BOX 6.1

Lyme Disease in the United States

First discovered in the early 1970s and described as an epidemic of juvenile arthritis confined to the coastal community of Lyme, Connecticut (Steere et al., 1977), Lyme disease is now known to be endemic in 19 states (Nadelman and Wormser, 2005). The force behind this epidemic has been environmental change, which has increased contact between humans and a spirochete bacteria (Borrelia burgdorferi) transmitted by a tick (Ixodes scapularis). Adult ticks feed primarily on white-tailed deer, and new populations of ticks have spread rapidly over the northeastern United States through deer movement. This was assisted by reforestation prompted by farm abandonment in many areas of the northeastern United States during the early twentieth century. Combined with a marked expansion of the human population, this sequence of events has resulted in wider exposure to natural tickborne pathogens, including B. burgdorferi (Falco et al., 1995). This has resulted in a continuous increase of Lyme disease cases reported to the Centers for Disease Control and Prevention (CDC) over the past 20 years, despite enormous efforts to educate the public on prevention measures and the development of a Lyme disease vaccine. Geological factors, particularly sedimentary bedrock and soil type, play a key role in determining tick habitat and the risk of acquiring Lyme disease (Guerra et al., 2002).

to all pathogens of nonhuman origin (zoonoses), whether vectorborne or directly transmissible. Humans are not required for their maintenance in nature, although zoonotic pathogens may ultimately adapt to direct human-to-human transmission modes.

Microbial agents causing infectious diseases in humans often originate from processes and events occurring in the natural environment. Most of the so-called emerging diseases are caused by infectious agents of wildlife that have either recently adapted to infect humans or are preadapted and have recently come into opportunistic contact with humans (Taylor et al., 2001). These include some of the most important pathogenic agents that have caused major epidemics in humans, such as HIV/AIDS, influenza A, Ebola, West Nile virus, and Lyme disease. The current epidemic of Lyme disease in the United States (see Box 6.1) provides a relevant example of how environmental change can result in epidemic disease in humans. The geographic distribution of the risk of Lyme disease for humans can be predicted based on vegetation and climate characteristics that determine the distribution of the vector, wildlife hosts, and the pathogen (Guerra et al., 2002; Brownstein et al., 2003). The extent of hu-



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