Malaria, which affects about 600 millions people in the world, is the most widespread of the transmissible parasitic diseases. The causative agent is a microscopic parasite of the genus Plasmodium. The species P. falciparum induces the most serious form of this disease. Since the 1980s, African countries where malaria is endemic have been seeing the emergence of parasite forms resistant to the most widely used treatments, especially to chloroquine. Monitoring of the extent and distribution of such resistance therefore appeared necessary, in order to devise combined treatments (bitherapies) for controlling the disease.
With this objective in mind, researchers from IRD and OCEAC, in conjunction with the Cameroon Ministry of Public Health, assessed the efficacy of the antimalarial treatments most commonly used in Cameroon. In this country, because of the great diversity of landscapes and climates, the patterns of transmission of Plasmodium by mosquitoes differ according to the region, which makes it difficult to track changes and development in resistance (1).
The researchers determined the response in children with malaria, from 12 towns and villages in Cameroon, to three different monotherapies: two habitually prescribed as first treatment in that country (chloroquine and amodiaquine) and one issued as second intention medicine (sulfadoxine-pyrimethamine). The ineffectiveness of chloroquine was rapidly confirmed, with a very high proportion of therapeutic failure (48.6% on average), greater in the south than in the north of the country. That signalled a pressing need to rectify the treatment of the children concerned as soon as possible. This drug can no longer be considered a reliable treatment in Cameroon and its withdrawal from the market by the country's health authorities (in 2002) is justified. However, amodiaquine and, to a lesser extent, sulfadoxine-pyrimethamine are still effective treatments, with low therapeutic failure rates (an average of 7.3 and 9.9% respectively) obtained for the whole set of sites studied. In order to avoid the development of new resistance, these treatments must however be combined with the most recent therapies using artemisinine derivatives, for which no form of resistance currently exists. Amodiaquine, which is generally administered as soon as the first malaria symptoms appear, seems to be the best candidate for these combinations of therapies.
The series of investigations was run from 1999 to 2004, following a protocol established by the WHO (World Health Organization), which advocates regular clinical examination and blood tests for each patient, for 14 days. This protocol enables clinicians to take into account the whole range of factors governing the parasite-host interaction (such as acquired immunity, pharmacokinetics, synergy between constituents, and also the degree of chemoresistance of Plasmodium). However, implementation of this has proved a long and intricate process in Cameroon, where transmission does not have the same pattern and rate of development everywhere. Faced with these constraints, the researchers turned to molecular markers, quick and practical to use for resistance assessment (2). They used the blood samples taken from the different sites under study to estimate the degree of resistance to pyrimethamine, one of the components of sulfadoxine-pyrimethamine. The sequence of a particular gene of Plasmodium (dhfr, dihydrofolate reductase gene) was analysed, in the search for occasional mutations that might provide the parasite with resistance to this agent. The proportion of strains of mutant Plasmodium in the samples is thus a direct expression of the resistance.
Geographical mapping of the occurrence of pyrimethamine resistance was subsequently conducted. The frequency of mutant resistant strains and the number of mutations were plotted for the different areas. The data obtained, such as the lower proportion of these resistant strains observed in the north than in the south of Cameroon, complement the results of clinical investigations. The research team has since then launched similar work, focused especially on the gene for resistance to sulfadoxine, the other constituent of the sulfadoxine-pyrimethamine mixture.
The molecular approach, adapted to the varied epidemiological backgrounds that prevail in Cameroon, is therefore highly promising as a tool for tracking the evolution of resistance to monotherapies and thus for helping to improve malaria control strategies.
(1) Cameroon, with a surface area of 475 442 km2, is situated deep in the Gulf of Guinea, a region where Central Africa and West Africa meet. This geographical situation explains the great variety of climates and landscapes which make the country a kind of "Africa in miniature" (www.diplomatie.gouv.fr/fr/payszones- geo_833/cameroun_361/presentation-ducameroun_ 946/geographie_8432.html). In the north, dominated by vast plains, malaria transmission is seasonal. In the south, characterized by the presence of vast forest, it occurs continuously and intensively.
(2) In the standard surveys on therapeutic efficiency, just one drug or a single combination was tested simultaneously, in each patient and each locality, by a specialist mobile team. In the north of Cameroon where transmission is seasonal, this team's expeditions into the field had to be carefully planned. However, in the genetic approach, the collection of capillary blood on absorbent paper does not require any specialized team. A local team of local care providers gathers all samples together and sends them to the laboratory. The search for specific resistance markers can be carried on at the same time for several substances in uniform conditions.
Marie Guillaume-Signoret - IRD Translation : Nicholas Flay