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Subject: search.filters.subject.Anopheles

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Now showing 1 - 10 of 14
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    The effects of low temperatures on metabolic rates in Anopheles quadriannulatus
    (2018) Ngwenya, Duduzile K
    The effects of low temperatures on the metabolic rates of Anopheles quadriannulatus were investigated by measuring the rate of carbon dioxide emission (VCO2) at different temperatures using a closed respirometry system. In general, the metabolic rates of A. quardiannulatus decreased with decreasing ambient temperatures. However, the decreases were not uniform over the temperature range of 30 °C and 10 °C. Mean VCO2 measured at 30 °C and sometimes at 25 °C, were statistically different from those measured at other temperatures. Surprisingly, at 15 °C and 10 °C, the mean VCO2 were statistically similar. It was found that metabolic rates of the mosquitoes were affected differently if a 5 °C reduction in temperature occurred at 30 °C and when it occurred at 15 °C. Thus, a 5 °C drop in temperature at 30 °C and 15 °C did not yield similar percentage decrease in metabolic rates. Age was found not to impact on the metabolism of mosquitoes except at 10 °C and 15 °C in blood fed mosquitoes (both mated and unmated). Lack of a correlation between age and metabolic rates in these mosquitoes was a result of constant body masses as mosquitoes aged. Mean VCO2 of four days old mosquitoes in different mating and feeding states were found to be significantly differently only at 10 °C. Mated and blood fed mosquitoes from this age group retained constant metabolic rates despite changes in temperature. The finding that A. quadriannulatus is not capable of significantly dropping its metabolic rates at low temperatures (15 °C-10 °C) is an indication that the species most probably lacks a physiological overwintering mechanism. Consequently, its survival and longevity is highly compromised during winter. Thus, it can be concluded that if adult A. quadriannulatus can overwinter in cooler parts of sub-Saharan regions, it does so using non-physiological mechanisms. Otherwise, this species and related members of Anopheles gambiae overwinters in developmental stages other than adulthood. In this way, these mosquitoes are able perpetuate their populations post winter season.
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    Evaluating alternative insecticides for use in malaria vector control
    (2017) Samuel, Michael
    Malaria is a vector-borne disease responsible for morbidity and mortality on a global scale, with particular severity in sub-Saharan Africa. Interventions targeting Anopheles malaria vectors rely heavily on synthetic insecticides. This may select for insecticide resistance in the target population. Exacerbating the resistance problem is the excessive use of the pyrethroid class of insecticide and the limited number of alternatives available for public health use. This raises an urgent need for new alternatives, such as naturally occurring biochemicals, to be investigated. In this study, the larvicidal capabilities of the food spice, black pepper (Piper nigrum), and its principle alkaloid, piperine when administered as a food source to anopheline malaria vector species were evaluated. Additionally, a straight-chain fatty acid mixture patented as “C8910” was also investigated as a larvicide and adulticide against several Anopheles species, with the adulticidal efficacy of C8910 being specifically evaluated against An. arabiensis and a single strain of An. funestus derived from a wild population. The results indicate that black pepper is an effective larvicide when administered as a food source, whereas piperine is not. This suggests that piperine is not the primary larvicidal constituent in black pepper. C8910 demonstrated larvicidal properties, inducing mortality in both An. arabiensis and An. funestus. C8910 was also adulticidal against An. arabiensis and a wild sample of An. funestus. It is concluded that, pending further development and assessment of compliance with WHO standards, black pepper and C8910 could be considered as potential tools for future use in vector control.
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    Influence of environmental characteristics on the habitat of and behavioural interactions between anopheles species in South Africa
    (2016) Davies, Craig
    This project explored the ecological conditions of aquatic breeding sites of Anopheles gambiae sensu lato immatures in the Lowveld region of eastern Mpumalanga Province, South Africa. The aim was to determine the environmental conditions influencing anopheline abundance as well as abiotic parameters which associated with vector productivity. In addition to this, the levels of insecticide resistance to the three dominant compounds used in vector control in the region were assessed. Taking into account the sympatric occurrence of the major malaria vector in South Africa (An. arabiensis Patton) and its sibling, non-vector species (An. quadriannulatus Theobald), a laboratory study was devised which investigated the outcome of intra- and inter-specific competition under constant and fluctuating temperature regimes. There was a heterogenous distribution of anophelines across aquatic habitats in Mpumalanga with small-scale variation in salinity and Total Dissolved Solids (TDS) influencing species composition and Anopheles arabiensis was found in all sites surveyed with low numbers occurring where salinity levels were elevated. Anopheles merus associated with high salinity and TDS (Pearson’s Product Moment, r = 0.922, p < 0.05) whilst An. quadriannulatus dominated in breeding sites within 50m of a building or road. Anopheles gambiae complex members were susceptible to the insecticides tested with possible resistance (97%) to DDT in An. merus. Under laboratory conditions, temperature and competitive scenarios affected the life-history traits of both species studied here. The treatment 18 - 35°C generally reduced survivorship except for An. arabiensis in mixed, larval species treatments where it was similar to values reported for 25°C. Survivorship of both species at 20 - 30°C was not significantly impacted and the adult production was high across species treatments. The development rates at 25°C and 20 - 30°C were significantly different between species when reared alone and in mixed species treatments from larvae and from eggs. The effect of temperature was more pronounced at 18 - 35°C with An. arabiensis developing faster under both competitive scenarios and An. quadriannulatus slower, notably when in the presence of its competitor (p < 0.05). In the field component of this study, Anopheles arabiensis exploited all the habitats surveyed. It is therefore recommended that larval control operations should include all available breeding sites, focusing efforts during the dry season when these sites are limited and discreet within the landscape. In the laboratory component, it was possible to test whether or not community composition of anophelines at the adult stage was regulated by different temperature and competitive conditions at the larval stage to better understand the ecological conditions that determine anopheline composition and relative abundance. Taken together, the results of each component emphasize the need for local scale studies, especially under conditions of changing temperatures and rainfall patterns. The results of responses to temperatures and biotic interactions are necessary data for use in models predicting the impact of climate change on malaria vector mosquitoes.
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    Bionomic and genetic characterization of Anopheles gambiae from Ghana
    (2015) Kaiser, Maria
    Abstract Malaria vector control relies principally on the use of insecticides. However, the increasing incidence of insecticide resistance threatens to undermine the effectiveness of this approach, necessitating resistance management strategies. Multiple insecticide resistance is becoming common in West Africa including Ghana. In particular, a population of the major malaria vector Anopheles gambiae, from the Ahafo region of Ghana, shows resistance to all classes of insecticide currently available for use in public health. Insecticide resistance is a primary adaptive characteristic of epidemiological importance although other adaptive traits such as staggered larval time-to-hatch may also be important. Typically, An. gambiae oviposits in small, temporary, sun-lit water bodies and eggs generally hatch 2-3 days post-oviposition. However, staggered distribution of hatching has previously been shown and was also observed in a newly colonized strain (GAH) from Ahafo. The broad aims of this project were therefore to assess and characterize multiple insecticide resistance in An. gambiae from Ahafo as well as to quantify and describe staggered time-to-hatch in a laboratory colony of this population in terms of its adaptive significance and pleiotropic effects on resistance.
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    Insecticide resistance charaterization, quantification, and transferal between life stages of the malaria vector Anopheles funestus giles (Diptera: Culicidae)
    (2015-04-22) Wood, Oliver Richard
    Southern African pyrethroid resistant and insecticide susceptible laboratory colonies of the malaria vector Anopheles funestus were investigated to further understand the phenotypic expression of pyrethroid resistance and to establish at which life stage resistance was selected. Pyrethroid resistance levels of larvae and adults were assessed at the larval and adult life stages using WHO larval and CDC bottle bioassays. Subsequent resistance levels were then assessed following targeted selections at each life stage. Tests for an association between cuticle thickness and pyrethroid resistance were based on cuticle thickness measurements using scanning electron microscope imaging of prepared tissue sections. It is concluded that pyrethroid resistance in southern African An. funestus is only expressed in the adult life stage, and that selection for this phenotype can only be achieved by exposing adults. It also concluded that pyrethroid tolerant or resistant females are likely to have thicker cuticles than less tolerant or susceptible females.
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    Fitness assessments of Anopheles arabienesis laboratory colonies from Southern Africa and their suitability for the sterile insect technique
    (2015-04-13) Essop, Leyya
    In order to employ the Sterile Insect Technique (SIT), biologically fit mosquitoes able to compete with their wild counterparts, suitable field sites for mass release of sterilized male mosquitoes, and appropriate methods of rearing genetic sex-separation (GSS) mosquito strains are required. The life history traits and biological fitness of four laboratory-reared, southern African Anopheles arabiensis strains were investigated. Despite being colonized at different times, the four strains demonstrated comparable levels of biological fitness. Three sites in the Kruger National Park were assessed as possible SIT field sites. Mosquito collections were conducted at each site during three summer months. Anopheles arabiensis was predominant at Malahlapanga during each collection period, establishing Malahlapanga as the most appropriate site for SIT field trials. A standard larval diet was shown to be appropriate for rearing An. arabiensis GSS. This work formed the laboratory basis for the evaluation of a SIT strategy for South Africa.
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    Impact of changed feeding behaviour of An. funestus on malaria transmission in southern Tanzania
    (2012) Azizi, Salum
    In Tanzania both Anopheles funestus and An. gambiae s.l. play a role as major vectors of malaria. Different species exist in the An. funestus group and the An. gambiae complex and play different roles in disease transmission. For malaria vector control programmes knowledge of vector species and their behaviour is key. A recent report on increased exophagy of An. funestus in southern rural Tanzania as a response to increased use of insecticide treated bed nets raised the question of whether there was misidentification of species and/or behavioural insecticide resistance playing a part. The present study used molecular tools to identify the species and determine human biting rates indoors and outdoors along with development and field evaluation of a new tool for sampling malaria vectors which is more effective than human landing catches. The results showed that the majority (96.2%) of the An. funestus group that were collected were An. funestus s.s. by PCR assay. Also, the exophagic proportion (45.9%) of An. funestus was lower than the endophagic proportion (54.1%), similar to other places in Africa, although in this study the difference was insignificant when untreated bed nets and treated bed nets were used. In addition, there was significant outdoor biting activity early in the evening that could lead to the malaria transmission cycle being unaffected by ITNs. The new trap, “Sticky Bucket Trap”, caught considerably fewer mosquitoes (14.2%) than that caught by human landing catches (85.8%), with statistical significance of P>0.05. These results imply that the sticky bucket trap is not a suitable substitute for human landing catches and some modifications are needed to make it more effective. Whereas indoor and outdoor proportions insignificant difference in feeding preference imply that both indoor and outdoor interventions should be used to control this vector.
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    Detoxification enzymes associated with insecticide resistance and exposure to entomopathogenic fungi in Anopheles arabiensis
    (2014-02-06) Nardini, Luisa
    Anopheles arabiensis is one of the major African malaria vectors, and DDT and pyrethroid resistance in this species is widespread. The aim of this study was to investigate, in detail, what detoxification enzymes are associated with insecticide resistance using the An. gambiae “detox chip”, a small-scale microarray based on genes that are putatively involved in metabolic detoxification of insecticides. The first part of the study focused on two DDT and pyrethroid resistant laboratory strains of An. arabiensis – one that originated from Sudan, and a second that originated from South Africa. One P450 was over-transcribed in the Sudanese strain, while 20 genes were over-transcribed in the South African strain. The majority of these were P450s although GSTs and redox genes were also over-transcribed. The use of synergist assays indicated that DDT and permethrin resistance were related to the presence of a kdr mutation (determined by PCR), while deltamethrin resistance was based on insecticide metabolism. In order to evaluate the role of enzymatic detoxification in permethrin resistance, a permethrin selected strain was used. No kdr mutations were present in this strain. Here, 29 genes were over-transcribed. Most of these were CYP genes (55%), followed by redox genes (21%), and GSTs (14%). A certain degree of overlap in the gene over-transcription was observed between the deltamethrin and permethrin resistant phenotypes. These genes are potentially functional against both pyrethroids, while those that differed were possibly more substrate specific. The final part of the study aimed to assess whether genes that are associated with insecticide resistance are also induced in mosquitoes infected with the entomopathogen, Beauveria bassiana. Using microarray data, a subset of important insecticide resistance genes was chosen for analysis following fungal infection. This study was based on the use of qPCR to detect changes in expression. None of the genes that were investigated were overtranscribed suggesting that virulence factors, such as toxins, produced by B. bassiana may not be inhibited by genes that are already over-expressed in insecticide resistant mosquito populations. This is promising for biological control and suggests that the fungi are viable alternatives to insecticides.