Reproductive success in Anopheles arabiensis and the M and S molecular forms of Anopheles gambiae: do natural sporozoite infection and body size matter?

Malaria parasites stages prior to sporozoite formation are known to affect the fecundity of several species of mosquitoes in the laboratory, but little is known about this phenomenon in natural conditions especially with sporozoite-infected anophelines. The reproductive success of wild-caught Anopheles arabiensis and the M and S molecular forms of Anopheles gambiae was investigated by comparing females infected with Plasmodium falciparum sporozoites to females free of sporozoites. Association between sporozoite-infected females' body size and their egg batch size was also measured. There was no significant reduction in egg production due to sporozoite infection among wild females An. arabiensis and the M and S form of An. gambiae. The infected groups and the controls laid similar numbers of eggs. A positive association was found between body size of females infected with P. falciparum and mean egg production. Infected females of the molecular forms of An. gambiae and their sibling species An. arabiensis invest similarly in egg batch size regardless of their body size although the expected egg batch size may differ among them because of differences in their mean body size. A reduction of egg production related to infection status was not observed among females harboring sporozoites. Therefore for the gonotrophic cycles that occur once sporozoites are present, natural infection of all three vectors we studied has no or minimal effect on their densities or their reproductive outputs.

Low infectivity of Plasmodium falciparum gametocytes to Anopheles gambiae following treatment with sulfadoxine-pyrimethamine in Mali.

Sulfadoxine-pyrimethamine (SP) treatment increases the rate of gametocyte carriage and selects SP resistance-conferring mutations in Plasmodium falciparum dihydrofolate reductase (DHFR) and dihydropteroate synthase (DHPS), raising concerns of increased malaria transmission and spread of drug resistance. In a setting in Mali where SP was highly efficacious, we measured the prevalence of DHFR and DHPS mutations in P. falciparum infections with microscopy-detected gametocytes following SP treatment, and used direct feeding to assess infectivity to Anopheles gambiae sensu lato. Children and young adults presenting with uncomplicated malaria were treated with SP or chloroquine and followed for 28 days. Gametocyte carriage peaked at 67% 1 week after treatment with a single dose of SP. Those post-SP gametocytes carried significantly more DHFR and DHPS mutations than pre-treatment asexual parasites from the same population. Only 0.5% of 1728 mosquitoes fed on SP-treated gametocyte carriers developed oocysts, while 11% of 198 mosquitoes fed on chloroquine-treated gametocyte carriers were positive for oocysts. This study shows that in an area of high SP efficacy, although SP treatment sharply increased gametocyte carriage, the infectiousness of these gametocytes to the vector may be very low. Accurate and robust methods for measuring infectivity are needed to guide malaria control interventions that affect transmission.

Evaluation and optimization of membrane feeding compared to direct feeding as an assay for infectivity.

BACKGROUND: Malaria parasite infectivity to mosquitoes has been measured in a variety of ways and setting, includind direct feeds of and/or membrane feeding blood collected from randomly selected or gametocytemic volunteers. Anopheles gambiae s.l is the main vector responsible of Plasmodium falciparum transmission in Bancoumana and represents about 90% of the laboratory findings, whereas Plasmodium malariae and Plasmodium ovale together represent only 10%. MATERIALS AND METHODS: Between August 1996 and December 1998, direct and membrane feeding methods were compared for the infectivity of children and adolescent gametocyte carriers to anopheline mosquitoes in the village of Bancoumana in Mali. Gametocyte carriers were recruited twice a month through a screening of members of 30 families using Giemsa-stained thick blood smears. F1 generation mosquitoes issued from individual female wild mosquitoes from Bancoumana were reared in a controlled insectary conditions and fed 5% sugar solution in the laboratory in Bamako, until the feeding day when they are starved 12 hours before the feeding experiment. These F1 generation mosquitoes were divided in two groups, one group fed directly on gametocyte carriers and the other fed using membrane feeding method. RESULTS: Results from 372 Plasmodium falciparum gametocyte carriers showed that children aged 4-9 years were more infectious than adolescents (p = 0.039), especially during the rainy season. Data from 35 carriers showed that mosquitoes which were used for direct feeding were about 1.5 times more likely to feed (p < 0.001) and two times more likely to become infected, if they fed (p < 0.001), than were those which were used for membrane feeding. Overall, infectivity was about three-times higher for direct feeding than for membrane feeding (p < 0.001). CONCLUSION: Although intensity of infectivity was lower for membrane feeding, it could be a surrogate to direct feeding for evaluating transmission-blocking activity of candidate malaria vaccines. An optimization of the method for future trials would involve using about three-times more mosquitoes than would be used for direct feeding.

Natural malaria infection in Anopheles gambiae is regulated by a single genomic control region.

We surveyed an Anopheles gambiae population in a West African malaria transmission zone for naturally occurring genetic loci that control mosquito infection with the human malaria parasite, Plasmodium falciparum. The strongest Plasmodium resistance loci cluster in a small region of chromosome 2L and each locus explains at least 89% of parasite-free mosquitoes in independent pedigrees. Together, the clustered loci form a genomic Plasmodium-resistance island that explains most of the genetic variation for malaria parasite infection of mosquitoes in nature. Among the candidate genes in this chromosome region, RNA interference knockdown assays confirm a role in Plasmodium resistance for Anopheles Plasmodium-responsive leucine-rich repeat 1 (APL1), encoding a leucine-rich repeat protein that is similar to molecules involved in natural pathogen resistance mechanisms in plants and mammals.

A simulation model of African Anopheles ecology and population dynamics for the analysis of malaria transmission.

BACKGROUND: Malaria is one of the oldest and deadliest infectious diseases in humans. Many mathematical models of malaria have been developed during the past century, and applied to potential interventions. However, malaria remains uncontrolled and is increasing in many areas, as are vector and parasite resistance to insecticides and drugs. METHODS: This study presents a simulation model of African malaria vectors. This individual-based model incorporates current knowledge of the mechanisms underlying Anopheles population dynamics and their relations to the environment. One of its main strengths is that it is based on both biological and environmental variables. RESULTS: The model made it possible to structure existing knowledge, assembled in a comprehensive review of the literature, and also pointed out important aspects of basic Anopheles biology about which knowledge is lacking. One simulation showed several patterns similar to those seen in the field, and made it possible to examine different analyses and hypotheses for these patterns; sensitivity analyses on temperature, moisture, predation and preliminary investigations of nutrient competition were also conducted. CONCLUSIONS: Although based on some mathematical formulae and parameters, this new tool has been developed in order to be as explicit as possible, transparent in use, close to reality and amenable to direct use by field workers. It allows a better understanding of the mechanisms underlying Anopheles population dynamics in general and also a better understanding of the dynamics in specific local geographic environments. It points out many important areas for new investigations that will be critical to effective, efficient, sustainable interventions.