Contribution of PCR-based methods to diagnosis and management of imported malaria

Since the first description, in 1990, of the diagnosis of Plasmodium falciparum infection by polymerase-chain-reaction (PCR), the role of this kind of molecular method in laboratory diagnosis of imported malaria is still a topical question. Various molecular assays have been used, the first of which was hybridization using labeled probes in 1984. When compared to thick blood smear, this test displayed a sensitivity ranging from 65% to 81% and specificity was close to 100%. The next technical improvement was the introduction of the so-called polymerase chain reaction (PCR), the principle of which was described in 1985. In 1993, a PCR-based assay detecting all four Plasmodium species was published, followed by different variants of this method. By the turn of the century, novel real-time PCR slashed workaround time, which dropped from 2 1/2 hours to less than 1 hour. Moreover, automatic reading with no human action on PCR products reduced the risks of contamination. The first application of real-time PCR to the diagnosis of malaria was published in 2001. PCR-based assays were found to be more sensitive than all conventional methods. Variations in sensitivity were probably due to different medical practices as well as to the proportion of various types of subjects (travelers under chemoprophylaxis, immigrants from malaria-endemic areas) in the population undergoing malaria diagnosis. The target of the primers was also of crucial importance: for the detection of P. falciparum, the most efficient assays amplified either the gene SSUrRNA, or Pf155/RESA, or Cox 1. Specificity of PCR results is guaranteed by the nature of the target for primers or probes, as determined by the studies of the Plasmodium genome whose results are available in GenBank. PCR use often corrected the results of Plasmodium species identification by microscopy and PCR-based methods were found to be the most efficient for the detection of mixed infections. Concerning the diagnosis of imported malaria, it appears clearly that PCR should be considered as second-line method which can be especially interesting, as a negative result rules out malaria in febrile patients. However, the use of PCR assays appears to be restricted to health centers, such as University Hospitals, for whom malaria identification is an important and routine problem. In the future, the detection of mutations related to drug resistance could be used to orient anti-malarial therapy.