134 research outputs found
Maternal environment shapes the life history and susceptibility to malaria of Anopheles gambiae mosquitoes.
No full textBACKGROUND: It is becoming generally recognized that an individual's phenotype can be shaped not only by its own genotype and environmental experience, but also by its mother's environment and condition. Maternal environmental factors can influence mosquitoes' population dynamics and susceptibility to malaria, and therefore directly and indirectly the epidemiology of malaria. METHODS: In a full factorial experiment, the effects of two environmental stressors - food availability and infection with the microsporidian parasite Vavraia culicis - of female mosquitoes (Anopheles gambiae sensu stricto) on their offspring's development, survival and susceptibility to malaria were studied. RESULTS: The offspring of A. gambiae s.s. mothers infected with V. culicis developed into adults more slowly than those of uninfected mothers. This effect was exacerbated when mothers were reared on low food. Maternal food availability had no effect on the survival of their offspring up to emergence, and microsporidian infection decreased survival only slightly. Low food availability for mothers increased and V. culicis-infection of mothers decreased the likelihood that the offspring fed on malaria-infected blood harboured malaria parasites (but neither maternal treatment influenced their survival up to dissection). CONCLUSIONS: Resource availability and infection with V. culicis of A. gambiae s.s. mosquitoes not only acted as direct environmental stimuli for changes in the success of one generation, but could also lead to maternal effects. Maternal V. culicis infection could make offspring more resistant and less likely to transmit malaria, thus enhancing the efficacy of the microsporidian for the biological control of malaria
Resisting infection by Plasmodium berghei increases the sensitivity of the malaria vector Anopheles gambiae to DDT
No full textBACKGROUND
The evolution of insecticide resistance threatens current malaria control methods, which rely heavily on chemical insecticides. The magnitude of the threat will be determined by the phenotypic expression of resistance in those mosquitoes that can transmit malaria. These differ from the majority of the mosquito population in two main ways; they carry sporozoites (the infectious stage of the Plasmodium parasite) and they are relatively old, as they need to survive the development period of the malaria parasite. This study examines the effects of infection by Plasmodium berghei and of mosquito age on the sensitivity to DDT in a DDT-resistant strain of Anopheles gambiae.
METHODS
DDT-resistant Anopheles gambiae (ZANU) mosquitoes received a blood meal from either a mouse infected with Plasmodium berghei or an uninfected mouse. 10 and 19 days post blood meal the mosquitoes were exposed to 2%, 1% or 0% DDT using WHO test kits. 24 hrs after exposure, mortality and Plasmodium infection status of the mosquitoes were recorded.
RESULTS
Sensitivity to DDT increased with the mosquitoes' age and was higher in mosquitoes that had fed on Plasmodium-infected mice than in those that had not been exposed to the parasite. The latter effect was mainly due to the high sensitivity of mosquitoes that had fed on an infected mouse but were not themselves infected, while the sensitivity to DDT was only slightly higher in mosquitoes infected by Plasmodium than in those that had fed on an uninfected mouse.
CONCLUSIONS
The observed pattern indicates a cost of parasite-resistance. It suggests that, in addition to the detrimental effect of insecticide-resistance on control, the continued use of insecticides in a population of insecticide-resistant mosquitoes could select mosquitoes to be more susceptible to Plasmodium infection, thus further decreasing the efficacy of the control
Evaluating insecticide-resistance in the malaria vector Anopheles gambiae and its implications for malaria transmission
No full textInsecticide resistance, in the mosquito vector, threatens the efficacy of current methods to control malaria. Yet evidence of control failure due to insecticide resistance is sparse, despite over 50 years since resistance was identified in the mosquito. In this thesis, laboratory experiments with mosquitoes, as well as mathematical modelling, are used to improve our understanding of how insecticide resistance might impact malaria transmission.
Firstly, demographic and environmental effects on the phenotypic expression of resistance are investigated. Decreasing expression of resistance with age and malaria infection, suggest resistance may not be as large a problem as once believed. Further factors that affect the phenotypic expression of resistance, such as infection by the microsporidian Vavria culicis and quantity of larval food, suggest that the phenotypic expression of resistance may even be manipulated to reduce its impact on disease transmission.
Secondly, costs of resistance are explored as they may reduce the ability of a mosquito to transmit malaria. It is demonstrated that, under environmental stress from parasites, costs to longevity can be increased. Mosquito longevity is a key parameter in malaria transmission so any reduction in longevity, due to costs of resistance, will reduce the ability of the mosquito to transmit malaria. Finally, the thesis examines if the behavioural avoidance of insecticides can be changed through environmental manipulation.
In summary, the phenotypic expression of resistance and the costs of resistance are two factors that will determine the threat insecticide-resistance poses to malaria control. It is demonstrated, in the laboratory, that these two factors can vary due to environmental and demographic factors, but to fully understand the threat of resistance these ideas have to be investigated in the field.Open Acces
Anthropogenic impacts on Aedes mosquito community dynamics in Côte d'Ivoire
No full textBackground: The recent emergence, re-emergence and spread of arboviral diseases (e.g. yellow fever, dengue, chikungunya and Zika) that are transmitted by Aedes mosquitoes have raised concerns worldwide, and especially in tropical and subtropical regions of the world. In the past several years, Côte d’Ivoire has experienced sporadic, single and dual outbreaks of yellow fever and dengue in the southeastern part of the country, partially explained by high rates of urbanization and intensified agriculture. However, the impacts of these anthropogenic changes (urbanization and transformation of rainforests to vast agricultural areas) on the ecology of Aedes arbovirus vectors remain unexplored. Understanding of the impacts of these anthropogenic factors on the ecology of Aedes mosquitoes is crucial for predicting and preventing arboviral outbreaks, and developing, optimizing and evaluating existing and novel vector control measures and tools aimed at reducing disease incidence.
Objectives: This PhD thesis aimed to assess the anthropogenic impacts, including effects of urbanization and agricultural land use changes, on Aedes mosquito community dynamics in yellow fever and dengue foci in southeastern Côte d’Ivoire. The thesis specifically sought to: (i) explore the oviposition ecology of Aedes mosquitoes and Aedes aegypti dynamics in variously urbanized settings; (ii) assess the larval ecology of Aedes alongside a rural-to-urban gradient; and (iii) evaluate the ecology of Aedes mosquitoes along an anthropogenic disturbance gradient in oil palm-dominated landscapes.
Research partnerships: This PhD thesis was carried out within the frame of an existing and productive partnership between the Swiss Tropical and Public Health Institute (Swiss TPH) and the University of Basel, Basel, Switzerland, the Centre Suisse de Recherches Scientifiques en Côte d’Ivoire (CSRS) and the Université Félix Houphouët-Boigny (UFHB), Abidjan, Côte d’Ivoire and the Liverpool School of Tropical Medicine (LSTM), Liverpool, United Kingdom. The work was funded by Swiss TPH, CSRS and a Scholarship for Foreign Students program (FCS), Bern, Switzerland.
Methods: Aedes mosquito eggs, larvae, pupae and adults were collected along a gradient of urbanization (rural, suburban and urban) and different agricultural land uses, including an oil palm-dominated landscape (rainforest, polyculture, oil palm monoculture and rural housing area). Ovitraps were employed, alongside larval surveys and human-baited double-net trap methods from January 2013 to December 2014. Aedes immatures were reared in the laboratory until adult stage emerged for subsequent species identification. Socio-ecological data were conjointly sampled.
Results: Aedes mosquito ecology significantly varied from rural-to-urban areas and among human-disturbed ecosystems in oil palm-dominated landscapes. A total of 51,439 specimens of Aedes mosquitoes belonging to 20 species (Ae. aegypti, Ae. africanus, Ae. albopictus, Ae. angustus, Ae. apicoargenteus, Ae. argenteopunctatus, Ae. dendrophilus, Ae. fraseri, Ae. furcifer, Ae. haworthi, Ae. lilii, Ae. longipalpis, Ae. luteocephalus, Ae. metallicus, Ae. opok, Ae. palpalis, Ae. stokesi, Ae. unilineatus, Ae. usambara and Ae. vittatus) in rural, suburban and urban areas. The highest Aedes species richness (18 species) was observed in rural areas. A considerably lower Aedes species richness was noted in suburban (seven species) and urban areas (three species). Conversely, the highest Aedes abundance was found in urban (n = 26,072 specimens), followed by suburban (n = 16,787 specimens) and rural (n = 8,580 specimens). Ae. aegypti was the predominant species in all three types of study areas, with the highest abundance in urban areas (n = 26,072; 99.4%).
Aedes mosquito breeding site positivity rate was higher in urban (2,136/3,374; 63.3%), followed by suburban (1,428/3,069; 46.5%) and rural (738/2,423; 30.5%) areas. Rural areas exhibited a larger array of Aedes breeding sites ranging from natural containers (tree holes, coconuts, etc.) to traditional containers (clay pots, calabashes, etc.), and industrial containers (cans, tires, etc.) that hosted several wild species. In contrast, the highest proportions of artificial breeding sites (cans, tires, vehicle bodies, building tools and water storage containers) were found in urban areas where human activities (water storage practices, tire selling and environment management) were favourable to the creation of the breeding sites and proliferation of Aedes mosquitoes, mainly Ae. aegypti. The predatory larvae of Eretmapodites, Toxorhynchites and Culex tigripes were frequently found associated with the larvae of Aedes mosquitoes in rural areas. In all areas, the diversity and proportion of Aedes breeding sites, specimens and species were higher in the peridomestic zones and during the rainy seasons.
Aedes mosquito diversity and distribution were strongly associated with agricultural land-use changes. For example, no Aedes were found in oil palm monocultures. Conversely, the highest Aedes species richness (11 species) was observed in the rainforests, while the highest Aedes abundance (n = 28,276; 60.9%) was found in the polycultures. Aedes females displayed higher anthropophagy tendency in the polycultures (21.5 females/person/day) and the rural housing areas (4.5 females/person/day), and poor anthropophagy (0.6 females/person/day) in the rainforest. Aedes females’ host-seeking activities showed bimodal feeding cycles, with interruption from 11:00 to 14:00 hours in the rural housing areas, and a continuous pattern in the polycultures.
Conclusions: The findings revealed that anthropogenic changes influence significantly the ecology of Aedes mosquitoes by shaping the breeding sites, and altering the species diversity and abundance towards a predominance of Ae. aegypti in urban areas, lack of species in oil palm monocultures, high prevalence of species in polycultures and restriction of wild species in rural areas and rainforests that may serve as bridge vectors. Such Aedes species segregation thus suggests a coexistence of several arbovirus transmission cycles: enzootic, epizootic and epidemic. Moreover, the identification of new Aedes species in rural and forested areas supports the existence of still unidentified enzootic sylvatic transmission cycles of arboviruses. The high abundance of natural breeding sites (e.g. tree holes) of Aedes mosquitoes in the rainforests and rural areas can strongly limit the effectiveness of the removals of discarded containers, and calls for integrated vector management strategies. The evidence generated by this PhD work provides an important contribution to the comprehension of the emergence of arboviral diseases (yellow fever and dengue), Aedes vector surveillance and control in the contexts of urbanization and transformation of rainforests into large industrial oil palm monocultures
Evolutionary ecology and adaptiveness of immune defenses of the social insect Bombus terrestris (L.) (Hymenoptera: apidae)
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Effects of shortened host life span on the evolution of parasite life history and virulence in a microbial host-parasite system
No full textAbstract Background Ecological factors play an important role in the evolution of parasite exploitation strategies. A common prediction is that, as shorter host life span reduces future opportunities of transmission, parasites compensate with an evolutionary shift towards earlier transmission. They may grow more rapidly within the host, have a shorter latency time and, consequently, be more virulent. Thus, increased extrinsic (i.e., not caused by the parasite) host mortality leads to the evolution of more virulent parasites. To test these predictions, we performed a serial transfer experiment, using the protozoan Paramecium caudatum and its bacterial parasite Holospora undulata. We simulated variation in host life span by killing hosts after 11 (early killing) or 14 (late killing) days post inoculation; after killing, parasite transmission stages were collected and used for a new infection cycle. Results After 13 cycles (≈ 300 generations), parasites from the early-killing treatment were less infectious, but had shorter latency time and higher virulence than those from the late-killing treatment. Overall, shorter latency time was associated with higher parasite loads and thus presumably with more rapid within-host replication. Conclusion The analysis of the means of the two treatments is thus consistent with theory, and suggests that evolution is constrained by trade-offs between virulence, transmission and within-host growth. In contrast, we found little evidence for such trade-offs across parasite selection lines within treatments; thus, to some extent, these traits may evolve independently. This study illustrates how environmental variation (experienced by the host) can lead to the evolution of distinct parasite strategies.</p
Experimental evolution of specialization by a microsporidian parasite
No full textAbstract Background Evolutionary theory predicts that the pressure for parasites to specialize on one host or to become generalists on a wide range of hosts is driven by the diversity or temporal variability of the host's population and by genetic trade-offs in the adaptation to different hosts. We give experimental evidence for this idea by letting the parasite Brachiola algerae evolve on one of four genetically homogeneous lines of the mosquito Aedes aegypti, on a mixture of the four lines or on an alternating sequence of the four lines. The first regime was expected to lead to specialists, the other two to generalists. After 13 generations, we tested the evolved parasites on each of the four lines of the mosquito. Results The specialized parasites were most infective on their own isofemale line and least infective on other isofemale lines, while the generalist parasites had intermediate infection success on all lines. The success of a specialist on its matched mosquito line was negatively correlated with its success on other lines, suggesting an evolutionary cost to specialization. This trade-off was corroborated by the observation that the generalists had higher average mean infectivity than the specialists over all isofemale lines. Conclusions Overall, our experiment reveals the potential for specialization of a parasite to individual genotypes of its host and provides experimental evidence of the cost associated with the evolution of specialization, an important feature for understanding the coevolutionary dynamics between hosts and parasites.</p
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