Introduction to Parasitic Helminths Lecture 4 Medical Parasitology Course (MLAB 362) Dr. Mohamed A. El-Sakhawy 1 Parasitic Helminths Eukaryotic, multicellular animals that usually have digestive, circulatory, nervous, excretory, and reproductive systems. Worms with bilateral symmetry, head and tail, and tissue differentiation (endoderm, mesoderm, and ectoderm). Type of parasites Protozoa Nematodes Parasites Helminths Trematodes Endo- Cestodes Arthropods ---------------- Ecto- 2 1
Taxonomic classification of helminths Sub kingdom Phylum Class Genus examples Metazoa Nematodes Round worms; appear round in cross section, they have body cavities, a straight alimentary canal and an anus Ascaris (roundworm) Trichuris (whipworm) Ancylostoma (hookworm) Necator (hookworm) Enterobius (pinworm or threadworm) Strongyloides Platyhelminthes Flat worms; dorsoventrally flattened, no body cavity and, if present, the alimentary canal is blind ending Cestodes Adult tapeworms are found in the intestine of their host They have a head (scolex) with sucking organs, a segmented body but no alimentary canal Each body segment is hermaphrodite Taenia (tapeworm) 3 Trematodes Non-segmented, usually leaf-shaped, with two suckers but no distinct head They have an alimentary canal and are usually hermaphrodite and leaf shaped Schistosomes are the exception. They are thread-like, and have separate sexes Fasciolopsis (liver fluke) Schistosoma (not leaf shaped!) Parasitic helminths spend most or all of their lives in host and usually have the following specializations: May lack a digestive system. Absorb nutrients from host s food, body fluids, or tissues. Have a reduced nervous system. Means of locomotion is reduced or absent. Complex reproductive system. Individuals produce many eggs that can infect another host. 4 2
5 Two main groups (phyla) I. Platyhelminths (Flatworms) II.Nematoda (Roundworms) Life Cycle Extremely complex Intermediate hosts harbor larval (developmental) stage. Definitive host harbors adult stage. Sexual reproduction strategies: Dioecious: Male and female reproductive organs are found in separate individuals. Monoecious (Hermaphroditic): One animal has both male and female sex organs. Most hermaphrodites copulate with other animals, a few copulate with themselves. I. Platyhelminths (Flatworms) Flattened from front to back. Include: 1. Trematodes or Flukes Leaf shaped bodies Suckers for attachment and sucking fluids from host. Some can absorb nutrients through their cuticle. Named for host tissues in which adult lives. Blood Fluke (Schistosoma spp.): Cause schistosomiasis which affects over 200 million people worldwide. Cause damage to blood vessels, liver, and many other organs. Live in waters contaminated with feces, burrow through skin of human and enter the circulatory system. 6 3
Schistosoma Life Cycle of Blood Fluke 7 2. Cestodes or Tapeworms Long flat bodies Intestinal parasites Lack a digestive system, absorb food through cuticle. Body Organization: Head or scolex has suckers for attachment. Body is made up of segments called proglottids. Each proglottid has both male and female reproductive organs. Proglottids farthest from head are mature and contain many fertilized eggs. 8 4
Cestode (Tapeworm) Body Structure: Scolex and Proglottids 9 Parasitic human tapeworms: Beef Tapeworm (Taenia saginata): Human is definitive host. Can reach up to 6 meters in length, scolex is 2 mm long with hundreds of proglottids. Infection occurs by ingestion of contaminated, undercooked beef. Pork Tapeworm (Taenia solium): Human is definitive host. Infection can occur from eating infected undercooked pork. 10 5
11 II. Nematodes (Roundworms) Cylindrical body tapered at each end. Have a complete digestive system: mouth, intestine, and anus. Body is covered by tough cuticle that resists drying and crushing. Most species are dioecious: separate males and females. Males are smaller than females. Over 90,000 known species. Most are free-living. Only about 50 are human parasites. Life cycle of parasitic nematodes is simpler than that of flatworms. Infections can be caused by eggs or larvae. 12 6
Infectious eggs Ascaris (Ascaris lumbricoides): Large nematode, up to 30 cm. Live in small intestines of humans, horses, and pigs. Eggs can survive in soil for long time. Infectious larvae Adult Hookworm (Necator americanus): Live in small intestine of humans, eggs are excreted in feces. Enter host by penetrating skin. Enters bloodstream, travels to lungs. Avoided by wearing shoes. 13 Ascaris lumbricoides Head with three lips 14 7
Ascaris 15 Hookworms 16 8