Lecture Notes on Protists and Malaria

Protists

Definition of Protists

  • Single cell eukaryotes that are not classified as fungi, plants, or animals.

  • May undergo encystment as a response to harsh environmental conditions.

  • Encystment is similar to sporulation.

  • Three major groups of Protista:
      - Motile heterotrophs (animal-like protists) example:Amoeba
    Moves using pseudopodia (false “arms”). It surrounds and engulfs food like bacteria.
      - Nonmotile autotrophs (plant-like protists) example:Green algae
    Contain chloroplasts like plants. Common in freshwater.
      - Nonmotile heterotrophs (fungi-like protists) example: Slime molds
    Form a blob-like mass that feeds on decaying matter.

Important Protist Pathogens

  • Plasmodium
      - Obligate parasites of vertebrates and insects.
      - Responsible for causing malaria.

  • Trypanosoma spp.
      - Parasitic flagellates.
      - Typically transmitted through vectors, mostly blood-feeding invertebrates.

  • Giardia lamblia
      - Found in contaminated food, water, and soil.
      - Protective outer shell allows survival against chlorine.
      - Causes giardiasis (also known as ‘beaver fever’).

  • Entamoeba histolytica
      - Anaerobic amoeba that can invade tissues, causing amoebic dysentery.
      - Can lead to liver lesions.

  • Toxoplasma gondii
      - Obligate intracellular parasite causing congenital toxoplasmosis in humans.

Protists in Public Health

  • Toxoplasmosis
      - Over 60 million individuals in the U.S. are chronically infected with Toxoplasma gondii.
      - Infections in pregnant women can vary in their effects on the fetus, including hydrocephalus, psychomotor and neurological disabilities, and fetal death.
      - Infections can be deadly in immunocompromised individuals.

  • Giardiasis
      - Approximately 280 million cases globally per year, resulting in about 2.5 million deaths.
      - Can lead to complications such as heart failure and death.

  • Chagas Disease
      - More than 300,000 people in the U.S. are infected with Trypanosoma cruzi.
      - Over 300 babies infected each year at birth.

Malaria: History

  • Malaria parasites found in mosquitoes preserved in amber indicate ancient origins, likely originating in Africa.

  • Co-evolution of malaria parasites with their hosts (mosquitos and non-human primates).

  • Early zoonotic events with gorillas (P. falciparum, P. vivax), chimpanzees (P. vivax, P. malariae), and Asian macaque monkeys (P. knowlesi).

  • Increased incidence of malaria over 10,000 years ago correlated with developments in agriculture.

  • Natural selection favored certain blood disorders such as:
      - Sickle cell anemia.
      - Thalassaemias.
      - Glucose-6-phosphate dehydrogenase deficiency.

Historical Context of Malaria

  • Malaria has been documented since ancient times:
      - Clay tablets from Mesopotamia describe symptoms.
      - High prevalence in Ancient Egypt; builders of pyramids were given garlic to deter disease.
      - Pharaohs utilized bed nets as early as 2600 BCE.
      - Malaria antigens detected in Egyptian remains dated to 3200 BCE and 1300 BCE.
      - Greek writers such as Homer, Aristophanes, Aristotle, and Plato referenced malaria.
      - In China, around 300 BCE, signs and symptoms of the disease were described, along with the use of artemisia herb.

  • The term ‘Roman fever’ was used for particularly deadly strains of malaria.
      - Epidemics likely contributed to declines in the Roman Empire during the 5th century AD.
      - The term mal aria means ‘bad air’ in medieval Italian, leading to the association with malaria.
      - Miasma theory of disease was prominent during this time.

Disease Burden of Malaria

  • Data from WHO in 2024 reports:
      - Approximately 282 million cases of malaria worldwide.
      - Roughly 610,000 deaths identified across 80 countries.
      - 95% of malaria cases and deaths are localized in Africa.
      - Four countries account for just over half of all malaria-related fatalities globally.
      - Children under 5 years of age account for 75% of malaria deaths in the affected regions.

Chain of Infection for Malaria

  • Organism:
      - Plasmodium parasites:
        - P. falciparum
        - P. malariae
        - P. ovale
        - P. vivax
        - P. knowlesi

  • Reservoir:
      - Humans and various vertebrates including reptiles and birds, as well as Anopheles mosquitoes.

  • Transmission:
      - Indirect transmission occurs through the arthropod vector, specifically the female Anopheles mosquito.

  • Portal of Exit/Entry:
      - Infection is introduced through an insect bite onto a susceptible host.

Characteristics of Anopheles Mosquito Vector

  • Over 400 species of Anopheles mosquitoes exist, with around 30 classified as major malaria vectors.

  • Typically bite during dusk and dawn.

  • Egg-laying occurs in a variety of aquatic habitats.
      - Preferences can range from small, shallow puddles (e.g., hoofprints) to larger pools.

  • Emerging larvae eventually mature into adult mosquitoes.

  • Adult mosquitoes require a blood meal to nourish their eggs.

Characteristics of Plasmodium spp.

  • Unicellular, eukaryotic, obligate parasite classified as an apicomplexan.

  • Contains specialized biological structures:
      - Mitochondrion
      - Rhoptry: organelles containing proteins that facilitate invasion and modification of host cells.
      - Micronemes: house proteins required for motility and recognition of host cells.
      - Dense granules: secretory structures that alter the membrane of the parasitophorous vacuole.
      - Apicoplast: organelle of endosymbiotic origin from red algae, involved in several metabolic pathways including heme biosynthesis.

Life Cycle of Plasmodium spp.

  • Requires two hosts for its life cycle:
      - Definitive Host: Anopheles mosquito, where sexual maturation occurs.
      - Secondary Host: Vertebrates, typically humans.

  • Life Cycle Steps:
      - Sporozoites are injected into a human through mosquito saliva during a bite; these are the motile infective forms that target hepatocytes (liver cells).

Hepatic Phase

  • Within the hepatocytes, sporozoites undergo a process called tissue schizogony:
      - The sporozoite increases in size and replicates its nucleus and other organelles multiple times.
      - Forms a schizont, a large multinuclear cell.
      - Cytokinesis results in the formation of multiple daughter cells known as merozoites—thousands per hepatocyte.
      - Upon hepatocyte rupture, merozoites are released into the bloodstream.

Erythrocytic Phase

  • Merozoites attach to and infect red blood cells (erythrocytes).
      - Inside the erythrocyte, a merozoite develops into a trophozoite.
      - The mature trophozoite undergoes a process called blood schizogony:
        - The schizont ruptures, resulting in the release of more merozoites (8 to 24 merozoites per erythrocyte).
      - Some merozoites develop into gametocytes in the red blood cells.

Gametocyte Stage

  • Merozoites from the red blood cells circulate through the spleen, where the immune system detects infections or cellular damage.
      - P. falciparum produces proteins on the surface of infected red blood cells that promote adhesion to blood vessel walls, decreasing clearance by the spleen and evading immune detection.
      - This adhesion can lead to blockages in small blood vessels.
      - The parasite digests hemoglobin in infected red blood cells, releasing the heme group, which is toxic to merozoites; it converts heme into an insoluble crystalline form known as hemozoin.

Mosquito Life Cycle and Infection Pathogenesis

  • When the mosquito ingests gametocytes, they mature into male and female gametes in the midgut.
      - Fertilization results in the formation of a zygote, which becomes a motile ookinete that penetrates the midgut wall and transforms into an oocyst.
      - The oocyst divides and produces sporozoites, which migrate to the salivary glands of the mosquito to be transmitted during the next blood meal.

  • Immune Evasion Mechanisms:
      - Merozoites can wrap themselves in the hepatocyte membrane, aiding in immune evasion.
      - The presence of P. falciparum proteins on erythrocytes prevents normal circulation, obstructing the immune system from detecting infected cells, and can result in thrombosis or cell debris accumulation from bursting erythrocytes.

  • Other Effects:
      - Destruction of liver and blood cells leads to complications, such as thrombosis and inflammation due to the production of inflammatory cytokines.

Course of Malaria Disease

  • Infection and Incubation:
      - Incubation period of approximately 10 - 15 days, shorter for P. falciparum and longer for P. malariae.
      - Early phases of disease show no signs or symptoms.
      - Initial signs occur once first blood schizogony is completed, prompting the release of merozoites into the bloodstream and subsequent rupturing of erythrocytes.

Uncomplicated Malaria

  • Characterized by paroxysm, a cyclical occurrence of chills, fever, and sweating lasting between 6 to 12 hours.
      - Attack patterns:
        - Every second day for P. falciparum, P. vivax, or P. ovale.
        - Every third day for P. malariae.
      - P. falciparum can induce recurrent fever episodes every 36-48 hours or continuous milder fevers.
      - Common signs and symptoms include fever, chills, sweats, headaches, nausea, vomiting, body aches, and malaise.

Severe Malaria

  • Almost exclusively caused by P. falciparum and arises when infections lead to organ failure or unusual blood metabolism.
      - Respiratory Distress: Up to 25% of adults and 40% of children may experience this.
      - Severe Anemia: Resulting from hemolysis, which can also cause hemoglobinuria (hemoglobin in urine), associated with kidney failure.
      - Cerebral Malaria: Manifested by coma and high mortality rates; surviving patients may endure long-term neurocognitive impairments.
      - Enlarged Spleen: Functions in cleaning cellular debris from the blood.

Course of Disease Continuation

  • Recrudescence: Return of disease symptoms after a symptom-free period, typically due to inadequate treatment allowing parasites to survive in the blood.

  • Relapse: Can take months or years to occur, stemming from dormant liver stage parasites referred to as hypnozoites.

  • Reinfection: New parasites can enter the body, usually occurring after some level of immunity has developed post-infection.

Diagnosis and Treatment of Malaria

  • Diagnosis Methods:
      - Microscopic analysis of blood samples.
      - Rapid diagnostic tests based on antigen detection.

  • Treatment Options:
      - Quinine: Derived from the bark of the cinchona tree; effective against blood schizonts and gametocytes of P. vivax and P. malariae. Inhibits biocrystallization of hemozoin across all Plasmodium species.
      - Chloroquine: Also inhibits hemozoin biocrystallization; effective only on the erythrocytic stage, no effect on liver stages. Resistance is common in certain regions.
      - Artemisinin-Based Combination Therapies: 90% effectiveness in treating uncomplicated malaria; combines artemisinin with another drug to mitigate resistance development. Resistance found in strains in some areas. New treatments in phase 3 trials, notably GanLum, have shown a 97.4% cure rate, outpacing existing treatments that cured 94% of participants.

Prevention Strategies for Malaria

  • Vaccines:
      - The WHO endorses two vaccines aimed at children:
        - RTS,S (Mosquirix): Approved for deployment in October 2021 for use in areas with moderate to high levels of P. falciparum infection. The vaccination regimen requires four doses with a fift


      - Implementation of mosquito nets and indoor insecticide spraying.

  • Medications:
      - Recommended prophylactic treatments begin two weeks before travel and are maintained for four weeks following return to reduce risk of infection.