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CHAPTER FOURTEEN The Geography of Disease Transmission Zoonotic diseases (i.e., diseases that circulate in the animal world, occasionally affecting humans or other species of interest) are by definition a phenomenon of interactions among species. That is, the pathogen itself is a virus, bacterium, fungus, protozoan, or other small-sized species. Another (usually) larger-bodied species often serves as the reservoir species for that pathogen, holding a long-term pool of pathogen populations in a cycle of transmission and infection. Finally, other species (often arthropods or mollusks) may serve to move the pathogen from one individual of the reservoir species to another, or from the reservoir to humans—these species are termed “vectors.” An example disease transmission cycle, that of plague (Yersinia pestis), is shown in figure 14.1. As such, disease applications often differ from other applications of ecological niche modeling in that interactions among species can be very important and complex—that is, the Grinnellian and Eltonian perspectives intermix in this arena very frequently (Peterson 2008a). It is important to note that each of the interacting species likely has a different scenopoetic existing fundamental niche EA, and differences of scale may be enormous (pathogens may be capable of movements of millimeters, whereas some reservoirs are capable of movements on global scales; Kilpatrick et al. 2006, Peterson et al. 2007a). Thus, disease applications of niche modeling present some very real complications that challenge effective modeling and predicting. These considerations lead to two different approaches to the challenge. First, it is certainly possible to integrate across the entire transmission system, treating it effectively as a “black box,” and simply analyze the ecological and geographic distribution of disease occurrence (in essence modeling the “niche” of the disease occurrence in humans or other species of interest as if it were a species; e.g., Yeshiwondim et al. 2009). This approach subsumes all of the ecological requirements of the individual component species, as well as any ecological biases in their interactions—as such, key details may be lost in the process. However, in some cases, illustrated later, human case locations are the only information that is available, so analysis of the “niche” of the entire GEOGRAPHY OF DISEASE TRANSMISSION 227 transmission system is the only option available (Peterson et al. 2004a, Ron 2005, Reed et al. 2008, Williams et al. 2008). Perhaps more satisfying, however, is the idea of parsing the overall transmission cycle into the ecological niches of the individual component species. This approach offers the opportunity to distinguish different reasons for presence or absence of disease transmission in an area: transmission may be absent for lack of the pathogen, for lack of an appropriate vector, or for lack of an appropriate reservoir (Peterson 2007a), or because of rarity of any one of them. For example, in situations in which appropriate vectors and reservoirs are in place, introduction of the pathogen can lead to immediate transmission and spread, as was the case with the arrival of West Nile virus in North America in 1999 (Komar 2003). Certainly, these techniques can also be applied to the Figure 14.1. An example disease transmission cycle, showing how plague (Yersinia pestis) is transmitted in North America, and illustrating the involvement of various elements of biodiversity in the transmission. Image courtesy of Neal R. Chamberlain, PhD, A.T. Still University/Kirksville College of Osteopathic Medicine. 228 CHAPTER 14 hypothetical situation of “bioterrorism” in the form of introduction of novel pathogens into a region with the intent to do harm to humans or other species of interest (Bhalla and Warheit 2004). Applications of ecological niche modeling approaches to the challenge of understanding the geography and ecology of disease transmission are in an early stage. In general, most present applications fall in the category of “black box” analyses (e.g., Peterson et al. 2006a), as the necessary occurrence data are more readily available. A few efforts, however, have treated component species independently (Peterson et al. 2002c, Peterson et al. 2004b), which has potential to offer considerable novel insight into the ecology and geography of the transmission of the diseases in question. Niche modeling has a lot to offer to the field of public health and epidemiology . Particularly relevant, the field of “spatial epidemiology” or “landscape epidemiology” has emerged in recent years, and a standard suite of tools and approaches has been achieved (Elliott et al. 2000). Typical spatial epidemiological applications include mapping geographic patterns of disease transmission risk, identification of risk factors (spatially or not), and assessment of...


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