Mathematical Modelling of Mosquito Dispersal in a Heterogeneous Environment.

Mosquito dispersal is a key behavioural factor that affects the persistence and resurgence of several vector-borne diseases. Spatial heterogeneity of mosquito resources, such as hosts and breeding sites, affects mosquito dispersal behaviour and consequently affects mosquito population structures, human exposure to vectors, and the ability to control disease transmission. In this paper, we develop and simulate a discrete-space continuous-time mathematical model to investigate the impact of dispersal and heterogeneous distribution of resources on the distribution and dynamics of mosquito populations. We build an ordinary differential equation model of the mosquito life cycle and replicate it across a hexagonal grid (multi-patch system) that represents two-dimensional space. We use the model to estimate mosquito dispersal distances and to evaluate the effect of spatial repellents as a vector control strategy. We find evidence of association between heterogeneity, dispersal, spatial distribution of resources, and mosquito population dynamics. Random distribution of repellents reduces the distance moved by mosquitoes, offering a promising strategy for disease control

Mosquito Net Coverage and Utilisation for Malaria\ud Control in Tanzania\ud

\ud In recent years malaria parasites have developed resistance to the most commonly used antimalarial drugs in Tanzania, posing a major challenge for its control. This has led to frequent changes of malaria treatment guidelines, more recently to expensive, yet more effective arthemether/lumefantrine. The use of insecticide treated mosquito nets (ITNs) and long lasting nets (LLINs) in Tanzania has increased slowly over the past few years. Despite the introduction of a voucher scheme to the vulnerable groups, the proportion of households with at least an ITN/LLIN in the country has not been able to achieve the Abuja Declaration of 60% net coverage. Statistics available on the utilisation of nets do not provide a good estimate of the coverage, because of the different study design used to collect the information. This survey was carried out in 21 districts of Tanzania to determine the coverage and utilisation of insecticide treated nets to provide baseline information of the net requirement to cover every sleeping bed in the country. Specifically, this study aimed to (i) determine the ITN coverage by and its distribution in the country; and (ii) determine knowledge, attitudes and practice of the householders as regards to malaria prevention and control Twenty one districts (one from each region) of Mainland Tanzania were selected for the survey. Selection of the district was random. In each district, two wards were selected, one urban (within the district capital) and one rural or sub‐urban. Households were selected randomly using a table of random numbers. At household level, the head or any adult who represented the head of household was interviewed. A structured pre‐tested questionnaire was used to collect information on knowledge, attitude and practices in malaria control, with emphasis on mosquito net ownership and utilisation. Of the 9549 targeted households, 9166 (96%) participated in the survey. Majority of the respondents (76.8%) were from rural district.The mean household size was 3.9 persons. On average, children <5 years old accounted for 39.3% of the members of the households. Respondents with no formal education accounted for 15.8‐37.4% of the interviewees. Most of them were from Mkuranga (55%), Kigoma‐Ujiji (44.2%) and Newala (37.9%). High literate rates were observed in Arumeru and Moshi districts. The majority of the respondents knew that the mosquito is the vector of the malaria parasite (92.6‐99.4%) and infection is through a mosquito bite (92.7‐99.8%). The knowledge of respondents on malaria transmission was generally high (94.0‐99.0%). The majority of the respondents (95.2%) considered the use of mosquito nets as the most effective way of malaria prevention. However, of these, only 66.7% said to have actually used nets in their life time. Knowledge on the use of mosquito nets in the control of malaria was highest and lowest in Eastern and Central zones, respectively.\ud Seventy‐seven percent (4457/8933) agreed to have the investigator entry into their houses and verify the\ud number of nets owned. On average, 62.9% (5,785/8933) of the households had at least a mosquito net. Majority of the respondents in Northern (76.5%) and Southern (76.5%) zones owned at least a mosquito net. The lowest mosquito net ownership was observed among respondents in Western Zone (39.6%). District‐wise, net ownership was highest in Lindi (94.5%), Kyela (91.3%) Arumeru (86.1%), Ilala (83.1%)\ud and Nyamagana (80.0%). Ownership of net was very low in Kilolo (34.8%), Kigoma (36.5%) and Musoma Rural (41.3%). Of the households with nets, 74.4% were using nets all year round. A larger proportion of respondents in Kilolo (68.5%), Mpwapwa (51.9%), Songea Rural (49.2%) and Shinyanga Rural (46.3%) were only using the nets during the rainy season. Out of 9,166 households visited, 3,610 (39.3%) had at least one under five child. Of these, in 1,939 (53.7%) of the households the child slept under a mosquito net during the previous night. Use of nets in children <5 years was most common in northern zone (74%); followed by eastern (66.9%) and southern zone (61.1%). Districts with the largest proportion of <5 year children sleeping under a mosquito nets were Lindi (90.0%), Kyela (85.2%), Ilala (83.2%) and Arumeru (78.2%). Only about a quarter (27%) of the children <5% in western zone were sleeping under a mosquito net. Lowest net coverage for <5 year was in Kigoma (22.7%), Kilolo (25%) and Bukoba Rural (31.2%). A total of 5,785 (62.9%) owned at least a mosquito net. Of these, 4,219 and 1,566 were from the rural and urban districts, respectively. More households in the urban districts (73.4%) than rural districts (59.7%) owned at least a mosquito net. Likewise, there were more households (64.9%) in the urban districts with <5years children sleeping under mosquito nets than in the rural districts (50.4%). More households in urban (32.8%) than in rural districts (25.1%) had at least one insecticide treated net. The number of households with mosquito nets enough for all members of the households ranged from 18.9% (in Urambo) to 37.4% (in Hanang). Households with at least 50% or more occupants using mosquito nets ranged from 16.4% (in Urambo) to 42.8% (in Arumeru). Districts with the largest proportion of ≥50% of the household members sleeping under mosquito nets were Arumeru (46.9%) and Lindi (46.7%). In Manyoni and Lindi, 3.1% and 5% of the households were found to have more nets than the number of household occupants. Only 9% (801/9196) of the households had all occupants sleeping under a mosquito net. Kyela district had about a quarter (23.9%) of the households with all occupants sleeping under nets. Only 29% of the households had at least one insecticide treated mosquito nets. All nets in 51.4% of the households surveyed were ITNs. The largest proportion of households with ITN was observed in northern zone (40.2%), with Arumeru (46%) and Hanang (44.1%) districts having the highest ITN coverage. The lowest proportion (15.5%) of households with ITN was found in the Western Zone. Districts which had the lowest ITN coverage were Musoma Rural (12.6%), Kigoma‐Ujiji (13.2%), and Shinyanga Rural (14.4%). On average, 90.7% (8,123/8,953) of the respondents would prefer using ITN than having their house sprayed with long lasting residual insecticide. More households in urban (32.8%) than in rural districts (25.1%) had at least an ITN> A total of 1939 children underfives were sleeping under mosquito net (any type). Of these, 1140 (58.8%) were using insecticide treated nets (ITN). Overall, 31.6% of the underfives slept under an insecticide treated net during the previous night. Highest coverage was reported in Kyela (47.7%), Nyamagana (47.7%) and Arumeru (46.4%). Lowest ITN in underfives was reported in Kigoma‐Ujiji (16.0%), Musoma (17.2%) and Urambo (17.7%). In Songea more underfives children were sleeping under ITN (43.6%) than in untreated nets (40.9%). Control of bedbugs, lice, fleas, mites and cockroaches was the major added advantage of using insecticide treated nets. On average, 30.8% and 19.6% of the respondents mentioned cockroach and bedbug control as the main advantage of using ITN, respectively. The majority (52.9%) preferred blue coloured net (Northern=45.6%; Central=59.2%; Eastern=56.4%; Lake= 54.4%; Southern= 60.3%, Western= 58.5%) and Southern Highlands= 49.1%). Other colour preferences were white (29.6%), green (14.1%), black (2.1%) and pink (1.2%). A strong preference for blue mosquito nets was observed among respondents in Musoma (77.3%) and Newala (75.5%) districts. On the other hand, the weakest preference (24.7%) for blue nets was observed among respondents in Arumeru district. The majority of the respondents (82%) preferred rectangular shaped net. A larger percent (61.8%) the respondents preferred to have the map of Tanzania as a national logo to identify nets distributed in the country. On average, 62.7% and 28.8% of the households in Tanzania own at least one mosquito net (any type) and insecticide treated net, respectively. Tanzania expects that ITN coverage of under fives in 2009, after the Under Five Catch‐up Campaign is complete, to be at least 80%. If this is to be achieved, there is a need for concerted effort in scaling up the distribution and demand for long lasting nets throughout the country. Moreover, the planned use of IRS in malaria control, currently considered unpopular should be accompanied by rigorous community health education to avoid resistance from household members.\u

Transition from endemic behavior to eradication of malaria due to combined drug therapies: an agent-model approach

We introduce an agent-based model describing a susceptible-infectious-susceptible (SIS) system of humans and mosquitoes to predict malaria epidemiological scenarios in realistic biological conditions. Emphasis is given to the transition from endemic behavior to eradication of malaria transmission induced by combined drug therapies acting on both the gametocytemia reduction and on the selective mosquito mortality during parasite development in the mosquito. Our mathematical framework enables to uncover the critical values of the parameters characterizing the effect of each drug therapy. Moreover, our results provide quantitative evidence of what is empirically known: interventions combining gametocytemia reduction through the use of gametocidal drugs, with the selective action of ivermectin during parasite development in the mosquito, may actively promote disease eradication in the long run. In the agent model, the main properties of human-mosquito interactions are implemented as parameters and the model is validated by comparing simulations with real data of malaria incidence collected in the endemic malaria region of Chimoio in Mozambique. Finally, we discuss our findings in light of current drug administration strategies for malaria prevention, that may interfere with human-to-mosquito transmission process.Comment: 12 pages, 6 figure

Live calcium imaging of Aedes aegypti neuronal tissues reveals differential importance of chemosensory systems for life-history-specific foraging strategies.

BackgroundThe mosquito Aedes aegypti has a wide variety of sensory pathways that have supported its success as a species as well as a highly competent vector of numerous debilitating infectious pathogens. Investigations into mosquito sensory systems and their effects on behavior are valuable resources for the advancement of mosquito control strategies. Numerous studies have elucidated key aspects of mosquito sensory systems, however there remains critical gaps within the field. In particular, compared to that of the adult form, there has been a lack of studies directed towards the immature life stages. Additionally, although numerous studies have pinpointed specific sensory receptors as well as responding motor outputs, there has been a lack of studies able to monitor both concurrently.ResultsTo begin filling aforementioned gaps, here we engineered Ae. aegypti to ubiquitously express a genetically encoded calcium indicator, GCaMP6s. Using this strain, combined with advanced microscopy, we simultaneously measured live stimulus-evoked calcium responses in both neuronal and muscle cells with a wide spatial range and resolution.ConclusionsBy coupling in vivo live calcium imaging with behavioral assays we were able to gain functional insights into how stimulus-evoked neural and muscle activities are represented, modulated, and transformed in mosquito larvae enabling us to elucidate mosquito sensorimotor properties important for life-history-specific foraging strategies

Sterile insect technology for control of Anopheles mosquito: a mathematical feasibility study

Anopheles mosquito is a vector responsible for the transmission of diseases like Malaria which a_ect many people. Hence its control is a major prevention strategy. Sterile Insect Technology (SIT) is a nonpolluting method of insect control that relies on the release of sterile males. Mating of the released sterile males with wild females leads to non hatching eggs. Thus, if sterile males are released in su_cient numbers or over a su_cient period of time, it can leads to the local reduction or elimination of the wild population. We study the e_ectiveness of the application of SIT for control of Anopheles mosquito via mathematical modeling. Our main result is that there exists a threshold release rate ^_ depending only on the basic o_spring number R and the wild mosquito equilibrium for males such that a release rate higher than ^_ results in elimination of the mosquito population irrespective of its initial size. A release rate _ which is lower than ^_ eliminates the mosquito populations only if it is su_ciently small. If the population is at the wild equilibrium it is reduced by a percentage depending on _ and R only. (Résumé d'auteur

Wolbachia versus dengue: Evolutionary forecasts.

A novel form of biological control is being applied to the dengue virus. The agent is the maternally transmitted bacterium Wolbachia, naturally absent from the main dengue vector, the mosquito Aedes aegypti. Three Wolbachia-based control strategies have been proposed. One is suppression of mosquito populations by large-scale releases of males incompatible with native females; this intervention requires ongoing releases. The other interventions transform wild mosquito populations with Wolbachia that spread via the frequency-dependent fitness advantage of Wolbachia-infected females; those interventions potentially require just a single, local release for area-wide disease control. One of these latter strategies uses Wolbachia that shortens mosquito life, indirectly preventing viral maturation/transmission. The other strategy uses Wolbachia that block viral transmission. All interventions can be undermined by viral, bacterial or mosquito evolution; viral virulence in humans may also evolve. We examine existing theory, experiments and comparative evidence to motivate predictions about evolutionary outcomes. (i) The life-shortening strategy seems the most likely to be thwarted by evolution. (ii) Mosquito suppression has a reasonable chance of working locally, at least in the short term, but long-term success over large areas is challenging. (iii) Dengue blocking faces strong selection for viral resistance but may well persist indefinitely at some level. Virulence evolution is not mathematically predictable, but comparative data provide no precedent for Wolbachia increasing dengue virulence. On balance, our analysis suggests that the considerable possible benefits of these technologies outweigh the known negatives, but the actual risk is largely unknown

Characterization of a broad-based mosquito yeast interfering RNA larvicide with a conserved target site in mosquito semaphorin-1a genes

BACKGROUND: RNA interference (RNAi), which has facilitated functional characterization of mosquito neural development genes such as the axon guidance regulator semaphorin-1a (sema1a), could one day be applied as a new means of vector control. Saccharomyces cerevisiae (baker's yeast) may represent an effective interfering RNA expression system that could be used directly for delivery of RNA pesticides to mosquito larvae. Here we describe characterization of a yeast larvicide developed through bioengineering of S. cerevisiae to express a short hairpin RNA (shRNA) targeting a conserved site in mosquito sema1a genes. RESULTS: Experiments conducted on Aedes aegypti larvae demonstrated that the yeast larvicide effectively silences sema1a expression, generates severe neural defects, and induces high levels of larval mortality in laboratory, simulated-field, and semi-field experiments. The larvicide was also found to induce high levels of Aedes albopictus, Anopheles gambiae and Culex quinquefasciatus mortality. CONCLUSIONS: The results of these studies indicate that use of yeast interfering RNA larvicides targeting mosquito sema1a genes may represent a new biorational tool for mosquito control

A Systematic Review of Mosquito Coils and Passive Emanators: Defining Recommendations for Spatial Repellency Testing Methodologies.

Mosquito coils, vaporizer mats and emanators confer protection against mosquito bites through the spatial action of emanated vapor or airborne pyrethroid particles. These products dominate the pest control market; therefore, it is vital to characterize mosquito responses elicited by the chemical actives and their potential for disease prevention. The aim of this review was to determine effects of mosquito coils and emanators on mosquito responses that reduce human-vector contact and to propose scientific consensus on terminologies and methodologies used for evaluation of product formats that could contain spatial chemical actives, including indoor residual spraying (IRS), long lasting insecticide treated nets (LLINs) and insecticide treated materials (ITMs). PubMed, (National Centre for Biotechnology Information (NCBI), U.S. National Library of Medicine, NIH), MEDLINE, LILAC, Cochrane library, IBECS and Armed Forces Pest Management Board Literature Retrieval System search engines were used to identify studies of pyrethroid based coils and emanators with key-words "Mosquito coils" "Mosquito emanators" and "Spatial repellents". It was concluded that there is need to improve statistical reporting of studies, and reach consensus in the methodologies and terminologies used through standardized testing guidelines. Despite differing evaluation methodologies, data showed that coils and emanators induce mortality, deterrence, repellency as well as reduce the ability of mosquitoes to feed on humans. Available data on efficacy outdoors, dose-response relationships and effective distance of coils and emanators is inadequate for developing a target product profile (TPP), which will be required for such chemicals before optimized implementation can occur for maximum benefits in disease control