Plasmodium

• Mosquito-borne protozoon that causes malaria

Background

Microbiology

• Intracellular protozoal parasite of red blood cells
• Species that cause human disease are: P. falciparum, P. vivax, P. ovale, P. malariae, and P. knowlesi (from the macaque monkey)
  • Most common cause of disease in humans is Plasmodium falciparum
  • P. knowlesi looks like P. malariae microscopically, but has a higher (>1%) parasitemia with a clinical course more like P. falciparum
• Identified on thick-and-thin Giemsa-stained blood films

Life Cycle

• Infected mosquito injects sporozoites into human
• Sporozoites infect the hepatocytes, which develop intracellular schizonts
  • P. vivax and P. ovale can have prolonged (months to years) liver stages during which the patient is asymptomatic
• The hepatocytes rupture and release trophozoites, which infect erythrocytes and mature into trophozoites
• Trophozoites develop into schizonts, then rupture the erythrocyte to release more merozoites
  • These cycles of merozoite to trophoziote to schizont to merozoite explain the periodic fevers
• Trophozoites can also develop into gametocytes (micro- or macro-gametocytes), which are taken up by the mosquito
• In the mosquito, the micro- and macro-gametocytes join to form a zygote, which matures into an ookinete then oocyst, which releases infective sporozoites

Pathophysiology

• Infected red blood cells adhere to endothelial cells, and clump, causing rosetting
• This causes microvascular obstruction and ischemia, which causes cerebral malaria and metabolic acidosis
• Can cause marrow suppression
• P. falciparum manages to avoid splenic sequestration
• Hypoglycemia
  • In children, hypermetabolic and consumes glucose
  • In adults, hyperinsulin state and quinine also contributes

Epidemiology

• Transmitted by female Anopheles mosquitoes, but can also be transmitted through blood transfusions
• Distribution is that of the Anopheles mosquito: tropical and subtropical regions worldwide with the exception of North America, Europe, and Australia
• Distribution varies by species
  • P. falciparum in tropical and subtropical Americas, Africa, and Southeast Asia
  • P. vivax in the Americas, India, and Southeast Asia
  • P. malariae in tropical and subtropical Americas, Africa, and Southeast Asia
  • P. ovale in sub-Saharan Africa
  • P. knowlesi in Southeast Asia
• Resistance varies geographically
  • Chloroquine
    • Chloroquine-resistant P. falciparum is widespread in sub-Saharan Africa, Asia, and the Americas
      • Chloroquine-susceptible is in Mexico, regions west/north of the Panama Canal, Haiti, and the Dominican Republic, and parts of Middle East
    • Chloroquine-resistant P. vivax is in Papua New Guinea and Indonesia, with case reports in many other countries
    • Chloroquine-resistant P. malariae is found in Sumatra and Indonesia
  • Amodiaquine-resistant P. falciparum can be found in Africa and Asia
  • Mefloquine-resistant P. falciparum is in Thailand, Cambodia, Myanmar, and Vietnam, with case reports in Brazil and Africa
  • Sulfadoxine-pyrimethamine resistance is widespread in Southeast Asia, the Amazon Basin, and Africa
  • Atovaquone-proguanil resistance is increasing but still rare
  • Reduced quinine susceptibility is reported in Southeast Asia, sub-Saharan Africa, and South America
  • Reduced artemisinin susceptibility is reported in Cambodia, Thailand, Vietnam, and Myanmar
  • Doxycycline has no known resistance

Clinical Manifestations

• History of travel to an endemic country
• Non-specific febrile illness with headache, myalgias, and malaise
• Incubation period can vary, but is generally 9 to 14 days for P. falciparum, 12 to 18 days P. vivax and P. ovale, and longer for others
• Fevers are often periodic, appearing based on rupture of schizonts (tertian and quartan fever)
  • q24h: P. falciparum, but wide variation
  • q48h: P. vivax or P. ovale
  • q72h: P. malariae
  • May vary by timepoint in infection, with early infection having daily or more frequent fevers from shedding from the liver stage
• Leukopenia more common
• May have concurrent bacterial or other infections

Severe Malaria

• Mostly caused by P. falciparum, though can also be caused by P. vivax

CATMAT Criteria (2019)

• Clinical
  • Prostration / impaired consciousness
  • Respiratory distress
  • Multiple convulsions, which can be from cerebral malaria, hypoglycemia, severe metabolic acidosis, etc
  • Circulatory collapse
  • Pulmonary edema
  • Abnormal bleeding
  • Jaundice
  • Hemoglobinuria
• Laboratory
  • Severe anemia (Hb ≤ 70 or Hct <20%)
  • Hypoglycemia (< 2.2)
  • Acidosis (pH < 7.25 or bicarb < 15)
  • Renal impairment (creatinine > 265)
  • Hyperlactatemia
  • Hyperparasitemia
    • ≥2% for children <5 years
    • ≥5% for non-immune adults and children ≥5 years
    • ≥10% for semi-immune adults and children ≥5 years
• Non-immune: born in non-endemic or low-transmission areas (e.g. as travellers), and those who are more than 6 to 12 months away from malaria exposure
• Semi-immune: birth and long-term residence in an endemic country and prior episodes of malaria

Cerebral Malaria

• Classically defined as coma not attributable to other cause such as post-ictal state, hypoglycemia, or another disease altogether1
• Seizures are common
• Most suggestive physical examination finding that helps to rule in malaria and rule out other causes of fever and coma is malarial retinopathy, which can include:
  • Patchy retinal whitening in the macula (especially peri-foveal) and/or in the peripheral retina
  • White or orange discolouration of retinal vessels
  • White-centred haemorrhages
  • Papilloedema
• Pathophysiology is the sequestration of erythrocytes in the cerebral microvessels

Blackwater Fever

• Caused by massive hemolysis leading to hemoglobinuria, usually in the context of severe malaria but with low or undetectable parasitemia
• Syndrome of loin pain, abdominal pain, restlessness, vomiting, diarrhea, and polyuria that is followed by oliguria and passage of dark red or black urine
• May have hepatosplenomegaly, profound anemia, and jaundice
• Unclear if it is triggered by exposure to quinine

Malaria in Pregnancy

• Accumulation of infected erythrocytes in the placenta, causing IUGR, prematurity, and neonatal mortality
• P. falciparum has a tropism for the placenta, and can form a reservoir for recurrence even after appropriate treatment

Late or Relapsing Malaria

• P. vivax and P. ovale can have liver stages (hypnozoites) that lie latent for months to years before causing relapses
• P. malariae can have a low-level asymptomatic parasitemia lasting for years before presentation

Differential Diagnosis

• Essentially any cause of undifferentiated fever
• See also fever in the returned traveller

Diagnosis

Thick and Thin Peripheral Blood Films

• Thick for detecting parasites, thin for parasitemia and species
• Usually done three times over three days for improved sensitivity
• Clues on microscopy:
  • Banana or crescent-shaped gametocytes: P. falciparum
  • Only ring forms, without trophozoites: P. falciparum more likely
  • Amoeboid trophozoite: P. vivax
  • Ring form in an enlarged erythrocyte: P. vivax
  • Band-shaped trophozoite: P. malariae
  • Ring form in an oval-shaped erythrocytes: P. ovale
  • Looks like P. malariae but clinically severe: P. knowlesi

Rapid Diagnostic Antigen Test (RDT)

• Detects Plasmodium antigen in circulating blood
• Good sensitivity and specificity for falciparum malaria, but lower sensitivity (66-88%) for non-falciparum or at low levels of parasitemia
  • Aldolase less sensitive for P. ovale and P. malariae and depends on parasitemia
  • Both may cross-react with ANA and RF, and with dengue, hepatitis C, leishmaniasis, trypanosomiasis, schistosomiasis, tuberculosis, and toxoplasmosis
• BinaxNOW is the only test in Canada
  • T1 band: histidine-rich protein-2 (HRP-2) of P. falciparum, which is fairly specific and sensitive
  • T2 band: aldolase, a common antigen of four species of human malaria parasites
  • C+ / T1+ / T2+: P. falciparum or mixed
  • C+ / T1+ / T2–: P. falciparum
  • C+ / T1– / T2+: non-falciparum
  • C+ / T1– / T2–: no malaria
• Can remain positive for up to 4 weeks due to detection of dead organisms, persistent gamecotyes, and slow antigen clearance, so are not used to document treatment success
• Because of the low specificity, every patient with a positive RDT must have a peripheral blood film

Molecular Testing

• PCR is available
• Done reflexively in Ontario to confirm species and detect a mixed infection

Management

• All returned travellers with fever should have thick and thin smears to rule out malaria
• Management depends on species and susceptibility (predicted by country of acquisition), and severity (including the level of parasitemia)
  • Most of the world has chloroquine-resistant P. falciparum, so when in doubt, treat all P. falciparum malaria as resistant
• All patients with P. falciparum malaria should be considered for hospital admission
  • If severe, advocate for ICU-level care
  • If severe, monitor for hypoglycemia
  • Monitor with daily peripheral blood films until they are negative

Concurrent Supportive Care

• Fluid resuscitation as needed (too much may be harmful in children)
• Rule out hypoglycemia if sudden change in clinical status (worsened with quinine)
• Avoid steroids, which are associated with worse outcomes in cerebral malaria
• Correct coagulopathy and bleeding with blood products and vitamin K
• If shock develops, treat empirically with antibiotics while getting blood cultures to rule out intercurrent bacteremia

Uncomplicated Malaria

• Chloroquine-sensitive P. falciparum (Mexico, Central America west of the Panama Canal, Haiti, the Dominican Republic, and most of the Middle East), P. vivax, P. ovale, P. malariae, and P. knowlesi
  • Oral chloroquine 600 mg base PO once, followed by 300 mg base PO at 6, 24, and 48 hours
    • The dose for salt is 1000 mg and 500 mg
  • If from Papua New Guinea or Indonesia adjacent to Papua New Guinea, treat with atovaquone-proguanil
• Chloroquine-resistant P. falciparum (most of the world) or chloroquine-resistant P. vivax (Papua New Guinea and Indonesia)
  • Atovaquone-proguanil 1000 mg/400 mg (4 tablets) PO daily for 3 days
  • Alternative: quinine 542 mg base (650 mg salt) PO q8h for 3 to 7 days, plus doxycycline 100 mg PO bid for 7 days
• Prevention of relapsing P. vivax and P. ovale with radical cure
  • Indicated for patients with prolonged exposure
  • Primaquine 30 mg base daily for 14 days started concurrent with chloroquine
    • First rule out G6PD deficiency and pregnancy
  • If pregnant, just treat intermittently until after delivery with once-weekly chloroquine

Severe Malaria

• Usually due to P. falciparum, though can also be caused by P. vivax or P. knowlesi
• Admit to hospital, ideally ICU
  • Frequent vitals and urine output
  • Capillary glucose at least q4h
  • Follow peripheral blood films q12-24h until cleared, and longer if pregnant
    • With P. falciparum, can have some fluctuations due to irregular releasing from sequestration
    • Parasitemia and clinical status should both improve by 48 to 72 hours
• Antimalarials
  • Artesunate 2.4 mg/kg IV bolus over 1-2 minutes at 0, 12, 24, and 48 hours
    • Artesunate is held in specific centres in Canada
    • Four hours after the last dose, add one of the following
      • Atovaquone-proguanil 1000 mg/400 mg PO daily for 3 days
      • Doxycycline 100 mg PO bid for 7 days
      • Clindamycin 10 mg/kg IV followed by 5 mg/kg IV q8h for 7 days
  • Quinine 5.8 mg/kg IV loading dose over 30 min followed by 8.3 mg/kg IV infused over 4 hours q8h for 7 days
    • Dose of quinine dihydrochloride would be 7 mg/kg and 10 mg/kg
    • Do not use loading dose if they had quinine within 24 hours or mefloquine within 2 weeks
    • Switch to oral tablets as soon as able to swallow
    • If no infusion pump, run the loading dose as quinine 16.7 mg/kg IV over 4 hours
    • Monitor for cardiovascular toxicity (hypotension and QTc prolongation), ototoxicity (tinnitus and hearing loss), and hypoglycemia (which is exacerbated by quinine)
    • Concurrent to last dose of quinine
      • Atovaquone-proguanil 1000 mg/400 mg PO daily for 3 days
      • Doxycycline 100 mg PO BID for 7 days
      • Clindamycin 10mg/kg IV load followed by 5 mg/kg q8h for 7 days
        • Clindamycin is the preferred treatment in pregnant women and children under 8 years
• Treat seizures with benzodiazepines; no role for seizure prophylaxis
• Avoid steroids in cerebral malaria (worse outcomes)
• Exchange transfusion has been investigated; it reduces parasitemia but has no clinically-important benefits
  • CATMAT still recommends considering it if parasitemia ≥10%
  • Usually 5 to 10 units of pRBC

Pregnancy

• Uncomplicated chloroquine-susceptible malaria:
  • Chloroquine, or artemether-lumefantrine after the first trimester
  • Rather than terminal prophylaxis, treat with once weekly chloroquine until delivery, then reassess for terminal prophylaxis at that point
• Uncomplicated chloroquine-resistant P. falciparum or P. vivax:
  • Mefloquine, quinine, and clindamycin, or artemether-lumefantrine after the first trimester
• Prevention of relapsing P. vivax and P. ovale:
  • Maintained chloroquine prophylaxis 300 mg base (500 mg salt) PO weekly for the duration of their pregnancy
  • Reassess for terminal with primaquine or tafenoquine prophylaxis after delivery
    • Primaquine preferred if breastfeeding
• Severe malaria:
  • Preferred is artesunate followed by clindamycin
  • Alternative is quinine followed by clindamycin
  • There are few data on artesunate in first trimester, but it appears safe, and the overall risk-benefit assessment favours treatment
  • Monitor peripheral blood films q12-24h until cleared, and then for a few more days, to confirm clearance and no relapse from parasites sequestered in the placenta
• Other antimalarials
  • Atovaquone-proguanil is likely safe and can be used after the first trimester for any of the above regimens
  • Doxycycline, primaquine, and tafenoquine should be avoided in pregnancy

Prevention

Behavioural Interventions

• Mosquito avoidance (Anopheles mosquitoes are evening biters)
  • Long sleeves & pants
  • Insecticide-treated clothing
  • Bed nets, screens on doors & windows

Chemoprophylaxis

• See Malaria chemoprophylaxis

Further Reading

• Boggild A, et al. Summary of recommendations for the diagnosis and treatment of malaria by the Committee to Advise on Tropical Medicine and Travel (CATMAT). CCDR 2014;40(7).
  • A continuously-updated version is maintained online.
• CDC Treatment of Malaria: Guidelines For Clinicians (United States)