Tropical Malaria Does Not Mean Hot Environments

Abstract

If global warming progresses, many consider that malaria in presently malaria-endemic areas will become more serious, with increasing development rates of the vector mosquito and malaria parasites. However, the correlation coefficients between the monthly malaria cases and the monthly mean of daily maximum temperature were negative, showing that the number of malaria cases in tropical areas of Africa decreases during the season when temperature was higher than normal. Moreover, an analysis of temperature and development rate using a thermodynamic model showed that the estimated intrinsic optimum temperatures for the development of the malaria parasites, Plasmodium falciparum and P. vivax, in the adult mosquito stage and that of the vector mosquito Anopheles gambiae s.s. were all approximately 23-24 degrees C. Here, the intrinsic optimum temperature is defined in the thermodynamic model as the temperature at which it is assumed that there are no or negligible adverse effects for development. Therefore, this study indicates that the development of malaria parasites in their mosquito hosts and the development of their vector mosquitoes are inhibited at temperatures higher than 23-24 degrees C. If global warming progresses further, the present center of malarial endemicity in sub-Saharan Africa will move to an area with an optimum temperature for both the vector and the parasite, migrating to avoid the hot environment.

Understanding the link between malaria risk and climate

“…Other alternative functions such as those proposed by Logan (70) and Lactin (71) are more complex and provide no better fit to the available data. While having equivalent fit alone does not ensure identical model behavior (61), exploring the effects of temperature fluctuations on EIP with an alternative thermodynamic model for Plasmodium falciparum (63), produces qualitatively similar results (Fig. S4A and B) due to the fundamental nature of the rate summation effect with nonlinear development functions (61).…”

Possible Effects of Future Climate Changes on the Maximum Number of Generations of Anopheles in Monsoon Asia

“…According to our projections, the future G max values in the all the tropical regions would increase by 2 generations. However, considering that high temperatures injure mosquitoes (Depinay et al, 2004;Ikemoto, 2005;2008), it is possible that these increases in G max in tropical regions will not occur. The possibility of a subsequent generation is only one of the factors for determining the magnitude of a population.…”

“…Other ecophysiological and entomological approaches have been employed to explain the temporal occurrence of vectors by describing their life histories. These models calculate development according to air temperature at each developmental stage of the insect, based on the assumption that the growth of an insect depends on the temperature of its habitat (Hopp & Foley, 2001;Ikemoto, 2003;2005;2008;Depinay et al, 2004;Pascual et al, 2006). Temperature affects the survival of the parasite only during the life cycle of Anopheles mosquitoes.…”

Using the loess method to describe the effect of temperature on development rate

“…However, this linear relationship between development rate and temperature does not fit if temperatures were not in this range. Several non-linear models have been developed for describing the effect of temperature on development rate (e.g., Logan et al 1976;Sharpe & DeMichele 1977;Schoolfield et al 1981;Taylor 1981;Wang et al 1982;Lactin et al 1995;Brière et al 1999;Ikemoto 2005Ikemoto , 2008Shi et al 2011a, b). These non-linear models are like a two-sided sword: (1) they represent abundant tools for describing the temperature-dependent development rates; (2) it takes to spend much time on choosing one among so many models.…”

“…It indicates that we need not to know the complex mechanism of the effect of temperature on development rate but we can use a non-parametric model to reflect the relationship between development data and temperature very well. Frankly speaking, although the Sharpe-Schoolfield-Ikemoto model (Sharpe & DeMichele 1977;Schoolfield et al 1981;Ikemoto 2005Ikemoto , 2008Shi et al 2011b) and another model proposed recently by Ratkowsky et al (2005) based on thermodynamics have perfectly combined the temperature-dependent development rates to the probability of an enzyme being in the active state, the mechanisms of temperature on the enzymes that control development and the types of these controlling enzymes are still unknown. Therefore, it is necessary to use a non-parametric fitting method.…”