Department of Clinical Science

Midway Evaluation - Gro Strøm

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Challenges in diagnosing pediatric malaria in Dar es Salaam, Tanzania

Gro Strøm, PhD student

Background: In the world today approximately 3.3 billion people live in areas where they are at risk of contracting a malaria infection. In Tanzania, 5.5 million cases of suspected and confirmed malaria were found in 2011. Clinically malaria cannot be differentiated from other febrile diseases. The gold standard for malaria diagnostics, blood film microscopy, appears to perform poorly in many low-resource areas. Rapid diagnostic tests (RDTs) are being used increasingly, but have certain limitations especially in detecting low-level parasitemia and in follow-up post-treatment. PCR is a method that is especially relevant for use in research. Asymptomatic malaria parasitemia can complicate the interpretation of malaria test results in a febrile child. Dried blood spots (DBS) on filter paper are a useful alternative to whole blood in research in rural and remote settings for retrospective PCR for malaria.

Aim: The aim of the studies was to compare various methods of malaria diagnostics used both for clinical and research purposes on febrile children in a malaria-endemic area. Also, a study on asymptomatic malaria aimed to investigate healthy children for malaria parasitemia.

Methods: Febrile pediatric admissions at Muhimbili National Hospital (MNH) in Dar es Salaam were recruited. Clinical and demographical data, routine blood film microscopy results were collected. Research blood films, whole blood, and dried blood spots (DBS) on filter paper were also collected from each child and analyzed retrospectively by microscopy, RDTs and PCR. Asymptomatic children were recruited at a health clinic and RDTs and DBS were collected from these patients. Whole blood and DBS were compared as sources for DNA extract for DBS. Before doing this comparison, seven different protocols of four commonly used methods for extracting DNA from filter paper were compared.

Results: Routine microscopy at MNH had a high level of false positives and high overtreatment of malaria. However, high parasitemia on routine slides was associated with true positivity for malaria as defined by PCR results. PCR was considered the gold standard in this study as it detected all cases found by the other methods as well as forty cases that were not identified by any of the other methods when several PCR runs were performed. RDTs detected more cases of malaria than both routine and research microscopy and positive RDT was also associated with signs of severe malaria. Anemia and thrombocytopenia were associated with RDT-, PCR- and research microscopy-positive malaria. All malaria positive cases by all methods had Plasmodium falciparum infection. No asymptomatic malaria was found among the children recruited at the health clinic. The method for DNA extraction from DBS that had the lowest limit of detection (0.5 parasites/µL) when tested on dilutions of a standardized malaria positive sample was also a cheap and easy-to-perform method. Only approximately half of those positive on whole blood PCR were positive on DBS PCR. All RDT positive samples were also DBS PCR positive.

Conclusion: Routine microscopy performed poorly in this setting and overtreatment of malaria was frequent. RDTs appear to be a useful supplement or alternative to microscopy for routine malaria diagnostics. There appears to be no asymptomatic malaria in young children in the area, however the study was small and only at one health facility was included. Clinical DBS for PCR appear to have similar sensitivity as RDTs and to be much less sensitive than whole blood PCR.