Chapter 57: Trematodes

GENERAL FEATURES OF TREMATODES

Of the many relationships that have developed between humans and helminths over the millennia of our mutual existence, perhaps the most destructive to our health and productivity is that forged by the trematodes, or “flukes.” Typically, the adults live for decades within human tissues and vasculature, where they resist immunologic attack and produce progressive damage to vital organs. Morphologically, trematodes are bilaterally symmetric, vary in length from a few millimeters to several centimeters, and possess two deep suckers from which they derive their name (“body with holes”). One surrounds the oral cavity, and the other is located on the ventral surface of the worm. These organs are used for both attachment and locomotion; movement is accomplished in a characteristic inchworm fashion.

The digestive tract begins at the oral sucker and continues as a muscular pharynx and esophagus before bifurcating to form bilateral ceca that end blindly near the posterior extremity of the worm. Undigested food is vomited through the oral cavity. The excretory system consists of a number of hollow, ciliated “flame” cells that excrete waste products into interconnecting ducts terminating in a posterior pore.

Trematodes are divided into two major categories, based on their reproductive systems: The hermaphrodites and the schistosomes (Table 57–1). The adult hermaphrodites contain both male and female gonads and produce operculated eggs (defined as having a lid). In contrast, the schistosomes have separate sexes, and the fertilized female deposits nonoperculated eggs. However, the two groups have similar life cycles. In both cases, eggs are excreted from the human host and—if they reach fresh water—hatch to release ciliated larvae called miracidia. These larvae find and penetrate a snail host specific for the trematode species. In this intermediate snail host, they are transformed by a process of asexual reproduction into thousands of tail-bearing larvae called cercariae, which are released from the snail over a period of weeks. The cercariae swim in fresh water, searching vigorously for their next host. In the case of schistosomal cercariae, this host is the human: When they contact the skin surface, they attach, discard their tails, and invade, thereby completing their life cycle. The cercariae of the hermaphroditic flukes, in contrast, encyst in or on an aquatic plant or animal, where they undergo a second transformation to become infective metacercariae. Their cycle is completed when this second intermediate host is ingested by a human.