The Human Infectious Reservoir of Falciparum Malaria

Malaria control strategies are often targeted at individuals who suffer most morbidity, not at those driving transmission. This is in part due to our limited understanding, described in Chapter 1, of the human infectious reservoir of falciparum malaria – i.e. individuals responsible for human-to-mosquito transmission of Plasmodium falciparum parasites in endemic areas. This work, whose objectives are listed in Chapter 2, assessed the prevalence of infectiousness in naturally exposed human populations during dry and wet seasons. Exposure to Anopheles mosquitoes, a measure of transmission opportunities, was also quantified; and to determine the value of infectiousness-reducing interventions, the use of primaquine to block transmission from infectious individuals was investigated. Experimental infections of mosquitoes were performed to determine malaria infectivity of randomly selected individuals in two villages in Burkina Faso. Molecular assays were used to quantify parasite, including gametocyte, densities. Less than 10% of the population was infectious to mosquitoes. These results are presented in a manuscript that included data from other study sites (Chapter 3). To assess exposure to malaria vectors, bloodfed mosquitoes were collected indoors in one of the study villages in Burkina Faso. A multiplex PCR assay targeting nine human microsatellites and a gender-specific marker was used to identify the human sources of mosquito blood meals. Although there was substantial variation in the number of mosquito bites each individual received (Chapter 4), on average adults received more mosquito bites than children. This suggests that, despite their lower infectiousness, adults are major contributors to malaria transmission in endemic areas. An efficacy trial of single low dose primaquine was performed in Burkina Faso and pre- and post-treatment infectiousness were quantified by mosquito feeding experiments to assess primaquine’s infectiousness-reducing activity (Chapter 5). Individuals receiving primaquine cleared gametocytes faster than individuals who received artemether-lumefantrine alone. Feeding assays, however, suggest that artemether-lumefantrine blocks most parasite transmission after treatment administration. In Chapter 6, these findings, and how they can inform future control strategies, are discussed

Higher gametocyte production and mosquito infectivity in chronic compared to incident Plasmodium falciparum infections.

Plasmodium falciparum gametocyte kinetics and infectivity may differ between chronic and incident infections. In the current study, we assess parasite kinetics and infectivity to mosquitoes among children (aged 5-10 years) from Burkina Faso with (a) incident infections following parasite clearance (n = 48) and (b) chronic asymptomatic infections (n = 60). In the incident infection cohort, 92% (44/48) of children develop symptoms within 35 days, compared to 23% (14/60) in the chronic cohort. All individuals with chronic infection carried gametocytes or developed them during follow-up, whereas only 35% (17/48) in the incident cohort produce gametocytes before becoming symptomatic and receiving treatment. Parasite multiplication rate (PMR) and the relative abundance of ap2-g and gexp-5 transcripts are positively associated with gametocyte production. Antibody responses are higher and PMR lower in chronic infections. The presence of symptoms and sexual stage immune responses are associated with reductions in gametocyte infectivity to mosquitoes. We observe that most incident infections require treatment before the density of mature gametocytes is sufficient to infect mosquitoes. In contrast, chronic, asymptomatic infections represent a significant source of mosquito infections. Our observations support the notion that malaria transmission reduction may be expedited by enhanced case management, involving both symptom-screening and infection detection

Characterisation of Plasmodium Sexual Stage Antigens as Targets of Transmission Blocking Immunity

A promising strategy to achieve malaria elimination is by interrupting transmission using transmission-blocking vaccines (TBVs). TBVs target the sexual or mosquito stages of the malaria parasite to inhibit parasite development within the mosquito. To date, however, there are only three lead TBV candidate antigens Pfs230, Pfs48/45 and Pfs25 in various stages of clinical development. This project sought to identify novel antigens expressed on gametocytes, gametes and ookinetes with potential as TBV candidates. Furthermore, naturally acquired immune responses (NAI) to gametocytes have been described and have the potential to guide the development and the implementation of TBVs Therefore, this work also sought to improve our understanding of NAI to gametocytes. This was achieved by (1) carrying out a systematic review and meta-analysis of studies investigating NAI to the lead gametocyte-stage TBV candidates, and (2) assessing the changing patterns of gametocyte carriage at the Kenyan coast over time. Key indicators of gametocytaemia and anti-gametocyte immunity were identified and evaluated against a novel panel of gametocyte antigens. These antigens, together with a separate set of gamete and ookinete stage antigens, were identified as potential TBV candidates using a combination of bioinformatic tools and laboratory investigations. Immunoprofiling of the identified gametocyte candidates provided evidence that stable responses can be generated to sexual stage antigens. Moreover, antigens that could serve as serological markers of recent gametocyte exposure, in particular PEB-P (PF3D7_0303900), were also identified. The ability of the sexual stage antigens to induce transmission-blocking immunity was also assessed. Promising candidates identified included PBCPP2 (PBANKA_0719100), a novel conserved and uncharacterised P. berghei protein), and SOAP (PBANKA_1037800). Further characterisation of these antigens may yield new candidates to add to the TBV development pipeline

A field-based modelling framework of the ecohydrology of schistosomiasis

Successful control of schistosomiasis, a water-borne parasitic disease, is challenged by the intricacy of the wormâs lifecycle, which depends on aquatic snail intermediate hosts, and involves environmental, ecologic, and socio-economic factors. Current strategies rely on deworming through mass drug administration which however do not protect against reinfection and the persistence of hotspots. It is recognized that multifaceted approaches will be necessary to reach elimination, whose development will require a renewed focus on the diseaseâs social-ecological drivers. Taking cue from the hydrological underpinning of these drivers, this Thesis aims at developing an ecohydrological approach to schistosomiasis with a view to identifying and exploiting the points in which its cycle can be broken. Schistosomiasis is a poverty-reinforcing disease affecting more than 150 million people in sub-Saharan Africa, being the parasitic disease causing the largest health burden after malaria. However, the impairing morbidity it causes has been undervalued in the past, qualifying it as a neglected tropical disease. Moreover, water resources development often exacerbate transmission, posing scientific and ethical challenges in addressing the ensuing trade-off between economic development and public health. The relevance of this Thesisâ work lies in furthering tools to offset this trade-off by unlocking the predictive appraisal of the social-ecological drivers of transmission. An integration of fieldwork applied in Burkina Faso (West Africa) and theoretical methods are employed to address this aim. This Thesis establishes the use of spatially explicit mathematical models of schistosomiasis at the national-scale, allowing to study the effect of human mobility and spatial heterogeneity of transmission parameters. Weekly ecological samplings of snail abundance and continuous environmental monitoring were preformed at three sites along the countryâs climatic gradient, leveraged through ecological modelling. A novel methodology for the large-scale prediction of river network ephemerality allowed for refined snail species distribution models, and the analysis of the diseaseâs geography in link with socio-economic covariates. Finally, surveys and participatory workshops shed light on local-scale water contact patterns. The obtained results substantiate the stance that hydrology is a first-order control of disease transmission. Stability analysis of the spatially explicit model generated additional insight into the impact of the expansion of suitable snail habitat due to water resources development, highlighting the interplay between local and country-wide effects driven by human mobility. Models of snail ecology revealed key hydrological drivers, and disputed density feedbacks. Uncovered phase shifts between permanent and ephemeral habitats were adequately reproduced at the national scale through model regionalization. Characterization and predictions of hydrological ephemerality improved the estimation of the snailsâ ecological range, mirroring the diseaseâs geography. Finally a national-scale association between ephemerality and disease risk was observed, possibly due to human-water contacts aggregation, as supported by preliminary results at village-level. The future incorporation of these ecohydrological findings into spatially explicit models of schistosomiasis is considered promising for optimizing control strategies and attaining disease elimination

The ecology and behaviour of insecticide resistant malaria vectors and implications for control in Burkina Faso

Long-Lasting Insecticide-Treated Nets (LLINs) and Indoor Residual Spraying (IRS) are the most common and successful methods for malaria vector control in Africa. There is growing evidence of shifts in mosquito vector biting and resting behaviours in several African settings where high LLIN coverage has been achieved. These changes, combined with growing insecticide resistance, may reduce intervention success by decreasing the contact between vectors and insecticide-treated surfaces. While insecticide resistance in malaria vectors has been widely investigated, less is known about the implications of mosquito behavioural changes to malaria control. In recent years, LLIN programmes appear to have a reducing impact in a small number of high burden African countries including Burkina Faso. This reducing effectiveness is hypothesized to be the result of insecticide resistance, but the potential additional contribution of mosquito behavioural avoidance strategies has not yet been investigated in Burkina Faso. The aim of this PhD was to investigate the contribution of insecticide resistance and mosquito behaviours to the persistence of malaria transmission in southwestern Burkina Faso following a national LLIN-distribution campaign. Specific objectives were to (i) evaluate the performance of a new mosquito sampling method, the Mosquito Electrocuting Trap (MET) to measure spatial and temporal variation in human exposure to malaria vectors; and characterize the spatial, seasonal and longer-term trends in (ii) vector ecology and behaviours, (iii) insecticide resistance within Anopheles gambiae sensu lato (s.l.) and (iv) malaria vector survival and transmission potential in rural Burkina Faso. A two-year programme of longitudinal mosquito vector surveillance was initiated within 12 villages of south-western Burkina Faso in 2016, shortly after completion of a mass LLIN distribution. Host seeking malaria vectors were sampled monthly using Human Landing Catches (HLC) and METs conducted inside houses and in the surrounding outdoor area (911 households in total). Resting bucket traps (RBTs) were used to sample indoor and outdoor resting vectors. In an initial study (Chapter 2), I evaluated the performance of the MET relative to the HLC for sampling host-seeking malaria vectors over 15 months in 12 villages. Overall, the MET caught proportionately fewer An. gambiae s.l. than the HLC (mean estimated number of 0.78 versus 1.82 indoors, and 1.05 versus 2.04 outdoors). However provided a consistent representation of vector species composition, seasonal and spatial dynamics, biting behaviour (e.g. location and time) and malaria infection rates relative. The MET slightly underestimated the proportion of bites that could be prevented by LLINs relative to the HLC (5%). However, given the major advantage of the MET of reducing human infection risk during sampling, I conclude these limitations are acceptable and that the MET presents a promising and safer alternative for monitoring human exposure to malaria vectors in outdoor environments. Vector sampling was extended (using HLCs and RBTs) to investigate longer-term temporal changes in vector ecology and behaviour (Chapter 3). Analysis of a subset (20%) of the An. gambiae s.l. (N= 7852) indicated that An. coluzzii (53.82%) and An. gambiae (45.9%) were the main vector species. There was substantial variation in vector abundance between sites and seasons, with a predicted ~23% reduction in An. gambiae s.l. biting density from start to end of study. A higher proportion of outdoor biting (~54%) was detected than expected from previous studies; but there was no evidence of spatial, seasonal or longer-term changes in exophagy. Species level analyses indicated that revealed moderate but statistically significant different in the exophagy and biting time between An. coluzzii and An. gambiae. Combining information on biting times and location (indoors versus outdoors), I estimated that ~85% of exposure could be prevented using good quality and effective LLINs during standard sleeping hours (10 pm – 5 am). Bioassays were conducted on the An. gambiae s.l. population at 9 out of the original 12 study villages to estimate spatial, seasonal and longer-term variation in insecticide resistance (IR) over the study period. Overall, only 23% of An. gambiae s.l. exposed to a diagnostic dose of deltamethrin were killed within 24 hours; indicating that all surveyed populations are resistant. Furthermore, IR increased over the study period, with significant reduction in mortality after exposure to deltamethrin in bioassays. There was no evidence of variation in IR between An. gambiae and An. coluzzii. Finally, the transmission potential of An. gambiae s.l. in this area was investigated through assessment of mosquito parity rates (a proxy of survival), malaria infection rates and estimation of annual Entomological Inoculation Rates (EIR; Chapter 5). The daily survival rate of malaria vectors in this area was > 90%), but with variation between villages and seasons. After controlling for this spatial and seasonal variation, there was evidence of a longer-term increase in vector survival over the study period. In contrast, both mosquito vector biting densities and their malaria infection rates declined over the study period. This resulted in a drop in the predicted EIR from 320 to 105 infective bites per person/year respectively in year 1 and 2. Considering the proportion of exposure estimated to be preventable by effective LLIN use (~85%, Chapter 2 &3), I estimated that residents in this area are still exposed to ~32 infective bites per person per year even when this intervention is used. This confirms that even with 100% coverage and usage of highly effective LLINs, high levels of transmission will persist in this setting. Taking the case of Burkina Faso as an example, results obtained here confirm that both IR and outdoor biting by malaria vectors are contributing to the persistence of transmission in high burden African countries. Consequently, a successful vector control programme in this context need a clear insecticide resistance management plan and supplementary tools that target vectors feeding and resting outdoors

Risk Factors for Soil-transmitted Helminth Infections in Schoolchildren from Rural Communities in Honduras

Background: Honduras is endemic for soil-transmitted helminth (STH) infections. However, knowledge gaps remain in terms of risk factors involved in STH transmission and infection intensity. Objectives: To determine the prevalence and intensity of STH infections in schoolchildren living in rural Honduras. Additionally, to investigate risk factors associated with STH infections. Methods: A cross-sectional study was done among Honduran rural schoolchildren, in 2011. Demographic and epidemiological data were obtained and STH infections were determined using Kato-Katz method. Results: A total of 320 children completed the study. Overall and specific prevalences for Ascaris lumbricoides, Trichuris trichiura and hookworms were 72.5%, 30%, 67% and 16%, respectively. Several risk factors associated with STH transmission and infection intensity were identified at the individual and familial level as well as at the schools. Conclusions: Improving hygienic conditions and providing semi-annual deworming treatment are feasible interventions that could enhance undergoing STH control activities

Intestinal and urinary schistosomiasis dynamics in sub-Saharan Africa

Schistosomiasis is a chronic infection by a digean trematode of the genus Schistosoma. More than 207 million people are infected with this parasite, of which 120 million are symptomatic. There are two main species infecting humans in sub-Saharan Africa: Schistosoma haematobium and S. mansoni, both occur in areas with similar socio-economic and environmental conditions and often have matching distribution patterns. The principle aims of the research presented in this thesis were to further our understanding of schistosome population genetics, associated human host morbidity and chemotherapeutic treatment of schistosomes in relation to mixed species infections. Structured sampling of parasites and/or host traits from school-aged children at baseline and post Mass Drug Administration (MDA) in Niger and Kenya were performed. The results presented provided evidence for S. haematobium - S. mansoni interactions and their impact on the human host and on the parasite population. In Kenya coinfections had lower S. haematobium related morbidity relative to single S. haematobium infections pre and post MDA. Additionally parasite infra-populations from coinfected children had higher genetic diversity levels compared to single infected children in mixed infection foci. In Niger, an impact of MDA on the population genetics of S. mansoni was detected in one mixed infection village, characterised as a noticeable bottleneck effect, but not in the other. There was no apparent impact of MDA on the population genetics of S. haematobium. Conversely, in Kenya, a significant impact of MDA on both species was detected, with a bottleneck effect occurring on the S. haematobium population and conversely, an increase in genetic diversity in the S. mansoni population. The results of this thesis are discussed in terms of their implications on schistosome epidemiology and evolution, and in relation to the control of schistosomiasis in sub-Saharan Africa