Fasciola hepatica lifecycle. (a) Graphical representation of the F. hepatica lifecycle (modified from ). a1 Definitive host - host range includes cattle, sheep and humans. a1.1 Parasite excysts in the intestine of the definitive host, releasing newly excysted juveniles (NEJ) that migrate across the intestinal wall, through the peritoneal cavity to the liver. a1.2 NEJ migrate through the liver parenchyma, increasing in size to juvenile flukes as they migrate a1.3 into the bile ducts a1.4, where they grow and develop into fully mature adults. a2 Eggs are released in the faeces and develop on pasture. a3 From each embryonated egg hatches a single miracidium, which infects the snail intermediate host (Galba truncatula). a4 Within the snail the parasite undergoes a clonal expansion, developing through the sporocyst, rediae and cercariae lifecycle stages. a5 Cercariae are released from the snail and encyst on vegetation as dormant metacercariae, which are ingested by the definitive host a1. (b, c, d) Graphical representation of the development of the parasite through the definitive host. (b) The parasite increases dramatically (approximately 1,000-fold) in size over the course of approximately 12 weeks, from NEJ to adult. (c) Expression of key enzymes of metabolism reveals how the growth of the parasite limits oxygen diffusion into the parasite tissue, switching from aerobic energy metabolism (Kreb’s cycle; PK: pyruvate kinase; SD: succinate dehydrogenase) to aerobic acetate production (ME: malic enzyme) to anaerobic dismutation (PEPCK: phosphoenolpyruvate carboxykinase), as shown by the log fold-change in expression between the lifecycle stages (expression is shown relative to metacercariae lifecycle stage). (d) In addition to the dramatic growth, maturation of the parasite occurs, with the fully mature adult digesting host blood, which provides the nutrient for massive egg production (approximately 20,000 eggs per day per parasite), as shown by the increased expression of the egg shell component, vitelline.