Malaria Notes

Sporozoa & Apicomplexa

  • Oocyst of Cryptosporidium
  • Belongs to the Phylum Apicomplexa, Class Sporozoa.
  • Produces sporozoites.
  • Complex life cycle.
  • Examples include Plasmodium and Toxoplasma.

Nobel Prize Discoveries

  • Alphonse Laveran (1907)
  • Ronald Ross (1902)
  • Julius Wagner (1927)
  • Youyou Tu (2015)

Malaria and Plasmodium

  • FULL-MALARIA: Malaria Full-Length cDNA Database.
  • Includes:
    • Plasmodium falciparum
    • Plasmodium yoelii
    • Plasmodium berghei
    • Plasmodium vivax

Plasmodium Species

  • Approximately 156 named species infect various vertebrates.
  • Four species infect humans:
    • Plasmodium vivax
    • Plasmodium falciparum
    • Plasmodium malariae
    • Plasmodium ovale

Erythrocytic Stage of Plasmodium

  • Basic structure includes:
    • Red nucleus
    • Blue cytoplasm
    • Brown pigment granules

Trophozoite Stages

  • Mature trophozoite
  • Immature trophozoite (ring form)
  • Features:
    • Food vacuole
    • Chromatin
    • Cytoplasm
    • Schuffner’s dots (in P. vivax)

Schizont Stages

  • Merozoite
  • Immature schizont
  • Mature schizont
  • A mature schizont about to rupture
  • Merozoites lying free

Gametocyte Stage

  • Male and female gametocytes (O+)

Sporogony and Life Cycle

  • Transformation of gametocytes to gametes.
  • Combination of male and female gametes to form sporozoites.
  • Sporozoites migrate to salivary glands.
  • Sexual cycle (Sporogony) occurs in mosquito.
  • Asexual cycle occurs in humans and liver.
    • Gametocytes ingested by mosquito.
    • Sporozoites injected into skin by mosquito, migrate to liver.
    • Hypnozoite formation (hepatic dormancy) in P. vivax and P. ovale.
    • Merozoites released into circulation, invade red blood cells.
    • Erythrocytic schizogony in red blood cells.
    • Merozoites escape from ruptured red blood cell, causing fever.

Malaria Invasion

  • Amerozite invading an erythrocyte.
  • Attachment:
    • Rhoptries
    • Micronemes
    • AMA1
    • RON4
    • Actin
    • MSP1
  • Invasion
  • Sealing
  • Aberrant rhoptry release
  • R1 inhibition
  • CytoD/PMSF inhibition
  • Surface release

Fertilization in Mosquito

  • Male and female gametes mature rapidly in the mosquito's stomach.
  • Flagella budding and breaking away takes about 2 minutes at the right temperature.
  • Zygote formation occurs when a flagellum fertilizes the female gamete.

Sporogony in Mosquito Stomach

  • Zygote transforms into a leaf-shaped mobile ookinete in 12-24 hours.
  • Ookinete penetrates the outer surface of the stomach.
  • Growth and eventual bursting releases thousands of sporozoites.
  • Structures:
    • Plasmodium oocyst
    • Gut epithelium
    • Gut lumen
    • Oocyst
    • Sporozoites

Erythrocytic Stage of P. falciparum

  • Multiple ring form
  • Gametocyte

Malaria Life Cycle Summary

  • In Human: Sporozoites (tachy-/brady-) → Exo-erythrocytic stage → Trophozoites → Merozoites → Trophozoites → Schizonts → ♀♂ gametocytes.
  • In Mosquito: ♀♂ gamete → Zygote → Oocyst → Sporozoites.
  • Schizogony occurs in humans, sporogony & gametogony in mosquitoes.
  • P. vivax: 48h erythrocytic stage, 8d exo-erythrocytic stage (6-8m if hypnozoite).
  • P. falciparum: 36-48h erythrocytic stage, 6d exo-erythrocytic stage.
  • Sporogony: 25 ℃ P. vivax 9d, P. falciparum 12d.

Malaria Transmission

  • Transmitted by: Bite of a mosquito, blood transfusion, sharing needles (drug addicts), laboratory accidents, congenital infection.
  • Infective stage: Sporozoites

Key Points in Life Cycle - Parasitic Stage

  • Exo-erythrocytic cycle: In liver cells (tachysporozoites TS, bradysporozoites BS, hypnozoite P.v & P.o)
  • Erythrocytic cycle: Specificity for RBCs
    • P. vivax, P. ovale: Reticulocytes
    • P. malariae: Older RBCs
    • P. falciparum: RBCs of all ages

Pathogenesis and Clinical Manifestations

  • Incubation period (Development in liver + early erythrocytic cycles)
  • Time vs. Parasite burden/threshold
    • P. vivax: 11-25 d (tachy-spo), 6-12 m (brady-spo), 10-500/μl threshold
    • P. falciparum: 7-27 d, 500-1300/μl threshold

Paroxysm in Malaria

  • Typical Pattern:
    • Cold phase: Abruptly and intense shivering (0.5-2h)
    • Hot phase: Hyperpyrexia (39 ℃ -40 ℃), headache, backache, nausea, muscle soreness (4-6h)
    • Sweating phase: Perspiration and defervesce; patient feels relieved
  • Periodic chill and fever: Shiver, fever, perspiration, defervesce

Mechanism of Paroxysm

  • Thermoregulatory Center affected
  • Destruction of RBCs / Liberation of merozoites
  • Pyrogenic cytokines (IL-1, TNF, IFN-γ) elevate set-point

Malaria Periodicity

  • P. vivax, P. ovale: 48 h
  • P. falciparum: 36-48h
  • P. malariae: 72h

Recrudescence and Relapse

  • Recrudescence: Erythrocytic plasmodia evading from immune response
  • Relapse: Hypnozoites in parenchymal cells of liver (Only for P. vivax & P. ovale)

Other Symptoms of Malaria

  • Jaundice: Mild jaundice due to hemolysis, severe jaundice in P. falciparum due to liver involvement.
  • Anemia: Hemolytic type, more severe in P. falciparum infections.
  • Splenomegaly: Spleen enlarges early; repeated attacks lead to enormous size and secondary hypersplenism.

Main Reasons of Anemia

  • Destruction of erythrocytes
  • Hypersplenism
  • Autoimmunity hemolysis
  • Hematopoietic disorder

Severe Forms of Malaria

  • Cerebral malaria:
    • Progressive headache, with non-specific fever
    • Impaired consciousness, nervous dysfunction, and coma
    • Generalized convulsion especially in children
  • Gastrointestinal malaria: Abdominal pain, diarrhea, vomiting caused by infection of plasmodium

Cerebral Malaria Pathology

  • Capillary with pigmented parasitized cells adhering to endothelium
  • Uninfected erythrocytes in the center
  • Adhesion damages endothelium; uninfected blood cells leak out.
  • Haemorrhage starts small, rapidly increases.

Other Forms of Malaria

  • Transfusional malaria: Caused by infected blood transfusion/contaminated needles.
    • Short incubation period (7-10d), no relapse.
  • Congenital malaria: Neonate infected through placenta/birth canal, usually without typical manifestation.

Diagnosis of Malaria

  • Microscopic identification (most frequent method).
  • Antibody detection (detects past infections).
  • Molecular diagnosis (complement microscopy, esp. in species identification).
  • Blood smear.

Immunity Against Malaria

  1. Natural resistance:
    • Species specificity
    • Duffy antigen (-)
    • Heritable anemia (sickle cell anemia)
  2. Acquired (protective) immunity:
    • Infants; young children; non-immune adults
    • Species or stages specific
    • PREMUNITION

Premunition

  • Non-sterilizing immunity, different from concomitant immunity.
  • Effective only with a small and residual population of parasite.
  • Resistance disappears and susceptibility returns if person is completely cured.

Links of Transmission

  • Patients/carriers with gametocytes in peripheral blood
  • Anopheles mosquitoes

Prevention and Control

  • "Suitable measures to local conditions"
  • Four basic technical elements of global strategy:
    • Early diagnosis and prompt treatment
    • Selective and sustainable preventive measures (vector control)
    • Early detection, containment, or prevention of epidemics
    • Strengthen local capacities in basic and applied research

Drugs for Malaria

  • Target to exo-erythrocytic merozoite, hypnozoite: primaquine;
  • Target to erythrocytic schizogony: chloroquine, artemisinin;
  • Target to gametocyte: primaquine, pyrimethamine;
  • Target to sporogony: pyrimethamine;
  • Elimination of mosquitoes

Malaria Life Cycle Revisited

  1. Anopheles mosquito bites skin
  2. Exoerythrocytic stage in infected hepatocyte (hypnozoite, sporozoites)
  3. Hepatic schizont ruptures
  4. Late RBCs trophozoite → merozoites
  5. Erythrocytic stage, early trophozoite (ring form) → Gametocyte

Prevention Measures

  • Artemisinin-based combination therapy (ACT)
  • Pyrethroids
  • Indoor residual spraying (IRS)
  • Insecticide Treated Nets (ITNS)

Malaria Prevalence

  • Countries and areas at risk of malaria transmission (2011)
  • Countries with potential to eliminate malaria by 2020

World Health Organization (WHO)

  • One or more indigenous cases
  • Zero cases in 2018-2019
  • Zero cases in 2019
  • Zero cases (≥3 years) in 2019
  • Certified malaria free after 2000
  • No malaria
  • Not applicable

Malaria Cases Worldwide (2010-2017)

  • 2010: 239,300
  • 2011: 229,500
  • 2012: 226,800
  • 2013: 221,400
  • 2014: 219,000
  • 2015: 217,500
  • 2016: 216,600
  • 2017: 214,100

Malaria Vaccine

  • Mosquirix (RTS, S/AS01)
  • Efficacy: 6-12 weeks (27%), 5-17 months (39%)
  • Targets:
    • Mosquito stage
    • Erythrocytic stage
    • Exo-erythrocytic stage
  • Mechanisms:
    • Agglutinate sporozoites (spz)
    • Prevent spz leaving skin/blood vessel
    • Prevent hepatocyte invasion/traversal/egress
    • Killing infected hepatocyte
    • Prevent maturation in hepatocytes/RBCs
    • Prevent RBC invasion/egress
    • Prevent gamete fusion/fertilization
    • Prevent ookinete migration/oocyst maturation/sporozoite emergence
    • Prevent male and female gametocyte formation and maturation
    • No transmission

Trypanosoma

  • T. brucei has three subspecies: T. b. gambiense, T. b. rhodesiense, and T. cruzi
  • T. b. gambiense and T. b. rhodesiense cause African sleeping sickness, transmitted by tsetse fly.
  • Trypanosoma cruzi is the pathogen of Chagas disease, transmitted by reduviid bugs.

Trypanosoma brucei Life Cycle

  • Tsetse fly bites human, ingests trypomastigotes.
  • Trypomastigotes in blood and cerebrospinal fluid.
  • Trypomastigotes injected into human by bite.
  • Possible migration to choroid plexus of brain
  • Metacyclic trypomastigote forms.
  • Development in gut of fly.
  • Epimastigote in salivary gland of fly.

Trypanosomal Chancre

  • Acute inflammatory local response seen a week after the bite.
  • Chancre is large, red, and rubbery, filled with parasites.
  • Painful, resolves spontaneously within weeks.

Hemolymphatic Stage of Trypanosomiasis

  • Symptoms: Fever, headache, weakness, joint pain.
  • Enlarged lymph glands.
  • Swollen cervical glands constitute “Winterbottom’s sign.”
  • Anemia, cardiovascular problems, and kidney disorders.

Meningoencephalitic Stage of Trypanosomiasis

  • Symptoms: Headaches, somnolence, abnormal behavior, coma, death if untreated.
  • Host responds with cellular and humoral immune reaction.
  • Immunoglobulin (IgM) secreted into the CSF.

Trypanosoma cruzi Life Cycle

  1. Triatomine bug takes blood meal, passes metacyclic trypomastigotes in feces, enter wound.
  2. Metacyclic trypomastigotes penetrate cells, transform into amastigotes.
  3. Amastigotes multiply by binary fission in cells of infected tissues.
  4. Intracellular amastigotes transform into trypomastigotes, burst out of cell, enter bloodstream.
  5. Triatomine bug takes blood meal (trypomastigotes ingested).
  6. Multiply in midgut.
  7. Epimastigotes in midgut.
  8. Metacyclic trypomastigotes in hindgut.

Pathogenesis and Clinical Manifestations of Chagas Disease

  • Acute local inflammatory reaction: Chagoma, swelling of regional lymph nodes. Romana’s sign.
  • Acute phase: Anemia, loss of strength, nervous disorders, chills, muscle and bone pain, heart failure.
  • Chronic Chagas disease: 10-20 years, complications depend on organs (heart, esophagus, colon, peripheral nervous system), leading to megaesophagus.

Diagnosis of Trypanosomiasis

  • T. brucei in blood smears, CSF.
  • T. cruzi in heart tissue.
  • Note the dividing forms in African trypanosomes, but not in American trypanosomes.
  • Note the typical C-shape of the trypomastigote.

Treatment and Control of Trypanosomiasis

  • Drugs depend on the stage.
  • Early stages: Pentamidine or Suramin.
  • Melarsoprol may be used at all stages, Eflornithine is safer and newer.
  • T. cruzi does not respond well to chemotherapy. Nitrofuran has been tried.
  • Surgery for megaesophagus or megacolon.
  • Insecticides to reduce vector population.