Gametocyte Sex Ratio Research Articles

A within-vector mathematical model of Plasmodium falciparum and implications of incomplete fertilization on optimal gametocyte sex ratio

A mathematical model that simulates the within-vector dynamics of Plasmodium falciparum in an Anopheles mosquito is developed, based on experimental data. The model takes a mosquito's blood meal as input and computes the salivary gland sporozoite load as the final output, a probable measure of mosquito infectivity. Computational model results are consistent with observed results in nature. Sensitivity analysis of the model parameters suggests that reducing the gametocyte density in the blood meal most significantly lowers sporozoite load in the salivary glands and hence mosquito infectivity, and is thus an attractive target for malaria control. The model is used to investigate the implication of incomplete fertilization on optimal gametocyte sex ratio. For a single strain, the transition from complete fertilization to increasingly incomplete fertilization shifts that ratio from 1 to N, where N is the number of viable male gametes produced by a single male gametocyte, towards 1 to 1, which is demonstrated to be the limiting ratio analytically. This ratio is then shown to be an evolutionarily stable strategy as well in the limiting case.

Effects of mefloquine and artesunate mefloquine on the emergence, clearance and sex ratio of Plasmodium falciparum gametocytes in malarious children

BackgroundThe gametocyte sex ratio of Plasmodium falciparum, defined as the proportion of gametocytes that are male, may influence transmission but little is known of the effects of mefloquine or artesunate-mefloquine on gametocyte sex ratio and on the sex ratio of first appearing gametocytes.Methods350 children with uncomplicated P. falciparum malaria were enrolled in prospective treatment trial of mefloquine or artesunate-mefloquine between 2007 and 2008. Gametocytaemia was quantified, and gametocytes were sexed by morphological appearance, before and following treatment. The area under curve of gametocyte density versus time (AUCgm) was calculated by linear trapezoidal method.Results91% and 96% of all gametocytes appeared by day 7 and day 14, respectively following treatment. The overall rate of gametocytaemia with both treatments was 31%, and was significantly higher in mefloquine than in artesunate-mefloquine treated children if no gametocyte was present a day after treatment began (25.3% v 12.8%, P = 0.01). Gametocyte clearance was significantly faster with artesunate-mefloquine (1.8 ± 0.22 [sem] v 5.6 ± 0.95 d; P = 0.001). AUCgm was significantly lower in the artesunate mefloquine group (P = 0.008). The pre-treatment sex ratio was male-biased, but post-treatment sex ratio or the sex ratio of first appearing gametocytes, was significantly lower and female-biased two or three days after beginning of treatment in children given artesunate-mefloquine.ConclusionAddition of artesunate to mefloquine significantly modified the emergence, clearance, and densities of gametocytes and has short-lived, but significant, sex ratio modifying effects in children from this endemic area.

Population structure of Plasmodium falciparum gametocyte sex ratios in malarious children in an endemic area

The gametocyte sex ratio (proportion of gametocytes that are male) of Plasmodium falciparum may influence transmission. The distribution of P. falciparum sex ratios, the extent of inbreeding, the relationship between clone multiplicity and sex ratio, and the pre- and post-treatment factors influencing a sex ratio of 0.5 were determined in 1609 children, with acute malaria. Gametocytes were sexed by morphological appearance and asexual clone multiplicity was determined by polymerase chain reaction (PCR) using polymorphic loci of merozoite surface proteins-1 and -2 (MSP-1, MSP-2) and glutamine-rich protein (GLURP). The weighted mean population sex ratio on presentation in 162 gametocyte carriers was 0.22, that is, 3.5 female to 1 male (95% CI 0.15–0.28), with an estimated inbreeding rate (f) (the proportion of a mother's daughters that is fertilized by her sons) of 0.56 (95% CI 0.44–0.70). Sex ratio was significantly higher when clone multiplicity was >1 infecting clone than when it was 1 (P=0.02). The frequency of a pre-treatment sex ratio of 0.5 was low (3%), and was significantly increased by non-artemisinin but not by artemisinin – mono or combination – drugs by day 7 after therapy commenced (P=0.03 and P=0.44, respectively). No factor was associated with a pre-treatment sex ratio of 0.5 but two factors were independent predictors of a sex ratio of 0.5 by day 7 after therapy commenced: an age ≥5years and anaemia. These population data provide some empirical support for the predictions of local mate competition (LMC) theory and, in conjunction with effects of antimalarials on a sex ratio of 0.5, may have implications for malaria control efforts in endemic settings.

Density-dependent impact of the human malaria parasite Plasmodium falciparum gametocyte sex ratio on mosquito infection rates

Malaria parasites produce male and female life cycle stages (gametocytes) that must fertilize to achieve successful colonization of the mosquito. Gametocyte sex ratios have been shown to be under strong selection pressure both as an adaptive response to a worsening blood environment for transmission and according to the number of co-infecting clones in the vertebrate. Evidence for an impact of sex ratio on the transmission success of Plasmodium falciparum has, however, been more controversial. Theoretical models of fertilization predict that increasingly male sex ratios will be favoured at low gametocyte densities to ensure fertilization. Here, we analyse in vitro transmission studies of P. falciparum to Anopheles gambiae mosquitoes and test this prediction. We find that there is a discernible effect of sex ratio on transmission but which is dependent upon the gametocyte density. While increasingly male sex ratios do give higher transmission success at low gametocyte densities, they reduce success at higher densities. This therefore provides empirical confirmation that sex ratio has an immediate impact on transmission success and that it is density-dependent. Identifying the signals used by the parasite to alter its sex ratio is essential to determine the success of transmission-blocking vaccines that aim to impede the fertilization process.

Some features of primary and recrudescent amodiaquine-resistant Plasmodium falciparum infections in Nigerian children

Characteristics of primary and recrudescent Plasmodium falciparum infections were evaluated in 25 children who did not recover after amodiaquine (AQ) treatment. Recrudescence was detected by a thick blood smear and confirmed by polymerase chain reaction. Over half of recrudescent events occurred after 14 days of initiation of treatment and were associated with relatively low asexual parasitaemia. We examined the gametocyte sex ratio (GSR) in these children and in age and gender-matched controls that had AQ-sensitive (AQ-S) infections (n = 50). In both AQ-S and AQ-resistant (AQ-R) infections, the GSR was female-biased pre-treatment and became male-biased by the third day after treatment initiation. However, gametocyte males persisted after this period in children with AQ-R infections. AQ-recrudescent infections are relatively low (25 of 612.4%) in children from this endemic area.

Plasmodium falciparum gametocyte sex ratios in symptomatic children treated with antimalarial drugs

The sex ratios of Plasmodium falciparum gametocytes, defined as the proportion of gametocytes in peripheral blood that were male, were evaluated in 1609 children with acute, symptomatic, uncomplicated malaria, pre- and post-treatment with various antimalarial drugs, over an 8-year period (1999–2006) in an endemic area of southwest Nigeria. Gametocyte carriage on presentation was 10% (162 children). In 162 children in whom 5797 gametocytes were sexed on presentation, the weighted mean sex ratio was 0.18 (95% confidence interval 0.13–0.25). Following therapy, in 446 children in whom 38,519 gametocytes were sexed, the weighted mean sex ratio was 0.38 (95% CI 0.33–0.43) on day 3 and 0.70 (95% CI 0.63–0.75) ( P < 0.000001) by day 7 after therapy commenced. Non-artemisinin monotherapy significantly increased sex ratio producing a male-biased ratio, but artemisinin combination therapy significantly reduced the sex ratio producing a female biased ratio. Pre-treatment sex ratio correlated negatively with haematocrit ( r = −0.229, P = 0.003) or gametocytaemia ( r = −0.435, P < 0.0001) but not with other clinical or parasitological parameters. The ratio of the sex-specific half lives male:female, the ‘gametocyte maleness index’ (GMI), was >1 with non-artemisinin monotherapy but <1 with artesunate and artemisinin-based combinations. In a multiple regression model, anaemia, low gametocytaemia, and enrolment before 2004 were independent predictors of a male-biased sex ratio pre-treatment. A pre-treatment haematocrit <25%, enrolment before 2004 and treatment with non-artemisinin monotherapy were independent predictors of a male-biased sex ratio 7 days postinitiation of therapy. These findings may have implications for malaria management efforts in sub-Saharan Africa.

Influence of Anemia on Plasmodium falciparum Gametocyte Sex Ratios in Acutely Symptomatic Children

Anemia is common in African children but little is known about how malarial anemia influences Plasmodium falciparum gametocyte sex ratios (PfGSR) and transmission in endemic areas in Africa. We investigated the changes in PfGSR in 1126 consecutive children with acute, symptomatic, uncomplicated falciparum malaria who did (n = 99) or did not (n = 1027) have anemia (defined as a hematocrit 3 d was associated with a male-biased sex ratio (defined as PfGSR > 0.5) (P = 0.029). Hematocrit correlated negatively with PfGSR in non-anemic but not in anemic children (r = -0.219, P = 0.027 and r = -0.106, P = 0.697, respectively) sug- gesting that the critical hematocrit producing 'all or none effect' on PfGSR was a value below 25% in this cohort of chil- dren. Temporal changes showed that, in general, in anemic children, PfGSR was significantly higher at enrolment than in non-anemic children treated with chloroquine (CQ), amodiaquine (AQ) and amodiaquine-sulfalene-pyrimethamine (ASP) (P < 0.0007 in all cases), and remained significantly higher by day 7 or 14 in those treated with AQ and pyrimethamine- sulfadoxine plus probenecid (PSP) (P < 0.007 in all cases). In children who received the same treatment, the ratio of the sex specific half-life male:female, the 'gametocyte maleness index', was one and a half to two folds higher in anemic than non-anemic children suggesting anemia prolongs the survival of microgametocytes and may encourage transmission. These findings have implications for malaria control efforts in endemic sub-Saharan countries where malarial anemia is common. (defined as a hematocrit < 25%) in an endemic area of south- west Nigeria. Our aims were to: determine the contribution of anemia to PfGSR on presentation; determine the factors associated with a male biased PfGSR in children who did or did not have anemia on presentation; and evaluate the tempo- ral changes in PfGSR in children following treatment with antimalarial drugs in these children.

Plasmodium falciparum gametocyte sex ratios in children with acute, symptomatic, uncomplicated infections treated with amodiaquine

BackgroundAmodiaquine is frequently used as a partner drug in combination therapy or in some setting as monotherapy, but little is known about its effects on gametocyte production and sex ratio and its potential influence on transmission in Africa. The effects of amodiaquine on sexual stage parasites and gametocyte sex ratio, and the factors associated with a male-biased sex ratio were evaluated in 612 children with uncomplicated Plasmodium falciparum malaria who were treated with amodiaquine during the period 2000 – 2006 in an endemic area.MethodsClinical, parasitological and laboratory parameters were evaluated before treatment and during follow-up for 28–42 days, and according to standard methods. Gametocyte sex ratio was defined as the proportion of peripheral gametocytes that are male.ResultsClinical recovery from illness occurred in all children. Gametocytaemia was detected in 66 patients (11%) before treatment and in another 56 patients (9%) after treatment. Gametocyte densities were significantly higher by days 3–7 following treatment compared with pre-treatment (P < 0.0001). Overall, mean gametocyte sex ratio increased significantly during follow-up and over the study periods from 2000–2006 (P < 0.001 in both cases), but was female-biased at enrolment throughout the study periods. Absence of fever, a haematocrit < 25%, asexual parasitaemia > 20,000/μL, gametocytaemia < 18/μL, and enrolment in 2006 were associated with a male-biased sex ratio pre-treatment. Anaemia and high parasitaemia were independent predictors of gametocyte maleness 7 days post treatment.ConclusionAmodiaquine may significantly increase gametocyte carriage, density and sex ratio, and may potentially influence transmission. It is possible that anaemia could have contributed to the increased sex ratio. These findings may have implications for malaria control efforts in Africa.

Activities of Artemether-Lumefantrine and Amodiaquine-Sulfalene-Pyrimethamine against Sexual-Stage Parasites in Falciparum Malaria in Children

The activities of artemether-lumefantrine and amodiaquine-sulfalene-pyrimethamine against sexual-stage parasites were evaluated in 42 of 181 Nigerian children with uncomplicated Plasmodium falciparum malaria who had gametocytaemia before, during or after treatment with the two combination therapies. The children were randomized to the standard dose regimens. Clinical recovery from illness occurred in all children who carried gametocytes. Gametocytaemia was detected in 20 patients (11%) before treatment and in another 22 patients (12.2%) after treatment. Game- tocyte carriage rates were similar in both combination treatment groups, but the area under the curve of gametocytaemia plotted against time was 8-fold higher in the amodiaquine-sulfalene-pyrimethamine-treated than in the artemether-lumefantrine-treated children. The pretreatment gametocyte sex ratio was female biased in both treatment groups. During follow-up, there was a short-lived but significant increase in the gametocyte sex ratio in children treated with amodiaquine-sulfalene-pyrimethamine but not in those treated with artemether-lumefantrine. These results indicate that both combination therapies had moderate effects on gametocyte carriage, but artemether-lumefantrine may be more potent at reducing transmissibility in P. falciparum malaria by exerting greater effects on post-treatment gametocyte density and gametocyte sex ratio.

Effects of artesunate-cotrimoxazole and amodiaquine-artesunate against asexual and sexual stages of Plasmodium falciparum malaria in Nigerian children

The activities of artesunate-cotrimoxazole and artesunate-amodiaquine combinations against asexual-and sexual-stage parasites were evaluated in 182 Nigerian children with uncomplicated Plasmodium falciparum malaria. One hundred and twenty-one children received artesunate-cotrimoxazole and 61 received artesunate-amodiaquine and all were followed up for 28 days. Clinical recovery from illness occurred in all children. There was no significant difference in fever clearance time (P = 0.35). Both treatment groups achieved a parasite clearance time of less than 2 days (1.84 +/- 0.66 days and 1.31 +/- 0.48 days); gametocyte carriage rates were comparable in the two treatment groups prior to and following treatment; both treatments appeared to reduce gametocyte carriage. The pretreatment gametocyte sex ratio, which was female-biased, was maintained throughout the period of follow up in both treatment groups. Reduction of gametocyte carriage by these two treatment regimens may reduce transmissibility in P. falciparum malaria, and this reduction is presumed to be related to the accelerated clearance of the asexual forms of the parasite.

Development of reverse-transcription PCR techniques to analyse the density and sex ratio of gametocytes in genetically diverse Plasmodium chabaudi infections

We have developed cross-genotype and genotype-specific quantitative reverse-transcription PCR (qRT-PCR) assays to detect and quantify the number of parasites, transmission stages (gametocytes) and male gametocytes in blood stage Plasmodium chabaudi infections. Our cross-genotype assays are reliable, repeatable and generate counts that correlate strongly (R2s>90%) with counts expected from blood smears. Our genotype-specific assays can distinguish and quantify different stages of genetically distinct parasite clones (genotypes) in mixed infections and are as sensitive as our cross-genotype assays. Using these assays we show that gametocyte density and gametocyte sex ratios vary during infections for two genetically distinct parasite lines (genotypes) and present the first data to reveal how sex ratio is affected when each genotype experiences competition in mixed-genotype infections. Successful infection of mosquito vectors depends on both gametocyte density and their sex ratio and we discuss the implications of competition in genetically diverse infections for transmission success.

Activities of Amodiaquine, Artesunate, and Artesunate-Amodiaquine against Asexual- and Sexual-Stage Parasites in Falciparum Malaria in Children

The activities of amodiaquine, artesunate, and artesunate-amodiaquine against asexual- and sexual-stage parasites were evaluated in 360 Nigerian children with uncomplicated Plasmodium falciparum malaria randomized to the standard dose regimens of the three drugs/combination. Clinical recovery from illness occurred in all children. There were no significant differences in fever clearance times. Patients treated with artesunate or artesunate-amodiaquine had significantly shorter parasite clearance times (1.4 +/- 0.5 days or 1.4 +/- 0.6 days versus 3.2 +/- 2.3 days, P = 0.0001) and lower gametocyte carriage rates (3.3 or 1.7% versus 11.7%, P = 0.001) than those treated with amodiaquine alone. Gametocytemia was detected in 62 patients (11.7% before treatment and 5.6% after treatment). The pretreatment gametocyte sex ratio, which was female biased, increased significantly during the course of treatment with amodiaquine but not with artesunate and artesunate-amodiaquine. These results suggest that artesunate and artesunate-amodiaquine reduce gametocyte carriage and may reduce transmissibility in P. falciparum malaria by accelerating asexual clearance and influencing gametocyte sex ratio.

Population dynamics of sporogony for Plasmodium vivax parasites from western Thailand developing within three species of colonized Anopheles mosquitoes

BackgroundThe population dynamics of Plasmodium sporogony within mosquitoes consists of an early phase where parasite abundance decreases during the transition from gametocyte to oocyst, an intermediate phase where parasite abundance remains static as oocysts, and a later phase where parasite abundance increases during the release of progeny sporozoites from oocysts. Sporogonic development is complete when sporozoites invade the mosquito salivary glands. The dynamics and efficiency of this developmental sequence were determined in laboratory strains of Anopheles dirus, Anopheles minimus and Anopheles sawadwongporni mosquitoes for Plasmodium vivax parasites circulating naturally in western Thailand.MethodsMosquitoes were fed blood from 20 symptomatic Thai adults via membrane feeders. Absolute densities were estimated for macrogametocytes, round stages (= female gametes/zygotes), ookinetes, oocysts, haemolymph sporozoites and salivary gland sporozoites. From these census data, five aspects of population dynamics were analysed; 1) changes in life-stage prevalence during early sporogony, 2) kinetics of life-stage formation, 3) efficiency of life-stage transitions, 4) density relationships between successive life-stages, and 5) parasite aggregation patterns.ResultsThere was no difference among the three mosquito species tested in total losses incurred by P. vivax populations during early sporogony. Averaged across all infections, parasite populations incurred a 68-fold loss in abundance, with losses of ca. 19-fold, 2-fold and 2-fold at the first (= gametogenesis/fertilization), second (= round stage transformation), and third (= ookinete migration) life-stage transitions, respectively. However, total losses varied widely among infections, ranging from 6-fold to over 2,000-fold loss. Losses during gametogenesis/fertilization accounted for most of this variability, indicating that gametocytes originating from some volunteers were more fertile than those from other volunteers. Although reasons for such variability were not determined, gametocyte fertility was not correlated with blood haematocrit, asexual parasitaemia, gametocyte density or gametocyte sex ratio. Round stages and ookinetes were present in mosquito midguts for up to 48 hours and development was asynchronous. Parasite losses during fertilization and round stage differentiation were more influenced by factors intrinsic to the parasite and/or factors in the blood, whereas ookinete losses were more strongly influenced by mosquito factors. Oocysts released sporozoites on days 12 to 14, but even by day 22 many oocysts were still present on the midgut. The per capita production was estimated to be approximately 500 sporozoites per oocyst and approximately 75% of the sporozoites released into the haemocoel successfully invaded the salivary glands.ConclusionThe major developmental bottleneck in early sporogony occurred during the transition from macrogametocyte to round stage. Sporozoite invasion into the salivary glands was very efficient. Information on the natural population dynamics of sporogony within malaria-endemic areas may benefit intervention strategies that target early sporogony (e.g., transmission blocking vaccines, transgenic mosquitoes).

Effects of antifolates - co-trimoxazole and pyrimethamine-sulfadoxine - on gametocytes in children with acute, symptomatic, uncomplicated, Plasmodium falciparum malaria

Antimalarial drugs including the antifolate, pyrimethamine-sulfadoxine (PS), can modulate the prevalence and intensities of gametocytaemia following treatment of acute malaria infections. They may also directly influence the transmission and spread of drug insensitivity. Little is known of the effects of co-trimoxazole (Co-T), another antifolate antimalarial, on gametocytes in children with acute malaria infections. We compared the effects of Co-T and PS on the prevalence and intensities of gametocytaemia and gametocyte sex ratios in 102 children aged 0.5-12 years presenting with acute and uncomplicated falciparum malaria. Compared to pre-treatment, both drugs significantly increased gametocyte carriage post-initiation of treatment. However, gametocyte carriage was significantly lower on day 14 in those treated with Co-T than PS. Significant increase in gametocytaemia with time occurred in PS--but not Co-T-treated children. Kaplan-Meier survival curve of the cumulative probability of remaining gametocyte-free in children who were agametocytaemic at enrollment showed that by day 7 of follow up, children treated with PS had a significantly higher propensity to have developed gametocytes than in Co-T-treated children (Log-rank statistic 5.35, df = 1, P = 0.02). Gametocyte sex ratio changes were similar following treatment with both drugs. PS and Co-T treatment of acute malaria infections in children from this endemic area is associated with significant increases in prevalence and intensities of gametocytaemia but these effects are more marked in those treated with PS than Co-T.

INFLUENCE OF CHEMOTHERAPY ON THE PLASMODIUM GAMETOCYTE SEX RATIO OF MICE AND HUMANS

Plasmodium species, the etiologic agents of malaria, are obligatory sexual organisms. Gametocytes, the precursors of gametes, are responsible for parasite transmission from human to mosquito. The sex ratio of gametocytes has been shown to have consequences for the success of this shift from vertebrate host to insect vector. We attempted to document the effect of chemotherapy on the sex ratio of two different Plasmodium species: Plasmodium falciparum in children from endemic area with uncomplicated malaria treated with chloroquine (CQ) or sulfadoxine-pyrimethamine (SP), and P. vinckei petteri in mice treated with CQ or untreated. The studies involved 53 patients without gametocytes at day 0 (13 CQ and 40 SP) followed for 14 days, and 15 mice (10 CQ and 5 controls) followed for five days. During the course of infection, a positive correlation was observed between the time of the length of infection and the proportion of male gametocytes in both Plasmodium species. No effects of treatment (CQ versus SP for P. falciparum or CQ versus controls for P. vinckei petteri) on the gametocyte sex ratio were found for either Plasmodium species. This indicates that parasites do not respond to chemotherapy by altering their sex allocation strategy, even though, in the case of P. falciparum, they apparently increase their overall investment in sexual stages. This suggests that malaria parasite species respond to different environmental cues for their sex differentiation and sex determination.

Gametocyte sex ratio of a malaria parasite: response to experimental manipulation of parasite clonal diversity.

Sex ratio theory posits that the adaptive proportion of male to female gametocytes of a malaria parasite within the vertebrate host depends on the degree of inbreeding within the vector. Gametocyte sex ratio could be phenotypically flexible, being altered based on the infection's clonal diversity, and thus likely inbreeding. This idea was tested by manipulating the clonal diversity of infections of Plasmodium mexicanum in its lizard host, Sceloporus occidentalis. Naive lizards were inoculated with infected blood from a single donor or 3 pooled donors. Donors varied in their gametocyte sex ratios (17-46%, male), and sex ratio theory allowed estimation of the clonal diversity within donor and recipient infections. Phenotypic plasticity would produce a correlation between donor and recipient infections for infections initiated from a single donor, and a less female-biased gametocyte sex ratio in recipients that received a mixed blood inoculum (with predicted higher clonal diversity) than recipients receiving blood from a single donor. Neither pattern was observed. Gametocyte sex ratio of most infections ranged from 35 to 42% male, expected if clonal diversity was high for all infections. Alternative explanations are suggested for the observed variation of gametocyte sex ratio among P. mexicanum infections.

Experimental evaluation of the relationship between lethal or non-lethal virulence and transmission success in malaria parasite infections

BackgroundEvolutionary theory suggests that the selection pressure on parasites to maximize their transmission determines their optimal host exploitation strategies and thus their virulence. Establishing the adaptive basis to parasite life history traits has important consequences for predicting parasite responses to public health interventions. In this study we examine the extent to which malaria parasites conform to the predicted adaptive trade-off between transmission and virulence, as defined by mortality. The majority of natural infections, however, result in sub-lethal virulent effects (e.g. anaemia) and are often composed of many strains. Both sub-lethal effects and pathogen population structure have been theoretically shown to have important consequences for virulence evolution. Thus, we additionally examine the relationship between anaemia and transmission in single and mixed clone infections.ResultsWhereas there was a trade-off between transmission success and virulence as defined by host mortality, contradictory clone-specific patterns occurred when defining virulence by anaemia. A negative relationship between anaemia and transmission success was found for one of the parasite clones, whereas there was no relationship for the other. Notably the two parasite clones also differed in a transmission phenotype (gametocyte sex ratio) that has previously been shown to respond adaptively to a changing blood environment. In addition, as predicted by evolutionary theory, mixed infections resulted in increased anaemia. The increased anaemia was, however, not correlated with any discernable parasite trait (e.g. parasite density) or with increased transmission.ConclusionsWe found some evidence supporting the hypothesis that there is an adaptive basis correlating virulence (as defined by host mortality) and transmission success in malaria parasites. This confirms the validity of applying evolutionary virulence theory to biomedical research and adds support to the prediction that partially effective vaccines may select for increasingly virulent malaria parasite strains. By contrast, there was no consistent correlation between transmission and sub-lethal anaemia, a more common outcome of malaria infection. However, overall, the data are not inconsistent with the recent proposal that sub-lethal effects may impose an upper limit on virulence. Moreover, clone specific differences in transmission phenotypes linked to anaemia do suggest that there is considerable adaptive potential relating anaemia and transmission that may lead to uncertain consequences following intervention strategies.

Changes in Plasmodium falciparum gametocytaemia in children with chloroquine-sensitive asexual infections.

A non-compartmental pharmacokinetic model was used to describe the changes in gametocytaemia in nine children with chloroquine-sensitive Plasmodium falciparum malaria in whom asexual parasitaemia cleared within 72 h of chloroquine treatment. Peak gametocytaemia was 74 +/- 19.9 (se), range 24-198, geometric mean 58 sf (sexual forms)/microliter. Time to peak gamelocytaemia was 43.2 +/- 14.4, range 0-120 h. Following peak gametocytaemia, gametocytes persisted in blood for a period of 168-504 h. The decline from peak gametocytaemia was exponential with a half-life of gametocytaemia of 43.2 +/- 20.4, range 13.1-206 h. The mean pre-treatment sex ratio was male-biased and remained so till complete elimination of gametocytaemia. Peak microgametocytaemia, area under the curve of microgametocytaemia versus time, and the half-life of microgametocytaemia were significantly higher than those of macrogametocytaemia. The volume of blood completely cleared of macrogametocytaemia per unit time was significantly higher than that of microgametocytaemia. Macrogometocytes are cleared from the circulation faster than microgametocytes but chloroquine treatment of chloroquine-sensitive infections has little or no significant effect on gametocyte sex ratios in this group of children.

Gametocyte sex ratios in children with asymptomatic, recrudescent, pyrimethamine-sulfadoxine-resistant, Plasmodium falciparum malaria.

The clinical features of the primary Plasmodium falciparum infections in 25 children, and of the recrudescent infections that emerged after pyrimethamine-sulfadoxine (PS) treatment of the children had failed, were evaluated. In addition, the gametocyte sex ratios in these children and in age- and gender-matched controls who had PS-sensitive (PS-S) infections were also examined. Compared with the primary infections, the recrudescent infections were accompanied by significantly fewer symptoms and lower levels of parasitaemia but significantly higher gametocytaemia:parasitaemia ratios. Although the mean gametocyte sex ratio was female-biased pre-treatment, in both the PS-resistant (PS-R) and PS-S infections it became male-biased on days 7 and 14 post-treatment. The times taken to attain a sex ratio of 1 were similar in both groups. The predominance of macrogametocytes seen 'early' post-treatment (on day 3) was later replaced by a predominance of microgametocytes (on days 7 and 14). Analysis of the disposition of gametocytaemia, from the time to attain a sex ratio of 1, showed that the area under the curve of the plot of the level of microgametocytaemia upsilon. time and the mean half-life of the microgametocytaemia were significantly greater and microgametocytaemia clearance was significantly slower than the corresponding values for macrogametocytaemia. Although sex ratios in Plasmodium may naturally become more male-biased as the infection progresses, it is possible that PS treatment may have contributed to the male-biased sex ratios observed post-treatment.

Sex ratio of Plasmodium falciparum gametocytes in inhabitants of Dielmo, Senegal.

An epidemiological survey was conducted during a 4-month period of intense malaria transmission in Dielmo, a holoendemic Senegalese village. Two thick blood smears per inhabitant were collected weekly. The sex ratio of Plasmodium falciparum gametocytes (gamete precursors) was studied in 50 gametocyte carriers. All age classes were represented (mean 19.7 years; range: 2 months-75 years); 42 (84%) of them did not receive antimalarial treatment. Overall 668 thick smears were examined until 100 gametocytes had been counted or for 40 min. A total of 11204 gametocytes were observed with a mean sex ratio of 0.346 (95% CI 0.317-0.374), i.e. 2.89 females per 1 male. Among the 284 thick smears in which at least 10 gametocytes were observed, the mean percentage of male gametocytes was 27.8%, with a range of 0-82%. Great variability was observed between gametocyte carriers and also between thick smears from the same gametocyte carrier. A multivariate analysis was performed which highlighted the fact that only 2 variables had a significant effect on the sex ratio. Anaemia was associated with an increased percentage of males (Prevalence Rate Ratio [PPR] of male gametocytes was multiplied by 1.65 if haematocrit rate < 32%) and a wave of gametocytes was associated with an increased percentage of female gametocytes (PRR was multiplied by 0.48 during the peak of gametocytaemia and for the 2 weeks following this peak). The variables without significant effect on sex ratio were: age, sex, clinical status and sickle cell trait status of the gametocyte carrier, density of asexual parasites, quinine treatment, and gametocyte density (when taking account of its waves). These results are discussed in regard of possible differential production, mortality or sequestration of one gametocyte sex and selective advantages for the transmission of parasites.