Dissertations / Theses on the topic 'Mosquito population dynamics'

Malaria remains one of the world’s most devastating vector-borne parasitic diseases and existing control tools may not be enough to meet the challenge of eliminating malaria in areas of high transmission. Understanding the population dynamics of Plasmodium within the mosquito vector is essential for developing, optimising, and evaluating novel control measures aimed at reducing transmission by targeting this important interface. Malaria research and mathematical models of transmission classically assume that the processes involved in the progression and development of the Plasmodium parasite within Anopheles mosquitoes are independent of parasite density. The research presented in this thesis challenges this assumption, investigating the impact of parasite density on population processes and regulation. A multidisciplinary approach has been taken, including statistical analyses, practical experimentation, and mathematical modelling. The results show that the progression of the rodent malaria Plasmodium berghei through Anopheles stephensi mosquitoes depends nonlinearly on parasite density, with the presence of both negative and positive density-dependent processes in operation. Analyses of other Plasmodium– Anopheles species combinations also indicate that the traditional assumption of density independence may be an oversimplification. Experimental investigation of mosquito mortality illustrates that the survival of a mosquito depends both on mosquito age and parasite density, again in contrast to the assumptions of malaria transmission modelling. A framework for a mathematical model tracking Plasmodium density within the mosquito has been developed as part of this thesis. Further investigation of sporogonic processes will allow this model to be further refined and extended for use in the future design and evaluation of interventions which target the mosquito or the parasite whilst within the vector.

Mosquitoes are vectors for many diseases that cause significant mortality and morbidity across the globe such as malaria, dengue fever and Zika. As mosquito populations expand their range into new areas, they may undergo mate-finding Allee effects such that their ability to successfully reproduce becomes difficult at low population densities. With new technology, creating target specific gene modification may now be a viable method for mosquito population control. We develop a mathematical model to investigate the effects of releasing transgenic mosquitoes into newly established low-density mosquito populations. Our model consists of two life stages (aquatic and adult), which are further divided into three genetically distinct groups: heterogeneous and homogeneous transgenic alleles that cause female infertility and a homogeneous wild type. We perform analytical and numerical analyses on the equilibria to determine the level of saturation needed to eliminate mosquitoes in a given area. This model demonstrates the potential for a gene drive system to reduce the spread of invading mosquito populations.
Master of Science

Meteorological  conditions  have  a  significant influence on the time of occurrence, abundance and activity of the mosquito vector. In the current context of climate change, it is of great importance to assess the  impact  of  shifts  in  climatic  conditions  on  the suitability for the establishment and annual activity of  the  vector  species.  Moreover,  changes  in  the variability  of  meteorological  elements  and  their extremes  can  generate  unexpected  changes  in  the mosquito  vector  population  which  in  turn  have  an  important effect on human health. One of the ways to put  these  causes  and  effects  into  perspective  is  to simulate the activity of the vector within a processbased framework which allows for the analysis of the contribution of individual factors on the different life stages of the vector. Such analysis is presented by use of  sophisticated  dynamical  models  simulating  the characteristics of the biological population, forced by observed  meteorological  data,  capturing  the  localmicro-environment  of  the  vector  habitat,  and validated by the observed entomology.Numerical  models  are  being  developed  to  model vector  population  dynamics  and  the  expected circulation of the virus within a closed system. Two modelling  approaches  are  standardly  applied  to modelling vector population dynamics: Mechanistic and  Stochastic.  The  advantage  of  mechanistic  overstatistical  models  is  that  they  can  provide  a deterministic  framework  allowing  for  the  isolated evaluation of each input parameter and their effect on the modelled system. Mechanistic dynamical models are used to describe the biophysical processes or partof  the  process  as  a  response  to  changes  in  the meteorological conditions.The  work  carried  out  in  this  thesis  can  be summarized as follows: (i) Analysis of the association between  the  most  important  abiotic  drivers influencing the population dynamics, annual activity and  time  of  occurrence  of Culex  pipiens and Aedes aegypti;  (ii)  Identifying  the  most  important  climatic factors and model settings as a function of climatic characteristics of the study region; (iii) Modelling the vector population dynamics and stability analysis of the  dynamical  system  (iv)  Analysis  of  different verification techniques and implications in terms of model  application;  (v)  Feasibility  analysis  ofimproving  the  model  with  a  Land-Surface Parametrization scheme and short-range forecasting of pest population dynamics.
Meteorološki  uslovi  bitno  utiču  na  vreme  pojave, brojnost  vektora  i  njihovu  aktivnost.  U  uslovima evidentnih promene klime, od ogromne je važnosti sagledati  uticaj  očekivanih  promena  klime  na pogodnost  uslova  na  pojavu  izabranih  vektora.Takodje, značajne promene kolebanja meteoroloških elemenata  u  odnosu  na  višegodišnji  prosek  i  sve češće pojave nepovoljnih vremenskih prilika dovode do neočekivanog ponašanja populacije komarca što značajno  utiče  na  kvalitet  života  i  zdravlje  ljudi. Jedini  način  da  se  sagledaju  uzroci  i  posledice navedenih pojava zasniva se na simulaciji aktivnosti i  brojnosti  vektora  uz  mogućnost  testiranja  uticaja svakog  pojedinačnog  faktora.  Ovu  mogućnost pružaju samo visoko sofistikovani dinamički modeli koju su prošli proces kalibracije i validacije zasnovanna izmerenim vrednostima meteoroloških elemenata i karakteristika biološke populacije.Sofistikovani  modeli  za  simulaciju  dinamike populacije vektora i očekivane cirkulacije vektorskih transmisivnih bolesti se koriste sa ciljem modeliranja potencijalnog rizika od zaraze i epidemije. Modeli zasimulaciju dinamike vektora mogu da se podele na dve  glavne  grupe:  Mehanističke  i  Statističke. Prednost  mehanističkih  modela  nad  statističkim  je što  mogu  da  se  koriste  za  evaluaciju  uticaja izolovanog  faktora  na  dinamički  sistem  i odgovarajuće promene brojnosti unutar svake faze u razvoju  vektora.  Mehanistički  dinamički  sistemi  se koriste  kako  bi  se  opisao  mehanizam  biofizičkog procesa  ili  dela  procesa  u  zavisnosti  od  forsirajuće veličine.Predmet  istraživanja  u  ovom  radu  jeste identifikovanje  najznačajnijih  bioloških  i  fizičkih procesa  kao  i  odgovarajućih  faktora  koji  utiču  na brojnost i aktivnost vektora roda Aedes i Culex. Ciljevi istraživanja mogu da se sumiraju na sledeći način: (i)analiza najznačajnijih meteoroloških parametara koji utiču na vreme pojave, brojnost i aktivnost vektora Aedes  i  Culex  roda;  (ii)  definisanje  najznačajnijih klimatskih  faktora  i  stepena  osetljivosti  procesa  na njih; (iii) modeliranje dinamike populacije vektora i analiza  stabilnosti  dinamičkog  sistema;   (iv) verifikacija  i  analiza  metoda  verifikacije  i  validacije dinamičkog  modela;  (v)  kratkoročna  prognoza dinamike  populacije  komarca  i  formulacija hidrološkog modula upotrebom SURFEX površinskešeme sa ECOCLIMAP fiziogeografskim podacima.

Over the last decade, the malaria burden has reduced drastically across many parts of sub-Saharan Africa. This is mainly due to effective implementation of integrated malaria control programmes that include large scale application of vector control in the form of long-lasting insecticidal nest (LLINs) and indoor residual spraying (IRS), both of which target the most efficient human-seeking malaria vector species. However, in spite of these efforts, malaria has yet to be eliminated from most of Africa. However, recent increases in the physiological resistance of vector populations, especially to the pyrethroids that remain the only active ingredients currently used on nets threaten these achievements. Furthermore, various forms of behavioural resilience and resistance exhibited by some vector species to LLIN and IRS delivery formats for insecticides respectively limit and undermine these valuable impacts upon malaria transmission. To monitor the impact that LLINs and IRS have on vector population dynamics and malaria transmission, more effective, practical and affordable entomological surveillance systems are required. Currently, surveillance of mosquito populations are conducted by the centralized specialist teams with limited personnel, resources and geographic outreach. None of these existing systems can adequately monitor vector population dynamics longitudinally across the vastness of entire countries. The overall goal of the study was to demonstrate how a community-based surveillance system can be applied to longitudinally monitor vector population dynamics and assess the impact that LLINs and IRS have on malaria transmission in rural Zambia. To achieve this overall goal, the following specific objectives were addressed: (1) To evaluate the efficacy of exposure-free mosquito trapping methods for measuring malaria vector density, as alternatives to human landing catch; (2) To assess the cost-effectiveness using a community-based (CB) mosquito trapping scheme for monitoring vector population dynamics; (3) To determine the extent to which a community-based mosquito trapping scheme captures trends in epidemiological indicators of malaria infection risk; (4) To determine the impact of indoor residual spraying with different classes of insecticides on malaria infection burden and vector abundances in an area of high coverage with insecticide treated nets using a community-based platform. To address objective 1, a 3 x 3 Latin square method was used to evaluate the sensitivity of the Center for Disease and Control and Prevention miniature light traps (LT), the Ifakara tent trap (ITT), window exit traps (WET) and the resting boxes (RB) using the golden standard human land catch (HLC) as the reference method. The mean catches of HLC indoor, HLC outdoor, CDC-LT, ITT, WET, RB indoor and RB outdoor, were 1.687, 1.004, 3.267, 0.088, 0.004, 0.000 and 0.008 for Anopheles quadriannulatus Theobald respectively, and 7.287, 6.784, 10.958, 5.875, 0.296, 0.158 and 0.458, for An. funestus Giles, respectively. The LT (Relative rate (RR) [95% Confidence Interval] = 1.532 [1.441, 1.628] P < 0.001) and ITT (RR = 0.821 [0.765, 0.881], P < 0.001), were the only exposure-free alternatives which had comparable sensitivities relative to HLC indoor for sampling An. funestus. To address objectives 2 and 3, the two most sensitive of these exposure-free trapping methods, the LT and ITT, were applied through a CB longitudinal entomological surveillance system implemented by local community health workers (CHW) trained in basic entomology. This surveillance platform was conducted using a monthly sampling cycle for over 2 years in 14 population clusters distributed across two rural districts covering over 4,000km2 of south-east Zambia. Parallel active surveillance of malaria parasite infection rates amongst humans was also conducted by CHWs in the same population clusters to determine the epidemiological relevance of these CB entomological surveys. Prior to the end of the study, a controlled quality assurance (QA) survey was conducted by a centrally supervised expert team using HLC, LT and ITT to evaluate accuracy of the CB trapping data. While the relative sampling efficiencies of both CB surveys were less than their QA counterparts, the costs of implementing per sampling night were far less expensive than any QA survey. The cost per specimen of Anopheles funestus captured was lowest for CB-LT ($5.3), followed by potentially hazardous QA-HLC ($10.5) and then CB-ITT ($28.0). Time-trends of malaria diagnostic positivity (DP) followed those of An. funestus density with a one-month lag and the wide range of mean DP across clusters was closely associated with mean densities of An. funestus caught by CB-LT (P<0.001). To address objective 4, the same 14 cluster populations, with pre-existing high coverage of pyrethroid-impregnated long-lasting insecticidal nets (LLINs), were quasi-randomly assigned to receive IRS with either of two pyrethroid formulations, namely Deltamethrin (Wettable granules (WG)) (DM-WG) and Lambdacyhalothrin (Capsule suspension (CS)) (LC-CS), or with an emulsifiable concentrate (EC) or CS formulation of the organophosphate pirimiphosmethyl (PM), or with no supplementary vector control measure. DP conducted is described in objective 2. Over the first 3 months, the PM-CS IRS supplement offered the greatest level of protection against malaria followed by LC-SC and then by DM-WG. Neither pyrethroid formulation provided protection beyond 3 months after spraying, but both PM CS and EC formulations persisted for 6 months and 12 months respectively. The CS formulation of PM provided greater protection than the combined pyrethroid IRS formulations throughout its effective life (Incremental protective efficacy (IPE) [95%CI] = 0.79 [0.75, 0.83]) over 6 months. The EC formulation of PM provided incremental protection for the first three months (IPE [95%CI] = 0.23 [0.15, 0.31]) that was approximately equivalent to the two pyrethroid formulations (LC-CS, IPE [95%CI] = 0.31 [0.10, 0.47] and DM-WG, IPE [95%CI] = 0.19 [-0.01, 0.35]) but the additional protection provided by the former, apparently lasted an entire year. There were no obvious differences in the densities of An. funestus during the first three months post-spraying for both pyrethroid formulations (DM-WG (IPE[95%CI]=0.01[-0.56,0.37],P=0.103) and LC-CS (IPE[95%CI]=-0.03[-0.88,0.44],P=0.195) and PM-EC (IPE[95%CI]=-0.04[-0.30,0.17], P=0.103). However, where PM-CS was applied, mosquito densities were dramatically reduced during the same period (IPE [95%CI] =0.93[0.87, 0.97], P<0.001). Between the fourth and the sixth month after spraying with DM-WG, there was an apparent, but presumably spurious, three-fold increase in An. funestus densities while LC-CS, PM-EC and PM-CS achieved 5, 3 and 71-fold reductions, respectively. However, from the seventh to twelfth months after spraying, DM-WG and PM-EC had no obvious effect on the An. funestus densities while insufficient data was available to examine the incremental impact of LC-CS or PM-CS. When applied at this pilot scale, this CB mosquito-trapping scheme provided entomological evidence that complements epidemiological monitoring data to demonstrate how supplementing LLINs with IRS can reduce malaria transmission beyond levels achieved with LLINs alone in this setting where physiological resistance to pyrethroids occurs, especially when a non-pyrethroid organophosphate insecticide is used. Overall, it appears that CB trapping schemes are affordable, cost-effective, and epidemiologically relevant. It also appears, based on the evidence from this pilot scale evaluation, that they may be applicable to routine programmatic monitoring of vector population dynamics on unprecedented national scales.

Apesar dos esforços desenvolvidos nas últimas décadas, a malária continua a ser um dos maiores problemas de saúde pública no mundo, sendo a principal causa de morbilidade e mortalidade principalmente na África Subsaariana. Fazer uma análise global, que integre todos os intervenientes deste sistema complexo, que engloba três entidades biológicas, fatores socioeconómicos e ambientais, não é fácil, mas pensamos ser um ponto fulcral para um maior conhecimento sobre esta doença. Neste estudo, utilizando um conjunto completo de amostras – sangue periférico e mosquitos – pretendeu-se analisar este complexo sistema de forma abrangente. Deste modo, este trabalho teve como principais objetivos: 1) Caraterizar as populações parasitárias circulantes nos dois hospedeiros – humano e mosquito vetor - através da identificação das espécies de Plasmodium presentes; marcadores moleculares de diversidade (Pfmsp2) e marcadores moleculares associados a resistência a fármacos (mutações pontuais nos genes Pfdhps, Pfdhfr, Pfcrt e Pfmdr1); 2) Analisar as pressões seletivas atuantes sobre os genes associados a resistência a fármacos e 3) Analisar a diversidade de genes do mosquito vetor - AgTG1 e AgTG2 - tentando relacioná-los com a presença/ausência de infeção. As amostras biológicas utilizadas para este trabalho foram recolhidas em três países diferentes: na Guiné Equatorial continental foram colhidas amostras de sangue e mosquitos adultos em duas localidades, Miyobo e Ngonamanga. Em Angola obtiveramse as amostras de sangue em quatro localidades diferentes (Gabela, Porto Amboim, Kissala – Sumbe e Praia – Sumbe) e foram ainda usados neste estudo mosquitos adultos provenientes de Antula, Guiné-Bissau. Em relação ao primeiro e segundo objetivos deste trabalho, foi possível constatar a presença das quatro espécies de Plasmodium em ambos os hospedeiros, com prevalências superiores às reportadas oficialmente, incluindo P. vivax, espécie que ainda não tinha sido detetada na Guiné Equatorial continental. Detetou-se igualmente indivíduos Duffy negativos infetados com duas estirpes diferentes de Plasmodium vivax (P. vivax clássico e o P. vivax VK247). Relativamente às mutações pontuais associadas à resistência aos antimaláricos, constatou-se que de um modo geral estas ocorriam em elevada prevalência. Verificou-se igualmente que a resistência à pirimetamina encontrase bem estabelecida neste país, enquanto a resistência à sulfadoxina terá tido uma introdução mais recente. Relativamente ao terceiro e último objetivo deste trabalho, constatou-se que os dois genes estudados - AgTG1 e AgTG2- apresentam fortes sinais de seleção positiva, podendo estar envolvidos no reconhecimento de organismos patogénicos, e por conseguinte envolvidos numa resposta contra a infeção. Por fim, este trabalho permitiu concluir que na Guiné Equatorial continental existem as quatro espécies de Plasmodium, incluindo a espécie P. vivax que até à data não estava descrita no país. Foi encontrada uma elevada prevalência de mutações associadas à resistência à sulfadoxina-pirimetamina, pelo que se recomenda uma contínua monitorização destas mutações. Por fim constatou-se que os genes AgTG1 e AgTG2 apresentam fortes sinais de de seleção positiva, podendo estar envolvidos no reconhecimento de organismos patogénicos, e por conseguinte envolvidos numa resposta contra a infeção. Por fim, este trabalho permitiu concluir que na Guiné Equatorial continental existem as quatro espécies de Plasmodium, incluindo a espécie P.vivax que até à data não estava descrita no país. Foi encontrada uma elevada prevalência de mutações associadas à resistência à sulfadoxina-pirimetamina, pelo que se recomenda uma contínua monitorização destas mutações. Por fim constatou-se que os genes AgTG1 e AgTG2 apresentam fortes sinais de seleção positiva, podendo estar envolvidos na resposta à infeção por Plasmodium.
Despite all efforts made over the past decades, malaria remains a major public health problem in the world, affecting mainly the Sub-Saharan Africa. A comprehensive analysis that integrates all factors in this complex system, which consists of three biological entities, socio-economic and environmental factors, is not easy, but it is crucial for a better understanding of this disease. In this study, using a complete set of peripheral blood samples and mosquitoes, we intended to analyse this complex system. So, the main objectives of this study were to: 1) Characterise the circulating parasite populations in the two hosts – human and mosquito vector - through the identification of Plasmodium species; molecular marker diversity (Pfmsp2) and drug resistance-associated markers (mutations in the genes Pfdhps , Pfdhfr , Pfcrt and Pfmdr1; 2) analyse the selective pressures acting on genes associated with drug resistance and 3 ) analyse the diversity of genes in the mosquito vector - AgTG1 and AgTG2 - trying to relate them to the presence / absence of infection. The biological samples used in this study were collected in three different countries: blood samples were collected in mainland Equatorial Guinea (in two villages: Miyobo and Ngonamanga) and in Angola (in four different villages: Gabela, Porto Amboim, Kissala - Sumbe and Praia - Sumbe); mosquitoes were collected, also, in the two villages of Equatorial Guinea and in Antula, Guinea Bissau. Regarding the first and second objectives of this study, it was possible to detect the presence of the four Plasmodium species in both hosts, with prevalence higher than officially reported, including Plasmodium vivax, a species that had not been previously described in this country. Duffy negative individuals infected with two different strains of P. vivax (VK247 and classic strains) were also found. Concerning the molecular markers associated to drug resistance, high prevalence was found. Results also demonstrated that pyrimethamine resistance has been established for a while in mainland Equatorial Guinea as shown by several selection signatures in the parasite genome, while sulphadoxine had a more recent introduction in this country.Finally and regarding the third and final objective of this study, it was found that the both genes studied - AgTG1AgTG2 - showed strong signs of positive selection. This study revealed that the four Plasmodium species are present in mainland Equatorial Guinea, including P. vivax, specie that had not been described yet in this country. High prevalence of mutation in genes associated with resistance to the sulphadoxine-pyrimethamine combination were found, so it is recommended a close and continuous monitoring of these mutations frequency, since there is the danger of an eventual reduction in the efficacy of combined therapy. Finally it was found that the AgTG1 and AgTG2 genes show strong positive selection signals, which may be involved in recognition and immune response triggered by the mosquito against the invading pathogens, like Plasmodium.

A population survey was conducted on the mosquito species recorded in Denton, Texas for the years of 2005 to 2015. Data used in this project were obtained from an ongoing, long-term surveillance program led by the City of Denton and conducted through the University of North Texas. Research focused on the population dynamics and community structure of mosquitoes collected within urban areas of Denton, Texas in relation to certain environmental variables. A total of 80,837 female mosquitoes were captured and represented 38 species found under the following genera: Aedes, Anopheles, Coquillettidia, Culex, Culiseta, Mansonia, Orthopodomyia, Psorophora, Toxorhynchites, and Uranotaenia. Culex quinquefasciatus was the most abundant species followed by Aedes vexans. Seasonal patterns of the most abundant species revealed high variability throughout the study. Container breeders were most abundant in August and those that breed in floodwaters were most abundant in the months of May and September. Samples were tested for arbovirus presence through the Texas Department of State Health Services in Austin, Texas and multiple pools tested positive for West Nile virus throughout the study. Stepwise multiple regression and Spearman's rank correlation analyses were performed to examine the relationship between the mosquito community and environmental variables. Data revealed that temperature, precipitation, and dew point were the most important variables influencing the mosquito population in the City of Denton.

A population survey was conducted from April through September 2002 on mosquito species occurring on the Ray Roberts Greenbelt, a riparian corridor used for public recreation on the Elm Fork of the Trinity River, in Denton County, Texas. ArcGIS software was used to set up a stratified random sampling design based on habitat parameters. Multivariate analyses of sampling data and climatic variables were used to describe spatial and temporal patterns of mosquito species. A total of 33 species were collected during this study belonging to the following genera: Aedes, Anopheles, Coquillettidia, Culex, Mansonia, Ochlerotatus, Orthopodomyia, Psorophora, Toxorhynchites, and Uranotaenia. Seasonal distributions of the dominant species revealed population fluctuations. Aedes vexans was the primary species collected in April and May, occurring in low numbers throughout the rest of the sampling period. Psorophora columbiae reached its highest population density in June, with a smaller peak occurring in late July. Present from May through the end of September, Culex erraticus was the most abundant species collected with major peaks in mid-June and the end of July. Abundance of Culex salinarius followed the same general trend as that for Cx. erraticus, but with smaller numbers. The specimens were tested for a variety of arboviruses by the Texas Department of Health. One pool of Cx. erraticus and Cx. salinarius, collected in August 2002, tested positive for West Nile virus. Variables that were important factors for determining dominant species abundance were temperature, wind speed, rain accumulation occurring one-week and two-weeks prior to sampling, number of day since last rain event, dew point, and average canopy coverage.

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CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
A incidência global dos vírus da Dengue e, mais recentemente, do Zika, Chikungunya e Febre Amarela, tem aumentado o interesse em estudar e compreender a dinâmica populacional do mosquito. Essas doenças são predominantemente disseminadas pelo Aedes aegypti nos países tropicais e subtropicais do mundo. Compreender essa dinâmica é importante para a saúde pública nos países, onde as condições climáticas e ambientais são favoráveis para a propagação destas doenças. Por essa razão, modelos que estudam a dinâmica populacional em uma cidade são de suma importância. Este trabalho discute a modelagem numérica da dinâmica populacional do mosquito Aedes aegypti em uma vizinhança urbana de uma cidade. Em um primeiro momento, apresentamos os resultados teóricos preliminares de modelos unidimensionais. Em seguida, propomos um modelo bidimensional utilizando equações diferenciais parciais. Este modelo permite incorporar fatores externos (vento e inseticidas químicos) e dados topográficos (ruas, blocos de construção, parques, florestas e praias). O modelo proposto foi testado em exemplos envolvendo duas cidades brasileiras (o centro da cidade de Juiz de Fora e a Praia de Copacabana no Rio de Janeiro).
The global incidence of the Dengue virus and, more recently, the Zika, Chikungunya and Yellow Fever, has increased interest in studying and understanding the population dynamics of the mosquito. These diseases are predominantly disseminated by Aedes aegypti in the tropical and subtropical countries of the world. Understanding this dynamics is important for public health in countries, where climatic and environmental conditions are favorable for the spread of these diseases. For this reason, models that study the population dynamics in a city are of short importance. This work discusses the numerical modeling of the population dynamics of the mosquito Aedes aegypti in an urban neighborhood of a city. First, we present the preliminary theoretical results of one-dimensional models. Next, we propose a two-dimensional model using partial differential equations. This model allows incorporating external factors (wind and chemical insecticides) and topographic data (streets, building blocks, parks, forests and beaches). The proposed model was tested in examples involving two Brazilian cities (the city center of Juiz de Fora and Copacabana Beach in Rio de Janeiro).

Mosquito-borne arboviruses are of considerable public health importance as they cause some of the most important emerging and re-emerging infectious diseases affecting humans and animals in many parts of the world including southern Africa. The threat of large epidemics of mosquito-borne arboviruses are often associated with climatic conditions, global warming, animal migrations, surface water, wind, topography, harbourage, vegetation, food supply and abundance of competent mosquito vectors. The goal of this project is to provide an in depth understanding of mosquito community dynamics and the importance of mosquito vector populations in the maintenance and transmission of mosquito-borne diseases in southern Africa. Firstly, a review of past and current literature was conducted to highlight: (a) the current state of knowledge regarding the most important mosquito-borne viruses of medical significance in southern Africa (b) lesser known mosquito-borne arboviruses with the potential of causing zoonotic health threats for humans in southern Africa. (c) key aspects of the ecology of mosquito vectors of medically significant mosquito-borne viruses in southern Africa. d) gaps in knowledge regarding southern African arbovirus mosquito vectors. Most of the studies on mosquito-borne viruses in southern Africa can be clustered into specific programmes led by Kokernot and Smithburn in the 1950s, McIntosh in the 1970s and 1980s, Swanepoel in the 1970s, Venter and others in more recent years, and have largely been restricted to South Africa, Mozambique and Zimbabwe. Twenty-six (26) arboviruses have been isolated from mosquitoes in southern Africa. Of these, Chikungunya (CHIK), Sindbis (SIN), West Nile (WN), Wesselsbron (WES), Spondweni (SPO), Banzi (BAN), Dengue (DEN), Bunyamwera (BUN), Germiston (GER) and Rift Valley fever (RVF) viruses are known to cause human illness. Middelburg (MID) and Shuni (SHUN) viruses are also important, causing neurological symptoms in animals with zoonotic potential for humans in South Africa. There are eight mosquito-borne arboviral infections most likely to impact humans in southern Africa (CHIK, MID, SIN, DEN, WES, WN, SHUN and RVF viruses). Mosquitoes in the subfamily Culicinae (mostly Aedes and Culex mosquitoes) are the most frequently associated with arbovirus transmission (115 and 105 types of arbovirus, respectively). Understanding the role of mosquito vector species in arbovirus transmission is vital for the development of new strategies to control the spread of arboviral diseases. In southern Africa, a few species in the genera Anopheles, Coquillettidia and Mansonia have also been implicated as vectors of arboviruses. Surveys over multiple decades across southern Africa have provided an insight regarding which species of mosquitoes are involved in the transmission of at least the most common of the mosquito-borne zoonotic arboviruses. These cluster within the genera Aedes and Culex, each representing a different transmission strategy. Aedes-borne viruses such as CHIK, DEN and WES tend to have primate or human reservoir hosts (McIntosh, 1986), while Culex-borne viruses often use birds as reservoir hosts, and these factors influence the distribution and epidemiology of the diseases they cause in humans and animals. Aedes and Culex have different breeding strategies and preferences which also represent fundamental differences. These mosquitoes are Aedes aegypti, Aedes furcifer/cordellieri, Aedes circumluteolus, Aedes unidentatus, Aedes mcintoshi, Aedes caballus, Aedes juppi, Culex theileri, Culex zombaensis, Culex univittatus, Culex neavei and Culex rubinotus. To determine mosquito community dynamics and mosquito vector distributions, sampling mosquito vectors at six sentinel sites in three provinces in the northern part of South Africa where recent cases had been detected in animals. Adult mosquitoes were collected from two horse properties in Gauteng Province; two wildlife reserves in Limpopo Province and at Orpen Gate in Kruger National Park and Mnisi Area in Mpumalanga Province between 2014–2017, using carbon dioxide-baited light and tent traps. Culex poicilipes, was the most abundant species caught during the study period. Highest diversity and species richness were found at Lapalala Wilderness Reserve, while the lowest diversity and abundances were at Orpen in Kruger National Park. Aedes aegypti, Ae. mcintoshi, Ae. metallicus, Ae. vittatus, Cx. pipiens sensu lato, Cx. theileri and Cx. univittatus, which are potential arbovirus vectors, had the widest geographical distribution in northern South Africa. Also collected were Anopheles arabiensis and An. vaneedeni, both known malaria vectors in South Africa. Therefore, arbovirus surveillance and vector control programs should be augmented in peri-urban and mixed rural settings where there is greater risk for arbovirus transmission to humans and domestic stock. Since climate has reportedly been associated with disease transmission, it’s important to understand the extent of its influence on mosquito abundance and distribution in northern South Africa. Thus, population composition, abundance and diversity of mosquitoes collected over a three-year period were determined and correlated to diverse climatic conditions during those years in order to determine seasonal trends in occurrence, abundance and distribution. Marked differences in the temporal distribution and seasonal abundances of the seven medically important mosquito vectors encountered from the two distinct geographic regions and climates. Statistical models have shown that climatic factors play a crucial role in shaping the population dynamics of Ae. mcintoshi, Ae. vittatus, An. arabiensis, Cx. pipiens s.l., Cx. poicilipes, Cx. theileri and Cx. univittatus both in Highveld Grassland and Middleveld Bushveld regions of northern South Africa. High summer temperatures and rainfall lead to increased vector density which might trigger outbreaks of RVF, SIN and WN viruses on the inland plateau of South Africa. This study also showed that abundances of RVF and WN virus vectors are related to elevation. These findings will be important in predicting the timing of onset and spread of future epidemics such as WN and RVF viruses, in southern Africa and other geographical settings with similar climates.
Thesis (PhD)--University of Pretoria, 2020.
University of Pretoria US Centers for Disease Control and Prevention
Medical Virology
PhD
Unrestricted

A new strategy to fight mosquito-borne disease is based on infections of the maternally-transmitted, intracellular bacterium Wolbachia pipientis. Estimates predict that Wolbachia infects nearly half of all insect species, as well as other arthropods and some nematodes. Wolbachia manipulates the reproduction of its host to promote infection, most commonly causing a form of conditional sterility known as cytoplasmic incompatibility. Generally, Wolbachia infections are benign and do not inflict significant costs upon its host. However, studies demonstrate that some infections are associated with substantial costs to its host. These same infections can also induce pathogen interference and decrease vector competency of important disease vectors. Theory predicts that organisms that incur costs relative to conspecifics are less competitive and their competitive exclusion is expected. In the case of Wolbachia, the bacterium can influence reproduction such that phenotypes with lower fitness may still reach fixation in natural populations. In this dissertation, I describe theoretical and empirical experiments that aim to understand the invasion and stability of Wolbachia infections that impose costs on their host. Particular attention is paid to immature insect lifestages, which have been previously marginalized. These results are discussed in relation to ongoing vector control strategies that would use Wolbachia to manipulate vector populations. Specifically, I discuss the cost of novel Wolbachia infections in Aedespolynesiensis, which decreases larval survival and overall fitness relative to wild-type mosquitoes. Then, a theoretical framework was developed to determine the significance of reductions in larval viability in relation to the population replacement disease control strategy. Further theoretical studies determined that Wolbachia infections, once established, resist re-invasion by uninfected individuals despite relatively high costs associated with infection so long as the infection produces reproductive manipulations. Additional studies determined that larvae hatched from old eggs experience reduced survival in mosquito strains with novel Wolbachia infections when compared to the wild-type. To validate the theoretical studies, model predictions were tested empirically to determine the importance of the larval viability. Finally, a COPAS PLUS machine was evaluated and its role in understanding early larval development in mosquitoes is discussed. The importance of integrated research in disease control is highlighted.

Thesis: S.M., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2014.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 89-96).
This thesis applies the HYDRology, Entomology and MAlaria Transmission Simulator (HYDREMATS) to the environment around a water resources reservoir in Ethiopia. HYDREMATS was modified to simulate the local hydrology and the mosquito population dynamics influenced by the reservoir system. The hydrology component of HYDREMATS including a representation of the groundwater was coupled with a reservoir model to describe the spatiotemporal variability of the groundwater table, and the variability in shoreline locations. The entomology component was modified to match the relatively humid environment. HYDREMATS was applied to two villages around the Koka Reservoir in Ethiopia, one adjacent to the reservoir, Ejersa, and the other located 12 km away from it, Gudedo. Meteorological data were collected from July 2011 to February 2013. Entomological data collection started in July 2012 and continued until February 2013. Adult mosquitoes were sampled from the two field sites and classified at the genus level, i.e., Anopheles or Culex. Because of their geographical proximity, the climatology in Ejersa and the climatology in Gudedo were comparable; however, entomological conditions in the two villages were distinct. Ejersa experienced an enhanced and prolonged mosquito season. HYDREMATS was able to simulate the hydrology and the population dynamics of Anopheles mosquitoes at both sites for the period, from January 2012 to February 2013. The model applied to Ejersa simulated a large mosquito population and a prolonged mosquito breeding season because of the existence of the reservoir, in agreement with observations. Especially, a large mosquito population in the post-rainy season was sustained in the simulation due to a large shoreline breeding area. The model applied to Gudedo simulated smaller mosquito population, but it failed to reproduce observed adult mosquito population dynamics correctly. However, the simulated adult mosquito population dynamics in Gudedo resembled those of the observed larvae samples. Further model calibration and validation will be conducted as more data become available. This study demonstrates that HYDREMATS can serve as an effective tool to simulate local hydrology and mosquito population dynamics at a reservoir environment, given hydrological and entomological parameters specified for the given field site.
by Noriko Endo.
S.M.

New and old mosquito-borne diseases have emerged and re-emerged in temperate regions over the recent past, but an understanding of mosquito population dynamics, a fundamental step toward disease control, remains elusive. In particular, we are still lacking reliable predictive models of mosquito abundance in temperate areas due to the subtle links between the fluctuation of mosquito population and highly heterogeneous environmental drivers. Hence, this doctoral dissertation presents an interdisciplinary approach towards an improved understanding and prediction of the fluctuations in mosquito abundance in temperate regions. In the first part of this dissertation a hierarchical Gompertz-based model is used to assess the relative importance of endogenous (density dependence) and exogenous (environmental forcings) controls and their interactions in regulating the dynamics of a West Nile Virus vector (Culex pipiens) in the Po River delta in Italy. The results clearly detect the effects of density-dependence in the observed population dynamics for the mosquito species analyzed and highlight the controls exerted by environmental forcings and habitat conditions. Subsequently, the characteristic scales of temporal variability in mosquito populations, and the representativeness of observations at different sampling resolutions, are investigated using a 10 year daily mosquito sample from Brunswick County, North Carolina. The species present in the sample (among which Aedes vexans and Culiseta melanura are addressed in greater detail, as vectors of East Equine Encephalitis and West Nile Virus) are investigated using a combination of time series analysis, individual based simulations, and density-dependent modeling approaches. Significant population fluctuations with characteristic periodicity between 2 days and several years are found in response to different regulation mechanisms. In particular, the observed fast fluctuations are importantly determined by a varying mosquito activity, rather than by reproduction/mortality processes, driven by rapid changes in meteorological conditions. Finally, in the third part of this study, a state space reconstruction (SSR) approach is used to understand how the predictability of mosquito abundance varies with aggregation time scale and with the prediction horizon, and how much can the prediction of mosquito abundance be improved by using daily observations compared to the commonly used once-per-week samples. The results show that the predictability of mosquito abundance decreases as the time scale of the models increases from one week to one month, while the predictability of per capita growth rate increases together with the modeling scale. It is also shown that the prediction of mosquito per capita growth rate can be improved using daily abundance observations. Furthermore, many mosquito models compare the observed and predicted abundance as a measure of model performance. However, my results suggest that short term forecasts of mosquito abundance may appear to have a significant capability due to the positive autocorrelation between abundance in subsequent time steps, even when the model's ability to predict the abundance change is low. Model capability should thus be evaluated comparing observed and modelled per capita rates of change.

Dissertation

This study investigated aspects of the breeding biology, ecophysiology, morphology, over-wintering and population dynamics of the pre-imaginal stages of members of the Anopheles gambiae complex in northern Natal, South Africa, Investigation of the breeding biology, concentrated on breeding site utilisation by the different members of the Anopheles gambiae complex. Surveillance personnel were unable to locate the breeding sites of Anopheles arabiensis and postulated that location and description of these would offer a unique opportunity for control. The difficulty in locating the breeding sites of An. arabiensis at certain localities was found to be a product of their low density, presumably as a result of the intra-domiciliary, residual insecticide spray programme. The effect of temperature on larval physiology and adult morphology was investigated. terms of their implications Anopheles gambiae complex. The findings are discussed in for anopheline taxonomy and the The effect of temperature on larval growth rates was investigated in both the field and laboratory. The finding of this part of the study indicate that the larval stages play an important role in the over-wintering of populations within the region. The control implications of these findings and winter breeding site localisation are discussed. A theory for the so called 'late season transmission i.e. Apri-May, within the province and southern Africa is proposed. The population dynamics of An. merus were investigated, together with the effect of abiotic factors such as temperature, salinity and rainfall. The effect of sampling bias due to factors such as behavioural avoidance were studied.
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1991.