Coupling ecology and evolution: malaria and the S-gene across time scales

Malaria has long been a scourge to humans. The exceptionally high mortality in some regions has led to strong selection for resistance, even at the cost of increased risk of potentially fatal red blood cell deformities in some offspring. In particular, genes that confers resistance to malaria when they appear in heterozygous individuals are known to lead to sickle-cell anemia, or other blood diseases, when they appear in homozygous form. Thus, there is balancing selection against the evolution of resistance, with the strength of that selection dependent upon malaria prevalence. Over longer time scales, the increased frequency of resistance in a population might be expected to decrease the frequency of malaria and reduce selection for resistance. However, possession of the sickle-cell gene leads to longer-lasting parasitaemia in heterozygote individuals, and therefore the presence of resistance may actually increase infection prevalence. In this paper, we explore the interplay among these processes, operating over very different time scales. In particular, we show that on the fast time scale of malarial dynamics, the disease level reaches an equilibrium; on the slower, evolutionary time scale, this equilibrium tracks gene frequency. We analyze the slow time scale dynamics to investigate the impact of malaria on the evolution of resistance.     

Sickle-cell Anaemia,Sickle-cell Thalassaemia,Sickle-cell Haemoglobin C Disease,and Asymptomatic Haemoglobin C Thalassaemia in one Ghanaian Family

A Ghanaian family is described in which a sickle-cell haemoglobin C man married to a sickle-cell thalassaemia woman produced 12 children (eight alive). Four children have sickle-cell anaemia, two sickle-cell haemoglobin C disease, one has sickle-cell thalassaemia, and one is asymptomatic haemoglobin C thalassaemia.It is emphasized that the contribution that adult sickle-cell disease patients make, through procreation, to the persistence of the S gene may be greater than is normally supposed, and that this contribution may soon outstrip that made by balanced polymorphism through falciparum malaria. Widespread haemoglobin genotyping in schools leading to genetic counselling is advocated to decrease the incidence of sickle-cell disease.     

Relationship between Malaria Incidence and IgG Levels to Plasmodium falciparum Merozoite Antigens in Malian Children: Impact of Hemoglobins S and C

Heterozygous hemoglobin (Hb) AS (sickle-cell trait) and HbAC are hypothesized to protect against Plasmodium falciparum malaria in part by enhancing naturally-acquired immunity to this disease. To investigate this hypothesis, we compared antibody levels to four merozoite antigens from the P. falciparum 3D7 clone (apical membrane antigen 1, AMA1-3D7; merozoite surface protein 1, MSP1-3D7; 175 kDa erythrocyte-binding antigen, EBA175-3D7; and merozoite surface protein 2, MSP2-3D7) in a cohort of 103 HbAA, 73 HbAS and 30 HbAC children aged 3 to 11 years in a malaria-endemic area of Mali. In the 2009 transmission season we found that HbAS, but not HbAC, significantly reduced the risk of malaria compared to HbAA. IgG levels to MSP1 and MSP2 at the start of this transmission season inversely correlated with malaria incidence after adjusting for age and Hb type. However, HbAS children had significantly lower IgG levels to EBA175 and MSP2 compared to HbAA children. On the other hand, HbAC children had similar IgG levels to all four antigens. The parasite growth-inhibitory activity of purified IgG samples did not differ significantly by Hb type. Changes in antigen-specific IgG levels during the 2009 transmission and 2010 dry seasons also did not differ by Hb type, and none of these IgG levels dropped significantly during the dry season. These data suggest that sickle-cell trait does not reduce the risk of malaria by enhancing the acquisition of IgG responses to merozoite antigens.     

Mosquito mortality and the evolution of malaria virulence

Abstract Several laboratory studies of malaria parasites (Plasmodium sp.) and some field observations suggest that parasite virulence, defined as the harm a parasite causes to its vertebrate host, is positively correlated with transmission. Given this advantage, what limits the continual evolution of higher parasite virulence? One possibility is that while more virulent strains are more infectious, they are also more lethal to mosquitoes. In this study, we tested whether the virulence of the rodent malaria parasite P. chabaudi in the laboratory mouse was correlated with the fitness of mosquitoes it subsequently infected. Mice were infected with one of seven genetically distinct clones of P. chabaudi that differ in virulence. Weight loss and anemia in infected mice were monitored for 16–17 days before Anopheles stephensi mosquitoes were allowed to take a blood meal from them. Infection virulence in mice was positively correlated with transmission to mosquitoes (infection rate) and weakly associated with parasite burden (number of oocysts). Mosquito survival fell with increasing oocyst burden, but there was no overall statistically significant relationship between virulence in mice and mosquito mortality. Thus, there was no evidence that more virulent strains are more lethal to mosquitoes. Both vector survival and fecundity depended on parasite clone, and contrary to expectations, mosquitoes fed on infections more virulent to mice were more fecund. The strong parasite genetic effects associated with both fecundity and survival suggests that vector fitness could be an important selective agent shaping malaria population genetics and the evolution of phenotypes such as virulence in the vector.     

Optimal Population-Level Infection Detection Strategies for Malaria Control and Elimination in a Spatial Model of Malaria Transmission

Mass campaigns with antimalarial drugs are potentially a powerful tool for local elimination of malaria, yet current diagnostic technologies are insufficiently sensitive to identify all individuals who harbor infections. At the same time, overtreatment of uninfected individuals increases the risk of accelerating emergence of drug resistance and losing community acceptance. Local heterogeneity in transmission intensity may allow campaign strategies that respond to index cases to successfully target subpatent infections while simultaneously limiting overtreatment. While selective targeting of hotspots of transmission has been proposed as a strategy for malaria control, such targeting has not been tested in the context of malaria elimination. Using household locations, demographics, and prevalence data from a survey of four health facility catchment areas in southern Zambia and an agent-based model of malaria transmission and immunity acquisition, a transmission intensity was fit to each household based on neighborhood age-dependent malaria prevalence. A set of individual infection trajectories was constructed for every household in each catchment area, accounting for heterogeneous exposure and immunity. Various campaign strategies—mass drug administration, mass screen and treat, focal mass drug administration, snowball reactive case detection, pooled sampling, and a hypothetical serological diagnostic—were simulated and evaluated for performance at finding infections, minimizing overtreatment, reducing clinical case counts, and interrupting transmission. For malaria control, presumptive treatment leads to substantial overtreatment without additional morbidity reduction under all but the highest transmission conditions. Compared with untargeted approaches, selective targeting of hotspots with drug campaigns is an ineffective tool for elimination due to limited sensitivity of available field diagnostics. Serological diagnosis is potentially an effective tool for malaria elimination but requires higher coverage to achieve similar results to mass distribution of presumptive treatment.     

Focus: Zoonotic Disease: An Ecologically Framed Comparison of The Potential for Zoonotic Transmission of Non-Human and Human-Infecting Species of Malaria Parasite

The threats, both real and perceived, surrounding the development of new and emerging infectious diseases of humans are of critical concern to public health and well-being. Among these risks is the potential for zoonotic transmission to humans of species of the malaria parasite, Plasmodium, that have been considered historically to infect exclusively non-human hosts. Recently observed shifts in the mode, transmission, and presentation of malaria among several species studied are evidenced by shared vectors, atypical symptoms, and novel host-seeking behavior. Collectively, these changes indicate the presence of environmental and ecological pressures that are likely to influence the dynamics of these parasite life cycles and physiological make-up. These may be further affected and amplified by such factors as increased urban development and accelerated rate of climate change. In particular, the extended host-seeking behavior of what were once considered non-human malaria species indicates the specialist niche of human malaria parasites is not a limiting factor that drives the success of blood-borne parasites. While zoonotic transmission of non-human malaria parasites is generally considered to not be possible for the vast majority of Plasmodium species, failure to consider the feasibility of its occurrence may lead to the emergence of a potentially life-threatening blood-borne disease of humans. Here, we argue that recent trends in behavior among what were hitherto considered to be non-human malaria parasites to infect humans call for a cross-disciplinary, ecologically-focused approach to understanding the complexities of the vertebrate host/mosquito vector/malaria parasite triangular relationship. This highlights a pressing need to conduct a multi-species investigation for which we recommend the construction of a database to determine ecological differences among all known Plasmodium species, vectors, and hosts. Closing this knowledge gap may help to inform alternative means of malaria prevention and control.     

Practical Implications of the Non-Linear Relationship between the Test Positivity Rate and Malaria Incidence

Background

The test positivity rate (TPR), defined as the number of laboratory-confirmed malaria tests per 100 suspected cases examined, is widely used by malaria surveillance programs as one of several key indicators of temporal trends in malaria incidence. However, there have been few studies using empiric data to examine the quantitative nature of this relationship.

Methods

To characterize the relationship between the test positivity rate and the incidence of malaria, we fit regression models using the confirmed malaria case rate as the outcome of interest and TPR as the predictor of interest. We varied the relationship between the two by alternating linear and polynomial terms for TPR, and compared the goodness of fit of each model.

Results

A total of 7,668 encounters for malaria diagnostic testing were recorded over the study period within a catchment area of 25,617 persons. The semi-annual TPR ranged from 4.5% to 59% and the case rates ranged from 0.5 to 560 per 1,000 persons. The best fitting model was an exponential growth model (R² = 0.80, AIC = 637). At low transmission levels (TPR<10%), the correlation between TPR and CMCR was poor, with large reductions in the TPR, for example from 10% to 1%, was associated with a minimal change in the CMCR (3.9 to 1.7 cases per 1,000 persons). At higher transmission levels, the exponential relationship made relatively small changes in TPR suggestive of sizeable change in estimated malaria incidence, suggesting that TPR remains a valuable surveillance indicator in such settings.

Conclusions

The TPR and the confirmed malaria case rate have a non-linear relationship, which is likely to have important implications for malaria surveillance programs, especially at the extremes of transmission.     

A mathematical model for the transmission of Plasmodium vivax malaria

We have proposed a mathematical model for the transmission of Plasmodium vivax malaria quantitatively, which is adjusted to the infected region, Guadalcanal, in the Solomon Islands. The simulation of a transmission model will be instrumental in planning the malaria control strategy. A characteristic of the life cycle of P. vivax is that a sporozoite injected into the blood stream by a mosquito bite may sometimes stay in a hepatocyte as a hypnozoite. Therefore, we have incorporated a phenomenon of renewed infections caused by a relapse into the transmission model. Also through the simulations we have attempted to evaluate the decline in prevalence caused by the programs of selective mass drug administration (MDA) and vector control such as the distribution of permethrin-treated bednets. The simulations have indicated that the concentrated repetition of MDA at 1-week intervals would reduce the prevalence of vivax malaria swiftly in the beginning and would keep the parasite rate below 1% for a few years but the prevalence would increase thereafter. In contrast, the parasite rate would remain below 1% for a long time if a trial of 1 or 2 times MDA is accompanied with some reduction of the vectorial capacity by the enforcement of vector control. In any case, it is important to beware of relapse cases because even after the execution of MDA it takes a long time to decrease the proportion of hypnozoite carriers.     

Effect of Maternal Sickle-cell Trait on Perinatal Mortality

Possession of the sickle-cell trait (Hb AS) by the African mother has been shown to be associated with a significant increase in perinatal mortality when there is anoxic stress. This observation should be taken into consideration in the management of labour of both the indigenous and immigrant mothers at risk. The findings do not influence the proposed explanations for the maintenance of high frequencies of the haemoglobin S gene in areas of endemic malaria.     

Child mortality and malaria transmission intensity in Africa

The desirability of controlling malaria transmission in the areas of highest endemicity of Plasmodium falciparum has long been debated. Most recently, it has been claimed that rates of malaria morbidity are no higher in areas of very high transmission in Africa than they are in places with lower inoculation rates. We now review the literature on the relationship of morbidity and mortality to malaria transmission intensity, and have linked published child mortality and malaria transmission rates to examine how age-specific mortality actually varies with the inoculation rate of P. falciparum.     

Importance of factors determining the effective lifetime of a mass, long-lasting, insecticidal net distribution: a sensitivity analysis

Background

Long-lasting insecticidal nets (LLINs) reduce malaria transmission by protecting individuals from infectious bites, and by reducing mosquito survival. In recent years, millions of LLINs have been distributed across sub-Saharan Africa (SSA). Over time, LLINs decay physically and chemically and are destroyed, making repeated interventions necessary to prevent a resurgence of malaria. Because its effects on transmission are important (more so than the effects of individual protection), estimates of the lifetime of mass distribution rounds should be based on the effective length of epidemiological protection.

Methods

Simulation models, parameterised using available field data, were used to analyse how the distribution's effective lifetime depends on the transmission setting and on LLIN characteristics. Factors considered were the pre-intervention transmission level, initial coverage, net attrition, and both physical and chemical decay. An ensemble of 14 stochastic individual-based model variants for malaria in humans was used, combined with a deterministic model for malaria in mosquitoes.

Results

The effective lifetime was most sensitive to the pre-intervention transmission level, with a lifetime of almost 10 years at an entomological inoculation rate of two infectious bites per adult per annum (ibpapa), but of little more than 2 years at 256 ibpapa. The LLIN attrition rate and the insecticide decay rate were the next most important parameters. The lifetime was surprisingly insensitive to physical decay parameters, but this could change as physical integrity gains importance with the emergence and spread of pyrethroid resistance.

Conclusions

The strong dependency of the effective lifetime on the pre-intervention transmission level indicated that the required distribution frequency may vary more with the local entomological situation than with LLIN quality or the characteristics of the distribution system. This highlights the need for malaria monitoring both before and during intervention programmes, particularly since there are likely to be strong variations between years and over short distances. The majority of SSA's population falls into exposure categories where the lifetime is relatively long, but because exposure estimates are highly uncertain, it is necessary to consider subsequent interventions before the end of the expected effective lifetime based on an imprecise transmission measure.     

Environmental, pharmacological and genetic influences on the spread of drug-resistant malaria

Plasmodium falciparum malaria is subject to artificial selection from antimalarial drugs that select for drug-resistant parasites. We describe and apply a flexible new approach to investigate how epistasis, inbreeding, selection heterogeneity and multiple simultaneous drug deployments interact to influence the spread of drug-resistant malaria. This framework recognizes that different human 'environments' within which treatment may occur (such as semi- and non-immune humans taking full or partial drug courses) influence the genetic interactions between parasite loci involved in resistance. Our model provides an explanation for how the rate of spread varies according to different malaria transmission intensities, why resistance might stabilize at intermediate frequencies and also identifies several factors that influence the decline of resistance after a drug is removed. Results suggest that studies based on clinical outcomes might overestimate the spread of resistant parasites, especially in high-transmission areas. We show that when transmission decreases, prevalence might decrease without a corresponding change in frequency of resistance and that this relationship is heavily influenced by the extent of linkage disequilibrium between loci. This has important consequences on the interpretation of data from areas where control is being successful and suggests that reducing transmission might have less impact on the spread of resistance than previously expected.     

Identifying Malaria Transmission Foci for Elimination Using Human Mobility Data

Humans move frequently and tend to carry parasites among areas with endemic malaria and into areas where local transmission is unsustainable. Human-mediated parasite mobility can thus sustain parasite populations in areas where they would otherwise be absent. Data describing human mobility and malaria epidemiology can help classify landscapes into parasite demographic sources and sinks, ecological concepts that have parallels in malaria control discussions of transmission foci. By linking transmission to parasite flow, it is possible to stratify landscapes for malaria control and elimination, as sources are disproportionately important to the regional persistence of malaria parasites. Here, we identify putative malaria sources and sinks for pre-elimination Namibia using malaria parasite rate (PR) maps and call data records from mobile phones, using a steady-state analysis of a malaria transmission model to infer where infections most likely occurred. We also examined how the landscape of transmission and burden changed from the pre-elimination setting by comparing the location and extent of predicted pre-elimination transmission foci with modeled incidence for 2009. This comparison suggests that while transmission was spatially focal pre-elimination, the spatial distribution of cases changed as burden declined. The changing spatial distribution of burden could be due to importation, with cases focused around importation hotspots, or due to heterogeneous application of elimination effort. While this framework is an important step towards understanding progressive changes in malaria distribution and the role of subnational transmission dynamics in a policy-relevant way, future work should account for international parasite movement, utilize real time surveillance data, and relax the steady state assumption required by the presented model.     

Limited recombination events in merozoite surface protein-1 alleles of Plasmodium falciparum on islands.

Intragenic recombination is a principal mechanism for the generation of allelic variation in the merozoite surface protein-1 gene (Msp-1) of the human malaria parasite Plasmodium falciparum. In the present study, linkage disequilibrium between the 5'- and 3'-polymorphic sites was analyzed to determine the frequency of recombination events in Msp-1 in parasite populations on four islands in Vanuatu, the southwestern Pacific, where malaria transmission is moderate and comparable to other mesoendemic areas. Of 141 isolates, whose 5'-haplotypes (Msp-1 blocks 2-6) were determined by PCR-based typing, 138 were successfully sequenced for the 3'-polymorphism (block 17). A total of four distinct 5'-haplotypes and three distinct 3'-sequence types were identified with apparently different frequency distribution among islands. The number of 5'-haplotypes in each island was one to four, far smaller than in other previously studied geographic areas (ten to 21). Associations between the 5'- and 3'-polymorphisms (here termed Msp-1 gene types) were subjected to the R(2) linkage disequilibrium test. The test revealed complete or very strong linkage disequilibrium in all four islands. Mixed infection was unusually rare (2.1%) and the mean number of Msp-1 alleles per person was nearly 1.0. The heterozygosity of the Msp-1 gene type calculated for each island (h=0.41-0.65) was significantly lower than that in other areas of comparable endemicity (h=0.81-0.89) (P<0.01). These results indicate that recombination events in Msp-1 would be extremely limited in Vanuatu, and stress that the frequency of recombination in Msp-1 is determined by not only the intensity of malaria transmission but the frequency of mixed clone infections, the mean number of clones per person and a repertoire of clones in a local area.     

The International Limits and Population at Risk of Plasmodium vivax Transmission in 2009

Background

A research priority for Plasmodium vivax malaria is to improve our understanding of the spatial distribution of risk and its relationship with the burden of P. vivax disease in human populations. The aim of the research outlined in this article is to provide a contemporary evidence-based map of the global spatial extent of P. vivax malaria, together with estimates of the human population at risk (PAR) of any level of transmission in 2009.

Methodology

The most recent P. vivax case-reporting data that could be obtained for all malaria endemic countries were used to classify risk into three classes: malaria free, unstable (<0.1 case per 1,000 people per annum (p.a.)) and stable (≥0.1 case per 1,000 p.a.) P. vivax malaria transmission. Risk areas were further constrained using temperature and aridity data based upon their relationship with parasite and vector bionomics. Medical intelligence was used to refine the spatial extent of risk in specific areas where transmission was reported to be absent (e.g., large urban areas and malaria-free islands). The PAR under each level of transmission was then derived by combining the categorical risk map with a high resolution population surface adjusted to 2009. The exclusion of large Duffy negative populations in Africa from the PAR totals was achieved using independent modelling of the gene frequency of this genetic trait. It was estimated that 2.85 billion people were exposed to some risk of P. vivax transmission in 2009, with 57.1% of them living in areas of unstable transmission. The vast majority (2.59 billion, 91.0%) were located in Central and South East (CSE) Asia, whilst the remainder were located in America (0.16 billion, 5.5%) and in the Africa+ region (0.10 billion, 3.5%). Despite evidence of ubiquitous risk of P. vivax infection in Africa, the very high prevalence of Duffy negativity throughout Central and West Africa reduced the PAR estimates substantially.

Conclusions

After more than a century of development and control, P. vivax remains more widely distributed than P. falciparum and is a potential cause of morbidity and mortality amongst the 2.85 billion people living at risk of infection, the majority of whom are in the tropical belt of CSE Asia. The probability of infection is reduced massively across Africa by the frequency of the Duffy negative trait, but transmission does occur on the continent and is a concern for Duffy positive locals and travellers. The final map provides the spatial limits on which the endemicity of P. vivax transmission can be mapped to support future cartographic-based burden estimations.     

An analytical model for genetic hitchhiking in the evolution of antimalarial drug resistance

We analytically study a deterministic model for the spread of drug resistance among human malaria parasites. The model incorporates all major characteristics of the complex malaria transmission cycle and accounts for the fact that only a fraction α of infected hosts receive drug treatment. Furthermore, the model incorporates that hosts can be co-infected. The number m of parasites co-infecting a host is either a constant or, more generally, follows a given frequency distribution.Although the model is formulated in a multilocus setup, for our results we assume that drug resistance is caused by a single locus with two alleles — a sensitive one and a resistant one. We assume that the resistant allele has a selective advantage only in treated hosts and pays metabolic costs, which causes this allele to be deleterious in untreated hosts. We provide necessary and sufficient conditions for the fixation of the resistant allele. Moreover, provided the resistant allele will sweep through the population, we derive a formula for the time until it reaches a given frequency and in particular for the time until quasi-fixation.Furthermore, we establish an analytical solution for allele frequency changes at a linked neutral biallelic locus due to the rapid increase in frequency of the resistant allele. Our solution describes a local reduction in heterozygosity among parasite chromosomes around the resistant allele, the effect commonly referred to as the hitchhiking effect, as a function of α and m. The result therefore allows the investigation of selective sweep patterns under specific demographic settings. We find that the hitchhiking effect is similar but different from the standard model of genetic hitchhiking that assumes random mating and homogeneous selection. In particular, the process of recombination and selection cannot be decoupled. We further explain why standard hitchhiking theory cannot be applied to drug resistance in malaria. Furthermore, we will show that a genome-wide reduction in relative heterozygosity can occur provided a fraction of hosts is infected by a single parasite haplotype.Finally, we show how to incorporate host heterogeneity, and generalize our results to this biologically more realistic case.     

Association of red cell glucose-6-phosphate dehydrogenase with haemoglobinopathies

A total of 1,112 randomly selected Saudi Arabs, of both sexes, living in Jeddah and the surrounding areas were screened for the phenotypic distribution of red cell glucose-6-phosphate dehydrogenase (G6PD) and 6-phosphogluconate dehydrogenase (6PGD). They were also investigated for haemoglobin and for thalassaemia. Phenotyping of the haemoglobins and the red cell enzymes was carried out by starch gel electrophoresis and the dye-decolouration screening test, while the investigation for thalassaemia was carried out by globin-chain biosynthesis, followed by column chromatography. The red cell Gd- alleles were significantly associated with the sickle-cell gene in both the males (chi 2(1): AS-28.80; SS-4.89) and females (chi 2(1): AS-10.99; SS-13.16). A similar association was also observed between G6PD deficiency and thalassaemias in males (chi 2(1): alpha-thalassaemia - 3.13; beta-thalassaemia - 11.06) and females (chi 2(1): alpha-thalassaemia - 6.63). However, no such association was detected between red cell 6PGD types and haemoglobin genes. The results suggest that the red cell G6PD deficiency, sickle-cell and thalassaemia genes might have evolved as a result of the same ecological factor, probably malaria.     

Plasmodium falciparum malaria and sickle cell gene in the popular Republic of Congo. I. Relationship between parasitemia and sicke cell trait in Djoumouna (region of Brazzaville) (author's transl)

The relationship between sickle cell trait and falciparum malaria was studied in the village of Djoumouna, twenty kilometers south west of Brazzaville. Malaria is characterized by a stable high intensity of transmission on the average one infective mosquito bite by night and by child contrasting with a relatively low malarial infection rate. The prevalence of carriers of an S gene (AS) does not change with age: 22.2% for children under 5 years, 22.1% for childrern between 5 and 15 years, and 22.9% in adults. Malarial infection rates are 32% in homozygous AA children under five years and 38% in AS children, an insignificant difference. Our data for this region of the Congo fail to confirm the hypothesis that the AS genotype protects the carrier against Plasmodium falciparum infection.     

温度对我国中华按蚊世代分布及其传疟有效季节的影响

中华按蚊是我国北纬25度以北地区传播疟疾的重要媒介。温度是影响按蚊繁殖和疟原虫发育的主要生态因子,对疟疾流行季节与流行程度影响甚大。本文从温度与中华按蚊各虫态发育历期的关系,分析不同纬度地区中华按蚊     

Imbalanced distribution of Plasmodium falciparum MSP-1 genotypes related to sickle-cell trait.

BACKGROUND: The sickle-cell trait protects against severe Plasmodium falciparum malaria and reduces susceptibility to mild malaria but does not prevent infection. The exact mechanism of this protection remains unclear. We have hypothesized that AS individuals are protected by virtue of being less susceptible to a subset of parasite strains; thus we compared some genetic characteristics of parasites infecting AS and AA subjects. MATERIALS AND METHODS: Blood was collected from asymptomatic individuals living in two different regions of Africa. The polymorphic MSP-1 and MSP-2 loci were genotyped using a PCR-based methodology. Individual alleles were identified by size polymorphism, amplification using family-specific primers, and hybridization using family-specific probes. Multivariate logistic regression was used to analyze allele distribution. RESULTS: In Senegalese carriers, age and hemoglobin type influenced differently the distribution of the three MSP-1 families and had an impact on distinct individual alleles, whereas the distribution of MSP-2 alleles was marginally affected. There was no influence of other genetic traits, including the HLA Bw53 genotype, or factors such as place of residence within the village. In a cohort of Gabonese schoolchildren in which the influence of age was abrogated, a similar imbalance in the MSP-1 allelic distribution but not of MSP-2 allelic distribution by hemoglobin type was observed. CONCLUSIONS: The influence of the host's hemoglobin type on P. falciparum genotypes suggests that parasite fitness for a specific host is strain-dependent, which is consistent with our hypothesis that innate resistance might result from reduced fitness of some parasite strains for individuals with sickle-cell traits.