Emerging-infectious diseases and travel medicine

In the 1992 report, Emerging Infections: Microbial Threats to Health in the United States,(43) produced by the Institute of Medicine of the National Academy of Sciences, expanding international travel and commerce was identified as one of six principal factors contributing to the global development and spread of emerging and reemerging pathogens. In recent years the number of travelers has risen sharply, and populations are now in motion to a degree never before seen in history. Mobile populations include leisure and business travelers, military personnel, long-term expatriates, and missionaries. Also included are legal and illegal immigrants and persons uprooted while seeking refuge from numerous regional conflicts. Members of these populations travel between countries more frequently and rapidly than previously possible. It has been estimated that during 1995 over 1.4 million tourists crossed international borders every day, an annual total of over 500 million persons (Fig. 1).(39) Virtually any destination can be reached from any other in only 36 hours of travel. This 36-hour window is well within the incubation period of most infectious diseases, affording ample opportunity for the unrecognized movement of pathogens from place to place and for rapid global spread of microbial agents. From the microbe's point of view, the global village of the late 20th century provides global opportunities for disease emergence and transmission. For several reasons travelers warrant special attention from those wishing to monitor, control, and prevent the spread of emerging pathogens. First, emerging infections may arise in areas frequented by tourists or other travelers, and these individuals may provide the first evidence fora new disease agent. Second, travelers often visit areas where the spectrum of infectious diseases includes agents that are nonendemic for their homeland. During travel, such persons may become exposed to these illnesses and transport them home, contributing to potential global spread; they may also bring disease vectors with them and expand their range. Third, travelers often engage in activities that increase their risk for disease exposure, such as outdoor activities during adventure travel. And finally, mass population movements associated with civil strife, particularly in developing countries, have proven to be high-risk settings for disease emergence. This article examines these issues and provides examples of emerging infectious diseases linked to international travel and discusses systems for surveillance, control, and prevention of travel-associated disease.