Platyhelminthes

1. Platyhelminthes Part 2: Parasitic Flatworms

2. Class Trematoda • ~10,000 species • All are parasitic, and often of considerable economic and/or medical importance • Adults usually parasitize vertebrates • Juveniles may parasitize verts or inverts, or live on plants • Body flat; oval to elongate in shape • Range in size from <1 mm to ~60 mm • Outer layer called the tegument • Layer of fused cells (syncytium) • Efficient at absorbing nutrients and resisting immune system of hosts

3. Trematoda: Digestive System • Attach to host with two suckers: oral sucker (anterior) and acetabulum (midbody) • Anterior mouth • Muscular, pumping pharynx • Digestive system divides into two blind pouches called cecae • Mostly feed on host cells and cell fragments

4. Trematoda: Life Cycle I • At least two different body forms (adult and one or more larval stages) • At least two different hosts • Adults live in bloodstream, digestive tract, or visceral organs of a definitive (final) host (usually a vertebrate)

5. Trematoda: Life Cycle II • Eggs exit the adult fluke’s host via the feces and hatch in freshwater • Ciliated larvae swim around until they find suitable intermediate hosts (often snails) • Asexual reproduction takes place in intermediate host

6. Trematoda: Life Cycle III • A second type of larvae exit the intermediate host • Swims freely until it encounters either a second intermediate host or definitive host, depending on the species • Some larvae penetrate host; some are consumed by hosts

8. Class Trematoda/(Flukes) • Blood flukes • Schistosoma mansoni, S. japonicum and S. hematobium. • Other trematodes of significance are • Intestinal fluke, Fasciolopsis buski • Liver fluke, Clonorchis sinensis • Lung fluke, Paragonimus westermani.

9. Schistosome Fluke(Schistosoma spp.) • Larvae directly penetrate skin of person • Adults live in bloodstream • Snail is the only intermediate host • >200 million people infected in Africa, South America, and Southeast Asia • Second only to malaria in human parasitic impact

10. Schistosoma mansoni • Definitive hosts: primates and some rodents • Site of infection: portal veins draining the large intestine • Typical size: males 12-15 mm long x 1 mm wide; females 18-20 mm long x 0.25 mm wide • Distribution: Africa; introduced into South America and the Caribbean • First intermediate host: a variety of snail species predomintely in the genus Bulinus (Old World) and Tropicorbis (New World) • Cercariae penetrate the skin of the definitive host

11. Schistosoma mansoni

12. Fasciolopsis buski

13. Chinese Liver Fluke(Clonorchis sinensis) • Humans contract these from eating raw or undercooked fish • Adults live in bile duct of liver • Snail and fish are intermediate hosts • >30 million people infected in Asia

14. Clonorchis sinensis • Definitive hosts: this species infects a variety of fish eating mammals, including humans, swine, canids, felids, rodents, lagomorphs, camelids • Site of infection: bile ducts • Typical size: 10-20 mm long x 1.5-4 mm wide • Distribution: Asia and southeast Asia • First intermediate host: a variety of snail species may be employed, where sporocyst and redial generations occur • Second intermediate hosts, predominately cyprinids (carp) • Metacercariae: metacercariae develop within the musculature of fish

18. Fasciola hepatica (Sheep liver fluke)

19. Fasciola hepatica • Definitive hosts: this species infects a variety of mammals, especially ruminants • Site of infection: bile ducts • Typical size: 20-30 mm long x 8-12 mm wide • Distribution: cosmopolitan • Intermediate host: a variety of lymnaeid snail species (about a dozen to date) may be employed, where one sporocyst and 2 redial generations occur. • Metacercariae: encystment on a variety of aquatic vegetation

20. Fasciola hepatica

21. Class Cestoda

22. Class Cestoidea (Tapeworms)

23. Cestoidea: General Characteristics • ~3,500 species • Internal parasites (endoparasites) of vertebrate digestive systems • Adults consist of long series of repeating units called proglottids • 3 main body regions • Scolex - holdfast structure containing system of suckers and sometimes hooks • Neck - immature proglottids • Strobila - series of proglottids, which produce eggs when mature

25. Cestoidea: Structural Adaptations • Many organs reduced compared to flatworms since environment is very stable • Lack mouth and digestive system, absorb nutrients across body wall • Tegument similar to flukes, but absorbs nutrients and enzymes from host • Reduced nervous system – nerve mass in scolex and lateral nerve cords • Simple protonephridial system

26. Cestoidea: Reproduction • Monoecious; testes and ovaries in every proglottid • Sperm and eggs produced at different times; fertilization between different proglottids on same tapeworm or different tapeworms in same host. • Organs degenerate as eggs accumulate • Gravid proglottids break away from tapeworm’s end and pass out of the body in the feces. • Eggs or proglottids consumed by intermediate hosts. • Larvae move via bloodstream and penetrate intermediate host muscle tissue, then consumed by definitive hosts

27. Beef and Pork Tapeworms • Larvae hatch in the intestine of cows or pigs, bore into their bloodstream, and are carried to muscle tissue where they encyst and become bladder worms (cysticerci) • Cysticerci of pork tapeworms often released in human intestines, can invade human muscle tissue, causing cysticercosis (can be fatal)

29. Cysticercus

34. Taenia solium (hooks) Taenia saginata (no hooks)

35. gravid proglottid typical of the distal end of the strobila mature proglottid young gravid proglottid

36. Taenia pisiformis(dog tapeworm) • Definitive hosts: canids; sometimes felids • Site of infection: small intestine • Distribution: cosmopolitan • Intermediate hosts: cysticerci predominately in peritoneal cavity of rabbits and hares

37. Eggs of three tapeworm species