The adaptation of Anopheles gambiae to humans and its environment involves an ongoing speciation process that can be best demonstrated by the existence of various chromosomal forms adapted to ...different environments and of two molecular forms known as incipient taxonomic units.
The aim of this study was to compare the epidemiologic role of Anopheles arabiens is and the molecular forms M and S of Anopheles gambiae in the transmission of Plasmodium in a rural areas of southern Senegal, Dielmo. The sampling of mosquitoes was carried out monthly between July and December 2004, during the rainy season, by human volunteers and pyrethrum spray catches.
Anopheles arabiensis, An. gambiae M and S forms coexisted during the rainy season with a predominance of the M form in September and the peak of density being observed in August for the S form. Similar parity rates were observed in An. arabiensis 70.9% (n = 86), An. gambiae M form 68.7% (n = 64) and An. gambiae S form 81.1% (n = 156). The circumsporozoite protein (CSP) rates were 2.82% (n = 177), 3.17% (n = 315) and 3.45% (n = 405), with the mean anthropophilic rates being 71.4% (n = 14), 86.3% (n = 22) and 91.6% (n = 24) respectively for An. arabiensis and An. gambiae M and S forms. No significant difference was observed either in host preference or in Plasmodium falciparum infection rates between sympatric M and S populations.
No difference was observed either in host preference or in Plasmodium falciparum infection rates between sympatric M and S populations, but they present different dynamics of population. These variations are probably attributable to different breeding conditions.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Despite a long history of attempts to model malaria epidemiology, the over-riding conclusion is that a detailed understanding of host-parasite interactions leading to immunity is required. It is ...still not known what governs the duration of an infection and how within-human parasite dynamics relate to malaria epidemiology.
Immunity to Plasmodium falciparum develops slowly and requires repeated exposure to the parasite, which thus generates age-structure in the host-parasite interaction. An age-structured degree of immunity would present the parasite with humans of highly variable quality. Evolutionary theory suggests that natural selection will mould adaptive phenotypes that are more precise (less variant) in "high quality" habitats, where lifetime reproductive success is best. Variability in malaria parasite gametocyte density is predicted to be less variable in those age groups who best infect mosquitoes. Thus, the extent to which variation in gametocyte density is a simple parasite phenotype reflecting the complex within-host parasite dynamics is addressed.
Gametocyte densities and corresponding infectiousness to mosquitoes from published data sets and studies in both rural and urban Cameroon are analysed. The mean and variation in gametocyte density according to age group are considered and compared with transmission success (proportion of mosquitoes infected). Across a wide range of settings endemic for malaria, the age group that infected most mosquitoes had the least variation in gametocyte density, i.e. there was a significant relationship between the variance rather than the mean gametocyte density and age-specific parasite transmission success. In these settings, the acquisition of immunity over time was evident as a decrease in asexual parasite densities with age. By contrast, in an urban setting, there were no such age-structured relationships either with variation in gametocyte density or asexual parasite density.
Gametocyte production is seemingly predicted by evolutionary theory, insofar as a reproductive phenotype (gametocyte density) is most precisely expressed (i.e. is most invariant) in the most infectious human age group. This human age group would thus be expected to be the habitat most suitable for the parasite. Comprehension of the immuno-epidemiology of malaria, a requisite for any vaccine strategies, remains poor. Immunological characterization of the human population stratified by parasite gametocyte allocation would be a step forward in identifying the salient immunological pathways of what makes a human a good habitat.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
In Plasmodium falciparum‐parasitized pregnant women, erythrocytes infected by mature stages of the parasite sequester into placental intervillous spaces. The presence of parasites in the placenta ...causes maternal anaemia and low birth weight of the infant. In‐vitro studies suggest placental sequestration may involve the cytoadherence of infected erythrocytes to chondroitin sulphate A (CSA) and/or intercellular adhesion molecule 1 (ICAM‐1) expressed by human placental syncytiotrophoblast. We identified P. falciparum receptors expressed on the surface of human syncytiotrophoblast using immunofluorescence of placental biopsies from Cameroon, a malaria‐endemic area. In all placentas, a strongly positive staining was observed on the syncytiotrophoblast for CSA, but not for ICAM‐1, vascular endothelium cell adhesion molecule‐1, E‐selectin, nor CD36. The cytoadherence ability of parasites from pregnant women and nonpregnant subjects was assessed on in‐vitro cultured syncytiotrophoblast. Parasites from pregnant women bound to the trophoblast via CSA but not ICAM‐1. Parasites from nonpregnant hosts either did not bind to the trophoblast culture or bound using ICAM‐1. Our data support the idea that placental sequestration may result from cytoadherence to placental trophoblast and that pregnant women are parasitized by parasites that differ from parasites derived from nonpregnant host by their cytoadherence ability.
In the context of generalization of insecticide resistance, the hypothesis that insecticide resistance has a positive impact on the capacity of mosquitoes to transmit malaria constitutes a hindrance ...for malaria elimination. The aim of this study was to investigated populations of Anopheles coluzzii and Anopheles gambiae S molecular form to assess whether different genotypes at the kdr locus are responsible for different susceptibility to Plasmodium falciparum infection.
F3 progeny of An. gambiae s.l. collected in Dielmo were infected by direct membrane feeding with P. falciparum gametocyte-containing blood sampled from volunteer patients. The presence of oocysts was determined by light microscopy after seven days, and the presence of sporozoites by ELISA after 14 days. Mosquito species and molecular forms were identified by PCR. Generalized linear models were performed using the R software to test the effect of explanatory variables including the genotype at the kdr locus on infection rate and density.
The odds of being infected with oocysts and sporozoites were greater in RS and RR groups than in SS groups (χ2 = 42.8, df = 1, P(>χ2) = 6.1e-11). The density of infection was also dependent on genotype, with RR and RS genotypes showing denser infection than SS genotypes. Pairwise comparisons of oocyst number and absorbance indicated sometime a small betwen species (i.e. between An. gambiae S form, and An. coluzzii), but the effect of genotype was much more important.
The presence of the resistance allele at the kdr locus increases susceptibility to Plasmodium not only at the oocyst stage but also at the sporozoite stage in non-genetically modified wild mosquitoes. These results have significant implications and should be taken into account in the development of strategies for malaria control.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Deciphering molecular interactions between the malaria parasite and its mosquito vector is an emerging area of research that will be greatly facilitated by the recent sequencing of the genomes of ...Anopheles gambiae mosquito and of various Plasmodium species. So far, most such studies have focused on Plasmodium berghei, a parasite species that infects rodents and is more amenable to studies. Here, we analysed the expression pattern of nine An.gambiae genes involved in immune surveillance during development of the human malaria parasite P.falciparum in mosquitoes fed on parasite‐containing blood from patients in Cameroon. We found that P.falciparum ingestion triggers a midgut‐associated, as well as a systemic, response in the mosquito, with three genes, NOS, defensin and GNBP, being regulated by ingestion of gametocytes, the infectious stage of the parasite. Surprisingly, we found a different pattern of expression of these genes in the An.gambiae–P.berghei model. Therefore, differences in mosquito reaction against various Plasmodium species may exist, which stresses the need to validate the main conclusions suggested by the P.berghei–An.gambiae model in the P.falciparum–An.gambiae system.
In areas where malaria is endemic, infected individuals generally harbor a mixture of genetically distinct
Plasmodium falciparum parasite populations. For the first time, we studied temporal ...variations of blood parasite densities and circulating genotypes in asymptomatic Senegalese children, at time intervals as short as 4–12
h. Twenty-one Senegalese children, presenting with an asymptomatic
P. falciparum infection, were sampled eight times within three days. Parasite density was assessed by thick blood smears, and all infecting genotypes were quantified by the fragment-analysis method. Parasite densities showed dramatic fluctuations up to a 1 to 1000 ratio, with at least one peak of parasite density. Polyclonal infections were detected in all children, with a multiplicity of infection of 5.2–6.8 genotypes per child. A single sample never reflected the full complexity of the parasite populations hosted by a given individual. Genotypes with different behaviors were detected in all children, some genotypes undergoing major fluctuations, while others were highly stable during the follow-up. A single peripheral blood sampling does not reflect the total parasite load. Repeated sampling is needed to have a more detailed scheme of parasite population dynamics and a better knowledge of the true complexity of an infection.
In areas endemic for malaria, pregnant women frequently present with a placenta that has been parasitized by Plasmodium falciparum, an infection associated with a reduction in the birth weight of the ...offspring. However, the impact of placental infection on malaria-related morbidity during the infant's first years of life has not been investigated. Between 1993 and 1995, 197 children in southern Cameroon were followed weekly clinically and monthly parasltologically. The dates of first positive blood smear and the evolution of the parasite prevalence rates were compared between infants bom to mothers presenting with (n = 42) and without (n = 155) P. falciparum infection of the placenta. Infants born to placenta-infected mothers were more likely to develop a malaria infection between 4 and 6 months of age; then the difference progressively disappeared. Similarly, parasite prevalence rates were higher in placenta-infected infants from 5 to 8 months of age. Thus, malarial infection of the placenta seems to result in a higher susceptibility of infants to the parasite. This was not related to maternally transmitted antibodies, as specific antibody levels were similar in both groups of infants. A better understanding of the involved mechanisms may have important implications for the development of malaria control strategies.
Summary
Quantitatively assessing the impact of naturally occurring transmission‐blocking (TB) immunity on malaria parasite sporogonic development may provide a useful interpretation of the underlying ...mechanisms. Here, we compare the effects of plasma derived from 23 naturally infected gametocyte carriers (OWN) with plasma from donors without previous malaria exposure (AB) on the early sporogonic development of Plasmodium falciparum in Anopheles gambiae. Reduced parasite development efficiency was associated with mosquitoes taking a blood meal mixed with the gametocyte carriers’ own plasma, whereas replacing autologous plasma with non‐immune resulted in the highest level of parasite survival. Seven days after an infective blood meal, 39.1% of the gametocyte carriers’ plasma tested showed TB activity as only a few macrogametocytes ingested along with immune plasma ended up as ookinetes but subsequent development was blocked in the presence of immune plasma. In other experiments (60.9%), the effective number of parasites declined dramatically from one developmental stage to the next, and resulted in an infection rate that was two‐fold lower in OWN than in AB infection group. These findings are in agreement with those in other reports and go further by quantitatively examining at which transition stages TB immunity exerts its action. The transitions from macrogametocytes to gamete/zygote and from gamete/zygote to ookinete were identified as main targets. However, the net contribution of host plasma factors to these interstage parasite reductions was low (5–20%), suggesting that irrespective of the host plasma factors, mosquito factors might also lower the survival level of parasites during the early sporogonic development.
Plasmodium falciparum transmission blocking immunity (TBI) was investigated in 3 different endemic areas. Reared Anopheles gambiae s.s. were experimentally infected with the blood of gametocyte ...carriers, either in the presence of autologous plasma (OWN) or after replacement of the OWN plasma with a nonimmune serum of AB blood group (control). Transmission reduction was defined by a lower level of mosquito infection in the OWN batch compared with the control. After controlling for the effect of gametocytemia, the proportion of "transmission reducers" was lower in the town of Yaounde in Cameroon (UC), (14%, N = 75) than in the two rural areas of South Cameroon (RC) (29%, N = 31) and Sénégal (RS) (44%, N = 32). The contribution of TBI relative to the total inhibition of the parasite development (including human, parasite, and mosquito factors) was higher in RS (49.6%) than in RC (12.6%) and UC (9.5%).
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