Introduction to Protozoa

• Symbiosis


  A relationship between unlike organisms which exist together
• Commensalism


  One species benefits while the other is not harmed
• Mutualism


  Two organisms mutually benefit each other
• Parasitism


  One benefits and the other is harmed
• Based on habitat


  • Ectoparasites (live outside the host's body, e.g. fleas, lice)
  • Endoparasites (live inside the body of the host, e.g. helminths or worms)

  
• Infestation


  Invasion of the body by parasites

  
• Based on ability to live independently of the host


  • Facultative parasites (live independently of the host, don't have to live inside a host to complete their life cycle)
  • Obligate parasites (must live inside a host, e.g. Plasmodium, hookworms)

  
• Based on mode of living


  • Permanent parasites (remains in host from early life to maturity, e.g. Plasmodium)
  • Intermittent parasites (simple visit the host during feeding, e.g. non-pathogenic parasites)
  • Incidental parasites (remains in an unusual host, e.g. dog tapeworm)
  • Transitory parasites (larva develops in a host while adult is free-living, e.g. dog tapeworm)
  • Erratic parasites (seen in an unusual organ, different from that which it ordinarily parasitizes, e.g. Ascaris lumbricoides in lungs or kidneys)

  
• Types of hosts


  • Definitive hosts (harbor the adult stage of parasite, e.g. humans for the intestinal worms)
  • Intermediate hosts (harbor the larval stage of parasites, where the asexual stage of the life cycle of the parasite)
  • Reservoir hosts (vertebrate hosts that harbor the parasite and may act as additional source of infection in man, e.g. migratory birds for Capillaria philippinensis)
  • Paratenic hosts (those that serve as a means of transport for the parasite so that the infective stage of a certain parasite may reach its final host, e.g. insect vectors)

  
• Sources of exposure to infection

  • Contaminated soil or water (most common, e.g. ascaris lumbricoides, human hookworms, tricuriasis)
  • Food containing the parasite's infective stage (e.g. cyst of parasitic amoeba, larvae of blood flukes, eggs of pork tapeworm)
  • A blood-sucking insect (e.g. female anopheles mosquito for malaria, culex mosquito for filariasis)
  • A domestic or wild animal harboring the parasite (e.g. dogs for hydatic cyst of dog tapeworm, pigs, cows and birds)
  • Another person and his or her clothing, bedding, or the immediate environment he or she has contaminated
  • One's self (auto-infection)

  
• Modes of transmission


  • Fecal-oral (most common mode of transmission of most intestinal parasites, e.g. ingestion of contaminated food and water, intestinal protozoa cyst stage, embryonated egg stage of the intestinal roundworms)
  • Penetration of the skin from the soil or contaminated water (e.g. blood flukes, hookworm)
  • Bite of blood-sucking insect vectors (e.g. malaria, filariasis, leishmaniasis)
  • Transplacental or congenital infection (e.g. Toxoplasma gondii, Plasmodium)
  • Transmammary (mother's milk) infection (e.g. Strongyloides, Ancylostoma)
  • Sexual intercourse (e.g. Trichomonas vaginalis)
• Portals of exit


  • Anus (most common)
  • Feces
  • Urine
  • Sputum
• Mechanism of disease production by parasites


  • Physical trauma (physical damage caused by the parasite in the organ it parasitizes or at the point of entry of the parasite)
  • Lytic necrosis (e.g. Entamoeba histolytica releases enzymes that lyse tissue for their nutritional needs and penetrate the tissues of the colon)
  • Stimulation of host tissue reaction (cellular proliferation and infiltration at the site of the parasite entry, e.g. helminths, destruction of RBC, stimulates RBC production by bone marrow)
  • Toxic and allergic phenomena (immunopathology) (proteins produced by the parasites may lead to hypersensitivity or allergic reactions due to stimulation of antibody production, e.g. female pinworm (Enterobius vermicularis) in anus)
  • Opening of pathways for entry of other pathogens into the tissues (presence of parasites may damage the tissues and favor the entry and proliferation of other organisms, especially bacteria, e.g. infection with pinworm leads to intense itchiness in the anus)

  
• Infective stage


  Stage that enters the host, stage that is present in the parasite's source of infection, non-motile cyst

  
• Pathogenic stage


  Stage responsible for producing the disease in the host leading to the clinical manifestations, motile trophozoite (stage when they are absorbing and feeding nutrients from the host and reproducing)

  
• Encystation


  Process by which trophozoites differentiate into cyst forms
• Excystation


  Process by which cysts differentiate into trophozoite forms
• Intestinal Protozoa Subphylum


  • Sarcodina (Entamoeba histolytica)
  • Mastigophora (Giardia lamblia, Giardia intestinalis, Cercomonas intestinalis, Giardia duodenalis)
• Entamoeba histolytica


  • Characteristic (only known pathogenic intestinal ameba)
  • Mode of Transmission (fecal-oral route, sexual transmission, water)
  • Reproduction/Life Cycle (cyst, trophozoite)
• Entamoeba histolytica


  • Disease (Acute Intestinal Amoebiasis, Extraintestinal Amoebiasis)
  • Laboratory Diagnostics (Fecalysis, Trophozoite in diarrheal stool, Cyst in formed stool, Serology)
  • Treatment (Metronidazole, Tinidazole, Diloxanide furoate, Paromomycin, Surgery)
  • Prevention (Good personal hygiene, Proper waste disposal, Avoid use of "night soil", Adequate washing and cooking of vegetables)
• Pericardium


  Membrane surrounding the heart
• Spleen


  Organ that filters blood and helps fight infection
• Skin


  Outer protective layer of the body
• Asymptomatic carrier state


  When a person is infected with a parasite but shows no symptoms

  
• Conditions for asymptomatic carrier state


  • Low-virulence strain
  • Low parasite load
  • Intact immune system of patient
  • No symptoms
  • Parasite reproduces
  • Parasite passed out in feces

  
• Fecalysis


  Examination of stool sample
• Trophozoite


  Active feeding stage of a protozoan parasite found in diarrheal stool

  
• Cyst


  Dormant, resistant stage of a protozoan parasite found in formed stool

  
• Serologic


  Relating to the serum component of blood, used for diagnosis
• Metronidazole


  Antiprotozoal drug
• Tinidazole


  Alternative antiprotozoal drug
• Drugs for asymptomatic infection


  • Diloxamide furoate
  • Metronidazole
  • Paromomycin
• Surgery


  Treatment for asymptomatic infection
• Prevention measures


  • Good personal hygiene
  • Proper waste disposal
  • Avoid use of "night soil" (human feces) for fertilization of crops
  • Adequate washing and cooking of vegetables
• Giardia lamblia


  Protozoan parasite that causes giardiasis
• Giardia intestinalis


  Alternate name for Giardia lamblia
• Cercomonas intestinalis


  Alternate name for Giardia lamblia
• Giardia duodenalis


  Alternate name for Giardia lamblia
• Giardiasis


  • Self-limiting, lasting 1-2 weeks
  • Transmitted by fecal-oral route and oral-anal contact
• Cyst


  Oval, thick-walled stage with 4 nuclei that gives rise to 2 trophozoites during excystation