Plasmodium falciparum: Differential Selection of Drug Resistance Alleles in Contiguous Urban and Peri-Urban Areas of Brazzaville, Republic of Congo

The African continent is currently experiencing rapid population growth, with rising urbanization increasing the percentage of the population living in large towns and cities. We studied the impact of the degree of urbanization on the population genetics of Plasmodium falciparum in urban and peri-urban areas in and around the city of Brazzaville, Republic of Congo. This field setting, which incorporates local health centers situated in areas of varying urbanization, is of interest as it allows the characterization of malaria parasites from areas where the human, parasite, and mosquito populations are shared, but where differences in the degree of urbanization (leading to dramatic differences in transmission intensity) cause the pattern of malaria transmission to differ greatly. We have investigated how these differences in transmission intensity affect parasite genetic diversity, including the amount of genetic polymorphism in each area, the degree of linkage disequilibrium within the populations, and the prevalence and frequency of drug resistance markers. To determine parasite population structure, heterozygosity and linkage disequilibrium, we typed eight microsatellite markers and performed haplotype analysis of the msp1 gene by PCR. Mutations known to be associated with resistance to the antimalarial drugs chloroquine and pyrimethamine were determined by sequencing the relevant portions of the crt and dhfr genes, respectively. We found that parasite genetic diversity was comparable between the two sites, with high levels of polymorphism being maintained in both areas despite dramatic differences in transmission intensity. Crucially, we found that the frequencies of genetic markers of drug resistance against pyrimethamine and chloroquine differed significantly between the sites, indicative of differing selection pressures in the two areas

Drug usage amongst all febrile patients from the urban and peri-urban regions presenting at health centers.

<p>Drug usage amongst all febrile patients from the urban and peri-urban regions presenting at health centers.</p

Multiplicities of infections (MOI) for the urban (n = 42) and peri-urban (n = 61) areas as assessed by <i>msp1</i> haplotyping.

<p>Multiplicities of infections (MOI) for the urban (n = 42) and peri-urban (n = 61) areas as assessed by <i>msp1</i> haplotyping.</p

<i>msp1</i> haplotype frequencies of <i>Plasmodium falciparum</i> parasites in the urban (a) and peri-urban (b) areas of Brazzaville, and in Gamboma (d) and Pointe-Noire (c).

<p><i>msp1</i> haplotype frequencies of <i>Plasmodium falciparum</i> parasites in the urban (a) and peri-urban (b) areas of Brazzaville, and in Gamboma (d) and Pointe-Noire (c).</p

Map of the study area in Brazzaville, Republic of Congo.

<p>Blood sample donors were recruited from two health centres, Madibou, which serves patients from the peri-urban area to the west of the Djoue river, and Tenrikyo, which predominantly serves those resident in the urban area to the east of the Djoue. Both centres are marked with a star symbol. Insets (a) and (b) show aerial photographs typical of the regions in which patients from the peri-urban (a) and urban (b) areas reside.</p

Allele frequencies at the <i>dhfr</i> and <i>crt</i> loci for the urban and peri-urban areas, estimated using MalHaploFreq based on prevalence data combined with the number of clones per sample (determined by haplotyping <i>msp1</i>).

<p><i>P</i>-values were determined using log-liklelihood tests.</p