Module 8

• Parasitism


  Association between two living organisms where the parasite has become dependent on its host for its nutritional requirements and sometimes for other metabolic needs, and the host does not benefit but gets actively injured by the parasite

  
• The parasite must not kill the host since it will also die
• Characteristics of parasites


  • Transmitted water-borne, food-borne, or vector-borne, and even directly transmitted
  • Can be zoonotic but not all
  • Opportunistic as they cause severe diseases in immunocompromised individuals

  
• Endoparasites


  Live inside the body of the host
• Ectoparasites


  Attach themselves to the outside of the host's body
• Permanent parasites


  Majority of the life of the parasite is spent in the host

  
• Temporary parasites


  Only comes in contact with the host when it needs to feed

  
• Facultative parasites


  Can survive as a parasite, or a free-living organism

  
• Obligate parasites


  Incapable of surviving without the host
• Types of hosts


  • Intermediate host
  • Principal/Definitive host
  • Reservoir host
  • Vector host
  • Paratenic host
  • Accidental/Dead-end host
• Biological vector


  The parasite develops inside the vector and is needed in its life cycle

  
• Mechanical vector


  Only carries the parasite; no development happening

  
• Endoparasites


  • Protozoa
  • Helminths

  
• Protozoa


  • Flagellates
  • Amoeba
  • Sporozoa
  • Ciliates

  
• Helminths


  • Nematodes
  • Trematoda
  • Cestoda

  
• Humans are not always the definitive hosts
• Main factors why parasites survive


  • Host
  • Environment
  • Parasite

  
• Specific factors why parasites survive


  • Susceptibility
  • Age
  • Sex
  • Immune Status
  • Nutritional Status
  • Behavior/Lifestyle
  • Occupation
  • Sanitation Practices
  • Temperature
  • Climate
  • Humidity
  • Bodies of water
  • Presence of Intermediate Host/Vectors
  • Animal Reservoir Hosts
  • Virulence
  • Reproduction
  • Immune Evasion Mechanisms
  • Genetic Material

  
• Stages for parasites to transfer to another host


  • The parasite must leave the first host
  • Transmitted to another host
  • Gain entry to the body

  
• Portals of exit for parasites


  • Faeces
  • Urine
  • Sputum
  • Blood
  • Skin
  • Ingestion
• Groups of parasitic protozoa based on motility organelles


  • Flagellates
  • Amoeba
  • Sporozoa
  • Ciliates

  
• Amoebiasis


  Caused by the parasite Entamoeba histolytica, which is the third leading parasitic cause of death

  
• Life cycle of amoebiasis


  1. Cysts in formed stool are the diagnostic stage

  2. Cysts mature in aquatic environment and cause infection when ingested

  3. Trophozoites stay in intestinal lumen and can invade intestinal mucosa, and can become extraintestinal amoebiasis

  4. Trophozoites divide by binary fission and produce cysts

  5. Cysts have a thick cell wall and can survive days to weeks in water

  
• Plasmodium species


  • Plasmodium falciparum
  • Plasmodium vivax
  • Plasmodium malariae
  • Plasmodium ovale
  • Plasmodium knowlesi
• Plasmodium falciparum


  • Most virulent species, shortest incubation period of 5 days, has 1 ring form for each red blood cell, produces 8-32 merozoites which invades the red blood cells and is a sign of bad prognosis

  
• Plasmodium vivax


  • Could cause relapse due to presence of hypnozoites in the liver, amyloid-like shape, RBCs are enlarged
• Plasmodium malariae


  • Zoonotic species obtained from monkeys, has a band form, ring forms occupy at least 1/3 diameter of RBC which are normal sized, produces 8-12 merozoites that forms a rosette formation

  
• Plasmodium ovale


  • Associated with Schuffner's dots, in ring forms RBC are in oval shape called ovalocytes, produces 8-12 merozoites
• Plasmodium knowlesi


  • Most recently discovered species in humans, difficult to distinguish from P. malariae microscopically, 24 hour asexual cycle, causes severe hemolysis in humans

  
• Malaria paroxysm


  Periodic and differential act of chills, fever, and sweating, usually occurs when the RBCs have ruptured and released merozoites

  
• Virulence factor of Plasmodium


  • Cytoadherence or sequestration - infected red blood cells will adhere to various cell types such as endothelial cells and uninfected red blood cells, causing the parasite to sequester in deep vascular cells and avoid splenic clearance

  
• Manifestations of severe malaria


  • Cerebral malaria
  • Severe anemia due to hemolysis
  • Hemoglobinuria
  • Acute respiratory distress syndrome (ARDS)
  • Abnormalities in blood coagulation
  • Low blood pressure
  • Acute kidney injury
  • Hyperparasitemia
  • Metabolic acidosis
• Diagnosis of malaria


  Gold standard is microscopic examination of Giemsa stained thick and thin blood smears, rapid-diagnostic tests (RDTs) are also used

  
• First line drugs for malaria treatment


  • Artemether-lumefantrine (Coartem) + Primaquine
• Second line drugs for malaria treatment


  • Oral Quinine (Pf, Pm)/ Chloroquine (Pv, Po) + Tetracycline, or Clindamycin or Doxycycline + Primaquine, Another ACT (artemisinin-based combination, WHO/GMP certified) + Primaquine
• Third line drugs for severe malaria treatment


  • Parenteral Artesunate, Parenteral Quinine + AL (when patient can tolerate oral medication) + Primaquine
• Artemisinin analogs


  Contain an unusual peroxide bridge that undergoes an Iron-catalyzed cleavage within the parasite's food vacuole, rapidly acting blood schizonticides with some activity against gametocytes

  
• Chloroquine and other quinolones


  Became concentrated in the parasite's food vacuole preventing the polymerization of hemoglobin products, causing toxicity in the build-up of heme causing the parasite to die

  
• Quinine and quinidine


  Inhibit nucleic acid synthesis, protein synthesis and glycolysis in P. falciparum, can be used as a malarial treatment
• Malaria


  Has a lot of line of drugs because Plasmodium is capable of drug resistance