The Life Cycle of Fasciola hepatica

Fasciolosis is an economically important disease of domestic livestock, in particular cattle and sheep, and occasionally man. The disease is caused by digenean trematodes of the genus Fasciola, commonly referred to as liver flukes. The two species most commonly implicated as the aetiological agents of fasciolosis are F. hepatica and F. gigantica (family Fasciolidae). F. hepatica has a worldwide distribution but predominates in temperate zones while F. gigantica is found on most continents, primarily in tropical regions. The class Trematoda belongs to the phylum Platyhelminthes, and can be divided into two subclasses, the Digenea and Aspidogastrea. The Digenea are characterized by a complex life cycle in which one or more intermediate hosts are involved. Many variations on the life cycle exist, but each typically includes a molluscan primary or intermediate host in which larval multiplication occurs, and a vertebrate final or definitive host in which sexual reproduction occurs. Members of the family Fasciolidae are hermaphroditic and self-fertilization can occur, although sexual reproduction is normally by cross-fertilization in the final host. The story of the discovery of the life cycle of F. hepatica has been recounted before; therefore only a short account will be given here. A list of the principal discoveries is shown in Table 1.1. Integral to the successful completion of the life cycle, the biological factors involved will be discussed in the second part of the chapter. Additional details on many of the aspects of the life cycle will be covered in more depth in subsequent chapters.