Travelers are an epidemiologically important population because of their mobility, their potential for exposure to diseases outside their home country, and the possibility that they may serve as a conduit for disease from one country to another. In the past 10 years, for example, travelers have faced newly emerging threats, including Ebola, chikungunya, and Zika. Evolving epidemiology of disease, the increasing prevalence of antimicrobial drug resistance, and the development of new vaccines and prophylactic treatments have each contributed to create the ongoing need for surveillance of international travelers.
The risk of travel-related illness varies depending on destination and traveler characteristics. Existing information regarding the actual risk for travelers (often expressed as number of events per 100,000 travelers) is limited for several reasons. It is difficult to obtain an accurate numerator (number of cases of disease among travelers) and denominator (number of travelers overall or travelers to a specific destination who are susceptible to infection). To calculate a true risk for a traveler, scientific studies would have to document the number of travelers susceptible to that disease or condition and the number of those affected during a specific period of time. If the illness is mild, the traveler may never seek health care, or clinicians might not perform diagnostic tests to identify the cause accurately. Furthermore, because travelers often visit multiple destinations, it could be difficult to determine the location where the exposure occurred and attribute risk to that location.
Frequently quoted studies on the incidence of infection in travelers use a variety of methodologic designs, each with its own strengths and weaknesses, making findings difficult to compare or combine. These studies have examined, for the most part, only a few key diseases or conditions, combining all travelers regardless of destination. Many have been single-clinic or single-destination studies that lead to conclusions that are not generalizable to groups of travelers with different local, national, or cultural backgrounds.
Health care providers must understand the epidemiologic features of the traveling population to guide pretravel recommendations and posttravel evaluations. The characteristics of travel-related diseases must be considered, including mode of transmission, incubation period, signs and symptoms, duration of illness, and diagnostic testing. The presence, frequency, seasonality, and geographic distribution of the disease need to be assessed; these might change over time because of outbreaks, emergence or reemergence in new areas or populations, successful public health interventions, or other factors.
Data on disease incidence in local populations could identify the most important diseases to monitor within a country, but the relevance of such data to travelers—who have different risk behaviors, eating habits, accommodations, knowledge of preventive measures, and activities—is usually limited. Surveillance data that focus on travelers or on illnesses that affect travelers are therefore more useful in describing travel-related disease patterns and risks.
Two existing networks provide data on the demographics of US international travelers and the acquisition of travel-related illness. Global TravEpiNet (GTEN) is a consortium of health clinics across the United States that provide pretravel health consultations; data from GTEN provide a snapshot of the types of travelers seeking pretravel health care and their travel practices, as well as longitudinal cohort data on risk and acquisition of travel-associated conditions. The GeoSentinel Global Surveillance Network, a worldwide data collection and communication network composed of International Society of Travel Medicine (ISTM) travel and tropical medicine clinics, collects posttravel illness surveillance data. GeoSentinel analyzes this data to describe the relationships between travel and travel-related illness in specific subpopulations of travelers.
Familiarity with the epidemiology and prevalence of travel-related infections, coupled with demographic information on travelers and their particular travel details, can help clinicians provide optimal health-related information and advice. Clinical networks and surveillance systems provide epidemiologic data on new and prevalent global infectious disease threats. Improved collaboration between travel health providers and the travel health clinical networks is needed to further expand and develop the evidence base in this field; this will allow for better-informed preparation before travel and enhanced clinical awareness of travel epidemiology for clinicians seeing patients before and after travel.
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Allison Taylor Walker, Regina C. LaRocque, Mark J. Sotir