Abstract. Parasite genotyping by the polymerase chain reaction was used to distinguish recrudescent from newly acquired Plasmodium falciparum infections in a Karen population resident on the northwestern border of Thailand where malaria transmission is low (one infection/person/year). Plasmodium falciparum infections were genotyped for allelic variation in three polymorphic antigen loci, merozoite surface proteins-1 and -2 (MSP-1 and -2) and glutamaterich protein (GLURP), before and after antimalarial drug treatment. Population genotype frequencies were measured to provide the baseline information to calculate the probability of a new infection with a different or the same genotype to the initial pretreatment isolate. Overall, 38% of the infections detected following treatment had an identical genotype before and up to 121 days after treatment. These post-treatment genotypes were considered recrudescent because of the low (Ͻ 5%) probability of repeated occurrence by chance in the same patient. This approach allows studies of antimalarial drug treatment to be conducted in areas of low transmission since recrudescences can be distinguished confidently from newly acquired infections.The development of resistance to antimalarial drugs continues to be a major problem in the treatment of Plasmodium falciparum infections. Resistance to virtually all available antimalarial drugs has been documented over the last two decades, 1,2 and nowhere is the problem more serious than on the northwestern border of Thailand. 3,4 Tracking the spread of resistance is imperative for the design of appropriate drug regimens, as well as assessing the efficacy of new antimalarial drugs. Resistance levels can be assessed using in vitro tests by measuring inhibition of parasite growth. 5 These tests, while useful for assessing geographic and temporal patterns of drug resistance, overestimate the degree of resistance in vivo, because this is modulated by naturally acquired immunity, age, and other complex factors relating to host defense and the pharmacokinetic properties of the different antimalarial drugs.The assessment of antimalarial treatment regimens in uncomplicated malaria is based on the clinical and parasitological response following treatment. This is best conducted in the community so as to target all age groups. Reoccurrence of parasitemia as detected by a positive blood slide within 28 days of treatment with aminoquinolones or quinine is considered a recrudescence in most endemic areas. 6 For mefloquine, a drug with a long elimination half-life, recrudescence can potentially occur many weeks after drug administration. [7][8][9] This parasitologic measure variably underestimates treatment efficacy in endemic areas because it has not been possible to distinguish recrudescences from newly acquired infections. Chemotherapeutic studies have therefore required isolation of the patients from exposure to malaria. This is costly and impractical especially for women and children. The alternative approach, based on an epidemiologic assessm...

Genetic analysis of the number of Plasmodium falciparum genotypes per infected person in regions of holoendemic and hyperendemic malaria suggest that in areas of lower transmission intensity, significantly fewer parasite genotypes per infected person should be found. A predominance of single clone infections in the human population could generate the controversial clonal population structure proposed for P. falciparum by Tibayrenc and others. Characterization of P. falciparum from individuals on the Thai-Burmese border, an area of hypoendemic transmission, revealed a higher number of genotypes per infected person than that predicted. Possible reasons for this observation are discussed, with particular attention paid to human migration and multidrug resistance.

Abstract. We have developed a typing system using natural sequence variation in the thrombospondin-related adhesive protein (TRAP) gene of Plasmodium falciparum. This method permits a haplotype to be assigned to any particular TRAP gene. We have applied this method to a hospital-based, case control-study in Mali. Previous sequence variation and conservation in TRAP has been confirmed. Particular TRAP haplotypes can be used as geographic hallmarks. Because of the high level of conflict between characters, we have examined the phylogenetic relationships between parasites using a network approach. Having received patient samples from urban and periurban areas of Bamako, the majority of haplotypes were closely related and distinct from TRAP sequences present in other continents. This suggests that the structure of TRAP can only tolerate a limited number of sequence variations to preserve its function but that this is sufficient to allow the parasite to evade the host's immune system until a long-lived immune response can be maintained. It may also reflect host genetics in that certain variants may escape the host immune response more efficiently than others. For vaccine design, sequences from the major regional variants may need to be considered in the production of effective subunit vaccines.The thrombospondin-related adhesive protein (TRAP) gene of Plasmodium falciparum was first described in 1988. 1 The gene derives its name from the deduced protein sequence that contains two amino acid motifs found in the adhesive host glycoprotein thrombospondin. 2 One of these motifs is based on the amino acid sequence Glu-Trp-Ser-ProCys-Ser-Val-Thr-Cys-Gly-Lys-Gly-Thr-Arg-Ser-Arg-LysArg, which has been demonstrated to allow TRAP to bind to sulfatide. 3 The protein is present in sporozoites and erythrocytic-stage parasites. 4, 5 Antibodies against TRAP inhibit sporozoite invasion. 3 Recent data demonstrate that TRAP is expressed in mature, salivary gland sporozoites but not in hemocoel sporozoites, suggesting that TRAP may be one of the unidentified accessory molecules required for infection of the vertebrate host. 6 Data from a longitudinal and prospective study showed that the immune response to TRAP is long-lived in adults and older children but short-lived in young children. 7 There were seasonal fluctuations of the levels of specific antibodies, as well as age-dependent quantitative differences. 7 These data, together with that for the TRAP homolog sporozoite surface protein 2 (SSP2) from P. yoelii, suggesting that adoptive transfer of a CD8 ϩ cytotoxic T cell clone recognizing SSP2 gave complete protection against P. yoelii infection, suggest that TRAP would be an important component of a multi-subunit malaria vaccine. 8 Further evidence for this has been the identification of a cytotoxic T lymphocyte response to certain conserved epitopes in TRAP. 9,10 A major problem in the design of any malaria vaccine is which antigens to choose and which variants of these antigens are relevant. We have previously shown that TRAP is ...