The ecology of mosquito vectors and malaria parasites affect the incidence, seasonal transmission and geographical range of malaria. Most malaria models to date assume constant or linear responses of mosquito and parasite life-history traits to temperature, predicting optimal transmission at 31 °C. These models are at odds with field observations of transmission dating back nearly a century. We build a model with more realistic ecological assumptions about the thermal physiology of insects. Our model, which includes empirically derived nonlinear thermal responses, predicts optimal malaria transmission at 25 ° C( 6°C lower than previous models). Moreover, the model predicts that transmission decreases dramatically at temperatures > 28 °C, altering predictions about how climate change will affect malaria. A large data set on malaria transmission risk in Africa validates both the 25 °C optimum and the decline above 28 °C. Using these more accurate nonlinear thermal-response models will aid in understanding the effects of current and future temperature regimes on disease transmission.

/pdf/optimal-temperature-for-malaria-transmission-is-dramatically-47ra14blcf.pdf

Optimal temperature for malaria transmission is dramatically lower than previously predicted.

The incubation period for malaria parasites within the mosquito is exquisitely temperature-sensitive, so that temperature is a major determinant of malaria risk. Epidemiological models are increasingly used to guide allocation of disease control resources and to assess the likely impact of climate change on global malaria burdens. Temperature-based malaria transmission is generally incorporated into these models using mean monthly temperatures, yet temperatures fluctuate throughout the diurnal cycle. Here we use a thermodynamic malaria development model to demonstrate that temperature fluctuation can substantially alter the incubation period of the parasite, and hence malaria transmission rates. We find that, in general, temperature fluctuation reduces the impact of increases in mean temperature. Diurnal temperature fluctuation around means >21°C slows parasite development compared with constant temperatures, whereas fluctuation around <21°C speeds development. Consequently, models which ignore diurnal variation overestimate malaria risk in warmer environments and underestimate risk in cooler environments. To illustrate the implications further, we explore the influence of diurnal temperature fluctuation on malaria transmission at a site in the Kenyan Highlands. Based on local meteorological data, we find that the annual epidemics of malaria at this site cannot be explained without invoking the influence of diurnal temperature fluctuation. Moreover, while temperature fluctuation reduces the relative influence of a subtle warming trend apparent over the last 20 years, it nonetheless makes the effects biologically more significant. Such effects of short-term temperature fluctuations have not previously been considered but are central to understanding current malaria transmission and the consequences of climate change.

https://www.pnas.org/content/pnas/106/33/13844.full.pdf

Understanding the link between malaria risk and climate.

Recent epidemics of Zika, dengue, and chikungunya have heightened the need to understand the seasonal and geographic range of transmission by Aedes aegypti and Ae. albopictus mosquitoes. We use mechanistic transmission models to derive predictions for how the probability and magnitude of transmission for Zika, chikungunya, and dengue change with mean temperature, and we show that these predictions are well matched by human case data. Across all three viruses, models and human case data both show that transmission occurs between 18-34°C with maximal transmission occurring in a range from 26-29°C. Controlling for population size and two socioeconomic factors, temperature-dependent transmission based on our mechanistic model is an important predictor of human transmission occurrence and incidence. Risk maps indicate that tropical and subtropical regions are suitable for extended seasonal or year-round transmission, but transmission in temperate areas is limited to at most three months per year even if vectors are present. Such brief transmission windows limit the likelihood of major epidemics following disease introduction in temperate zones.

/pdf/detecting-the-impact-of-temperature-on-transmission-of-zika-3g17u21e99.pdf

Detecting the impact of temperature on transmission of Zika, dengue, and chikungunya using mechanistic models.

Dengue is a mosquito-borne viral disease that occurs mainly in the tropics and subtropics but has a high potential to spread to new areas. Dengue infections are climate sensitive, so it is important to better understand how changing climate factors affect the potential for geographic spread and future dengue epidemics. Vectorial capacity (VC) describes a vector's propensity to transmit dengue taking into account human, virus, and vector interactions. VC is highly temperature dependent, but most dengue models only take mean temperature values into account. Recent evidence shows that diurnal temperature range (DTR) plays an important role in influencing the behavior of the primary dengue vector Aedes aegypti. In this study, we used relative VC to estimate dengue epidemic potential (DEP) based on the temperature and DTR dependence of the parameters of A. aegypti. We found a strong temperature dependence of DEP; it peaked at a mean temperature of 29.3°C when DTR was 0°C and at 20°C when DTR was 20°C. Increasing average temperatures up to 29°C led to an increased DEP, but temperatures above 29°C reduced DEP. In tropical areas where the mean temperatures are close to 29°C, a small DTR increased DEP while a large DTR reduced it. In cold to temperate or extremely hot climates where the mean temperatures are far from 29°C, increasing DTR was associated with increasing DEP. Incorporating these findings using historical and predicted temperature and DTR over a two hundred year period (1901–2099), we found an increasing trend of global DEP in temperate regions. Small increases in DEP were observed over the last 100 years and large increases are expected by the end of this century in temperate Northern Hemisphere regions using climate change projections. These findings illustrate the importance of including DTR when mapping DEP based on VC.

/pdf/vectorial-capacity-of-aedes-aegypti-effects-of-temperature-2y7t5m6436.pdf

Vectorial capacity of Aedes aegypti: effects of temperature and implications for global dengue epidemic potential.

https://www.fs.fed.us/rm/pubs_other/rmrs_2012_regniere_j001.pdf

Effects of temperature on development, survival and reproduction of insects: experimental design, data analysis and modeling.

Two novel and simple mathematical models of arthropod temperature-dependent development are proposed. These models are easy to use and have 3 (equation 1) and 4 (equation 2) ecologically meaningful parameters, respectively. Each parameter can be estimated using nonlinear regression. These models were used to compare developmental rates at constant temperatures for our own experiments on Lobesia botrana (Dennis & Schiffermuller) and for data from 6 insect species described (a total of 13 stages). In all cases, we obtained an accurate nonlinear description of the rate of development against temperature given by the adjusted R 2 ([Kvalseth, 1985][1]). The adjusted R 2 calculated extended from 0.86 to 0.99 and were identical for our equations 1 and 2. In all cases, equation 2 provided the lowest residual sums of squares. The models gave upper T L and lower T temperature threshold estimations, and the estimations obtained were better by using equation 1 rather equation 2. Confidence intervals for each parameter were given and a comparison between estimated and observed temperature thresholds were presented.

 [1]: #ref-11

A Novel Rate Model of Temperature-Dependent Development for Arthropods

Resume On propose une nouvelle methode pour traiter les problemes de conditions aux limites dans un modele de Saint-Venant barotrope linearise. Elle combine un decouplage des equations avec une methode de relaxation newtonienne. On demontre theoriquement qu'elle permet d'evacuer les ondes sortantes sans modifier l'ecoulement dans le domaine physique etudie, contrairement aux methodes habituelles. Les resultats exposes valident cette methode.

Conditions limites non réfléchissantes pour un modèle de Saint-Venant bidimensionnel barotrope linéarisé

Numerical solution of a nonlinear reaction diffusion equation

Numerical solution of a Cauchy problem for nonlinear reaction diffusion processes

Resume Une nouvelle methode de correction des effets de diffusion numerique est presentee et la convergence en est demontree. Sans utiliser de test ni restreindre davantage la condition CFL, cette methode reste active au niveau des extrema de la solution approchee et compense la perte d'amplitude des ondes. L'idee consiste a controler la diffusion numerique par une projection sur la premiere fonction propre du Laplacien, puis de convecter ce terme a l'aide d'une equation de convection annexe, et de l'utiliser comme terme source dans l'equation principale.

Alain-Yves Le Roux

Papers

A Novel Rate Model of Temperature-Dependent Development for Arthropods

Conditions limites non réfléchissantes pour un modèle de Saint-Venant bidimensionnel barotrope linéarisé

Numerical solution of a nonlinear reaction diffusion equation

Numerical solution of a Cauchy problem for nonlinear reaction diffusion processes

Une nouvelle méthode d'antidiffusion dynamique pour la convection linéaire