Parasitic Infections of Humans

• Parasitology


  A branch of microbiology; the scientific study of parasites
• Parasitism


  A symbiotic relationship that is of benefit to one party or symbiont (the parasite) and usually detrimental to the other party (the host)
• Parasite


  An organism that lives on or in other living organisms (hosts), at whose expense they gain some advantage
• Ectoparasite


  Parasites that live on the outside of the host's body
• Endoparasite


  Parasites that live inside the host
• Definitive host


  The host that harbors the adult or sexual stage of the parasite
• Intermediate host


  The host that harbors the larval or asexual stage of the parasite
• Accidental host


  One that can serve as a host, but is not the usual host in the parasite's life cycle
• Dead-end host


  One in which the parasite cannot continue its life cycle
• Facultative parasite


  An organism that can be parasitic, but does not have to live as a parasite; it is capable of an independent life
• Obligate parasite


  A parasite that has no choice; it must inhabit a host(s)
• How parasites cause disease

  • They produce toxins; they produce harmful enzymes; invasive and migratory parasites cause physical damage to tissues and organs; they cause the destruction of individual cells; they cause occlusion of blood vessels and other tubular structures
• Protozoal infections


  Most often diagnosed by microscopic examination of body fluids, tissue specimens, or feces—specimens are examined for motile trophozoites and dormant cyst stages
• Most protozoa are unicellular
• Protozoa classified by mode of locomotion


  Amebae move by means of pseudopodia (false feet); flagellates move by means of flagella; ciliates move by means of cilia; sporozoans do not move
• Not all protozoa are parasitic
• Leishmaniasis


  Caused by various species of flagellated protozoa in the genus Leishmania; usually transmitted via the bite of an infected sand fly; there are three forms: cutaneous, mucocutaneous, visceral
• Amebic conjunctivitis and keratoconjunctivitis

  Caused by several species of amebae in the genus Acanthamoeba (facultative parasites); can lead to loss of vision or enucleation
• Toxoplasmosis


  Caused by Toxoplasma gondii, an intracellular sporozoan; can involve the CNS, lungs, muscles, or heart, as well as the eyes; acquired by ingesting cysts or oocysts
• Protozoal infections of the gastrointestinal tract

  • Amebiasis (Entamoeba histolytica)
  • Balantidiasis (Balantidium coli)
  • Cryptosporidiosis (Cryptosporidium parvum)
  • Cyclosporiasis (Cyclospora cayetanensis)
  • Giardiasis (Giardia lamblia)
• Trichomoniasis


  Caused by Trichomonas vaginalis, a flagellate; a sexually transmitted disease of men and women; usually symptomatic in women and asymptomatic in men
• Protozoal infections of the circulatory system

  • African trypanosomiasis (Trypanosoma brucei)
  • American trypanosomiasis (Chagas disease, Trypanosoma cruzi)
  • Babesiosis (Babesia microti and other Babesia spp.)
  • Malaria (Plasmodium spp.)
• Primary amebic meningoencephalitis


  Caused by Naegleria fowleri, an ameboflagellate; Acanthamoeba and Balamuthia spp. can cause similar conditions; persons often become infected by swimming and/or diving in ameba-contaminated water
• Helminth


  Parasitic worm; two major divisions: round worms or nematodes, and flatworms or platyhelminthes (tapeworms and flukes)
• Helminth life cycle


  Has three stages: egg, larva, and the adult worm; helminth infections are primarily acquired by ingesting the larval stage; in some helminth diseases, larvae enter by penetration of the skin
• Helminth infections of humans

  • Onchocerciasis (Onchocerca volvulus)
  • Trichinosis (Trichinella spiralis)
  • Dracunculiasis (Dracunculus medinensis)
  • Loiasis (Loa loa)
  • Paragonimiasis (Paragonimus westermani)
  • Filariasis (Wuchereria bancrofti/Brugia malayi)
  • Schistosomiasis (Schistosoma spp.)
  • Cysticercosis (Taenia solium)
  • Hydatid cyst disease (Echinococcus granulosis or E. multilocularis)
  • Ascariasis (Ascaris lumbricoides)
  • Hookworm infection (Ancylostoma duodenale or Necator americanus)
  • Pinworm (Enterobius vermicularis)
  • Whipworm (Trichuris trichiura)
  • Strongyloidiasis (Strongyloides stercoralis)
  • Beef tapeworm (Taenia saginata)
  • Dog tapeworm (Dipylidium caninum)
  • Dwarf tapeworm (Hymenolepis nana)
  • Fish tapeworm (Diphyllobothrium latum)
  • Pork tapeworm (Taenia solium)
  • Rat tapeworm (Hymenolepis diminuta)
  • Fasciolopsiasis (Fasciolopsis buski)
  • Fascioliasis (Fasciola hepatica)
  • Clonorchiasis (Clonorchis sinensis)
• Fish tapeworm life cycle

  Three-host life cycle
• Medically important arthropods


  • Insects (lice, fleas, mosquitoes)
  • Arachnids (mites and ticks)
  • Crustaceans (crabs, crayfish, Cyclops species)
• Mechanical vector


  An arthropod that picks up a parasite at point A and drops it off at point B
• Biologic vector


  An arthropod that harbors the parasite in its body, where the parasite matures and/or multiplies