In certain filaria-mosquito combinations, the number of infective, third-stage larvae (L(3)) that develop in a mosquito is not proportional to the number of microfilariae (mff) ingested by that mosquito. As the number of mff ingested increases, the yield of L(3) per microfilaria may either increase (in a process known as 'facilitation') or decrease (in a process known as 'limitation'). Each ingested microfilaria that is successful (in terms of reaching the haemocoel) increases (facilitation) or decreases (limitation) the 'permeability' of the stomach wall for the next microfilaria. Limitation is seen in some culicine mosquitoes, especially the Aedes spp. that transmit Wuchereria bancrofti, which, in consequence, become relatively more efficient as vectors as they ingest fewer mff. This phenomenon makes the interruption of filarial transmission by Aedes spp. particularly difficult. As the survival of anopheline mosquitoes is adversely affected by filarial infection, the use of mass drug administrations (MDA) to reduce the prevalence and intensity of microfilaraemias may increase the mean lifespan of some of the local Anopheles species. If these same species also act as vectors of malarial parasites, effective, drug-based control of W. bancrofti may worsen the problem posed by malaria. Therefore, wherever malaria and bancroftian filariasis are co-endemic and caused by parasites transmitted by the same species of mosquito, MDA should be augmented by interventions (use of bednets or house-spraying) against adult Anopheles.
To investigate the quantitative distribution of Plasmodiumfalciparum gametocytes into the vector bloodmeal, Anopheles arabiensis mosquitoes were fed on 3 volunteers naturally infected with gametocytes. The content of each mosquito midgut was smeared on a microscope slide and Giemsa stained. The distribution of gametocytes ingested by mosquitoes followed a negative binomial distribution, with apparently constant overdispersion (parameter k +/- S.E. = 3.105 +/- 0.392) for the 3 series. This aggregation of gametocytes in some midguts probably facilitates the conjunction of gametes and fertilization. This suggests that P. falciparum gametocytes in the peripheral blood flow of infected man do not follow an independent, homogeneous pattern but show a significant aggregation.
Adult sandflies (Diptera: Psychodidae) of the genus Phlebotomus and of the subgenus Larroussius were collected with sticky traps from thirty-four localities in central and south-western Morocco. Four Larroussius species were present in this area, Phlebotomus perniciosus Newstead, being the most widespread and abundant species particularly in the mountainous area, was sympatric with the closely related species Phlebotomus longicuspis Nitzulescu. Only one male of P. perniciosus displayed the bifid copulatory valves characteristic of the species. All specimens showed copulatory valves curved at their apex. The number of coxite hairs is a taxonomical tool that differentiates this morph of P. perniciosus from P. longicuspis. Phlebotomus ariasi Tonnoir was collected from six mountainous stations. Phlebotomus mariae Rioux et al. was scarce; only five males were collected and were often associated with P. ariasi.
Previous experimental gametocyte infections of Anopheles arabiensis on 3 volunteers naturally infected with Plasmodium falciparum were conducted in Senegal. They showed that gametocyte counts in the mosquitoes are, like macroparasite intakes, heterogeneous (overdispersed). They followed a negative binomial distribution, the overdispersion coefficient seeming constant (k = 3.1). To try to explain this heterogeneity, we used an individual-based model (IBM), simulating the behaviour of gametocytes in the human blood circulation and their ingestion by mosquitoes. The hypothesis was that there exists a clustering of the gametocytes in the capillaries. From a series of simulations, in the case of clustering the following results were obtained: (i) the distribution of the gametocytes ingested by the mosquitoes followed a negative binomial, (ii) the k coefficient significantly increased with the density of circulating gametocytes. To validate this model result, 2 more experiments were conducted in Cameroon. Pooled experiments showed a distinct density dependency of the k-values. The simulation results and the experimental results were thus in agreement and suggested that an aggregation process at the microscopic level might produce the density-dependent overdispersion at the macroscopic level. Simulations also suggested that the clustering of gametocytes might facilitate fertilization of gametes.
A follow-up of Loa loa and Mansonella perstans microfilaremia was carried out in an adult population living in a highly endemic area of the Congo. Infection rates and parasite loads were found to be stable in the general population, both in the short-term (two months) and long-term (3-4 years) followup. The microfilarial status of most of the subjects examined did not change between tests. At the individual level, the microfilarial densities of L. loa and M. perstans also remained remarkably constant over time. This results in a qualitative and quantitative stability of the parasitic material available for the vectors.
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