Bacterial Diseases

Two newly recognized vector-borne bacterial diseases, Lyme disease, caused by Borrelia burgdorferi, and ehrlichiosis, caused by Ehrlichia chaffeensis, Anaplasma phagocytophilum, and Ehrlichia ewingui, have emerged as important public health problems in the past three decades (Dumler et al. 2007; Steere et al. 2004). Both have small rodents as their natural vertebrate reservoir host, with hard ticks as their principal vectors. Both diseases are found primarily in temperate regions of the world, where emergence has been associated with environmental change. Figure 1 shows the dramatic increase in reported cases of Lyme disease in the United States since the Centers for Disease Control and Prevention (CDC) began surveillance in 1982. The increased transmission in the United States is directly related to reforestation of the northeastern United States, allowing the mouse and deer populations to increase unchecked, which in turn has allowed the tick population to increase. A final factor has been the trend in recent decades to build houses in woodlots where humans share the ecology with deer, mice, and ticks; thus most transmission to humans in the northeastern United States where the majority of cases of Lyme disease occur, is residential (Steere et al. 2004).

Plague, caused by Yersinia pestis, is the most important reemergent bacterial vector-borne disease. The current global increase in case reports of plague is primarily due to outbreaks in Africa. However, it is the potential of plague to cause explosive epidemics of pneumonic disease, transmitted person-to-person and with high mortality that makes it important as a reemergent infectious disease and as a potential bioterrorist threat. This was illustrated in 1994 when an outbreak of plague occurred in Surat, Gujarat, India (WHO 1994). Although this was a small outbreak (most likely less than 50 cases) that should have been a relatively unimportant local public health event, it became a global public health emergency. The reasons for this are complicated and beyond the scope of this article, but it is a

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Fig. 1 Reported Lyme disease cases by year, United States, 1982-2005. Source: Adapted from Gubler (1998) and CDC (2006), courtesy, Division of Vector-Borne Infectious Diseases, CDC, Fort Collins, CO

classic case of "success breeding failure." Briefly, because the Indian Health Service had successfully controlled epidemic plague in India for over 30 years (the last confirmed human plague case prior to 1994 was in 1966), laboratory, clinical, and epidemiologic capacity to diagnose and control plague had deteriorated. Thus, when the Surat outbreak occurred, the clinical and laboratory diagnosis was confused, creating lack of confidence in public health agencies and ultimately panic when it was finally announced that the disease was pneumonic plague. Within a few weeks in early October 1994, an estimated 500,000 people fled Surat, a city of about 2 million people at that time. Many of these people traveled to other urban areas in India, and within days, newspapers were reporting plague cases in other cities. The World Health Organization implemented Article 11 of the International Health Regulations (WHO 1983) for the first time in 33 years because it was thought that people with pneumonic plague might board airplanes in India and transport the disease to other urban centers around the world (Fig. 2). Many countries stopped air travel and trade with India and most implemented enhanced surveillance for imported plague cases via airplane travel. This was the first global emerging infectious disease epidemic that impacted the global economy since infectious diseases were controlled in the 1950s. It is estimated that this small outbreak cost India US$3 billion (John 1999) and the global economy US$5-$6 billion. Fortunately, there were no cases of plague exported from India (Fritz et al. 1996), but this epidemic was the "wake-up call" that modern transportation and globalization were major drivers of pandemic infectious diseases. It was this epidemic that helped stimulate in the first funding of CDC's Emerging Infectious Disease Program.

Fig. 2 Suspected spread of pneumonic plague from India, 1994. Source: Courtesy, Division of Vector-Borne Infectious Diseases, CDC, Fort Collins, CO

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