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Global warming and expansion of geographic range of T infestans

The Intergovernmental Panel on Climate Change (IPCC) synthesis report of 2007 concluded that local maximum temperatures will only modestly increase while minimum temperatures will increase dramatically under climate change scenarios.106 Because of the predicted increase in temperature and the known effect of temperature on insect development and reproduction, a number of authors have predicted a global increase in the transmission of vector-borne diseases (VBDs).107,108 However, not all experts agree on these predictions because of the simplistic linear relationship that underlies the former argument. In the latter case, authors argue that the epidemiology of each VBD is a system-specific product of complex, commonly nonlinear interactions between many disparate environmental factors. These factors include climate and other abiotic conditions (e.g., land cover), the physical structure of the environment, host abundance and diversity, socioeconomic factors driving human living conditions and behavior that determines the degree of exposure to vectorial transmission risk, and the nutritional status and concomitant immunity that determine resistance to infection.109 In addition to the complexity argument, these authors state that there is no single infectious disease with increased incidence over recent decades that can be reliably attributed to climate change.110 For the case of Chagas disease vectors, there are no specific studies on their relationship with the expected climate change, except for an early article by Gorla et al.111 which discussed the potential changes in the geographic distribution of T. infestans and Chagas disease transmission under a global temperature increase scenario. Based on 3-year field data on the population ecology of T. infestans under the natural climate conditions of the southern Gran Chaco (reported by Gorla112), Gorla et al.111 estimated the effect of a generalized temperature increase on the maximum potential population growth rate (r) of T. infestans and T. cruzi transmission risk (based on the relationship between temperature and T. infestans biting frequency) over the Argentinian territory. At a hypothesized 2°C or 4°C temperature increase, the vector populations may potentially expand the southern limit of the geographic distribution and have a greater capacity to increase its population

abundance in the Gran Chaco region, which corresponds with a parallel increase in the T. cruzi transmission risk. The authors stated that the distribution and abundance of T. infestans and the vectorial transmission of T. cruzi depend on factors other than just temperature. Although the study predicts the species-potential expansion of the southern limit and an eventually increased transmission risk, the vector control activities and the improved living conditions of rural communities in previously infested areas were able to eliminate domestic infestations by T. infestans. Additionally, interruption of the vectorial transmission of T. cruzi in several provinces outside the Gran Chaco region by 2001, as certified by the Pan American Health Organization,113 has significantly contributed to a decrease in disease prevalence.

A recent study of the impact of climate change on the geographic distribution and its influence on the force of T. cruzi infection by T. infestans and R. prolixus was published.114 The authors assessed the implications of climatic projections for 2050 on the geographical distribution of the two vector species. They estimated the epidemiological implications of current to future transitions in the climatic niche in terms of changes in the force of infection (FOI) on the rural population of Venezuela (tropical) and Argentina (temperate). They report a decreasing trend of suitability of areas that are currently at high-to-moderate transmission risk. As the analysis is based exclusively on temperature and rainfall (from the Worldclim database), the projection for 2050 produces a spurious result in central Argentina, where in spite of the climatic suitability for T. infestans, T. cruzi transmission no longer occurs because of the change in the lifestyle of rural communities after the land use change promoted by the advance of soy bean production. As the authors mention, the effect of other factors, besides climate, should be considered in a modeling effort to reliable project the epidemiological scenario for T. cruzi transmission.

 
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