with trematodes that migrate through or reside in host tissues are associated with a moderate to high degree of peripheral blood eosinophilia; this association is of significance in protective and immunopathologic sequelae and is a useful clinical indicator of infection.

Approach to the Patient: Trematode Infection The approach to individuals with suspected trematode infection begins with a question: Where have you been? Details of geographic history, exposure to freshwater bodies, and indulgence in local eating habits without ensuring safety of food and drink are all essential elements in the history. The workup plan must include a detailed physical examination and tests appropriate for the suspected infection. Diagnosis is based either on detection of the relevant stage of the parasite in excreta, sputum, or (rarely) tissue samples or on sensitive and specific serologic tests. Consultation with physicians familiar with these infections or with the U.S. Centers for Disease Control and Prevention (CDC) is helpful in guiding diagnosis and selecting therapy. Reff: Harrison's Internal Medicine > Chapter 212. Schistosomiasis and Other Trematode Infections