5. PROTOZOA - sporozoa plasmodium

• Malaria parasite (Plasmodium)


  Pathogen of malaria
• Malaria parasite species infecting humans

  • Plasmodium vivax
  • Plasmodium falciparum
  • Plasmodium malariae
  • Plasmodium ovale
  • Plasmodium knowlesi
• Route of transmission


  • Naturally acquired infections via bite of infected female Anopheles mosquitoes
  • Malaria also transmitted via blood transfusion, sharing of contaminated needles among IV drug abusers, congenital transmission
• Morphological forms of Plasmodium

  • Ring form (early trophozoite)
  • Developing trophozoite
  • Immature schizont
  • Mature schizont
  • Microgametocyte
  • Macrogametocyte
• Infected red blood cells

  • Plasmodium falciparum: Same size and shape as non-parasitized RBC, contain 6-12 coarse brick red dots (Maurer's cleft), some show basophilic stipling
  • Plasmodium vivax: Larger than non-parasitized ones, contains red granules (Schüffner's dots) on the surface
  • Plasmodium malariae: Same size as non-parasitized ones or slightly smaller, contain fine stipling (Ziemann's stipling)
  • Plasmodium ovale: Enlarged and most of them are oval in shape with fimbriated margins, contain prominent Schüffner's dots
• Trophozoite - ring form

  • Plasmodium falciparum: Small rings with a red staining nuclear dot and a small amount of blue-staining cytoplasm, sometimes double dots or multiple rings
  • Plasmodium vivax: Ring form with a nuclear dot and a crescent of cytoplasm
  • Plasmodium malariae: Ring form is well-defined, with a prominent central vacuole
  • Plasmodium ovale: Ring form resembles P. vivax with a nuclear dot and a crescent of cytoplasm but thicker and more-intensely stained
• Trophozoite - developing


  • Plasmodium falciparum: Pigments group together in one compact form, rarely amoeboid form
  • Plasmodium vivax: Larger and have an irregular amoeboid form, solid blue cytoplasm like thick band across the RBC
  • Plasmodium malariae: Resemble P. vivax but more compact and less amoeboid appearance, large round nucleus and often has a blob of brown malarial pigment in the RBC
  • Plasmodium ovale: Large and irregularly distributed cytoplasm, big nucleus
• Schizonts


  • Plasmodium falciparum: Only present in blood in pre-terminal phase of infection, contain 12-24 merozoites (nuclear dots), RBC with schizont is not enlarged
  • Plasmodium vivax: Contains 12-24 merozoites, infected RBC is enlarged
  • Plasmodium malariae: Contains 6-12 merozoites, infected RBC is not enlarged, fill the whole RBC with rosette appearance
  • Plasmodium ovale: Contains 6-12 merozoites, infected red blood cell is enlarged and is an oval shape, resemble P. malariae but pigment is darker
• Comparison of Plasmodium species

  • RBC: Plasmodium falciparum - normal, rarely stipling; Plasmodium vivax - enlarged, stipling; Plasmodium malariae - normal, no stipling; Plasmodium ovale - enlarged, stipling
  • Present in blood: Plasmodium falciparum - ring and gametocyte; Plasmodium vivax, Plasmodium malariae, Plasmodium ovale - all stages
  • Trophozoite: Plasmodium falciparum - small ring, sometimes 2 dots & multiple rings, may adhere to red cells; Plasmodium vivax - ring and amoeboid; Plasmodium malariae - ring and band form; Plasmodium ovale - ring and irregular shape
  • Pigment in developing trophozoite: Plasmodium falciparum - coarse, black, few clumps; Plasmodium vivax - fine, light brown, scattered; Plasmodium malariae - coarse, dark brown, scattered clumps, abundant; Plasmodium ovale - coarse, dark yellow-brown, scattered
  • Pigment in older trophozoite: Plasmodium falciparum - compact and rounded; Plasmodium vivax - very pleomorphic; Plasmodium malariae - occasionally band form; Plasmodium ovale - compact and rounded
  • Schizont: Plasmodium falciparum - rarely seen in peripheral blood, >12 merozoites; Plasmodium vivax - >12 merozoites; Plasmodium malariae - <12 merozoites (often in rosette); Plasmodium ovale - <12 merozoites (often in rosette)
  • Gametocyte: Plasmodium falciparum - banana shaped (crescentic); Plasmodium vivax, Plasmodium malariae, Plasmodium ovale - round or oval
• Plasmodium ovale


  • Oval shape
  • Resembles P. malariae but pigment is darker
• Comparison of Plasmodium species

  • Plasmodium falciparum
  • Plasmodium vivax
  • Plasmodium malariae
  • Plasmodium ovale
• RBC characteristics


  • Normal; Rarely stipling
  • Enlarge; stipling
  • Normal; No stipling
  • Enlarge; stipling
• Stages present in blood

  • Ring and gametocyte
  • All stages
  • All stages
  • All stages
• Trophozoite


  • Pigment in developing trop: Small ring; sometimes 2 dots & multiple rings. May adhere to red cells
  • Pigment in older trop: coarse, black, few clumps. Compact and rounded
  • Pigment in developing trop: Ring and amoeboid
  • Pigment in developing trop: Fine, light brown, scattered. Very pleomorphic
  • Pigment in developing trop: Ring and band form
  • Pigment in older trop: coarse, dark brown, scattered clumps, abundant. Occasionally band form
  • Pigment in developing trop: Ring and irregular shape
  • Pigment in older trop: coarse, dark yellow-brown, scattered. Compact and rounded
• Schizont


  • Rarely seen in peripheral blood; > 12 merozoite
  • > 12 merozoite
  • < 12 merozoite (Often in rosette)
  • < 12 merozoite (Often in rosette)
• Gametocyte


  • Banana shaped (crescentic)
  • Round or oval
  • Round or oval
  • Round or oval
• Life cycle


  1. Asexual phase (occurring in humans)

  2. Sexual phase (occurring in the mosquitoes)

  3. Mosquitoes are the definitive host and humans are the intermediate host

  4. Infective stage: sporozoite

  5. Infective mode: mosquito bite skin of human

  6. Parasitic position: liver and red blood cells

  7. Transmitted stage: gametocytes

  8. Schizogonic cycle in red cells: 48 hrs/. P vivax; 36 -48 hrs/. P falciparum

  9. Sporozoite: tachysporozoite and bradysporozoite

  
• Tachysporozoite and Bradysporozoite


  • Genetically distinct at the time of maturation when they enter the hepatic cells at the same time
  • Tachysporozoite grow in the hepatic cell and multiply to form exoerythrocytic schizonts and then invade to form RBCs to clinic malaria
  • Bradysporozoite is the cause of relapse of malaria. Bradysporozoite stay in the hepatic cells and will multiply later
• Erythrocytic stage


  1. Early trophozoite

  2. Later trophozoite

  3. Merozoite

  4. Immature schizont

  5. Mature schizont

  6. The process from trophozoite to merozoite is called schizogony
• Gametogenesis


  After completing a few schizogonic cycles, some merozoites develop into sexual cells, the male and female gametocytes. They continue their development in the mosquito.
• The average incubation periods are 8-11 days (P. falciparum), 10 -12 days (P. vivax and P. ovale) and 18 -40 days (P. malariae)
• The incubation period is the interval between the entry of the parasites into the host and the time when they first become detectable in blood
• Clinical disease develops when after a number of further cycles of multiplication, the level of parasitaemia rises high enough to cause fever
• Typical symptoms of malaria

  • Periodic fever with rigor
  • Anaemia
  • Splenomegaly
  • Severe headache
  • Nausea
  • Vomiting
  • Sweating stage
  • Blackwater fever
  • Cerebral malaria
• Pathogenesis


  1. Liberation of merozoites and malarial pigment; RBC debris into the blood stream

  2. Rupture of the infected RBCs and destruction of normal RBCs enhance phagocytosis; stimulate phagocytes to grow in number and enhance in function; Finally, lead to anemia and enlargement of the spleen
• Relapse


  A specific attack that occurs up to months or even years after the primary attacks
• Malignant malaria


  Malaria caused by P. falciparum is more severe than that caused by other Plasmodia. Serious complications involve cerebral malaria, massive haemoglobinuria (blackwater fever), acute respiratory distress syndrome, severe gastrointestinal symptoms, shock and renal failure which may cause death.