Vector Population Size Research Articles

Effectiveness evaluation of mosquito suppression strategies on dengue transmission under changing temperature and precipitation

Widespread resurgence of dengue outbreaks has seriously threatened the global health. Due to lack of treatments and vaccines, one key strategy in dengue control is to reduce the vector population size. As an environment-friendly mosquito control approach, releasing male mosquitoes transinfected with specific Wolbachia strain into the field to suppress the wild mosquito population size has become wildly accepted. The current study evaluates the effectiveness of this suppression strategy on dengue control under changing temperature and precipitation profiles. We formulate a mathematical model which includes larval intra-specific competition, the maturation period for mosquitoes, the extrinsic incubation period (EIP) and intrinsic incubation period (IIP). The persistence of mosquitoes and disease is discussed in terms of two basic reproduction numbers (RM and R0) and the release ratio pw. Further numerical simulations are carried out to not only validate theoretical results, but also provide interesting quantitative observations. Sensitivity analysis on the reproduction numbers, peak size, peak time and the final epidemic size is performed with respect to model parameters, which highlights effective control measures against dengue transmission. Moreover, by assuming temperature and precipitation dependent mosquito-related parameters, the model can be used to project the effectiveness of releasing Wolbachia-carrying males under climatic variations. It is shown that the effectiveness of various control strategies is highly dependent on the changing temperature and precipitation profiles. In particular, the model projects that it is most challenging to control the disease at the favorable temperature (around 27∼30∘C) and precipitation (5∼8mm/day) range, during which the basic reproduction number R0 is very high and more Wolbachia-infected males should be released.

RNA interference-mediated knockdown of genes involved in sugar transport and metabolism disrupts psyllid Bactericera cockerelli (Order: Hemiptera) gut physiology and results in high mortality.

The causal agent of zebra chip of potato and vein-greening diseases of tomato is "Candidatus Liberibacter solanacearum" (CLso), a fastidious bacterium transmitted by the potato psyllid. In the absence of disease-resistant cultivars, disease management has relied on minimizing vector population size to reduce CLso transmission, which requires frequent insecticide applications. There is growing interest in the use of RNA interference (RNAi) technology to supplant traditional insecticides with biopesticides. This requires knowledge of genes essential for insect livelihood whose knockdown leads to significant mortality or other phenotypes. Such candidate genes can be evaluated by reverse genetics approaches to further corroborate predicted gene function. Here, five potato psyllid genes involved in sugar homeostasis in the potato psyllid gut, α-glucosidase1 (AGLU1), aquaporin2 (AQP2), facilitated trehalose transporter1 (TRET1), Trehalase1 (TRE1), and Trehalase2 (TRE2), were investigated as candidates for effective gene silencing. Potato psyllid dsRNAs were designed to optimize knockdown of gene targets. Third instar PoP nymphs were given a 48-hr ingestion-access period (IAP) on individual or groups of dsRNA in 20% sucrose. Mortality was recorded 0, 3, 5, 7, and 9 days post-IAP. Gene knockdown was analyzed 9 days post-IAP by quantitative real-time reverse-transcriptase polymerase chain reaction amplification. The individual or stacked dsRNA combinations resulted in 20-60% and 20-40% knockdown, respectively, while subsequent psyllid mortality ranged from 20-40% to >60% for single and stacked dsRNA combinations, respectively. Reverse genetics analysis showed that simultaneous knockdown of the five selected candidate genes with predicted functions in pathways involved in sugar-homeostasis, metabolism, and -transport yielded the highest mortality, when compared with single or combinations of targets. Results confirmed the functions afforded by psyllid gut genes responsible for osmotic homeostasis and sugar metabolism/transport are essential for livelihood, identifying them as potentially lucrative RNAi biopesticide targets and highlighted the translational relevance of targeting multiple nodes in a physiological pathway simultaneously.

Does the corn leafhopper Dalbulus maidis (DeLong & Wolcott) (Hemiptera: Cicadellidae) reproduce in hosts other than maize?

The corn leafhopper Dalbulus maidis (DeLong & Wolcott) (Hemiptera: Cicadellidae) is a specialist insect vector of the corn stunting disease complex. This leafhopper has been reported from different overwintering plants, and more information is required regarding other potential Brazilian plants that may serve as reproductive hosts for this specie. The present study aimed to evaluate possible plants for leafhopper reproduction usually found near the maize fields and the D. maidis reproductive differences among maize genotypes. Three trials were conducted to assess D. maidis survival and oviposition on different maize genotypes and other potential host plants. Survival, nymph presence, oviposition, and nymph eclosion rates were observed. No nymphs were observed in plants other than maize. The number of D. maidis adults was influenced by the different maize inbreds, and the mean number of laid eggs varied across the tested maize materials. Oviposition and nymph emergence was observed in pearl millet [Pennisetum glaucum (L.) R. Br., Poaceae] when D. maidis adults were offered only this plant as substrate, although significantly lower than those recorded on maize. Our results suggest that eliminating maize plants in the off-season could be used as a tool to reduce D. maidis reproduction, thereby minimizing the vector population size in the following season.

Modelling the Potential Human Exposure to Japanese Encephalitis Virus (JEV) in Case of Introduction into Reunion Island

Japanese encephalitis virus (JEV) is a vector-borne zoonotic virus and the leading cause of human acute encephalitis in Asia. Continuous human and commercial exchanges between Southeast Asia where JE is endemic and Reunion Island increase the risk of introducing JEV on the island, where putative vectors of JEV such as Culex quinquefasciatus and amplifying hosts such as pigs are present. Each of the 255 Reunionese pig farms was assumed to harbor a Cx. quinquefasciatus population and, together with the competent hosts: pigs and poultry and noncompetent hosts: humans, dogs, and cattle, located within a radius of 1 km, formed an epidemiological unit. We used a deterministic compartmental model to investigate whether these epidemiological units could be invaded by JEV in the event of an introduction. Since the vector population size changes seasonally, we computed the basic reproduction number (R0) using vector population sizes ranging from 100 to 100,000 vectors for each of the 255 epidemiological units. The size of the potentially exposed human population was calculated in the case where the virus would be introduced in a single epidemiological unit and in the extreme case where the virus would have spread over the whole island. For a vector population of 1,000 vectors per unit, 2 out of 255 units had an R0 ≥ 1. With 50,000 vectors per unit, more than 75% (193/255) of the units had an estimated R0 ≥ 1, representing a median of approximately 2,500 potentially exposed people if JEV was introduced in a single unit, and about 140,000 potentially exposed people if JEV had expanded throughout the island. The unit located a few kilometers from the large port area of Reunion Island had an estimated R0 ≥ 1 with at least 10,000 vectors, making it a potential gateway to JEV given a virus introduction of infected vectors.

Integrating Community Ecology into Models of Vector-Borne Virus Transmission.

Vector-borne plant viruses are a diverse and dynamic threat to agriculture with hundreds of economically damaging viruses and insect vector species. Mathematical models have greatly increased our understanding of how alterations of vector life history and host-vector-pathogen interactions can affect virus transmission. However, insect vectors also interact with species such as predators and competitors in food webs, and these interactions affect vector population size and behaviors in ways that mediate virus transmission. Studies assessing how species' interactions affect vector-borne pathogen transmission are limited in both number and scale, hampering the development of models that appropriately capture community-level effects on virus prevalence. Here, we review vector traits and community factors that affect virus transmission, explore the existing models of vector-borne virus transmission and areas where the principles of community ecology could improve the models and management, and finally evaluate virus transmission in agricultural systems. We conclude that models have expanded our understanding of disease dynamics through simulations of transmission but are limited in their ability to reflect the complexity of ecological interactions in real systems. We also document a need for experiments in agroecosystems, where the high availability of historical and remote-sensing data could serve to validate and improve vector-borne virus transmission models.

Global dynamics in a model for anthrax transmission in animal populations

In this paper, we propose a deterministic model to study the transmission dynamics of anthrax disease, which includes live animals, carcasses, spores in the environment and vectors. We derive three biologically plausible and insightful quantities (reproduction numbers) that determine the stability of the equilibria. We carry out rigorous mathematical analysis on the model dynamics, the global stability of the disease-free and vector-free equilibrium, the disease-free equilibrium and the vector-free disease equilibrium is proved. The global stability of the endemic equilibrium as the basic reproduction number is greater than one is derived in the special case in which the disease-related death rate is zero. The possibility of backward bifurcation is briefly discussed. Numerical analyses are carried out to understand the transmission dynamics of anthrax and investigate effective control strategies for the outbreaks of the disease. Our studies suggest that the larval vector control measure should be taken as early as possible to control the vector population size, a vaccination policy and an animal carcass removal policy are useful methods to control the prevalence of the diseases in infected animal populations, the adult vector control measure is also necessary to prevent the transmission of anthrax.

No net effect of host density on tick-borne disease hazard due to opposing roles of vector amplification and pathogen dilution.

To better understand vector‐borne disease dynamics, knowledge of the ecological interactions between animal hosts, vectors, and pathogens is needed. The effects of hosts on disease hazard depends on their role in driving vector abundance and their ability to transmit pathogens. Theoretically, a host that cannot transmit a pathogen could dilute pathogen prevalence but increase disease hazard if it increases vector population size. In the case of Lyme disease, caused by Borrelia burgdorferi s.l. and vectored by Ixodid ticks, deer may have dual opposing effects on vectors and pathogen: deer drive tick population densities but do not transmit B. burgdorferi s.l. and could thus decrease or increase disease hazard. We aimed to test for the role of deer in shaping Lyme disease hazard by using a wide range of deer densities while taking transmission host abundance into account. We predicted that deer increase nymphal tick abundance while reducing pathogen prevalence. The resulting impact of deer on disease hazard will depend on the relative strengths of these opposing effects. We conducted a cross‐sectional survey across 24 woodlands in Scotland between 2017 and 2019, estimating host (deer, rodents) abundance, questing Ixodes ricinus nymph density, and B. burgdorferi s.l. prevalence at each site. As predicted, deer density was positively associated with nymph density and negatively with nymphal infection prevalence. Overall, these two opposite effects canceled each other out: Lyme disease hazard did not vary with increasing deer density. This demonstrates that, across a wide range of deer and rodent densities, the role of deer in amplifying tick densities cancels their effect of reducing pathogen prevalence. We demonstrate how noncompetent host density has little effect on disease hazard even though they reduce pathogen prevalence, because of their role in increasing vector populations. These results have implications for informing disease mitigation strategies, especially through host management.

'Candidatus Liberibacter' Pathosystems at the Forefront of Agricultural and Biological Research Challenges.

'Candidatus Liberibacter' Pathosystems at the Forefront of Agricultural and Biological Research Challenges.

Estimating disease vector population size from citizen science data.

Citizen science projects have the potential to address hypotheses requiring extremely large datasets that cannot be collected with the financial and labour constraints of most scientific projects. Data collection by the general public could expand the scope of scientific enquiry if these data accurately capture the system under study. However, data collection inconsistencies by the untrained public may result in biased datasets that do not accurately represent the natural world. In this paper, we harness the availability of scientific and public datasets of the Lyme disease tick vector to identify and account for biases in citizen science tick collections. Estimates of tick abundance from the citizen science dataset correspond moderately with estimates from direct surveillance but exhibit consistent biases. These biases can be mitigated by including factors that may impact collector participation or effort in statistical models, which, in turn, result in more accurate estimates of tick population sizes. Accounting for collection biases within large-scale, public participation datasets could update species abundance maps and facilitate using the wealth of citizen science data to answer scientific questions at scales that are not feasible with traditional datasets.

Transgenesis of Bioluminescence (BL) Gene and Its Potential Use for Malaria Residual Transmission Issue and Other Vector Borne Diseases: New Considerations for Vector and Vector Borne Diseases (VBD) Control

The present article considers the potential of transgenesis of the bioluminescent gene for malaria and other vector borne diseases (VBD) control. Vector control is an important component of every vector control operations for the vector borne disease control. Actually the bioluminescence phenomena and the green fluorescent protein GPF open great field of researches and “On December 10, 2008 Osamu Shimomura, Martin Chalfie and Roger Tsien were awarded the Nobel Prize in Chemistry for "the discovery and development of the green fluorescent protein, GFP". Bio-molecular technologies and transgenesis open the field for getting BL mosquitoes such as Plasmodium (several references) for “Transmission Reducing Activity” (TRA) without any hazard on human beings and ecological level. For exemple BL mosquitoes would be of paramount importance for mark-release-recapture becoming easier to implement and giving more relevant and reliable data on relation between density (size of the population) and distance (and wind) as the vector population size decrease with increasing distance from the source of production (breeding site) or release point. BL appeared already of great interest to understand the relation vector/parasite and to assess transmission intensity. Our idea is to genetically produce “Bioluminescent Mosquitoes” allowing a better identification of their presence, behavior, densities, infected specimens, risk of transmission before/after vector control operations which could be greatly improved thanks to the targeting of mosquitoes resting site or flight range and any other biological component.

Wolbachia

Ary Hoffman introduces Wolbachia.

Economic injury levels for the potato yellow vein disease and its vector, Trialeurodes vaporariorum (Hemiptera: Aleyrodidae), affecting potato crops in the Andes

The Economic Injury Level (EIL) for pest insects is conventionally expressed in terms of the number of insects, because the level of pest injury to plants is usually difficult to quantify. However, when insect injury is mainly caused indirectly by transmission of plant pathogens, the number of insects may not be an ideal measure to calculate the EIL. The greenhouse whitefly (GWF) Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae) is the only known vector of the Potato yellow vein virus (PYVV), the causal agent of potato yellow vein disease (PYVD), but the direct injury caused by insect feeding may not be significant since potato is not a preferred host for the GWF. Here, we quantified the effect of GWF feeding and PYVD incidence on potato yield, as compared to potato market value and control costs to determine the EIL for this system. We found that the direct injury caused by GWF feeding does not affect potato yield. However, an exponential reduction in crop yield was observed with the increase of PYVD incidence. Thus, the criteria to decide upon applying a control measure ranges between 100 and 1200 infected plants/ha depending mainly on potato market price. We suggest that disease incidence should be used to calculate the EIL for PYVD management, instead of focusing on estimations of vector population size. Furthermore, we stress that potato market price is fundamental in defining an appropriate, case specific EIL for decision-making in PYVD management.

Disentangling Effects of Vector Birth Rate, Mortality Rate, and Abundance on Spread of Plant Pathogens.

Models on the spread of insect-transmitted plant pathogens often fix vector population size by assuming that deaths are offset by births. Although such mathematical simplifications are often justified, deemphasizing parameters that govern vector population size is problematic, as reproductive biology and mortality schedules of vectors of plant pathogens receive little empirical attention. Here, the importance of explicitly including parameters for vector birth and death rates was evaluated by comparing results from models with fixed vector population size with models with logistic vector population growth. In fixed vector population size models, increasing vector mortality decreased percentage of inoculative vectors, but had no effect on vector population size, as deaths were offset by births. In models with logistic vector population growth, increasing vector mortality decreased percentage of inoculative vectors and decreased vector population size. Consequently, vector mortality had a greater effect on pathogen spread in models with logistic vector population growth than in models with fixed vector population size. Further, in models with logistic vector population growth, magnitude of vector birth rate determined time required for vector populations to reach large size, thereby determining when pathogen spread occurred quickly. Assumptions regarding timing of vector mortality within a time step also affected model outcome. A greater emphasis of vector entomologists on studying reproductive biology and mortality schedules of insect species that transmit plant pathogens will facilitate identification of conditions associated with rapid growth of vector populations and could lead to development of novel control strategies.

Reproductive and developmental costs of deltamethrin resistance in the Chagas disease vector Triatoma infestans.

Effective chemical control relies on reducing vector population size. However, insecticide selection pressure is often associated with the development of resistant populations that reduce control success. In treated areas, these resistant individuals present an adaptive advantage due to enhanced survival. Resistance can also lead to negative effects when the insecticide pressure ceases. In this study, the biological effects of deltamethrin resistance were assessed in the Chagas disease vector Triatoma infestans. The length of each developmental stage and complete life cycle, mating rate, and fecundity were evaluated. Susceptible and resistant insects presented similar mating rates. A reproductive cost of resistance was expressed as a lower fecundity in the resistant colony. Developmental costs in the resistant colony were in the form of a shortening of the second and third nymph stage duration and an extension of the fifth stage. A maternal effect of deltamethrin resistance is suggested as these effects were identified in resistant females and their progeny independently of the mated male's deltamethrin response. Our results suggest the presence of pleiotropic effects of deltamethrin resistance. Possible associations of these characters to other traits such as developmental delays and behavioral resistance are discussed.

Determinants of the population growth of the West Nile virus mosquito vector Culex pipiens in a repeatedly affected area in Italy

BackgroundThe recent spread of West Nile Virus in temperate countries has raised concern. Predicting the likelihood of transmission is crucial to ascertain the threat to Public and Veterinary Health. However, accurate models of West Nile Virus (WNV) expansion in Europe may be hampered by limited understanding of the population dynamics of their primary mosquito vectors and their response to environmental changes.MethodsWe used data collected in north-eastern Italy (2009–2011) to analyze the determinants of the population growth rate of the primary WNV vector Culex pipiens. A series of alternative growth models were fitted to longitudinal data on mosquito abundance to evaluate the strength of evidence for regulation by intrinsic density-dependent and/or extrinsic environmental factors. Model-averaging algorithms were then used to estimate the relative importance of intrinsic and extrinsic variables in describing the variations of per-capita growth rates.ResultsResults indicate a much greater contribution of density-dependence in regulating vector population growth rates than of any environmental factor on its own. Analysis of an average model of Cx. pipiens growth revealed that the most significant predictors of their population dynamics was the length of daylight, estimated population size and temperature conditions in the 15 day period prior to sampling. Other extrinsic variables (including measures of precipitation, number of rainy days, and humidity) had only a minor influence on Cx. pipiens growth rates.ConclusionsThese results indicate the need to incorporate density dependence in combination with key environmental factors for robust prediction of Cx. pipiens population expansion and WNV transmission risk. We hypothesize that detailed analysis of the determinants of mosquito vector growth rate as conducted here can help identify when and where an increase in vector population size and associated WNV transmission risk should be expected.

How entomological studies can help the control of mosquito-borne diseases: a five-years experience in north-eastern Italy

North-eastern Italy is particularly suitable for mosquito survival, due to its climate, landscape and abundance of wild/domestic animals. After the emergence of West Nile Virus (WNV) in 2008, entomological studies were implemented. Here we describe how entomological data were managed to optimize surveillance and control of mosquito-borne pathogens. CDC-CO2 traps were used (May-October, 2009-2013). In one site, captures with CDC and gravid traps were organized every 2hrs for 24hrs. In other three sites pre- and post-disinfestation captures were done. The mosquitoes were screened by RT-PCR for Flaviviridae and a sub-sample for Bunyaviridae. Host preference of Culex pipiens was assessed by PCR blood meal analysis. Mapping, modelling and spatial analyses were done using entomological data to identify correlations with climate, landscape, animal and human infections. More than 700,000 mosquitoes were collected, with Cx.pipiens the most abundant (80%) and the only vector of WNV and USUV. Tahyna virus was isolated once in Ochlerotatus caspius. Cx.pipiens fed preferentially on birds (76%), mainly blackbird, sparrow, magpie and collared dove, but not on humans. Diel activity showed that Cx.pipiens changed its host searching activity according to the season and defined the favorite period for oviposition. The success of disinfestation measures in reducing Cx.pipiens density varied according to the methods used. The contribution of density-dependence in regulating vector population growth resulted greater than any environmental factor on its own. Overall the most significant predictors of Cx.pipiens dynamics included length of daylight, population density and temperature in the 15 days prior to sampling. Precipitation, number of rainy days and humidity had limited importance. Linear models detected significant relationships between WNV in humans and mosquitoes. Spatial analysis detected clusters of WNV occurrences for all the hosts, identifying an area where to focus surveillance and promptly detect WNV re-activation. In long term studies the mosquito species composition, seasonality, distribution, abundance and pathogen rate of infections were assessed. The blood-meal analyses indicated possible bird targets for surveillance. The control of the efficacy of disinfestations highlighted the need for harmonic guidelines. Modelling indicated the need to incorporate density dependence in combination with key environmental factors for robust prediction of Cx.pipiens population expansion, helping to identify when and where an increase in vector population size and WNV transmission risk should be expected. The results stressed also the necessity to improve the density and the frequency of mosquito captures.

Environmental changes can produce shifts in chagas disease infection risk.

An epidemiological network contains all the organisms involved (types) in the transmission of a parasite. The nodes of the network represent reservoirs, hosts, and vectors, while the links between the nodes represent the strength and direction of parasite movement. Networks that contain humans are of special interest because they are of concern to public health authorities. Under these circumstances, it is possible, in principle, to identify cycles (closed paths in the network) that include humans and select the ones that carry the maximum probability of human infection. The basic reproduction number R0 in such a network gives the average number of new infections of any type after the introduction of one individual infected by any type. To obtain R0 for complex networks, one can use the next-generation matrix (NGM) approach. Every entry in NGM will average the contribution of each link that connects two types. To tease the contribution of every cycle apart, we define the virulence as the geometric mean of the NGM entries corresponding to the links therein. This approach allows for the quantification of specific cycles of interest while it also makes the computation of the sensitivity and elasticity of the parameters easier. In this work, we compute the virulence for the transmission dynamics of Chagas disease for a typical rural area in Colombia incorporating the effect of environmental changes on the vector population size. We concluded that the highest contribution to human infection comes from humans themselves, which is a surprising and interesting result. In addition, sensitivity analysis revealed that increasing vector population size increases the risk of human infection.

Fast and Slow dynamics of Malaria model with relapse

Two mathematical models of malaria with relapse are studied. When the vector population size is constant, complete analyses of the dynamics are conducted. The geometric singular perturbation theory is used to analyze the full dynamics. On the critical manifold, from next generation matrix method, we obtain the basic reproduction number. The global stability of disease-free equilibrium and the uniformly persistence of malaria have also been analyzed. While the vector population size is variable, the basic reproduction number and the stability of disease-free as well as the malaria-infected equilibrium have been obtained in a similar way. Some numerical simulations are also given.

Interactions in a host plant-virus–vector–parasitoid system: Modelling the consequences for virus transmission and disease dynamics

A full understanding of plant virus epidemiology requires studies at different scales of integration: from within-plant cell processes to vector population dynamics, behaviour and broader ecological interactions. Vectors respond to cues derived from plants (both healthy and virus-infected), from natural enemies and from other environmental influences, and these directly affect the temporal and spatial patterns of disease development. The key element in linking these scales is the transmission process and the determining factors involved. We use a mathematical model to show how the presence of natural enemies, by increasing virus transmission, can increase the rate of virus disease development while at the same time reducing vector population size, supporting recent empirical evidence obtained in microcosm studies. The implication of this work is that biological control of arthropod pests, which are also virus vectors, using parasitoid wasps, may have unanticipated and negative effects in terms of increased incidence of virus disease.

Integration of irradiation with cytoplasmic incompatibility to facilitate a lymphatic filariasis vector elimination approach

BackgroundMass drug administration (MDA) is the emphasis of an ongoing global lymphatic filariasis (LF) elimination program by the World Health Organization, in which the entire 'at risk' human population is treated annually with anti-filarial drugs. However, there is evidence that the MDA strategy may not be equally appropriate in all areas of LF transmission, leading to calls for the augmentation of MDA with anti-vector interventions. One potential augmentative intervention is the elimination of vectors via repeated inundative releases of male mosquitoes made cytoplasmically incompatible via an infection with Wolbachia bacteria. However, with a reduction in the vector population size, there is the risk that an accidental female release would permit the establishment of the incompatible Wolbachia infection type, resulting in population replacement instead of population elimination. To avoid the release of fertile females, we propose the exposure of release individuals to low doses of radiation to sterilize any accidentally released females, reducing the risk of population replacement.ResultsAedes polynesiensis pupae of differing ages were irradiated to determine a radiation dose that results in sterility but that does not affect the survival and competitiveness of males. Laboratory assays demonstrate that males irradiated at a female sterilizing dosage of 40 Gy are equally competitive with un-irradiated males. No effect of irradiation on the ability of Wolbachia to affect egg hatch was observed.ConclusionAn irradiation dose of 40 Gy is sufficient to cause female sterility, but has no observed negative effect on male fitness. The results support further development of this approach as a preventative measure against accidental population replacement.