Chapter 231: Malaria¶
Chapter 231 | Part 5: Infectious Diseases
KEY CLINICAL POINTS¶
- Malaria is caused by Plasmodium species, with P. falciparum being the most lethal.
- Transmission occurs via Anopheles mosquitoes, with global endemicity in tropical regions.
- Severe falciparum malaria is a medical emergency requiring immediate parenteral antimalarial therapy.
- Diagnosis relies on microscopy, rapid diagnostic tests (RDTs), and molecular methods like PCR.
- Artemisinin-based combination therapies (ACTs) are the first-line treatment for uncomplicated P. falciparum malaria.
1. DEFINITION & OVERVIEW¶
Malaria is a protozoan disease transmitted by the bite of infected female Anopheles mosquitoes. It is the most important parasitic disease of humans, causing significant morbidity and mortality in tropical regions. The disease is characterized by cyclical fever, chills, and sweating, with severe complications including cerebral malaria, acute renal failure, and hypoglycemia.
Table 231-1: Characteristics of Plasmodium Species Infecting Humans¶
| Characteristic | P. falciparum | P. vivax | P. ovale | P. malariae | P. knowlesi |
|---|---|---|---|---|---|
| Duration of intrahepatic phase (days) | 5.5 | 8 | 9 | 15 | 5.5 |
| Number of merozoites released per infected hepatocyte | 30,000 | 10,000 | 15,000 | 15,000 | |
| Approximate duration of erythrocytic cycle (h) | 48 | 48 | 50 | 72 | 24 |
| Red cell preference | Younger cells (can invade all ages) | Reticulocytes and cells up to 2 weeks old | Reticulocytes | Older cells | Younger cells |
| Morphology | Usually only ring forms; banana-shaped gametocytes | Irregularly shaped large rings and trophozoites; enlarged erythrocytes; Schüffner’s dots | Infected erythrocytes, enlarged and oval with tufted ends; Schüffner’s dots | Band or rectangular forms of trophozoites; common | Resembles P. falciparum (early trophozoites) or P. malariae (later trophozoites, including band forms) |
| Characteristic | P. falciparum | P. vivax | P. ovale | P. malariae | P. knowlesi |
|---|---|---|---|---|---|
| Pigment color | Black | Yellow-brown | Dark brown | Brown-black | Dark brown |
| Ability to cause relapses | No | Yes | Yes | No | No |
1.1 Pathogenesis¶
The Plasmodium parasite undergoes a complex life cycle involving both human and mosquito hosts. In humans, the intraerythrocytic stage causes symptoms, while gametocytes are transmitted to mosquitoes. P. falciparum is particularly virulent due to its ability to sequester in vital organs and induce severe complications.
1.2 Global Impact¶
In 2022, there were 249 million malaria cases and 608,000 deaths globally. P. falciparum predominates in Africa, while P. vivax is common in Central/South America and Asia. Resistance to antimalarials and insecticides poses a growing threat to control efforts.
2. EPIDEMIOLOGY¶
Malaria is endemic in tropical regions, with P. falciparum predominating in Africa, New Guinea, and Hispaniola. P. vivax is common in Central/South America and Asia. Global incidence has declined since 2000, but resistance and climate change threaten progress. Submicroscopic infections are common in endemic areas.
Table 231-4: Relative Incidence of Severe Complications of Falciparum Malaria¶
| Complication | Nonpregnant Adults | Pregnant Women | Children |
|---|---|---|---|
| Anemia | + | ++ | +++ |
| Convulsions | + | ++ | +++ |
| Hypoglycemia | + | +++ | +++ |
| Jaundice | ++ | +++ | + |
| Renal failure | +++ | +++ | + |
| Pulmonary edema | ++ | +++ | + |
2.1 Incidence and Prevalence¶
In 2022, 249 million cases were reported in 85 endemic countries. P. falciparum accounts for ~90% of deaths, with the highest burden in sub-Saharan Africa. Transmission is seasonal in many regions, peaking during the rainy season.
2.2 Risk Factors¶
Risk factors include living in endemic areas, lack of immunity, and poor access to healthcare. Children under 5 and pregnant women are most vulnerable. Travelers to endemic regions are at risk of acquiring malaria.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Malaria is caused by Plasmodium species, with P. falciparum being the most lethal. The parasite undergoes a complex life cycle involving sporogony in mosquitoes and schizogony in humans. Pathophysiology includes erythrocyte invasion, sequestration in organs, and immune-mediated complications.
3.1 Erythrocyte Changes¶
The parasite consumes hemoglobin, producing hemozoin (malaria pigment). Infected erythrocytes become irregular, less deformable, and express adhesins like PfEMP1, leading to cytoadherence and sequestration in organs.
3.2 Host Response¶
The immune response includes splenic clearance, cytokine release, and complement activation. Severe malaria is associated with immune dysregulation, endothelial damage, and multiorgan failure.
4. CLINICAL FEATURES¶
Clinical manifestations include fever, chills, headache, and anemia. Severe malaria presents with cerebral malaria, hypoglycemia, acute renal failure, and pulmonary edema. Nonimmune individuals may develop severe symptoms with low parasitemia.
Table 231-2: Manifestations of Severe Falciparum Malaria¶
| Signs | Manifestations |
|---|---|
| Major | Unarousable coma/cerebral malaria |
| Failure to localize or respond appropriately to noxious stimuli; coma persisting >30 min after generalized convulsion; Glasgow Coma Score <11 | |
| Bleeding, acidemia, anuria, shock |
4.1 Severe Falciparum Malaria¶
Characterized by coma, hypoglycemia, acidosis, and organ failure. Cerebral malaria is a leading cause of death, with a mortality rate of ~20% in adults and 15% in children.
4.2 Other Presentations¶
P. vivax and P. ovale may cause relapsing fever. P. knowlesi infections can present with high parasitemia and severe disease, even in nonendemic areas.
5. DIFFERENTIAL DIAGNOSIS¶
Differential diagnoses include other parasitic infections (e.g., babesiosis), bacterial sepsis, viral infections, and autoimmune disorders. In endemic areas, malaria must be considered in febrile patients, especially with travel history or exposure to mosquitoes.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnosis relies on microscopy of blood smears, RDTs, and PCR. Parasitemia levels and species identification are critical for treatment decisions. Molecular methods are used for genotyping and detecting low-level infections.
Table 231-5: Standard Methods for the Diagnosis of Malaria¶
| Method | Procedure | Advantages | Disadvantages |
|---|---|---|---|
| Thick blood film | Uneven thickness; stain with Giemsa, Field’s, or Romanowsky stain | Sensitive (0.001% parasitemia); species-specific | Requires experience; underestimates true count |
| Thin blood film | Stain fixed smear; count RBCs with asexual parasites | Rapid; species-specific; provides prognostic info | Insensitive (<0.05% parasitemia) |
| PfHRP2 dipstick | Blood placed on card; monoclonal antibody capture | Robust; rapid; detects P. falciparum | Remains positive after high-density infections; no quantitation |
| Microtube concentration | Acridine orange staining; fluorescence microscopy | Sensitivity similar to thick films | No speciation; requires fluorescence microscopy |
6.1 Diagnostic Methods¶
Thick and thin blood smears are the gold standard for species identification. RDTs detect specific antigens (e.g., PfHRP2, lactate dehydrogenase). PCR is used for confirmation and genotyping in endemic areas.
6.2 Laboratory Findings¶
Anemia, thrombocytopenia, and elevated lactate levels are common. Acidosis and metabolic disturbances are frequent in severe cases. Liver dysfunction may occur due to sequestration and immune-mediated damage.
7. MANAGEMENT & TREATMENT¶
Treatment depends on severity and species. Uncomplicated malaria is treated with ACTs, while severe cases require parenteral artemisinin derivatives. Radical cure for P. vivax/ovale requires primaquine. Drug resistance and G6PD deficiency must be considered.
Table 231-6: Regimens for the Treatment of Malaria¶
| Type of Disease/Treatment | Regimen |
|---|---|
| Uncomplicated P. falciparum | Artesunate (4 mg/kg qd for 3 days) + sulfadoxine/pyrimethamine |
| Severe falciparum malaria | Artesunate (2.4 mg/kg stat IV) + maintenance doses |
| P. vivax/ovale radical cure | Primaquine (0.5 mg/kg qd for 14 days) |
7.1 Uncomplicated Malaria¶
Treated with ACTs (e.g., artemether-lumefantrine, artesunate-mefloquine). Primaquine is used for radical cure of P. vivax/ovale. Chloroquine is used in areas with sensitive strains.
7.2 Severe Malaria¶
Artesunate is the drug of choice. Treatment includes IV/IM administration, with close monitoring for complications. Quinine is used if artemisinin resistance is suspected.
8. PROGNOSIS & COMPLICATIONS¶
Mortality is highest in children and pregnant women. Complications include cerebral malaria, acute renal failure, and hypoglycemia. Long-term sequelae may include neurological deficits and anemia.
8.1 Mortality¶
Mortality rates vary by region and species. In Africa, severe malaria mortality is ~20% in adults and 15% in children. Early treatment with ACTs reduces mortality significantly.
8.2 Long-Term Effects¶
Chronic anemia, neurocognitive deficits, and organ damage may persist. P. vivax relapses can lead to chronic hemolysis and splenomegaly.
9. SPECIAL CONSIDERATIONS¶
Malaria in pregnancy, children, and travelers requires tailored management. Prevention strategies include ITNs, chemoprophylaxis, and seasonal malaria chemoprevention (SMC). Drug resistance and G6PD deficiency are critical considerations.
Table 231-8: Drugs Used in the Prophylaxis of Malaria¶
| Drug | Usage | Adult Dose | Pediatric Dose | Comments |
|---|---|---|---|---|
| Atovaquone-proguan il | Prophylaxis in areas with chloroquine-resistant P. falciparum | 1 adult tablet daily | 5–8 kg: ½ pediatric tablet daily | Contraindicated in severe renal impairment |
| Chloroquine | Prophylaxis in chloro quine-sensitive areas | 300 mg PO weekly | 5 mg/kg PO weekly | Exacerbates psoriasis |
| Doxycycline | Prophylaxis in resistant areas | 100 mg PO daily | 2 mg/kg PO daily | Contraindicated in children <8 years |
| Mefloquine | Prophylaxis in resistant areas | 228 mg PO weekly | 4.6 mg/kg PO weekly | Contraindicated in psychiatric disorders |
9.1 Malaria in Pregnancy¶
Pregnant women are at higher risk for severe malaria. Primaquine is contraindicated in G6PD deficiency. IPTp with sulfadoxine-pyrimethamine is recommended in endemic areas.
9.2 Travelers¶
Travelers should take chemoprophylaxis (e.g., atovaquone-proguanil, doxycycline) and avoid mosquito bites. Post-travel monitoring is essential for early detection.
10. KEY POINTS & CLINICAL PEARLS¶
- Early diagnosis and treatment are critical to prevent severe complications. 2. ACTs are the first-line treatment for uncomplicated P. falciparum. 3. Primaquine is essential for radical cure of P. vivax/ovale but contraindicated in G6PD deficiency. 4. Monitor for hypoglycemia and acidosis in severe cases. 5. Use ITNs and chemoprophylaxis to prevent malaria in endemic areas.