Plasmodium

Snapshot

• A 25-year-old woman presents to the clinic with a one-week history of continuous high-grade fevers, myalgia, chills, and night sweats. She recently returned to the United States after volunteering with the Peace Corps in Nigeria. Her temperature is 103.2°F (39.6°C) and physical reveals a palpable spleen. A peripheral blood smear is shown

Introduction

• Epidemiology
  • geography
    • endemic throughout most of the tropics
    • disproportionately high burden of malaria in Sub-Saharan Africa
  • prevalence
    • WHO estimated 216 million cases of malaria in 91 countries in 2016
  • risk factors
    • exposure to Anopheles mosquitos, particularly at dusk and dawn
• Pathogenesis
  • transmission
    • through bites of female Anopheles mosquitos
  • Plasmodium life cycle involves two hosts: human and Anopheles mosquito 
    • human host
      • sporozoite stage
        • the Plasmodium-infected Anopheles mosquito inoculates sporozoites into the human host during a blood meal
        • sporozoites infect hepatocytes and mature into schizonts, which rupture and release merozoites  
      • hypnozoite stage
        • for Plasmodium vivax and Plasmodium ovale, a dormant stage (hypnozoites) can persist in the liver and cause relapses
      • merozoite and trophozoite stage
        • merozoites undergo asexual multiplication in erythrocytes
        • merozoites reform to become trophozoites, which are ring-shaped
      • schizont stage
        • trophozoites undergo division to form large multi-nucleated schizonts
        • the schizont ruptures releasing merozoites
          • rupture of erythrocytes correlates with fever spikes
      • gametocyte stage
        • some parasites differentiate into sexual gametocytes, which are the form of the parasite that is ingested by an Anopheles mosquito
    • Anopheles mosquito host
      • oocyst stage
        • male and female gametocytes fuse in the stomach of the mosquito to form an oocyst
      • sporozoite stage
        • the oocyst divides into many sporozoites, which are then inoculated into the human host to begin the cycle anew
  • 4 main speces of Plasmodium cause malaria
    • Plasmodium falciparum 
      • irregular fever patterns
    • Plasmodium vivax
      • 48-hour fever cycle
    • Plasmodium ovale
      • 48-hour fever cycle
    • Plasmodium malariae
      • 72-hour fever cycle
• Associated conditions
  • occlusion of capillaries in the brain (cerebral malaria), kidneys, and lungs (Plasmodium falciparum)

Presentation

• Symptoms
  • fever
  • headache
  • anemia
  • splenomegaly
• Physical exam
  • palpable spleen
  • conjunctival pallor

Studies

• Labs
  • Peripheral blood smear shows trophozoites and schizonts within erythrocytes 
    • trophozoite ring form within erythrocyte
    • schizont containing merozoites
    • red granules throughout cytoplasm in erythrocytes seen with Plasmodium vivax and Plasmodium ovale
• Making the diagnosis
  • most cases are clinically diagnosed

Differential

• Babesiosis
  • differentiating factor
    • predominantly in northeastern United States
• Trypanosomiasis
  • differentiating factors
    • lymphadenopathy, somnolence, coma
• Borrelia recurrentis
  • differentiating factor
    • history of tick exposure

Treatment

• Medical
  • chloroquine
    • used in areas with low drug resistance
  • mefloquine  
    • used in areas with high rates of chloroquine resistance
  • atovaquone/proguanil or artemether-lumefantrine
    • used in areas with high rates of chloroquine resistance
      • first-line treatment for P. falciparum resistant to chloroquine
  • primaquine
    • used to kill latent hypnozoites in Plasmodium vivax and Plasmodium ovale infection
  • intravenous quinidine or artesunate
    • used in life-threatening situations

Complications

• Cerebral malaria
  • complication of Plasmodium falciparum infection
  • parasitized erythrocytes occlude capillaries in the brain
• Renal failure
  • complication of Plasmodium falciparum infection
  • parasitized erythrocytes occlude capillaries in the kidney