Kacey C. Ernst

Climate and dengue transmission: evidence and implications.

Background: Climate influences dengue ecology by affecting vector dynamics, agent development, and mosquito/human interactions. Although these relationships are known, the impact climate change will have on transmission is unclear. Climate-driven statistical and process-based models are being used to refine our knowledge of these relationships and predict the effects of projected climate change on dengue fever occurrence, but results have been inconsistent. Objective: We sought to identify major climatic influences on dengue virus ecology and to evaluate the ability of climate-based dengue models to describe associations between climate and dengue, simulate outbreaks, and project the impacts of climate change. Methods: We reviewed the evidence for direct and indirect relationships between climate and dengue generated from laboratory studies, field studies, and statistical analyses of associations between vectors, dengue fever incidence, and climate conditions. We assessed the potential contribution of climate-driven, process-based dengue models and provide suggestions to improve their performance. Results and Discussion: Relationships between climate variables and factors that influence dengue transmission are complex. A climate variable may increase dengue transmission potential through one aspect of the system while simultaneously decreasing transmission potential through another. This complexity may at least partly explain inconsistencies in statistical associations between dengue and climate. Process-based models can account for the complex dynamics but often omit important aspects of dengue ecology, notably virus development and host–species interactions. Conclusion: Synthesizing and applying current knowledge of climatic effects on all aspects of dengue virus ecology will help direct future research and enable better projections of climate change effects on dengue incidence. Citation: Morin CW, Comrie AC, Ernst KC. 2013. Climate and dengue transmission: evidence and implications. Environ Health Perspect 121:1264–1272; http://dx.doi.org/10.1289/ehp.1306556

On the Seasonal Occurrence and Abundance of the Zika Virus Vector Mosquito Aedes Aegypti in the Contiguous United States.

Introduction: An ongoing Zika virus pandemic in Latin America and the Caribbean has raised concerns that travel-related introduction of Zika virus could initiate local transmission in the United States (U.S.) by its primary vector, the mosquito Aedes aegypti. Methods: We employed meteorologically driven models for 2006-2015 to simulate the potential seasonal abundance of adult Aedes aegypti for fifty cities within or near the margins of its known U.S. range. Mosquito abundance results were analyzed alongside travel and socioeconomic factors that are proxies of viral introduction and vulnerability to human-vector contact. Results: Meteorological conditions are largely unsuitable for Aedes aegypti over the U.S. during winter months (December-March), except in southern Florida and south Texas where comparatively warm conditions can sustain low-to-moderate potential mosquito abundance. Meteorological conditions are suitable for Aedes aegypti across all fifty cities during peak summer months (July-September), though the mosquito has not been documented in all cities. Simulations indicate the highest mosquito abundance occurs in the Southeast and south Texas where locally acquired cases of Aedes-transmitted viruses have been reported previously. Cities in southern Florida and south Texas are at the nexus of high seasonal suitability for Aedes aegypti and strong potential for travel-related virus introduction. Higher poverty rates in cities along the U.S.-Mexico border may correlate with factors that increase human exposure to Aedes aegypti. Discussion: Our results can inform baseline risk for local Zika virus transmission in the U.S. and the optimal timing of vector control activities, and underscore the need for enhanced surveillance for Aedes mosquitoes and Aedes-transmitted viruses.

Local topographic wetness indices predict household malaria risk better than land-use and land-cover in the western Kenya highlands

BackgroundIdentification of high-risk malaria foci can help enhance surveillance or control activities in regions where they are most needed. Associations between malaria risk and land-use/land-cover are well-recognized, but these environmental characteristics are closely interrelated with the lands topography (e.g., hills, valleys, elevation), which also influences malaria risk strongly. Parsing the individual contributions of land-cover/land-use variables to malaria risk requires examining these associations in the context of their topographic landscape. This study examined whether environmental factors like land-cover, land-use, and urban density improved malaria risk prediction based solely on the topographically-determined context, as measured by the topographic wetness index.MethodsThe topographic wetness index, an estimate of predicted water accumulation in a defined area, was generated from a digital terrain model of the landscape surrounding households in two neighbouring western Kenyan highland communities. Variables determined to best encompass the variance in this topographic wetness surface were calculated at a household level. Land-cover/land-use information was extracted from a high-resolution satellite image using an object-based classification method. Topographic and land-cover variables were used individually and in combination to predict household-level malaria in the communities through an iterative split-sample model fitting and testing procedure. Models with only topographic variables were compared to those with additional predictive factors related to land-cover/land-use to investigate whether these environmental factors improved prediction of malaria based on the shape of the land alone.ResultsVariables related to topographic wetness proved most useful in predicting the households of individuals contracting malaria in this region of rugged terrain. Other variables related to human modification of the environment also demonstrated clear associations with household malaria. However, these land-cover/land-use variables failed to produce unambiguous improvements in statistical predictive models controlling for important topographic factors, with none improving prediction of household-level malaria more than 75% of the time.ConclusionsTopographic wetness values in this region of highly varied terrain more accurately predicted houses at greater risk of malaria than did consideration of land-cover/land-use characteristics. As such, those planning control or local elimination strategies in similar highland regions may use topographic and geographic characteristics to effectively identify high-receptivity regions that may require enhanced vigilance.

Environmental, socio-demographic and behavioural determinants of malaria risk in the western Kenyan highlands: a case–control study

Objective  To identify risk factors for uncomplicated malaria in highland areas of East Africa at higher risk of malaria epidemics, in order to design appropriate interventions.

Correlates of high vaccination exemption rates among kindergartens.

OBJECTIVES The present study was designed to characterize Arizona schools with high rates of permanent PBE among kindergartners, and to determine the degree to which they aggregate across the state. METHODS Data for permanent personal belief exemptions (PBE) were accessed through the 2010-2011 kindergarten Immunization Data Report (IDR) from the Arizona Department of Health Services (AZDHS), and were linked to the 2009-2010 data from the National Center of Education Statistics (NCES). Incidence rate ratios (IRR) were calculated using negative binomial regression, and hotspots were identified using Getis-Ord Gi*. RESULTS Schools with highest proportion of white students compared to the lowest had the highest exemption rates (IRR=14.11; 95% confidence interval [CI], 9.47-21.03); furthermore charter schools and those with low prevalence of free and reduced lunches had significantly higher rates of PBE. Statewide analyses of PBE identified higher rates of permanent PBE in northern vs. southern Arizona, while a more focused examination of the central Arizona region demonstrated a pattern of increased PBE from west to east. CONCLUSION In Arizona, the profile of a high PBE school is that of a charter school attended by predominantly white, higher-income students. The local and statewide hotspots serve as a challenge that requires a multi-faceted approach that calls upon all healthcare professionals. It is important that both local and statewide pockets be targeted by local and state officials either to improve vaccination uptake or to employ careful monitoring to identify outbreaks at their onset.

Implications of philosophical and personal belief exemptions on re-emergence of vaccine-preventable disease: The role of spatial clustering in under-vaccination

Vaccine exemption policies vary by state, and it has been demonstrated that easier protocols for personal/ philosophical belief exemptions are associated with lower vaccine coverage at the state level. However, this does not lead to ubiquitously lower immunization rates across the state, as shown by variability in county-level exemption rates. Rather, there is geographical clustering of areas which are then more vulnerable to outbreaks of vaccine-preventable diseases. Understanding both state and local patterns of vaccine exemptions and developing policies and public health interventions to increase coverage in high-risk areas is critical. At all levels of healthcare, efforts must be made to encourage vaccination education and legislation to protect the public’s health.

Meteorologically Driven Simulations of Dengue Epidemics in San Juan, PR

Meteorological factors influence dengue virus ecology by modulating vector mosquito population dynamics, viral replication, and transmission. Dynamic modeling techniques can be used to examine how interactions among meteorological variables, vectors and the dengue virus influence transmission. We developed a dengue fever simulation model by coupling a dynamic simulation model for Aedes aegypti, the primary mosquito vector for dengue, with a basic epidemiological Susceptible-Exposed-Infectious-Recovered (SEIR) model. Employing a Monte Carlo approach, we simulated dengue transmission during the period of 2010–2013 in San Juan, PR, where dengue fever is endemic. The results of 9600 simulations using varied model parameters were evaluated by statistical comparison (r2) with surveillance data of dengue cases reported to the Centers for Disease Control and Prevention. To identify the most influential parameters associated with dengue virus transmission for each period the top 1% of best-fit model simulations were retained and compared. Using the top simulations, dengue cases were simulated well for 2010 (r2 = 0.90, p = 0.03), 2011 (r2 = 0.83, p = 0.05), and 2012 (r2 = 0.94, p = 0.01); however, simulations were weaker for 2013 (r2 = 0.25, p = 0.25) and the entire four-year period (r2 = 0.44, p = 0.002). Analysis of parameter values from retained simulations revealed that rain dependent container habitats were more prevalent in best-fitting simulations during the wetter 2010 and 2011 years, while human managed (i.e. manually filled) container habitats were more prevalent in best-fitting simulations during the drier 2012 and 2013 years. The simulations further indicate that rainfall strongly modulates the timing of dengue (e.g., epidemics occurred earlier during rainy years) while temperature modulates the annual number of dengue fever cases. Our results suggest that meteorological factors have a time-variable influence on dengue transmission relative to other important environmental and human factors.

Gender differences in HIV-related stigma in Kenya

Stigma associated with HIV/AIDS directly and indirectly drives HIV transmission. We examined how factors associated with HIV-related stigma differed by gender, using data from the 2008–2009 Kenya Demographic and Health Survey (KDHS). Descriptive, bivariate and multinomial logistic regression analyses were conducted on selected HIV-related stigma indicators for men and women. Bivariate analyses showed significant gender differences in the overall HIV Stigma index with a higher proportion of women than men presented at the highest stigma level (4.9% vs 2.7%, p < 0.01). Women were more likely to express higher stigmatic attitudes for all components of stigma measured than men. Multivariate analyses showed that HIV-related knowledge had significant inverse dose-response for both men and women. For instance, compared to women in the first HIV-related knowledge quartile, a 1 unit increase in HIV-related knowledge among women at the third HIV-related knowledge quartile was expected to lead to a 63.8% decrease in HIV-related stigma (95% CI [0.21, 0.63]) for women with high stigma, 57.8% decrease for similar women with medium stigma (95% CI [0.33, 0.55]) and 28.4% decrease for those with low stigma (95% CI [0.57, 0.90]). Acceptance with the statement “a husband is justified to hit or beat his wife if she refuses to have sex with him” was a significant risk factor for expression of stigmatising attitudes at all levels for women (High: OR = 1.49, 95% CI [1.02, 2.17]), Medium: OR = 1.47, 95% CI [1.18, 1.82], Low: OR = 1.38, 95% CI [1.10, 1.73]) and men at medium stigma (OR = 2.02, 95% CI [1.38, 2.95]). Other notable gender differences were found in employment, marital status, ethnicity, region of residence, wealth and media exposure. Our results showed that women in the general Kenyan population had higher stigmatic attitudes than men. This was associated with differences in risk factor profile and confirmed previous literature on complexity of social-cultural factors associated with HIV-related stigma.

Aging Field Collected Aedes aegypti to Determine Their Capacity for Dengue Transmission in the Southwestern United States

Aedes aegypti, the primary vector of dengue virus, is well established throughout urban areas of the Southwestern US, including Tucson, AZ. Local transmission of the dengue virus, however, has not been reported in this area. Although many factors influence the distribution of the dengue virus, we hypothesize that one contributing factor is that the lifespan of female Ae. aegypti mosquitoes in the Southwestern US is too short for the virus to complete development and be transmitted to a new host. To test this we utilized two age grading techniques. First, we determined parity by analyzing ovarian tracheation and found that only 40% of Ae. aegypti females collected in Tucson, AZ were parous. The second technique determined transcript levels of an age-associated gene, Sarcoplasmic calcium-binding protein 1 (SCP-1). SCP-1 expression decreased in a predictable manner as the age of mosquitoes increased regardless of rearing conditions and reproductive status. We developed statistical models based on parity and SCP-1 expression to determine the age of individual, field collected mosquitoes within three age brackets: nonvectors (0–5 days post-emergence), unlikely vectors (6–14 days post-emergence), and potential vectors (15+ days post-emergence). The statistical models allowed us to accurately group individual wild mosquitoes into the three age brackets with high confidence. SCP-1 expression levels of individual, field collected mosquitoes were analyzed in conjunction with parity status. Based on SCP-1 transcript levels and parity data, 9% of collected mosquitoes survived more than 15 days post emergence.

A country bug in the city: urban infestation by the Chagas disease vector Triatoma infestans in Arequipa, Peru

BackgroundInterruption of vector-borne transmission of Trypanosoma cruzi remains an unrealized objective in many Latin American countries. The task of vector control is complicated by the emergence of vector insects in urban areas.MethodsUtilizing data from a large-scale vector control program in Arequipa, Peru, we explored the spatial patterns of infestation by Triatoma infestans in an urban and peri-urban landscape. Multilevel logistic regression was utilized to assess the associations between household infestation and household- and locality-level socio-environmental measures.ResultsOf 37,229 households inspected for infestation, 6,982 (18.8%; 95% CI: 18.4 – 19.2%) were infested by T. infestans. Eighty clusters of infestation were identified, ranging in area from 0.1 to 68.7 hectares and containing as few as one and as many as 1,139 infested households. Spatial dependence between infested households was significant at distances up to 2,000 meters. Household T. infestans infestation was associated with household- and locality-level factors, including housing density, elevation, land surface temperature, and locality type.ConclusionsHigh levels of T. infestans infestation, characterized by spatial heterogeneity, were found across extensive urban and peri-urban areas prior to vector control. Several environmental and social factors, which may directly or indirectly influence the biology and behavior of T. infestans, were associated with infestation. Spatial clustering of infestation in the urban context may both challenge and inform surveillance and control of vector reemergence after insecticide intervention.