What are the microscopic features of Plasmodium (P.) malariae?

Microscopic Features of Plasmodium malariae

P. malariae is best identified on Giemsa-stained thin blood films at pH 7.2, where it displays distinctive morphological features that differentiate it from other Plasmodium species, though it is frequently misidentified as P. falciparum on initial microscopy. 1

Specimen Preparation and Staining

• Both thick and thin blood films should be prepared and examined, with thin films fixed in ethanol before staining to preserve red blood cell (RBC) structure and intraerythrocytic parasite morphology 1
• Giemsa stain at pH 7.2 is optimal for highlighting microscopic features of malaria parasites, though Wright-Giemsa and rapid Field stains are acceptable alternatives 1
• Blood films should be made within 1 hour of collection, as prolonged EDTA exposure alters parasite morphology 1

Distinctive Morphological Features of P. malariae

Trophozoites (Growing Stages)

• Late and mature trophozoites appear compact and band-like, often stretching across the RBC in a characteristic "band form" that is relatively specific for P. malariae 2
• The cytoplasm is dense and compact compared to other species 2
• Infected RBCs are not enlarged (unlike P. vivax or P. ovale) and may appear slightly smaller than uninfected cells 2
• Occasionally shows double chromatin dots, though this is less common than in P. knowlesi 2

Schizonts (Dividing Stages)

• Mature schizonts typically contain 8-12 merozoites (most commonly 8-10), arranged in a rosette pattern around central pigment 2, 3
• This merozoite count is lower than P. falciparum (16-32) and helps distinguish the species 2
• Schizonts may be detected at densities up to 2,080 parasites/µL in clinical infections 3

Gametocytes (Sexual Stages)

• Gametocytes are round to oval and fill most of the RBC 2
• They comprise only 1.2-2.8% of infected erythrocytes when present 2

Critical Examination Technique

• Examine at least 100 microscopic fields using the 100× oil immersion objective on both thick and thin films before reporting negative 1
• For patients without previous malaria exposure, examine at least 300 fields as they may be symptomatic at lower parasite levels 1
• Screen first at low power (10× objective) for microfilariae, then proceed to oil immersion examination 1

Common Diagnostic Pitfalls

• P. malariae is frequently misidentified as P. falciparum on initial microscopy, particularly when only early trophozoites are visible 2, 3
• The infection is usually not highly synchronous, so multiple erythrocytic stages (early trophozoites, late trophozoites, schizonts) may be present simultaneously 2
• Rapid diagnostic tests detecting only P. falciparum antigens will miss P. malariae infections, leading to underdiagnosis in areas of intense P. falciparum transmission 3
• If laboratory expertise for species identification is lacking, report as "Plasmodium or Babesia parasites" and send for confirmatory testing at a reference laboratory, as P. falciparum cannot be excluded without expert review 1

Quantification and Clinical Correlation

• Parasitemia percentage is calculated by counting infected RBCs divided by total RBCs on thin film, which guides treatment decisions and monitors response 1
• P. malariae infections can cause clinical disease with fever and mild anemia, even when initially mistaken for other species 3
• Three blood films over 72 hours are necessary to confidently exclude malaria when clinical suspicion remains high 4