Dana A. Focks

Temporal and Geographic Patterns of Aedes aegypti (Diptera: Culicidae) Production in Iquitos, Peru

Abstract Large-scale longitudinal cohort studies are necessary to characterize temporal and geographic variation in Aedes aegypti (L.) (Diptera: Culicidae) production patterns and to develop targeted dengue control strategies that will reduce disease. We carried out pupal/demographic surveys in a circuit of ≈6,000 houses, 10 separate times, between January 1999 and August 2002 in the Amazonian city of Iquitos, Peru. We quantified the number of containers positive for Ae. aegypti larvae and/or pupae, containers holding pupae, and the absolute number of pupae by 4-mo sampling circuits and spatially by geographic area by using a geographic information system developed for the city. A total of 289,941 water-holding containers were characterized, of which 7.3% were positive for Ae. aegypti. Temporal and geographic variations were detected for all variables examined, and the relative importance of different container types for production of Ae. aegypti was calculated. Ae. aegypti larvae and pupae were detected in 64 types of containers. Consistent production patterns were observed for the lid status (lids: 32% wet containers, 2% pupal production), container location (outdoor: 43% wet containers, 85% pupal production), and method by which the container was filled with water (rain filled: 15% wet containers, 88.3% pupal production); these patterns were consistent temporally and geographically. We describe a new container category (nontraditional) that includes transient puddles, which were rare but capable of producing large numbers of pupae. Because of high variable pupal counts, four container categories (large tank, medium storage, miscellaneous, and nontraditional) should be targeted in addition to outdoor rain-filled containers that are not covered by a lid. The utility of targeted Ae. aegypti control is discussed, as well as the ability to achieve control objectives based on published but untested threshold values.

Epidemiology of Dengue Virus in Iquitos, Peru 1999 to 2005: Interepidemic and Epidemic Patterns of Transmission

Background Comprehensive, longitudinal field studies that monitor both disease and vector populations for dengue viruses are urgently needed as a pre-requisite for developing locally adaptable prevention programs or to appropriately test and license new vaccines. Methodology and Principal Findings We report the results from such a study spanning 5 years in the Amazonian city of Iquitos, Peru where DENV infection was monitored serologically among ∼2,400 members of a neighborhood-based cohort and through school-based absenteeism surveillance for active febrile illness among a subset of this cohort. At baseline, 80% of the study population had DENV antibodies, seroprevalence increased with age, and significant geographic variation was observed, with neighborhood-specific age-adjusted rates ranging from 67.1 to 89.9%. During the first 15 months, when DENV-1 and DENV-2 were co-circulating, population-based incidence rates ranged from 2–3 infections/100 person-years (p-years). The introduction of DENV-3 during the last half of 2001 was characterized by 3 distinct periods: amplification over at least 5–6 months, replacement of previously circulating serotypes, and epidemic transmission when incidence peaked at 89 infections/100 p-years. Conclusions/Significance Neighborhood-specific baseline seroprevalence rates were not predictive of geographic incidence patterns prior to the DENV-3 introduction, but were closely mirrored during the invasion of this serotype. Transmission varied geographically, with peak incidence occurring at different times among the 8 geographic zones in ∼16 km2 of the city. The lag from novel serotype introduction to epidemic transmission and knowledge of spatially explicit areas of elevated risk should be considered for more effective application of limited resources for dengue prevention.

Towards a global dengue research agenda.

Dengue is the most rapidly advancing vector-borne disease with an estimated 50 million dengue infections occurring annually. As a result of major demographic changes rapid urbanization on a massive scale global travel and environmental change the world - particularly the tropical world - faces enormous challenges from emerging infectious diseases. Dengue epitomizes these challenges. In the early years of the 21st century we are collectively failing to meet the threat posed by dengue as the disease spreads unabated and almost 40% of the worlds population now live at risk of contracting it. Because of the rapidly increasing public health importance of dengue the 2002 World Health Assembly Resolution (WHA55.17) urged greater commitment among Member States and WHO to dengue control; of particular significance is the 2005 Revision of the International Health Regulations (WHA58.3) which includes dengue fever as an example of a disease that may constitute an international public health emergency. (excerpt)

Skeeter Buster: a stochastic, spatially explicit modeling tool for studying Aedes aegypti population replacement and population suppression strategies.

Background Dengue is the most important mosquito-borne viral disease affecting humans. The only prevention measure currently available is the control of its vectors, primarily Aedes aegypti. Recent advances in genetic engineering have opened the possibility for a new range of control strategies based on genetically modified mosquitoes. Assessing the potential efficacy of genetic (and conventional) strategies requires the availability of modeling tools that accurately describe the dynamics and genetics of Ae. aegypti populations. Methodology/Principal findings We describe in this paper a new modeling tool of Ae. aegypti population dynamics and genetics named Skeeter Buster. This model operates at the scale of individual water-filled containers for immature stages and individual properties (houses) for adults. The biology of cohorts of mosquitoes is modeled based on the algorithms used in the non-spatial Container Inhabiting Mosquitoes Simulation Model (CIMSiM). Additional features incorporated into Skeeter Buster include stochasticity, spatial structure and detailed population genetics. We observe that the stochastic modeling of individual containers in Skeeter Buster is associated with a strongly reduced temporal variation in stage-specific population densities. We show that heterogeneity in container composition of individual properties has a major impact on spatial heterogeneity in population density between properties. We detail how adult dispersal reduces this spatial heterogeneity. Finally, we present the predicted genetic structure of the population by calculating FST values and isolation by distance patterns, and examine the effects of adult dispersal and container movement between properties. Conclusions/Significance We demonstrate that the incorporated stochasticity and level of spatial detail have major impacts on the simulated population dynamics, which could potentially impact predictions in terms of control measures. The capacity to describe population genetics confers the ability to model the outcome of genetic control methods. Skeeter Buster is therefore an important tool to model Ae. aegypti populations and the outcome of vector control measures.

Editorial: Towards a global dengue research agenda

Dengue is the most rapidly advancing vector-borne disease with an estimated 50 million dengue infections occurring annually. As a result of major demographic changes rapid urbanization on a massive scale global travel and environmental change the world - particularly the tropical world - faces enormous challenges from emerging infectious diseases. Dengue epitomizes these challenges. In the early years of the 21st century we are collectively failing to meet the threat posed by dengue as the disease spreads unabated and almost 40% of the worlds population now live at risk of contracting it. Because of the rapidly increasing public health importance of dengue the 2002 World Health Assembly Resolution (WHA55.17) urged greater commitment among Member States and WHO to dengue control; of particular significance is the 2005 Revision of the International Health Regulations (WHA58.3) which includes dengue fever as an example of a disease that may constitute an international public health emergency. (excerpt)

Pupal/demographic surveys to inform dengue-vector control

‘Standard’ entomological indicators of the abundance of the urban yellow-fever and dengue vector were developed primarily for monitoring the progress of the Aedes aegypti eradication campaign in the Americas, which was launched by the Pan American Health Organization in the late 1940s. The three classic Stegomyia indices — the house (or premises) index, the container index, and the Breteau index — are all based on the presence or absence of immature stages of the vector in water-holding containers and confined natural habitats close to or inside dwellings or other buildings. With eradication as the public-health objective, the success of the national and international campaigns was measured in terms of the complete absence of Ae. aegypti. Subsequent surveillance and vector-control responses were necessary to ensure the rapid detection and elimination of any re-infestations. For the purposes of campaign management, the Stegomyia indices were largely satisfactory, although ovitraps were later introduced as an additional and more sensitive method for detecting the presence of Ae. aegypti in neighbourhoods where vector densities had been reduced to very low levels. With the abandonment of the hemispheric eradication campaign in 1985, the overwhelming majority of Ae. aegypti programmes in the Americas became de-facto control programmes, having much in common with those being carried out elsewhere in the tropics and subtropics. The responsibility of such programmes thus became, and remains, the cost-effective utilization of available resources to reduce vector populations to levels at which they are no longer of significant public-health importance. It became increasingly evident, however, that the Stegomyia indices were largely inadequate to the task of defining those levels and answering the question, for a given epidemiological situation, of how much vector control would be needed to reduce, interrupt or prevent viral transmission. That the traditional indices are a poor proxy for measuring adult abundance and are of limited use in assessing transmission risk was acknowledged in 1999, at a World Health Organization informal consultation on strengthening the implementation of the global strategy for the prevention and control of dengue fever and dengue haemorrhagic fever (WHO, 2000). The attendees at this consultation recommended ‘the refinement of existing indicators and/or the development of new indicators that better reflect transmission potential’. There followed a review, commissioned by the UNICEF/ UNDP/World Bank/WHO Special Programme for Research and Training in Tropical Diseases (TDR), of the entomological sampling methods and indicators for dengue vectors (Focks, 2003). The pupalsurvey technique was considered to be a promising method for identifying the most adult-productive container habitats in various ecological settings where dengue is an important public-health problem, and one with the potential for guiding programme managers in the application of more costeffective, targeted approaches to vector control. Moreover, it was envisaged that pupal surveys, if combined with the collection of demographic data, could help elucidate the transmission dynamics of dengue, identify transmission risk, and quantify and validate target levels of vector abundance. The TDR subsequently funded a multicountry project to validate the reliability and practicality of the pupal-survey technique. Theproject involvedstudies inninecountries on Annals of Tropical Medicine & Parasitology, Vol. 100, Supplement No. 1, S1–S3 (2006)

Parameterization and sensitivity analysis of a complex simulation model for mosquito population dynamics, dengue transmission, and their control.

Models can be useful tools for understanding the dynamics and control of mosquito-borne disease. More detailed models may be more realistic and better suited for understanding local disease dynamics; however, evaluating model suitability, accuracy, and performance becomes increasingly difficult with greater model complexity. Sensitivity analysis is a technique that permits exploration of complex models by evaluating the sensitivity of the model to changes in parameters. Here, we present results of sensitivity analyses of two interrelated complex simulation models of mosquito population dynamics and dengue transmission. We found that dengue transmission may be influenced most by survival in each life stage of the mosquito, mosquito biting behavior, and duration of the infectious period in humans. The importance of these biological processes for vector-borne disease models and the overwhelming lack of knowledge about them make acquisition of relevant field data on these biological processes a top research priority.

TOXORHYNCHITES AS BIOCONTROL AGENTS

ABSTRACT Toxorhynchites is an unusual and interesting genus of large, non-biting mosquitoes. In spite of their size, they are – like many species of mosquitoes – completely harmless to man. The larvae are predaceous on other mosquitoes and aquatic organisms that inhabit both natural and artificial containers. Because this habitat is the source of several medically important species of mosquitoes, there is warrant for evaluating the potential of Toxorhynchites as a biological control agent under various conditions. Toxorhynchites is not seen as a panacea for the control of all container-inhabiting mosquitoes. However, it has demonstrated practical potential in certain restricted but important situations.

Sample sizes for identifying the key types of container occupied by dengue-vector pupae: the use of entropy in analyses of compositional data

Abstract A method has been developed for estimating the sample sizes needed to identify categories that comprise a large proportion of a compositional data-set. The method is to be used in the design of surveys of mosquito pupae, for identifying the key container types from which the majority of adult dengue vectors emerge. Although a finite-population correction was devised for estimating the mean of a negative binomial distribution, other complications of parametric approaches make them unlikely to yield methods simple enough to be practically applicable. The Shannon–Wiener index was therefore investigated as a more useful alternative, at the cost of theoretical generalizability, in an approach based on resampling methods in conjunction with the use of entropy. This index can be used to summarize the degree to which pupae are either concentrated in a few container types, or dispersed among many. An empirical relationship between the index and the repeatability of surveys of differing sample sizes was observed. A step-wise rule, based on the entropy of the cumulative data, was devised for determining the sample size, in terms of the number of houses positive for pupae, at which a pupal survey might reasonably be stopped.

Aedes aegypti in Jamaica, West Indies : container productivity profiles to inform control strategies

Objective  To describe the Aedes aegypti container profile in the three parishes of Portland, St. Anns and St. Catherine, Jamaica.