Evolution of drug-resistance genes in Plasmodium falciparum in an area of seasonal malaria transmission in Eastern Sudan.

We investigated the evolution of drug-resistant Plasmodium falciparum in a village in eastern Sudan. The frequencies of alleles of 4 genes thought to be determinants of drug resistance were monitored from 1990 through 2001. Changes in frequencies of drug-resistance genes between wet and dry seasons were monitored from 1998 through 2000. Parasites were also typed for 3 putatively neutral microsatellite loci. No significant variation in frequencies was observed for the microsatellite loci over the whole study period or between seasons. However, genes involved in resistance to chloroquine showed consistent, significant increases in frequencies over time (rate of annual increase, 0.027/year for pfcrt and 0.018/year for pfmdr1). Genes involved in resistance to the second-line drug used in the area (Fansidar) remained at low frequencies between 1990 and 1993 but increased dramatically between 1998 and 2000, which is consistent with the advent of Fansidar usage during this period. For mutant alleles of the primary drug-resistance targets for chloroquine and pyrimethamine, higher frequencies were seen during the dry season than during the wet season. This cyclical fluctuation in drug-resistance genes most likely reflects seasonal variation in drug pressure and differences in the fitness of resistant and sensitive parasites.

Detection of mutations in the Plasmodium falciparum dihydrofolate reductase (dhfr) gene by dot-blot hybridization.

There is a need for a specific, sensitive, robust, and large-scale method for diagnosis of drug resistance genes in natural Plasmodium falciparum infections. Established polymerase chain reaction (PCR)-based methods may be compromised by the multiplicity of P. falciparum genotypes in natural infections. Here we adopt a dot-blot method to detect point mutations at nucleotide 323 (residue 108) in the P. falciparum dihydrofolate reductase (dhfr) gene using allele-specific oligonucleotide probes. Serine (Ser) or threonine (Thr) at this position are associated with sensitivity to pyrimethamine while asparagine (Asn) is associated with resistance. The method combines PCR amplification and hybridization of amplified products with radiolabeled allele-specific probes. This technique is specific and sensitive; it detects parasitemia of less than 100 parasites/microl of blood, and can identify a minority parasite genotype down to 1% in a mixture. Analysis of P. falciparum isolates from Sudan, of known response to pyrimethamine, has demonstrated the sensitivity and specificity of the method and its ability to detect multiple genotypes in single infections. Furthermore, it has confirmed the association between pyrimethamine responses and dhfr alleles. The method has been successfully extended for analysis of other point mutations in dhfr at residues 51 and 59, which are associated with a high level of pyrimethamine resistance.

Dynamics of gametocytes among Plasmodium falciparum clones in natural infections in an area of highly seasonal transmission.

The dynamics of gametocyte production in Plasmodium falciparum clones were studied in inhabitants of an area of highly seasonal malaria transmission in eastern Sudan. Reverse-transcriptase polymerase chain reaction was used to detect expression of 2 genes that encode gametocyte-specific proteins, pfs25 and pfg377, in parasites sampled from individuals throughout one year. Some patients who acquired infections during the wet season were found to harbor subpatent gametocytemia through the following dry season in the apparent absence of mosquito transmission. Genotyping of parasites in multiclonal infections showed considerable fluctuation of gametocyte production by individual clones. The gametocytes present at the end of the dry season provide the most probable source of the genetically complex cyclical malaria outbreaks following the rainy season in this region.