Comprehensive Zoology Notes on Protozoa and Plasmodium Life Cycles

Hemixis in Paramecium

  • Definition: Hemixis, also known as the purification act, is the reconstitution of the macronucleus in Paramecium without any change occurring in its micronucleus.

Overview and Discoveries of Plasmodium

  • Parasitic Nature: Plasmodium is an endoparasite, commonly known as the malarial parasite.
  • Historical Discoveries and Contributions:
    • Ronald Ross: Discovered the oocyst stage of Plasmodium on the gut walls of the female Anopheles mosquito (malaria mosquito relations). [Mnemonic: @R=R]
    • Charles Laveran: Discovered the malarial parasite (Plasmodium) in the red blood cells (RBCs) of man.
    • Grassi: Described the digenetic life cycle of Plasmodium in FAM.

Plasmodium Life Cycle Characteristics and Species Comparison

  • Host Relations and Digenetic Nature:
    • Plasmodium is a digenetic parasite requiring two hosts to complete its life cycle:
    • Intermediate / Secondary Host: Man.
    • Definitive / Primary Host: Female Anopheles mosquito, because the sexual life cycle takes place within it.
    • Medical Distinction: Medically, man is designated as the primary host and the female Anopheles mosquito is designated as the secondary host.
  • Comparative Table of Plasmodium Species:
    • Plasmodium vivax:
    • Disease: Benign tertian malaria (most common species).
    • Fever Pattern: Fever occurs after 48 hrs48\,\text{hrs} (every 3rd3^{\text{rd}} day).
    • Incubation Period: 14 days14\,\text{days}.
    • Characteristic Granules: Schuffner's granules [Mnemonic: @VS].
    • Plasmodium falciparum:
    • Disease: Malignant tertian malaria / cerebral malaria / severe malaria / black water fever [IOM].
    • Fever Pattern: Fever occurs every 2nd2^{\text{nd}} or 3rd3^{\text{rd}} day, i.e., after 48 hrs48\,\text{hrs}.
    • Incubation Period: 12 days12\,\text{days}.
    • Characteristic Granules: Maurer's dots [Mnemonic: @FM].
    • Virulence: Most dangerous species of Plasmodium because its incubation period is the least among all species (12 days12\,\text{days}), meaning its cycle is completed faster.
    • Plasmodium malariae:
    • Disease: Benign quartan malaria.
    • Fever Pattern: Fever occurs after 72 hrs72\,\text{hrs}.
    • Incubation Period: 28 days28\,\text{days} (longest incubation period among all species).
    • Characteristic Granules: Ziemann's dots [Mnemonic: @MaZi].
    • Plasmodium ovale:
    • Disease: Benign sub-tertian malaria (rarest species).
    • Fever Pattern: Fever occurs every 3rd3^{\text{rd}} day, i.e., after 48 hrs48\,\text{hrs}.
    • Incubation Period: 14 days14\,\text{days}.
    • Characteristic Granules: Jame's dots [Mnemonic: @OJa].
  • General Rules of Species Classification:
    • All Plasmodium species cause benign malaria except Plasmodium falciparum (which causes malignant malaria).
    • All Plasmodium species cause tertian malaria except Plasmodium malariae (which causes quartan malaria).

Asexual Life Cycle in Humans (Schizogony)

  • General Properties:
    • The asexual cycle (Schizogony) is completed in man (the secondary host).
    • Schizogony is a type of multiple fission.
  • Infective Stage and Host Liver Invasion:
    • Infective Stage: The sporozoite is the infective stage of Plasmodium, formed from a sporoblast (which develops from a sporont).
    • Sporozoite Structure: Sickle-shaped and S-shaped.
    • Liver Cell Penetration: Sporozoites break the cell membrane of liver cells using lytic substances. They possess rhopteries that secrete proteolytic enzymes.
    • Tissue Preference: Plasmodium initiates its life cycle in liver cells to gain protection against white blood cells (WBCs) and to utilize the rich source of glycogen present in liver tissue.
  • Pre-erythrocytic Cycle (1st1^{\text{st}} Liver Schizogony / Schizont Stage):
    • Sporozoites feed upon the cytoplasm of liver cells; this active feeding stage is called the Trophozoite.
    • The nucleus of the Schizont divides repeatedly to form numerous nuclei, each surrounded by a small amount of cytoplasm, forming cryptomerozoites.
    • Duration: Takes 6–9 days6\text{--}9\,\text{days}.
  • Exo-erythrocytic Cycle (2nd2^{\text{nd}} Liver Schizogony):
    • Absence in Specific Species: Does not occur in Plasmodium falciparum ("P. falciparum ma hudaina").
    • Duration: Takes 4 days4\,\text{days}.
    • Process: Each cryptomerozoite re-enters a liver cell and a second liver schizogony occurs according to the pathway: Cryptomerozoite →\rightarrow Metacryptozoite →\rightarrow Schizont $ ightarrow$ Metacryptomerozoite.
    • Types of Metacryptomerozoites:
    • Micrometacryptomerozoite: Attacks RBCs and feeds on hemoglobin.
    • Macrometacryptomerozoite: Attacks liver cells.
  • Erythrocytic Cycle (Golgi Cycle):
    • Takes place in RBCs following the attack by micrometacryptomerozoites.
    • Sequence of Stages [Mnemonic: @TaSi ASR Me Gaya]:
    1. Micrometacryptozoite / Merozoite Entry: Merozoite enters the RBC and forms a rounded disc-like structure known as the trophozoite stage.
    2. Trophozoite Stage: Active feeding stage.
    3. Signet Ring Stage: Increases in size and pushes the nucleus to the periphery, forming the signet ring trophozoite.
    4. Amoeboid Stage: The signet ring trophozoite throws out pseudopodial processes and assumes an amoeba-like shape (amoeboid trophozoite).
    5. Schizont Stage: The amoeboid trophozoite grows large and spherical, becoming a schizont.
    6. Rosette Stage: The schizont stage gives rise to the rosette stage, which exhibits a petals-like appearance.
    7. Merozoites Release: The schizont ruptures, releasing RBC merozoites into the blood along with haemozoin.
    8. Gametocyte Formation: Merozoites develop into gametocytes.
  • Pathophysiology and Hemoglobin Decomposition:
    • Ghost Cells: Ruptured RBCs and schizonts are termed ghost cells, which are cleared and destroyed by the spleen.
    • Hemoglobin Breakdown: Ingested hemoglobin decomposes into haematin and globulin.
    • Protein globulin is digested and used as food by the parasite.
    • Haematin (Fe\text{Fe}) is converted into a toxic pigment named haemozoin.
    • Pathogenesis of Fever: The release of haemozoin into the bloodstream triggers the paroxysmal fever in malaria patients.
    • Spleen Function: The spleen acts as the storehouse of malarial parasites in the human body and is referred to as the graveyard of RBCs.

Sexual Life Cycle in Mosquitoes (Gamogony & Sporogony)

  • Escape from Digestion: The gametocyte stage of the malarial parasite escapes digestion inside the stomach of the mosquito.
  • Types of Gametocytes:
    • Micro-gametocyte: Male gametocyte; gives rise to 6–86\text{--}8 haploid microgametes.
    • Macro-gametocyte: Female gametocyte; gives rise to 11 haploid macrogamete.
  • Ex-flagellation: The biological process of forming whip-like micro-gametes or sperms.
  • Fertilization and Ookinete Formation:
    • Fusion: Microgamete + Macrogamete →\rightarrow Fusion →\rightarrow Zygote.
    • The zygote transforms into a motile stage called the Ookinete.
  • Oocyst Development and Sporogony:
    • The motile ookinete penetrates the peritrophic membrane of the mosquito's stomach.
    • It forms an Oocyst / sporont on the outer wall of the mosquito's stomach.
    • Oocysts multiply asexually (sporogony) to produce sporozoites.
    • Sporozoites migrate to and colonize the salivary glands of the mosquito, ready to repeat the life cycle.

Anti-Malarial Pharmacology and Commemorations

  • Commemorative Dates:
    • 20th August20^{\text{th}}\,\text{August}: Mosquito Day.
    • 25th April25^{\text{th}}\,\text{April}: Malaria Day.
  • Anti-Malarial Pharmacotherapy:
    • Quinine: An important alkaloid used in malaria treatment, extracted from the bark of the Cinchona tree.
    • Other Anti-Malarial Synthetic Drugs: Basoquine, Camoquine, Chloroquine, Paludrine, Primaquine.

Comparative Notes on Other Protozoa

  • Giardia lamblia:
    • Anatomical Site: Resides in the duodenum.
    • Morphological Forms:
    • Trophozoite: Binucleated active feeding stage.
    • Cyst: Quadrinucleated infective stage.
    • Clinical Manifestation: Causes steatorrhea.
  • Entamoeba histolytica:
    • Discovery: First discovered by Losch.
    • Pathology: Causes Amoebiasis / Amoebic dysentery.
    • Cellular Features: Contractile vacuole is absent in Entamoeba.
    • Phases / Forms:
    • Minuta Form: Non-pathogenic form.
    • Magna / Trophic Form: Pathogenic form.
  • Amoeba proteus:
    • Cell Boundary: Plasmalemma (diffuses out).
    • Feeding & Digestion: Omnivorous; digestion is intracellular.
    • Contractile Vacuole Behavior in Variable Salinity:
    • When a fresh water amoeba is placed in salt water, its contractile vacuole disappears.
    • When placed in distilled/fresh water, its contractile vacuole develops and works faster.