Increasing Habitat Suitability in the United States for the Tick That Transmits Lyme Disease: A Remote Sensing Approach. (Research Articles)

Article excerpt

The warnings about the spread of Ixodes scapularis one of the vectors of Lyme disease, into the United States are based on reports about regional distribution and increasing local abundance. In a modeling approach, I used the recorded, current distribution of this tick and remotely sensed bioclimatic factors over the United States to establish the changes of habitat for this tick since 1982 and to detect the areas with factors adequate to support tick colonization. Results indicate the geographic expansion of areas with adequate habitat suitability in the period 1982-2000. A discriminant analysis of counties with different degrees of habitat suitability shows that the increase in winter temperatures and in vegetation vitality (as a direct consequence of higher rainfall) is key to habitat switch from unsuitable to suitable. Key words: climate, habitat suitability, Ixodes scapularis, Lyme disease, remote sensing.

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Arthropods are extremely sensitive to climate. Throughout the 20th century, public health researchers have understood that climate circumscribes the distribution of mosquito-borne diseases, whereas weather affects the timing and intensity of outbreaks (1). A growing number of investigators propose that vector-borne diseases shift their range in response to climate change (2). Lyme disease has become the most prevalent vector-borne illnesses in the United States, and its distribution is expanding (3). One of the tick species involved in Lyme disease transmission is Ixodes scapularis (Say), which is currently well established in several areas of the northeastern, southeastern, and midwestern United States. Knowledge of the environmental factors that influence the survival of this vector has acquired special importance, because the regional distribution and local abundance of I. scapularis in North America have increased over the past two decades, apparently at an unprecedented rate (4).

Considerable effort has been devoted to studying the ecologic and social factors that influence where and in what abundance this tick is found. However, the integration of these findings into a paradigm that explains tick colonization of new regions and changes in local abundance is still being developed. Added to this is concern that global climatic changes may create widespread conditions that permit expansion into areas that are not currently infested. We now have evidence that I. ricinus, an important tick species of the western Palearctic, has increased its density and geographic range (latitude and altitude shifts) in Sweden because of changes in the medium-term climate (10-15-year period) in that region (5). The aim of the present study was to detect changes in the habitat for I. scapularis in the United States in the period 1982-2000. I used a modeling approach together with remotely sensed climate and vegetation features at a medium resolution of 8 km to elucidate the temporal drift of these abiotic variables and how they influence the forecasted habitat suitability for the tick.

Materials and Methods

I took the tick distribution data for this study from the distribution of I. scapularis published by U.S. counties (6). This compilation covers the known range of the tick from published records and collections as of 1998. In this compilation, I considered I. scapularis ticks to be "reported" in a county if at least one tick at any life stage had been identified. I also considered tick populations "established" in a county if at least six ticks or two of the three tick life stages had been identified in a single collection period. Both definitions have a biologic meaning. More than one stage represents a reproductive population, whereas the collection of just a single tick implies the initial stages in the event sequence from invasion to colonization. In both cases, the county is the geographic unit, because it provides the ecologic background of collections (collecting point and its surroundings). …