Hematological Diagnosis of Malaria

Various parasites may be found in the blood stream, e.g., trypanosomes and filariae. Among the parasitic diseases, probably only malaria is of practical diagnostic relevance in the northern hemisphere, while at the same time malarial involvement of erythrocytes may confuse the interpretation of erythrocyte morphology. For these reasons, a knowledge of the principal different morphological forms of malarial plasmodia is advisable.

Recurrent fever and influenza-like symptoms after a stay in tropical regions suggest malaria. The diagnosis may be confirmed from normal blood smears or thick smears; in the latter the erythrocytes have been hemolyzed and the pathogens exposed. Depending on the stage in the life cycle of the plasmodia, a variety of morphologically completely different forms may be found in the erythrocytes, sometimes even next to each other. The different types of pathogens show subtle specific differences that, once the referring physician suspects malaria, are best left to the specialist in tropical medicine, who will determine which of the following is the causative organism: Plasmodium vivax (tertian malaria), Plasmodium falciparum (falciparum or malignant tertian malaria) and Plasmodium malariae (quartan malaria) (Table 27, Fig.56). Most cases of malaria are caused by P. vivax (42%) and P. falciparum (43%).

The key morphological characteristics in all forms of malaria can be summarized as the following basic forms: The first developmental stage of the pathogen, generally found in quite large erythrocytes, appears as small ring-shaped bodies with a central vacuole, called trophozoites (or the signet-ring stage) (Fig. 57). The point-like center is usually most noticeable, as it is reminiscent of a Howell-Jolly body, and only a very careful search for the delicate ring form will supply the diagnosis. Occasionally, there are several signet-ring entities in one erythrocyte. All invaded cells may show reddish stippling, known as malarial stippling or Schuffner's dots. The pathogens keep dividing, progressively filling the vacuoles, and then develop into schizonts (Fig. 56), which soon fill the entire erythrocyte with an average of 10-15 nuclei. Some schizonts have brown-black pigment inclusions. Finally the schizonts disintegrate, the erythrocyte ruptures, and the host runs a fever. The separate parts (merozoites) then start a new invasion of erythrocytes.

In parallel with asexual reproduction, merozoites develop into ga-monts, which always remain mononuclear and can completely fill the erythrocyte with their stippled cell bodies. The large, dark blue-stained forms (macrogametes) are female cells (Fig. 57d), the smaller, light blue-stained forms (microgametes) are male cells. Macrogametes usually predominate. They continue their development in the stomach of the infected Anopheles mosquito.

Plasmodium falciparum

Plasmodium vivax

Plasmodium ovale

Plasmodium malariae

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Fig. 56 Differential diagnosis of malaria plasmodia in a blood smear (from Kay-ser, F., et al., Medizinische Mikrobiologie (Medical Microbiology), Thieme, Stuttgart, 1993). 15g

The example shown here is of the erythrocytic phases of P. vivax, because tertian malaria is the most common form of malaria and best shows the basic morphological characteristics of a plasmodia infection.

Table 27 Parasitology of malaria and clinical characteristics (from Diesfeld, H., G. Krause: Praktische Tropenmedizin und Reisemedizin. Thieme, Stuttgart 1997)

Disease

Pathogen Exoerythro- Erythrocytic Clinical charac-cytic phase phase teristics = incubation

Falci-

parum malaria

Plasmodium 7-15 days, does 48 hours, Potentially lethal falciparum not form liver recurring disease, widely hypnozoites fever is rare therapy-resistant, there is usually no recurrence after recovery

Tertian malaria

P. vivax P. ovale

Benign form recurrences up to 2 years after infection

12-18 days, forms liver 48 hours hypnozoites

Benign form recurrences up to 5 years after infection

Quartan malaria

P. malariae 18-40 days, 72 hours Benign form recur-does not form rences possible up liverhypno- to 30 years after zoites infection

Conspicuous erythrocyte inclusions suggest malaria a b d

Hematology Malaria

Fig. 57 Blood analysis in malaria. a Trophozoites in Plasmodium falciparum (falciparum or malignant tertian malaria) infection. Simple signet-ring form (1), double-invaded erythrocyte with basophilic stippling (2). Erythrocyte with basophilic stippling without plasmodium (3) and segmented neutrophilic granulocyte with toxic granulation (4). b and c Plasmodium falciparum: single invasion with delicate trophozoites (1), multiple invasion (2). d and e In the gametocyte stage of falciform malaria, the pathogens appear to reside outside the erythrocyte, but remnants of the erythrocyte membrane may be seen (arrow, e). For a systematic overview of malarial inclusions, see p. 159.

Fig. 57 Blood analysis in malaria. a Trophozoites in Plasmodium falciparum (falciparum or malignant tertian malaria) infection. Simple signet-ring form (1), double-invaded erythrocyte with basophilic stippling (2). Erythrocyte with basophilic stippling without plasmodium (3) and segmented neutrophilic granulocyte with toxic granulation (4). b and c Plasmodium falciparum: single invasion with delicate trophozoites (1), multiple invasion (2). d and e In the gametocyte stage of falciform malaria, the pathogens appear to reside outside the erythrocyte, but remnants of the erythrocyte membrane may be seen (arrow, e). For a systematic overview of malarial inclusions, see p. 159.

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  • timoteo
    Can malaria recur after 30 years?
    20 days ago

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