III. Protozoa (Kingdom Protista)

A. Characteristics

1. Life Cycle

a. asexual reproduction and sexual reproduction (varies with species)

b. gametes - haploid sex cells

c. trophozoites – adult, vegetative(feeding), reproducing stage

d. cyst with a protective cyst wall - an immature, non-feeding stage, specialized for survival outside a host

2. Nutrition

a. mostly aerobic heterotrophs ; intestinal protozoa are capable of anaerobic growth

b. live in areas with a large supply of water

B. Medically Important Phyla

Amoeba - move by pseudopods

  • Entamoeba histolytica - only pathogenic amoeba in human intestine; causes amoebic Dysentery
  • Nagleria - free-living; found in warm waters; infect brain of swimmers  fatal encephalitis
  • Acanthamoeba - free-living; tap water; corneal infections (keratitis) in contact lens wearers

Flagellates – have 2 or more flagella

  • Trichomonas vaginalis - transferred directly from host to host; found in human vagina, male urinary tract ; important sexually transmitted disease (STD)
  • Giardia lamblia - vegetative trophozoite or cyst stage; found in small intestine of humans and other mammals; transmitted by fecal-contaminated food or water; causes gastrointestinal disease (giardiasis)

Hemoflagellates (blood parasites) - transmitted by bites of blood-feeding insects; found in circulatory system of bitten host

  • Trypanosoma brucei-gambiense - African sleeping sickness – transmitted by tsetse fly
  • Trypansoma cruzi - Chagas’ disease(South America) - kissing bug
  • Leishmania – tranmitted by sand flies; cutaneous or visceral disease

Ciliates - move by cilia

  • Balantidium coli - only ciliate that is a human parasite  dysentery; associated with pigs

Sporozoa

  • adult forms are not motile; obligate intracellular parasites
  • complex life cycle - transmission between several hosts
  • definitive host - host in which parasite undergoes sexual reproduction
  • intermediate host - host in which parasite undergoes asexual reproduction

Plasmodium species - causes malaria; transmitted by Anopheles mosquito

  • Plasmodium Life Cycle

Toxoplasma gondii  toxoplasmosis

  • cats harbor trophozoites; sexual and asexual reproduction occurs; oocysts containing sporozoites passed in cat feces & ingested by humans or other hosts;
  • sporozoites emerge  trophozoites ; infect tissue of new host & reproduce;
  • dangerous to pregnant women; infect fetus in uterus; congenital birth defects (severe - retardation and other neurological damage)

Cryptosporidium - AIDS patients; immunosuppressed people

  • respiratory, gallbladder, diarrheal disease
  • transmitted through feces of cows, rodents, dogs, cats, waterborne, and nosocomial routes

VI. Helminths (Kingdom Animalia)

Characteristics

  • multicellular; possess digestive, circulatory, nervous, excretory, reproductive systems
  • can be free-living or parasitic ; parasitic forms
  • May lack a digestive system ; Nervous system is reduced
  • Means of locomotion - greatly reduced or lost entirely
  • Complex reproductive system  large numbers of fertilized eggs to infect new host
  • Complex life cycle; a series of intermediate hosts for larval stages; definitive host for adult stage.
  • some species haveseparate sexes - male & female
  • some species are hermaphroditic - one animal has both male & female reproductive organs - can self-fertilize or cross-fertilize with another hermaphrodite.

Platyhelminths (flatworms) – includes trematodes ( flukes) and cestodes (tapeworms)

Trematodes - flukes

  • flat, leaf-shaped bodies; ventral sucker; oral sucker - attach & suck fluids from host
  • Fasciola ( liver fluke), Clonorchis (liver fluke), Paragonimus (lung fluke)
  • human is definitive host; passes eggs in feces into water; eggs develop into larva & hatch
  • larva enters intermediate host #1 ( usu. freshwater snail) & develops
  • larva bores out of snail and penetrates intermediate host #2 ( a crustacean, fish or aquatic plant like watercress) encysts in muscle or on plant
  • human eats material; larva is freed in small intestine; burrows out & reaches the organ where it develops into adult
  • Schistosoma sp. - blood fluke - After hatching out of the snail, the free-swimming larva burrows through skin around the ankle of human who is walking or bathing in infected waters.

Cestodes - tapeworms (intestinal parasites)

  • Scolex (head) with suckers - some have hooks; attach to intestinal mucosa of definitive host
  • segments (proglottids) contain eggs – hermaphrodites with complex reproductive systems
  • lack their own digestive system -- absorb nutrients from host
  • Taenia saginatta - beef tapeworm; human is definitive host; adults live & reproduce in human intestine; proglottids w/ fertilized eggs passed in feces; cows eat eggs; larva hatch from eggs, migrate to muscle of cow & encyst; viable cysts are eaten in undercooked beef - scolex survives digestion - attaches to intestine - produces proglottids.
  • Taenia solium - pork tapeworm - humans are only known definitive host; human - pig - human cycle is common in Latin America, Africa, Asia.
  • In US pigs are free from T. solium - Spread is person to person ; person eats eggs (not larva in pork); larva hatches & encysts in human muscle or brain - often fatal. In this case, one human is the definitive host, and the other (with the larva) is the intermediate host.

Nematodes (roundworms)

  • cylindrical & tapered at end; complete digestive system; separate sexes; free-living or parasites
  • Enterobius vermicularis (pinworm) - entire life is spent in human host - egg is infective. Female lays eggs on perianal region - Diagnosed by sticky tape method.
  • Ascaris lumbricoides - large, adult lives in small intestine of humans, pigs, horses - eggs are infective and survive in soil - often eaten on unwashed vegetables; eggs in small intestine hatch; larvae migrate to lungs& mature; get swallowed & migrate back to intestine
  • Necator americanus - hookworm - adult lives in small intestine; eggs passed in feces will hatch in the soil; Larvae enter host by penetrating intact skin; carried by blood to lungs; coughed up, swallowed, carried to small intestine. Avoid infection by wearing shoes.
  • Trichinella spiralis - trichinosis - acquired by eating encysted larva in undercooked pork. Larva mature in small intestine; produce eggs; eggs hatch in intestine; new larvae enter blood vessels, migrate through body - encyst in muscles & other tissues. Prevention - Cook pork thoroughly.
  • Anasakines (wriggly worms) - transmitted to humans from infected fish & squid - larva in fish migrate to muscles during refrigerated storage - freezing or thorough cooking kills larvae.

VII. Arthropods as Vectors: arachnids; crustacea; insecta (biological or mechanical vectors)