Today ticks inhabit almost every continent, with the number of species worldwide topping 850. Ticks have proven resilient and persistent in the environment, and the fossil records suggest that they originated 65–146 million years ago (see Olsen and Patz; Paddock and Telford, Appendix A).
The recognized number of important diseases transmitted by ticks has been growing over the past 30 years (see Paddock and Telford, Appendix A). The emergence and increased incidence of several major tick-borne diseases (TBDs) has been attributed to specific human activities and behaviors that disrupt ecosytems (see Paddock and Telford, Appendix A). Increases in human population and demographic shifts have brought dramatic changes in the distribution and composition of natural habitats, as people modify the land to create living spaces for agriculture or for recreation (see Munderloh and Kurtii, Appendix A). These changes mean that people and animals interact at many more interfaces, creating new opportunities for the transmission of zoonotic diseases, including TBDs. For example, habitat fragmentation can alter the movement of hosts that carry TBDs, the dynamics of disease transmission, and biodiversity (see Appendix A). Global environmental changes and other abiotic and biotic factors also help shape the ecology of TBDs and their emergence and reemergence.
Fortunately, new molecular tools and analytical techniques such as gene sequencing and analysis have enabled scientists to gain insights into tick biology and have resulted in a better understanding of TBDs. New technologies have also revealed a diverse microbial community associated with ticks that include viruses, bacteria, protozoans, and fungi. These microbes may act as symbionts (interacting closely, often to the benefit of the tick), pathogens, and transient commensals (colonizing the tick without marked detrimental effects), or as pathogens (see Clay and Fuqua, Appendix A).
In this chapter, five scientists examined the natural history of ticks and their wildlife and domestic hosts; outlined the contributions of animal health experts to understanding human TBD; explored genetic diversity among pathogens, vectors, and hosts; and showed how scientists investigate the microbial community found within the ticks to better understand the human risk for tick-borne diseases.