Comprehensive Study Notes on Phylum Apicomplexa and Genus Plasmodium

Characteristics of Phylum Apicomplexa

• General Overview

  • Phylum Apicomplexa is a large and diverse group containing $> 5000$ species.

  • Examples include: $Plasmodium$ spp., $Cryptosporidium$, $Toxoplasma$, $Babesia$, $Theilera$, and $Eimeria$.

  • The majority are obligate intracellular parasites, with exceptions such as $Cryptosporidium$ and certain gregarines.

  • They are part of the Alveolata lineage.

• Evolutionary Origin and the Apicoplast

  • They evolved from predatory flagellates/dinoflagellates.

  • They retained a chloroplast-like remnant known as the apicoplast.

  • The Apicoplast:

    • It is essential to the survival of the parasite.

    • It is implicated in host cell invasion and nutrient synthesis.

    • Hypothesized Mechanism: Origin via secondary endosymbiosis. An ancestral cell engulfed an algal cell. The algal cell lost its nucleus (leaving a diminished nucleus or DN) but the chloroplast remained. The algal cell became smaller, and the resulting chloroplast was surrounded by four membranes, becoming the apicoplast.

• The Apical Complex

  • This characteristic structure is used for the attachment and invasion of host cells.

  • It is composed of five secretory/structural organelles:

    1. Polar ring: Electron-dense structures; function remains unknown.

    2. Conoid: Microtubules arranged in a spiral configuration.

    3. Rhoptries: Located at the anterior end; secretory in function. Involved in host cell invasion.

    4. Micronemes: Small elliptical vesicles that connect to rhoptries. Involved in host cell invasion.

    5. Subpellicular microtubule network: Provides structural support.

• Reproduction and Life Cycle Phases

  • Replication results in the formation of invasive zoites.

  • Asexual replication: Involves numerous rounds of nuclear division without cytoplasmic division, resulting in multinucleated schizonts or meronts.

  • Internal Budding: In some species (e.g., $Toxoplasma\,gondii$, $Plasmodium$), progeny are formed by internal budding into invasive stages.

  • Three Distinct Phases:

    1. Sporogony: Asexual phase immediately following the sexual phase (spore formation).

    2. Merogony: Asexual phase usually with multiple rounds of replication; results in invasive merozoites.

    3. Gametogony: The development of merozoites into gametes.

  • Spore Formation:

    • Many produce a resistant spore-like stage called oocysts (e.g., $Toxoplasma$).

    • In vector-transmitted parasites (e.g., $Plasmodium$), the spore wall is reduced or absent.

Introduction to Genus Plasmodium

• Epidemiological Significance

  • Causative agent of malaria in humans and animals.

  • Evolutionary history: $Plasmodium$ species evolved long before vertebrates.

  • $> 200$ identified species infect birds, reptiles, rodents, and primates.

  • Transmission occurs via a mosquito vector ($Anopheles$ sp.).

  • Global Burden:

    • Endemic in $90$ countries.

    • $> 90\%$ of cases originate from Sub-Saharan Africa.

    • Other endemic regions: India, Brazil, Sri Lanka, Vietnam, Columbia, and the Solomon Islands.

    • $40\%$ of the world's population lives in endemic zones.

    • Annual prevalence: $300-500 \times 10^6$ people.

    • Annual death toll: approximately $500,000$.

• Species Causing Human Malaria

  1. $P. falciparum$: The most deadly; prevalent in Sub-Saharan Africa, SE Asia, and South America.

  2. $P. vivax$: Historically the most common form; found in Africa, Asia, Mediterranean-bordering countries, and South America.

  3. $P. malariae$: Infects humans and chimps (reservoir hosts); found in Africa, Asia, Western Pacific, and South America; typically benign.

  4. $P. ovale curtisi$: Found in Sub-Saharan Africa, SE Asia, and Western Pacific.

  5. $P. ovale wallikeri$: Found in Sub-Saharan Africa, SE Asia, and Western Pacific.

  6. $P. knowlesi$: Primarily a parasite of monkeys; causes serious infections in humans in SE Asia.

• Historical Impact

  • Hippocrates provided the first clinical observations of symptoms.

  • Contributed to the fall of the Roman Empire.

  • Medieval crusaders fell victim to the disease.

  • Spanish conquistadors and slaves likely introduced it to the New World.

  • Malaria epidemics incapacitated Allied and Japanese forces during WWII and entire platoons during the Vietnam War.

The Plasmodium Life Cycle and Host Invasion

• Main Stages

  1. Liver Stage (Exoerythrocytic Stage)

  2. Blood Stage (Erythrocytic Stage)

  3. Sexual Stage (Gametogony)

  4. Sporogony (In the mosquito)

• Mechanism of Host Cell Invasion

  • Attachment: The apical end of the merozoite attaches to the host cell.

  • Protein Secretion: Rhoptries, micronemes, and dense granule proteins are expelled into the host cell cytoplasm.

  • Adhesion Proteins (Adhesins): Micronemal proteins belonging to the thrombospondin-related anonymous protein (TRAP) family, epidermal growth factor (EGF)-like, APPLE domains, and lectin domains are used.

  • Specific Recognition:

    • $P. vivax$: Uses Duffy binding protein to recognize the Duffy antigen (glycosylated protein) on the erythrocyte surface.

    • $P. falciparum$: Uses erythrocyte binding antigen (EBA)-175 to recognize sialic acid groups on the erythrocyte surface.

  • Entry: A subpellicular microtubule network of motor proteins (actin, myosin) facilitates motility and active invasion.

  • The Parasitophorous Vacuole (PV): As the parasite enters the RBC, tight junctions become annular and pull back over the parasite. A new membrane is formed = parasitophorous vacuole membrane (PVM), composed of parasite-derived lipids and/or host cell membrane. The PVM ruptures just prior to merozoite release.

• Liver Stage Details

  • Sporozoites (from mosquito saliva) enter via the circulatory system ($30\,\text{min}$ window).

  • They pass through Kupffer cell macrophages ($CD68$) in the liver sinusoids to reach hepatocytes.

  • Exoerythrocytic schizogony: Multiple rounds of division without cytokinesis, followed by budding.

  • Hypnozoites: In $P. vivax$ and $P. ovale$, some sporozoites become dormant forms (hypnozoites) in the liver.

    • Relapse: Reactivation of hypnozoites (due to epigenetic control, subsequent infections like typhoid, or immunosuppression) causing the return of symptoms.

    • Recrudescence: Return of symptoms after merozoite quiescence (not hypnozoites), often due to ineffective immune response or incomplete drug treatment.

• Blood Stage Details

  • Merozoites invade RBCs.

  • Ring stage: Early trophozoite.

  • Trophozoite: Ingests host hemoglobin ($Hb$), producing diagnostic hemozoin (malarial pigment).

  • Synchronous erythrocytic schizogony: $3-5$ rounds of nuclear replication and budding.

  • RBC Rupture: Releases parasite antigens, waste products, and pyrogens (fever-inducing chemicals), leading to malarial paroxysms.

  • Sequestration: $P. falciparum$-infected RBCs adhere to capillary endothelium of major organs.

• Sexual Stage and Sporogony

  • Gametogony: Merozoites differentiate into microgametocytes (male) or macrogametocytes (female). These do not sequester.

  • In Mosquito Gut: Ingested gametocytes form microgametes and macrogametes. Flagellated microgametes fuse with macrogametes to form a zygote.

  • Ookinete: A motile stage that penetrates the insect gut wall.

  • Oocyst: Develops on the basal side of the gut epithelium; undergoes sporogony.

  • Sporozoites: Formed $10-28$ days post-infection; released into the hemocoel; invade salivary glands.

Behavioral and Chemical Ecology

• Vector Manipulation

  • Infected mosquitoes probe hosts longer and bite more frequently.

  • Mosquitoes are preferentially drawn to infected persons.

  • HMBPP ($(E)-4-hydroxy-3-methyl-but-2-enylpyrophosphate$): Produced by the apicoplast via the isoprenoid precursor pathway.

  • HMBPP induces the release of aldehydes, monoterpenes, and other volatile chemicals from infected host RBCs, enhancing vector attraction and stimulating vector feeding.

Clinical Manifestations and Fever

• Malarial Paroxysms

  1. Cold Stage (Rigor): Intense cold, vigorous shivering ($15-60\,\text{min}$).

  2. Hot Stage: Fever, severe headache, fatigue, nausea, dizziness ($2-6\,\text{h}$). Temperature can exceed $42^{\circ}C$ ($107.6^{\circ}F$).

  3. Sweating Stage: Profuse sweating, fever declines ($2-4\,\text{h}$).

• Mechanism of Fever

  • Phagocytosis of waste and hemozoin causes the release of endogenous pyrogens like Tumor Necrosis Factor ($TNF-\alpha$) and prostaglandins.

  • Pyrogens bind to the hypothalamic thermoregulatory center.

  • Neuronal Shift: Reducing the activity of warm-sensitive neurons and increasing cold-sensitive neurons.

  • The brain perceives the body is too cool, increasing heat production (shivering) and conservation (vasoconstriction), elevating the thermoregulatory set point.

Pathogenesis of $P. falciparum$

• General Pathophysiology

  • Severe malarial anemia: Caused by the destruction of parasitized and non-parasitized RBCs. Bilirubin formation exceeds excretion (jaundice). Host cannot recycle iron bound to hemozoin.

  • Sequestration: Parasitized erythrocytes express $PfEMP1$ (Plasmodium falciparum erythrocyte membrane protein 1) on surface "knobs."

  • Host Receptors for Sequestration:

    • $CD36$: Endothelial lining (binds $CIDR$ domain).

    • $ICAM-1$: Brain vessels (binds $DBL-\beta$ domain).

    • $CSA$ (Chondroitin Sulfate A): Placenta vessels (binds $DBL-\gamma$ domain).

    • $CD31$: Endothelial junctions.

    • VCAM, ELAM1, Thrombospondin.

  • Rosetting: Infected RBCs bind to uninfected RBCs, clumping and clogging vessels (leads to ischemia, gangrene, edema).

• Immune Evasion and Antigenic Variation

  • PfEMP1 Variants: Belong to the $var$ gene family (~$60$ genes). High genetic recombination allows millions of variants.

  • Antigenic Variation: Parasite switches surface variants at $> 2\%$ per generation to stay ahead of the host immune system.

  • Allelic Exclusion: Only a single $var$ gene is expressed at any one time.

• Cerebral Malaria

  • Characterized by convulsions, psychosis, unrousable coma.

  • Mechanism A (Sequestration): Obstructed flow, ischemia, hypoxia, and local high glycolysis leading to lactic acidosis and hypoglycemia.

  • Mechanism B (Cytokines): $TNF-\alpha$ upregulates Nitric Oxide (NO), causing vasodilation (intracranial hypertension) and aberrant neurotransmission.

• Renal Failure and Black Water Fever

  • Black water fever: Severe fever, jaundice, vomiting, and hemoglobinuria (black urine) due to massive RBC rupture.

  • Renal Failure: Acute tubular necrosis from impaired blood flow and metabolic breakdown products.

  • Oliguria: $< 400\,\text{ml}$ urine in $24\,\text{h}$.

• Modifications of the Host Erythrocyte

  • PTEX (Plasmodium Translocon of Exported proteins): A gatekeeper on the PV membrane that exports $> 10\%$ of parasite proteins into the host cell.

  • Maurer’s clefts: Membrane-bound compartments for protein trafficking.

  • Knobs: Formed by KAHRP (Knob-associated histidine-rich protein) and $PfEMP2$. They reorganize the sub-membrane cytoskeleton.

Comparison of Disease Severity (Untreated)

• Severity of Initial Paroxysms

  • $P. falciparum$: Severe.

  • $P. vivax$: Moderate to severe.

  • $P. ovale$: Mild.

  • $P. malariae$: Mild to moderate.

• Parasitemia (per $mm^3$)

  • $P. falciparum$: Avg: $50,000-500,000$; Max: $2,500,000$.

  • $P. vivax$: Avg: $20,000$; Max: $50,000$.

  • $P. ovale$: Avg: $9,000$; Max: $30,000$.

  • $P. malariae$: Avg: $6,000$; Max: $20,000$.

• Maximum Infection Duration

  • $P. malariae$: $20-50+$ years.

  • $P. vivax$: $5-8$ years (includes relapses).

  • $P. falciparum$: $6-17$ months.

  • $P. ovale$: $12-20$ months.

Immunity and Resistance

• Premunition

  • A pre-existing infection offers protection against newly arriving conspecifics and reduces severity.

  • Required to maintain immunity via repeated and regular exposure.

  • Older individuals exhibit lower parasitemia and fewer complications.

• Genetic Resistance

  • Sickle Cell Allele: Offers protection against $P. falciparum$; correlation between allele prevalence and malaria endemicity.

  • G6PD (Glucose-6-phosphate dehydrogenase): Variants confer some protection.

  • Duffy Negative: West Africans lacking Duffy antigens are resistant to $P. vivax$.

Epidemiology, Control, and Treatment

• Chemotherapy and Resistance

  • Chloroquine (CQ): Blocks heme dimerization. Heme is toxic; the parasite normally converts it to non-toxic hemozoin. If blocked, toxic heme causes cell lysis.

  • $PfCRT$ (P. falciparum Chloroquine Resistance Transporter): Resistant strains express an altered transporter that exports CQ from the food vacuole.

  • Drug Classes:

    • Fast-acting blood schizontocides: Chloroquine, Quinine, Mefloquine, Artemisinin derivatives.

    • Slow-acting: Doxycycline.

    • Tissue schizontocides (Anti-relapsing): Primaquine, Tafenoquine.

    • Combination Therapies: $Fansidar^{\text{\textregistered}}$ (Pyrimethamine + Sulfadoxine), $Malarone^{\text{\textregistered}}$ (Atovaquone + Proguanil), Artemisinin combination treatment (ACT).

• Vaccines and Control

  • Mosquirix (RTS, S/AS01): First malaria vaccine approved by WHO (Dec 2021). Developed by PATH and GSK. Recombinant vaccine directed against the sporozoite protein.

  • Vector Control: Genetically-modified mosquitoes, $Gambusia$ (mosquito-eating fish), $DDT$ treatment of habitats, and insecticide-treated sleeping nets.

Questions & Discussion

• Question: Why do $Plasmodium$ species occurring in temperate zones tend to have a longer hypnozoite stage than those in tropical zones?

• Discussion: The transcript highlights that reactivation of the hypnozoite is under epigenetic control. In temperate zones, a longer dormant stage likely synchronizes the return of the parasite to the bloodstream with the seasonal emergence of the mosquito vector during warmer months, ensuring transmission continues.