American trypanosomiasis, Chagas disease, is caused by the hemoflagellate Trypanosoma cruzi, and is primarily transmitted to mammal vertebrate hosts through the faeces of hematophagous insects of the subfamily Triatominae . The geographic distribution of triatomine species extends mainly through the Neotropical and Neoartic regions . Triatomine bug species such as Microtriatoma trinidadensis, Eratyrus mucronatus, Belminus herreri, Panstrongylus lignarius, and Triatoma tibiamaculata are excellently adapted to specialist niches . However, intrusion of human dwellings into sylvan areas has allowed some triatomine species to adapt to the domestic habitat, putting people at greater risk of contracting the disease. This suggests a long evolutionary history, as well as the recent dramatic spread of a few eclectic, domiciled triatomine species. In fact, in Southern Cone countries, the main human Chagas vector, Triatoma infestans, is almost exclusively domestic, meaning all stages of life are completed in close proximity to humans in rural or peri-urban environments, increasing the likelihood of human-vector interaction. Peri-domestic populations (those living in surrounding outbuildings, including animal pens and fences) provide a ready re-infesting population following pesticide treatment of houses. Some sylvan populations are interbreeding with domestic populations , whereas others seasonally move between houses and the extra-domicile , and still others remain in the wild . It is clear that as the development of human settlements continues into previously uninhabited areas, the risk of human infection increases . In addition, migration of people from highly endemic rural areas has brought the disease and vectors to cities .
There are four triatomine species in Chile: Triatoma infestans, Mepraia spinolai, Mepraia parapatrica and Mepraia gajardoi. Our study zone has only reported the first two species. Mepraia spinolai has been described as a mainly sylvatic species, but there are reports of individuals entering houses . Its described habitat is mainly rocky places and rock piles, but has also been found in terrestrial bromeliads . The prevalence of M. spinolai, detected by molecular techniques, varies between 42.7% to 76.1% [6, 11, 12] in sylvatic foci; the few individuals of this species that were captured in dwellings and sent to the National Health Service presented 22.8% of infection in the study area . Human blood has been detected as part of the blood meal of M. spinolai. Mepraia spinolai is the main vector of T. cruzi in the sylvatic cycle of transmission in its area of distribution. However, Triatoma infestans is a more efficient vector, compared to M. spinolai. The main differences between both species were in their alimentary profile and in their behaviour. They are particularly evident in the activity rhythm - M. spinolai is diurnal -, the time that its bite takes- shorter in M. spinolai -, and the delay in the defecation - longer in the latter species. These facts would explain its low epidemiological impact regarding human vectorial transmission . Triatoma infestans was found recently in sylvatic environments, associated to endemic terrestrial bromeliads in the Metropolitana Region, with a prevalence of 40.9% , and rock piles in the Valparaíso Region, with 36.5% of infection , along with periodic findings inside human dwellings, which mainly correspond to winged adults; the infection in the study area was reported to be 48.4% . Almost all domestic colonies have been eliminated by the Chagas disease vector control program; in fact, in 1999 vectorial T. cruzi transmission to human population was declared interrupted in Chile . Recent reports on human seroprevalence of T. cruzi infection in a national health survey indicated 1.8%, 0.9% and 0.7% in Coquimbo Region, Valparaíso Region and Metropolitana Region, respectively .
The vectorial disease transmission depends on ecological and environmental parameters of the ecological niche, which defines the ecologic space within which a species can maintain populations without immigration . The understanding of this complex environmental dependency, by means of ecological niche modeling, would help in answering the spatial and temporal issues relevant to domestic transmission control. Given the lack of local data in most of the study zone, there is a need to find predictive models that will allow extrapolation of the actual data of ecological niche to areas that have similar characteristics. Current statistical models can be grouped into two domains: data modeling and algorithm approaches . Data modeling approach starts with assuming a stochastic data model, such as linear or logistic regression which will be fitted and used to both predict what responses are going to be to future input variables and to extract information about how nature is associating the responses with input variables. Algorithm modeling approach focuses on prediction and uses predictive accuracy to validate the models. In general, the latter approach involves a machine-learning algorithm, decision trees or neural networks, to discover associations between point-occurrence data and sets of electronic maps summarizing environmental/ecologic dimensions that may or may not be important in limiting species’ geographic distributions. This methodology will provide distribution predictive models to be subsequently validated with new or independent data .
Triatoma infestans is very closely associated to domestic and peridomestic structures. The success of this species in particular is related to the ability to efficiently use the available resources in human environments. However, environmental variables do describe their geographical distribution on a regional scale. This indicates that there is an adequate environmental profile to permit the existence of this vector that is not necessarily associated with the availability of human dwellings . Analyzing the relation between the temperature and the population’s intrinsic rate of natural growth (r) of T. infestans, even though temperature was not the only climatic variable that limited the growth capacity of this species’ populations, and hence, its geographical distribution, the prediction of the regression model closely matched the known distribution of this species . The relation between geographical distribution and temperature, humidity, precipitation and altitude was shown for some species of Triatominae [25, 26].
Usually there is an indirect association between hematophagous insects and vegetation because the plants shelter warm blooded hosts, which are their feeding sources . Vegetation is a variable that includes temperature effects, precipitation and edaphic properties; because of this, it becomes an indicator related to variables that directly influence the demographic processes (mortality, birth rate, dispersion). What frequently occurs is that animal distribution is not associated to the classic distribution patterns of vegetal communities, but to spatio-temporal variability indicators of photosynthetic active biomass, as NDVI, acquired by remote sensors. So, vegetation can be characterized by its spatio-temporal change patterns .
Satellite-based remote sensing offers significant benefits for many applications because it provides historical data for comparison and analysis ; as in Medical Entomology, where environmental variables obtained by remote sensors can be used to elaborate the predictive models of the geographic distribution of several disease vectors [29, 30]. This tool has been applied to study Chagas disease vectors in a few instances, mainly of different species of Triatoma[5, 20, 22].
Our objective was to determine the distribution pattern of Triatoma infestans and Mepraia spinolai at a regional level, generating a predictive spatial model of their distribution that incorporates quantifiable macro-environmental variables.