ARCHIVED - Supplement: Canadian Recommendations for the Prevention and Treatment of Malaria Among International Travellers

6. Malaria Diagnosis

Falciparum malaria can be rapidly fatal, particularly in the non-immune host, and is the most urgent diagnosis to confirm or exclude in the febrile traveller who has been in a malarial zone. Although other signs and symptoms may be present in patients with malaria, they are neither sensitive nor specific. For example, fever is frequently not cyclic, and splenomegaly is rarely present early in the course of P. Falciparum malaria⁽¹⁴⁵⁾. Clinical assessment, even by experts, cannot reliably confirm or exclude a diagnosis of malaria⁽¹⁴⁶⁾. Most cases of P. Falciparum present within 2 to 3 months of exposure, although clinical presentation may be delayed in travellers who have taken chemoprophylaxis. Other malaria species may present as late as several years after exposure.

It is critical that any traveller who has been in a malaria-endemic area, particularly within the previous 3 months, and who develops an unexplained fever should be strongly advised to present for medical and laboratory assessment within 24 hours and should inform his or her health care provider of the relevant travel history. Travellers should be informed of this as part of their pre-travel assessment. Physicians should include a travel history in the assessment of febrile patients.

Confirmation or exclusion of a diagnosis of malaria depends upon laboratory examination of a blood sample. The standard test involves microscopic examination of thick and thin blood smears. Accurate blood smear examination requires considerable training and experience, particularly in the interpretation of the thick smear and in the speciation of identified parasites. Lack of experienced personnel may limit the accuracy of malaria diagnosis in laboratories in Canada⁽¹⁴⁷⁾, while diagnosis in low-income countries is often unreliable because of problems with the quality of the microscope or the stains, and supervision and quality control of the laboratory⁽¹⁴⁸⁾. For example, among Peace Corps volunteers whose malaria was diagnosed by blood smear in local clinics in Africa, the diagnosis could be confirmed in only 25% of cases⁽¹¹²⁾.

When malaria is suspected in Canada, a result indicating the presence or absence of malaria and, in most cases, the species should be available within 1-2 hours of the receipt of a blood specimen by the laboratory⁽¹⁴⁸⁾. In a minority of cases, when the level of parasitemia is low, an initial smear may be falsely negative, requiring one or two additional smears at 12- to 24-hour intervals to confirm or exclude the diagnosis. It is important to obtain repeat smears at regular intervals rather than potentially delay the diagnosis by attempting to time sample taking with the fever cycle⁽¹⁴⁹⁾.

An essential element of malaria smear interpretation is the speciation of the parasite. Correct speciation may be critical to the correct choice of life-saving treatment and other decisions, such as hospitalization. Quantitation of parasitemia is also important for initial determination of the need for parenteral treatment or, exceptionally, the use of exchange transfusion and the need for admission to an intensive care unit. Finally it is important for monitoring treatment of P. Falciparum infections.

Rapid Diagnostic Tests

RDTs do not require microscopy or specialized laboratory skills and can play a valuable adjunctive role in the diagnosis of malaria⁽¹⁵⁰⁾. A variety of rapid diagnostic tests are licensed by Health Canada for use in Canada⁽¹⁵¹⁾. RDTs are immunochromatographic assays that use monoclonal antibodies to capture malaria antigens in a patient sample, producing a visible colour change. All tests include a positive control band that becomes visible as the sample migrates along the strip. The absence of a control band is indicative of an invalid test, but the presence of a visible control band does not assure reliability⁽¹⁵²⁾.

These tests require small volumes of blood (2-50 µL) and can be done on fingerstick specimens or on anticoagulated blood or plasma. The current targets for RDTs are histidine-rich protein-2 of P. Falciparum (PfHRP-2) or enzymes from the parasite glycolytic pathway (e.g., parasite-specific lactate dehydrogenase [pLDH] or Plasmodium aldolase, also called pan malarial antigen). Lactate dehydrogenasebased tests may detect all species of malaria or may be specific for P. Falciparum or P. vivax. Combinations of target antigens can be used to detect infection due to P. Falciparum, P. vivax, mixed P. Falciparum/P. vivaxor mixed P. Falciparum with non-Falciparum species. To date, tests specific for P. malariae and P. ovale are not available⁽¹⁵³⁾.

Although some of these tests were originally developed for use by travellers who would not have access to effective malaria diagnosis during travel, their reliability in this setting has proved suboptimal. Significant proportions of travellers are unable to complete the test procedure or interpret the results correctly^{(154, 155)}, and rates of false-negative results are unacceptable⁽¹⁵⁶⁾. However, when used by trained laboratory staff, these tests can contribute to the rapid diagnosis of malaria pending confirmatory testing with microscopy and/or polymerase chain reaction (PCR)⁽¹⁵⁰⁾.

Operating characteristics of RDTs reported in the literature show significant variability. In general, RDTs perform best in the detection of P. Falciparum with sensitivities in the range of 88%-100% and specificities of 92%-95%⁽¹⁵⁷⁾. At parasite densities below 100/µL, sensitivity is decreased, with sensitivities of < 70% at parasite densities less than 50/µL⁽¹³⁸⁾. The sensitivity for the detection of P. vivaxis inferior to that of P. Falciparum. For P. vivax, the data are limited, but the threshold for satisfactory detection of parasitemia may be higher (> 1,000 parasites/µL)⁽¹⁵⁴⁾.

RDTs cannot be recommended as a means of assessing the response to antimalarial therapy. PfHRP-2 persists for prolonged periods after clearance of asexual stage parasites from blood with 68% positivity at 7 days and 27% positivity at 28 days after initiation of therapy⁽¹⁵⁸⁾. Parasite aldolase-based tests also remain positive after clearance of asexual stage parasites and may remain positive even longer than PfHRP-2 based tests.

The advantage of RDTs is that they are simple to use, can be performed by laboratory staff who are untrained in malaria microscopy and require no equipment. However, inaccurate results can occur if instructions are not followed carefully. Results must be read according to the time frame specified by the manufacturer, as test lines may become positive several hours after the test is performed in the absence of true parasitemia. Heat and humidity can damage the test system; therefore, test packages must be opened immediately before use.

The presence of autoantibodies such as rheumatoid factor, heterophile antibodies and anti-mouse antibodies have been shown to give false-positive results in some test kits. The likelihood of a false-positive result in the presence of rheumatoid factor varies with the test antibody. Occasional reports of negative RDTs in patients with high grade parasitemias are likely due to prozone phenomenon in which an excess of antigen masks the test antibody^{(150, 159)}. Occasionally there may be cross-reaction between species, for example, the aldolase- or pLDH-based tests with Falciparum and non-Falciparum bands may give a positive reaction on both bands when only P. Falciparum infection is present, making accurate diagnosis of mixed infections difficult⁽¹⁵⁷⁾. Geographic variation between P. Falciparum stains could also affect test sensitivity⁽¹⁶⁰⁾.

Polymerase Chain Reaction (PCR)

Although not practical for immediate patient care because of limited availability, PCR is emerging as the gold standard for both high sensitivity and specificity, as well as speciation (Table 2). It is increasingly used for «quality control». PCR techniques (e.g., real-time PCR) providing more rapid results are likely to become more available in the near future^{(131,162,163).}