Clinical practice: the diagnosis of imported malaria in children

Abstract

The present paper reviews the diagnosis of imported malaria in children. Malaria is caused by a parasite called Plasmodium and occurs in over 100 countries worldwide. Children account for 10-15% of all patients with imported malaria and are at risk to develop severe and life-threatening complications especially when infected with Plasmodium falciparum. Case-fatality ratios vary between 0.2% and 0.4%. Children visiting friends and relatives in malaria endemic areas and immigrants and refugees account for the vast majority of cases. Symptoms are non-specific and delayed infections (more than 3 months after return from an endemic country) may occur. Microscopic analysis of the thick blood film is the cornerstone of laboratory diagnosis. For pragmatic reasons, EDTA-anticoagulated blood is accepted, provided that slides are prepared within 1 h after collection. Information about the Plasmodium species (in particular P. falciparum versus the non-falciparum species) and the parasite density is essential for patient management. Molecular methods in reference settings are an adjunct for species differentiation. Signals generated by automated hematology analyzers may trigger the diagnosis of malaria in non-suspected cases. Malaria rapid diagnostic tests are reliable in the diagnosis of P. falciparum but not for the detection of the non-falciparum species. They do not provide information about parasite density and should be used as an adjunct (and not a substitute) to microscopy. In case of persistent suspicion and negative microscopy results, repeat testing every 8-12 h for at least three consecutive samplings is recommended. A high index of suspicion and a close interaction with the laboratory may assure timely diagnosis of imported malaria.

Fig. 1

Plasmodium life cycle. Retrieved from Centers for Disease Control and Prevention (CDC). Available at http://www.cdc.gov/malaria/about/biology/

Fig. 2

Giemsa-stained thin blood film of a patient with P. falciparum malaria: the patient was sick for 2 weeks before diagnosis. Numerous red blood cells are infected with small trophozoites (ring forms); some red blood cells contain multiple trophozoites. Three banana-shaped gametocytes (visible only after 1 or 2 weeks of clinical infection) are present and the white blood cell (center) contains black-brown hemozoin pigment

Fig. 3

Two- and three-band (below) malaria rapid diagnostic tests (MRDTs) with blood transfer devices (pipette and loop). Control and test lines are cherry-red colored. The two-band MRDT (upper) displays a control line and a test line which targets P. falciparum-specific histidine-rich protein-2 (HRP-2) The three-band MRDT (below) displays a control line and two test lines, one targeting HRP-2 and another line targeting pan-parasite lactate dehydrogenase

Fig. 4

Schematic drawing of the malaria rapid diagnostic test. Sequence of events when performing a MRDT: blood and buffer are applied, respectively, to the sample and buffer pad. They are attracted by the capillary action of the absorption pad and start to migrate. First, they pass the conjugate pad, which contains a detection antibody targeting a Plasmodium antigen, such as HRP-2, Pf-pLDH, Pv-pLDH, pan-pLDH, or aldolase (for abbreviations see text). This detection antibody is a mouse antibody that is conjugated to a signal, mostly colloidal gold. If present in the sample, the Plasmodium antigen is bound to this detection antibody–conjugate. Next, the antigen–antibody–conjugate complex migrates further until it is bound to the capture antibody, which binds to another site of the Plasmodium target antigen. As the capture antibody is applied on a narrow section of the strip, the complex with the conjugated signal will be concentrated and by virtue of the colloidal gold will become visible as a cherry-red line. The excess of detection antibody–conjugate that was not bound by the antigen and the capture antibody moves further until it is bound to a goat-raised anti-mouse antibody, thereby generating a control line