Author(s): Allison Walker, Regina LaRocque

Travelers are an important population because of their mobility, their potential for exposure to infectious diseases outside their home country, and the possibility that they could bring those diseases from one country to another. The coronavirus disease 2019 (COVID-19) pandemic is the most recent example of the role travelers can play in the global spread of infectious diseases. Ebola virus, Zika virus, and antimicrobial-resistant pathogens are other examples of health threats whose geographic distribution has been facilitated by international travelers over the past several years. Travelers consequently should be included in general and targeted epidemiologic surveillance—including the use of molecular genomic approaches—to better understand both the exposure risk and impact of current and novel prevention recommendations.

The ability to provide appropriate pretravel guidance—and, when necessary, optimal posttravel evaluation and treatment—is predicated on understanding the epidemiologic features (disease patterns) among different traveling populations. Accounting for behaviors that can influence and potentially increase risk for travel-associated infections and diseases (e.g., attendance at a mass gathering, long-term or adventure travel, visiting friends and family) helps the astute clinician make directed travel health recommendations and focus their attention on the more likely diagnoses from among the lengthy list of travel-associated infections and diseases. An understanding of the epidemiology of the diseases themselves, including modes of transmission, incubation periods, signs and symptoms, duration of infectiousness, and accuracy of diagnostic testing, is also crucial. Including international travelers in epidemiologic surveillance provides additional information about the presence, frequency, seasonality, and geographic distribution of diseases, which might shift over time due to outbreaks, changes in climate and vector habitat, emergence or reemergence in new areas or populations, successful public health interventions, or other factors.

The risk for travel-related infection can, however, be difficult to ascertain precisely for several reasons. Existing information regarding disease risk for travelers is limited because of the difficulty in obtaining accurate numerators (i.e., number of cases of infection among travelers) and denominators (i.e., number of overall travelers or number of travelers to a specific destination who are susceptible to infection). In cases of mild illness, travelers might never seek health care, or clinicians might not perform diagnostic tests to identify the cause. Travelers often visit multiple destinations, complicating identification of the location of exposure. Data on disease incidence in local populations might be available, but the relevance of such data to travelers—who have different risk behaviors, eating habits, accommodations, knowledge of and access to preventive measures, and activities—might be limited. In addition, epidemiologic investigations involving travelers use various methodologic designs, each with their own strengths and weaknesses, making findings difficult to compare or combine. Many single-clinic or single-destination investigations draw conclusions that might not be generalizable to travelers from different local, national, or cultural backgrounds.

Two existing networks provide epidemiologic data on international travelers from the United States and acquisition of travel-related illness. The GeoSentinel Global Surveillance Network is a worldwide data collection and communication network composed of International Society of Travel Medicine–associated travel and tropical medicine clinics that collect posttravel illness surveillance data. GeoSentinel scientists analyze these data to describe travel-related illness in specific populations of travelers.

Global TravEpiNet (GTEN) is a consortium of health clinics across the United States that deliver pretravel health consultations. Data from GTEN provide a snapshot of travelers seeking pretravel health care, and longitudinal cohort data on risk for and acquisition of travel-associated conditions, including for a subset of travelers who self-collect biological samples for microbiologic and genomic testing.

These travel medicine networks, and travel medicine researchers, increasingly are implementing next-generation sequencing tools to delineate the epidemiology of travel-associated infections and the role of travelers in the global spread of infectious diseases. Advances in the field of genomic sequencing enable high-resolution surveillance that can identify previously unrecognized geographic and epidemiologic associations. These molecular tools are becoming essential to understanding the spread of disease, the emergence of new pathogens or variants of existing ones, and the evolution of antimicrobial resistance. Combining these molecular techniques with traditional surveillance, epidemiologic approaches, and community-based participatory research represents a promising approach to expanding the evidence base underpinning the guidance and recommendations in the field of travel medicine. A broader evidence base will enable better-informed pretravel preparation for the individual traveler, and development of new approaches to mitigating the impact of travel on the global spread of disease.

Resources

HealthMap uses online informal sources and real-time surveillance to provide information on emerging public health threats for diverse audiences.