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Environmental variables and the distribution of T. infestans

Gorla87 described the maximum potential expansion of the geographic distribution of T. infestans using a 20-year time series of satellite imagery produced by the advanced very high resolution radiometer (AVHRR). From these images, statistics of a temporal Fourier analysis (amplitude and phase of 1-, 2-, 3-annual cycles, maximum, minimum, average, variance) of the normalized difference vegetation index (NDVI), estimations of land surface temperature (LST), air temperature (AT), vapor pressure deficit (VPD), and a measure of middle infrared radiation (MIR) were estimated. The maximum potential distribution was derived from a multivariate discriminant analysis and then compared with the T. infestans distribution determined using data collected by field teams of vector control programs of the Southern Cone countries and academic reports. This analysis resulted in an overall 90% correct

classification of either presence or absence of T. infestans. The model identified the variability of the AT, air temperature average, and amplitude of the AT annual cycle as the main variables defining the geographic distribution of T. infestans.

The maximum expansion of T. infestans geographic distribution probably occurred between 1970 and 1980 when the presence of the species was first reported to have crossed the Sao Francisco River in northeastern Brazil.88,89 Although some discussions remain, current knowledge of the species distribution originated from studies using different methodological approaches (i.e., population genetics, morphometric geometry, cytogenetics, antennal phenotypes) supports the hypothesis that the Andean valleys in Bolivia were the center of origin and expansion of T. infestans throughout the southern countries of South America, along the trade routes of the Incas first, and then along the trade routes of the new European empires of Spain and Portugal.70,90,91 The present geographic distribution of domestic populations significantly decreased their habitat ranges compared with the maximum geographic distribution of the species,87,92 and they are patchily distributed over the arid Chaco and the inter-Andean valleys of Bolivia. This reduction was caused by concerted actions of the vector control programs of the Southern Cone countries, living conditions improvement of previously affected communities and the generalized migration from rural to urban settings in many areas.

Because of T. infestans importance as vector of T. cruzi, the biology and population ecology of their domestic and peridomestic populations have been studied intensively from the 1970s. Although frequently debated during the 1980s, the existence of syl- vatic populations were minimized (sylvatic populations were underreported during the period, e.g., findings of T. infestans adults in bird nests up to 1.2 km from the nearest house,93 but their relative importance was increased during the last 15 years). Recent studies showed sylvatic populations of T. infestans occurring more frequently than previously expected in the Andean and the Chaco regions of Bolivia and Argentina.94,95

Taking into account the existence of the Andean and non-Andean (Chaco) cyto- types described by Panzera,91 Gorla and Noireau96 analyzed the distribution of Andean and Chaco sylvatic populations of T. infestans, using field-collected data and environmental variables to identify potential areas of the geographic distribution of both types of sylvatic populations. The derived geographic distribution models showed that the Andean population of sylvatic T. infestans is restricted mainly to the southern part of La Paz department, southern Cochabamba, and the boundary between the departments of Chuquisaca and Tarija with Potosi. The Chaco populations of sylvatic T. infestans have a wider area of potential distribution around the Chaco region of Argentina, Bolivia, and Paraguay (Fig. 9.3).

Population genetics, geometric morphometry, and cytogenetics data show that T. infestans is a heterogeneous entity that can colonize domestic, peridomestic, and sylvatic ecotopes. In the domestic and/or peridomestic ecotopes, the species is able to constitute highly frequent and abundant populations, but in the sylvatic ecotopes, the populations are rare and always constituted by a few individuals. The entity we call T. infestans includes an Andean form, living in the Bolivian Montane Dry Forests, mainly associated with rodents and a non-Andean form97 and later discovered in several other places of the arid Gran Chaco and also in colonies near

Potential distribution of sylvatic T. infestans populations of the Andean and Chaco regions. In the print version

Figure 9.3 Potential distribution of sylvatic T. infestans populations of the Andean and Chaco regions. In the print version: Dark and light grey shades to the left of the map are predictions of presence and absence of Andean populations, respectively. Dark and light grey shades to the right of the map are predictions of presence and absence of Chaco populations of T. infestans, respectively.

Santiago city in Chile,98 frequently found associated with bird nests. Where the non-Andean form constitutes peridomestic populations, it frequently constitutes abundant populations associated with chicken nests. It is still under debate whether all these sylvatic populations are of truly sylvatic origin, or they are in fact domestic populations that migrated to the sylvatic ecotopes. Panzera et al.99 described a hybrid T. infestans population group geographical restricted to the border between Argentina and Bolivia, that was recently associated with the highest resistant populations to pyrethroid insecticides.100

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