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. The dependence on one drug—praziquantel—for treatment of most infections caused by trematodes raises the specter of developing resistance in these worms; several instances of reduced drug efficacy have already been reported. The widespread use of oxamniquine in the 1970s to reduce the impact of schistosomiasis resulted in the development of significant resistance. Recently, a single quantitative trait locus on schistosomal chromosome 6 was identified as the genetic basis for resistance.
ETIOLOGIC AGENTS AND THEIR LIFE CYCLES
For clinical purposes, significant trematode infections of humans may be divided according to the tissues invaded by the adult stage of the fluke, whether bloodstream, biliary tree, intestines, or lungs (Table 259-1). Trematodes share some common morphologic features, including macroscopic size (from one to several centimeters); dorsoventral, flattened, bilaterally symmetric bodies (adult worms); and the prominence of two suckers. Except for schistosomes, all human parasitic trematodes are hermaphroditic. Their life cycles involve a definitive host (mammalian/human), in which adult worms initiate sexual reproduction, and an intermediate host (snail), 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 for humans.
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 statistical distribution 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. Recent evidence indicates that the prevalence of morbidity in infected populations is greater than was previously thought. 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 ...