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Medically Important Protozoa 1. Amoeboflagellates (Phylum Sarcomastigophora) Move using pseudopods (false feet) or flagella. A. Amoebas (Subphylum Sarcodina) Move by extending blunt, lobelike projections (pseudopods). Amoebas engulf food with pseudopods and phagocytize it. Several species cause amoebic dysenteries of varying degrees of severity. Entamoeba hystolytica: Feeds on red blood cells. Produces dysentery and extraintestinal cysts. Dientamoeba fragilis: Found in 4% of humans. Usually commensal. Can cause chronic, mild diarrhea.

Other diseases include: Meningoencephalitis: Caused by Naegleria fowleri. Penetrate nasal mucosa of swimmers in warm waters. Mortality rate almost 100%. Keratitis: Caused by Acanthamoeba. Can cause blindness. Associated with use of contact lenses.

B. Flagellates (Subphylum Mastigophora) Move by one or more whiplike flagella. Some parasitic flagellates have up to eight flagella. Most are spindle shaped with flagella projecting from anterior end. Outer membrane is a tough pellicle. Food is ingested through an oral groove or cytosotome. Important pathogens: Trichomonas vaginalis: Causes genital and urinary infections. Has undulating membrane. Lacks a cyst stage. Transmitted sexually or by fomites. Giardia lamblia: Causes a persistent intestinal infection (giardiasis) with diarrhea, nausea, flatulence, and cramps. In U.S. most common cause of waterborne diarrhea. About 7% of U.S. population are healthy carriers. Trypanosoma brucei gambiense: Hemoflagellate (blood parasite). Causes African sleeping sickness. Trypanosoma cruzi: Hemoflagellate that causes Chaga’s disease, a cardiovascular disease common in Texas and Latin America.

Medically Important Protozoa (Continued) 2. Apicomplexans (Phylum Apicomplexa) Not motile in their mature form. Obligate intracellular parasites. Have specialized organelles at tip (apex) of cells that penetrate host tissues. Complex life cycles. May have more than one host. Definitive host: Harbors sexually reproducing form. Intermediate host: In which asexual reproduction occurs.

Medically Important Protozoa (Continued) 2. Apicomplexans (Phylum Apicomplexa) Important pathogens: Plasmodium vivax and falciparum: Cause malaria in humans (intermediate host). Initially treated with quinine, drug resistance is a major problem today. Major cause of worldwide mortality: Kill 3 million people/year and infect 500 million. Transmitted by Anopheles mosquito (definitive host). DDT was used extensively in 1960s in an attempt to eradicate the mosquito vector. Successful vaccine not available yet.

Life Cycle of Plasmodium spp. the Infectious Agent of Malaria

Medically Important Protozoa (Continued) 2. Apicomplexans (Phylum Apicomplexa) Important pathogens: Toxoplasma gondii: Causes toxoplasmosis in humans. Causes blindness and lymphatic infections in adults. Dangerous to pregnant women, causes severe neurological defects in unborn children. Cats are part of life cycle, oocysts excreted in feces. Contact with infected feces or meat are means of transmission. Cryptosporidium: Causes respiratory and gallbladder infections in immunosuppressed individuals. Found in intestines of mammals and water. Major cause of death in AIDS patients. Cyclospora cayetensis: New parasite (1996) caused diarrhea associated with raspberries.

Medically Important Protozoa (Continued) 3. Ciliates (Phylum Ciliophora) Move and obtain food using cilia. Only known human pathogen is Balantidium coli, which causes a severe intestinal infection in pigs and humans. 4. Microsporans (Phylum Mycrospora) Obligate intracellular parasites, lack mitochondria and microtubules. Discovered in 1984 to cause chronic diarrhea and conjunctivitis, mainly in AIDS patients.

Paramecium caudatum is a Ciliated Protozoan Conjugation Between Opposite Mating Strains

VI. HELMINTHS (WORMS) Characteristics 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). 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.

VI. HELMINTHS (Continued) Two main groups (phyla) Platyhelminths (Flatworms) 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.

VI. HELMINTHS (Continued) I. Platyhelminths (Flatworms) Flattened from front to back. Include: 1. Trematodes or Flukes Leaf shaped bodies Ventral and oral 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 400,000 immigrants in U.S. and 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, particularly abdominal and pelvic veins.

Free-Living Flatworm Planaria

Life Cycle of Blood Fluke (Schistosoma)

VI. HELMINTHS (Continued) I. Platyhelminths (Flatworms) 1. Trematodes or Flukes Asian Liver Fluke (Clonorchis sinensis): Infests gallbladder, bile ducts, pancreatic ducts, causes biliary cirrhosis and jaundice. Cannot be transmitted in U.S. because intermediate hosts are not available. Lung Fluke (Paragonius westermani): Lives in bronchioles of humans and other animals. 12 mm long. Infection from eating undercooked crayfish.

Asian Liver Fluke (Clonorchis sinensis)


VI. HELMINTHS (Continued) I. Platyhelminths (Flatworms) 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.

Cestode (Tapeworm) Body Structure: Scolex and Proglottids

VI. HELMINTHS (Continued) I. Platyhelminths (Flatworms) Cestodes or Tapeworms (Continued) 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 (“measly beef�). Pork Tapeworm (Taenia solium): Human is definitive host. Infection can occur from eating infected undercooked pork (rare in the U.S.) or from human to human contact. Echinococcus granulosus: Dogs and coyotes are definitive hosts. Humans may become infected by contact with dog feces or saliva.

VI. HELMINTHS (Continued) 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 and have one or two spicules on posterior end.

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.

Comparison of Body Organization of Flatworms, Roundworms, and Earthworms Flatworm



II. Nematodes (Roundworms) Continued Infectious eggs Pinworm (Enterobius vermicularis): Spends entire life in human host. Adults live in large intestine. Female lays eggs in perianal region which causes itching. Up to 90% of children are infected through contaminated clothes or bedding. Infection usually disappears after a few years. Ascaris (Ascaris lumbricoides): Large nematode, up to 30 cm. Dioecious with sexual dimorphism. 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, swallowed in sputum. Avoided by wearing shoes. Trichinosis (Trichinella spiralis): Reproduce sexually in small intestine of humans. Obtained from eating undercooked pork. Larvae enter blood vessels and form cysts throughout body. Anisakines (Wriggly worms): Infected fish and squid. Killed by freezing and cooking.

Pinworm (Enterobius vermicularis)


Pinworm Eggs (Enterobius vermicularis)


Ascaris (Ascaris lumbricoides)


Head of Ascaris (Ascaris lumbricoides)

Notice three lips characteristic of Ascaris Source: www.

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