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. 2018 Apr 25;11(1):267.
doi: 10.1186/s13071-018-2808-6.

Effects of desiccation stress on adult female longevity in Aedes aegypti and Ae. albopictus (Diptera: Culicidae): results of a systematic review and pooled survival analysis

Affiliations

Effects of desiccation stress on adult female longevity in Aedes aegypti and Ae. albopictus (Diptera: Culicidae): results of a systematic review and pooled survival analysis

Chris A Schmidt et al. Parasit Vectors. .

Abstract

Background: Transmission dynamics of mosquito-borne viruses such as dengue, Zika and chikungunya are affected by the longevity of the adult female mosquito. Environmental conditions influence the survival of adult female Aedes mosquitoes, the primary vectors of these viruses. While the association of temperature with Aedes mortality has been relatively well-explored, the role of humidity is less established. The current study's goals were to compile knowledge of the influence of humidity on adult survival in the important vector species Aedes aegypti and Ae. albopictus, and to quantify this relationship while accounting for the modifying effect of temperature.

Methods: We performed a systematic literature review to identify studies reporting experimental results informing the relationships among temperature, humidity and adult survival in Ae. aegypti and Ae. albopictus. Using a novel simulation approach to harmonize disparate survival data, we conducted pooled survival analyses via stratified and mixed effects Cox regression to estimate temperature-dependent associations between humidity and mortality risk for these species across a broad range of temperatures and vapor pressure deficits.

Results: After screening 1517 articles, 17 studies (one in semi-field and 16 in laboratory settings) met inclusion criteria and collectively reported results for 192 survival experiments. We review and synthesize relevant findings from these studies. Our stratified model estimated a strong temperature-dependent association of humidity with mortality in both species, though associations were not significant for Ae. albopictus in the mixed effects model. Lowest mortality risks were estimated around 27.5 °C and 21.5 °C for Ae. aegypti and Ae. albopictus, respectively, and mortality increased non-linearly with decreasing humidity. Aedes aegypti had a survival advantage relative to Ae. albopictus in the stratified model under most conditions, but species differences were not significant in the mixed effects model.

Conclusions: Humidity is associated with mortality risk in adult female Ae. aegypti in controlled settings. Data are limited at low humidities, temperature extremes, and for Ae. albopictus, and further studies should be conducted to reduce model uncertainty in these contexts. Desiccation is likely an important factor in Aedes population dynamics and viral transmission in arid regions. Models of Aedes-borne virus transmission may be improved by more comprehensively representing humidity effects.

Keywords: Aedes aegypti; Aedes albopictus; Humidity; Longevity; Pooled analysis; Review; Survival; Temperature; Vapor pressure.

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Conflict of interest statement

Competing interests

The authors declare that they have no competing interests.

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Figures

Fig. 1
Fig. 1
Directed acyclic graph (DAG) of hypothesized relationships among modeled exposure and outcome variables. Some hypothesized mediators (red text) were not directly modeled but are included here for explicatory purposes. Boxes reflect conditioning on variables in the statistical models to address potential confounding and colliding bias in estimating the association between SVPD and mortality, with variable names along paths reflecting modeled interactions
Fig. 2
Fig. 2
Summary flow diagram of survival time simulations. Steps within the gray box were repeated to generate 500 simulated data sets, which were individually analyzed via stratified and mixed effects Cox regression prior to pooling. (*) One study reported some experiments as survival curves and some as median longevities
Fig. 3
Fig. 3
Example simulated survival curves for individual experiments. a A right-censored experiment with reported survival observations at numerous time points (Aedes aegypti at 24.1 °C and 75% RH, with water but no nutrition supplied; Alto et al. [70]). b A right-censored experiment with sparse reported survival observations (Ae. albopictus at 32.2 °C and 60% RH, with sugar solution supplied; Hylton [98]). c An experiment with only median longevity reported (Ae. aegypti at 0.5 °C and 85% RH, with no water or nutrition provided; Bar-Zeev [66]; x-axis has been truncated for display purposes). All 500 simulated data sets are shown for each experiment. Survival curves are colored according to their fitted model: Weibull (red), log-logistic (blue), or aggregate (black; all simulated data). Open circles indicate reported observations
Fig. 4
Fig. 4
Pooled marginal effects estimates (mean log hazard ratios, with 95% CI) from stratified model analysis for temperature (a) and saturation vapor pressure deficit (b), and from mixed effects model analysis for temperature (c) and saturation vapor pressure deficit (d). Results for Aedes aegypti are indicated with solid lines, and for Ae. albopictus by dashed lines. Estimates in a and c are relative to a reference of Ae. aegypti at 27.5 °C, and in b and d are relative to a reference of Ae. aegypti at full saturation (100% RH). Plots in c and d provide median (blue lines) and 95% CI (blue shading) from the mixed effects model, with mean estimates from stratified analyses provided for comparison (gray lines). Estimates are restricted to the range of values present in the data for each species
Fig. 5
Fig. 5
Pooled joint effects estimates (mean log hazard ratios, with 95% CI) from stratified model analysis for temperature and select relative humidities (%) for Aedes aegypti (a) and Ae. albopictus (b), illustrating the interacting effects of temperature and humidity. In order to facilitate direct comparisons between species, estimates for both species are relative to a reference of Ae. aegypti at 27.5 °C and full saturation (100% RH)

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