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Chapter 1Introduction

Travel Epidemiology

David O. Freedman

To prescribe optimal pre-travel advice, preventive measures, and education, travel health providers must be aware of the absolute and relative magnitude of the many travel-related health risks. Such knowledge allows travel health care providers to perform an epidemiologic and traveler-specific risk assessment so that these measures can be appropriately prioritized for each traveler. Travel-related health problems are self-reported by 22%–64% of travelers to the developing world; most of these problems are mild, self-limited illnesses such as diarrhea, respiratory infections, and skin disorders. Approximately 8% of the more than 50 million travelers to developing regions, or 4 million people, are ill enough to seek health care, either while abroad or upon returning home.

LIMITATIONS OF CURRENT EPIDEMIOLOGIC KNOWLEDGE

Knowledge of the precise risk for a specific disease in a specific location has proved elusive, despite several decades of interest and investigation. (For additional discussion, see Chapter 2, Perspectives: Risks Travelers Face.) A reasonably accurate estimate of the number of cases of a disease or infection in all travelers over a time period at a location is difficult to determine, as many will have returned to their home countries by the time the disease manifests symptoms. Similarly difficult to obtain is an exact denominator reflecting the total number of travelers to that location. An accurate numerator must be divided by an accurate denominator to calculate a true incidence rate or risk. Even this standard population-based approach assumes that past experience predicts future risk. In addition, disease risks are not stable over time, and current or real-time data are rarely available. Much of the frequently quoted numerical data regarding the incidence of infection in travelers are based on extrapolations of limited data, collected in limited samples of travelers anywhere from a few to >20 years ago. This knowledge base includes morbidity studies of various methodologic designs, each with its own set of strengths and weaknesses. These studies have mostly examined a few key individual diseases in all travelers regardless of destination, profiles of disease occurrence at a few specific high-risk destinations, and disease occurrence in certain types of travelers with certain behaviors. Many have been single-clinic or single-destination studies that can lead to conclusions that are not generalizable to groups of travelers with different local, national, or cultural backgrounds.

INCIDENCE RATES AND ESTIMATES OF RISK

A compilation of best available incidence rate estimates, given the above limitations, is available and has been updated over the years (Figure 1-01). With the notable exception of malaria, the major preventable travel-related diseases are associated with relatively low risks, ranging from 1 in 100 for influenza to <1 in 100,000 for several diseases that often concern travelers. Hepatitis A may be taken as an example of a prototypical vaccine-preventable disease, with an estimated overall uncorrected incidence of approximately 1 in 5,000 travelers to the developing world. Thus, the odds against acquiring hepatitis A on a single short trip are greatly in the traveler’s favor, as many travelers realize. Any considered vaccination should be presented in context as insurance against a relatively uncommon event, but one that may result in illness or other consequences.

For diseases with poor or fatal outcomes, such as meningococcal meningitis, rabies, or Japanese encephalitis, the context of less tolerance of even small risks needs to be communicated to travelers to help them make informed decisions about all available interventions. The incidence rates in Figure 1-01 reflect aggregate data and studies, and do not consider variations in risk behaviors, destination, season, duration of travel, or general style of travel. For many diseases, research into increased or decreased risk according to these variables is still in its infancy because of difficulties in tracking outcomes at remote destinations.

A more recent and novel approach to defining disease epidemiology in travelers has involved the use of collaborative networks of specialized travel medicine clinics to collect and aggregate data on large samples of travelers (see the next section in this chapter, Travel Medicine Data Collection: GeoSentinel & Global TravEpiNet).

Figure 1-01. Estimated incidence rate per month of infections and fatal accidents among travelers in developing countries, 20101

Estimated incidence rate per month of infections and fatal accidents among
travelers in developing

 

View Larger Figure

1Unpublished; used with permission of Robert Steffen, Zurich, Switzerland.

 

FUTURE CHALLENGES AND PRIORITIES FOR TRAVEL EPIDEMIOLOGY

Issues surrounding the relative merits of different methodologic approaches to defining travel-associated disease risk have recently been reviewed at length. In order to better characterize health risks and provide guidance to travelers, some epidemiologic priorities include the following:

  • Obtaining travel-related data for many existing and potentially vaccine-preventable diseases. Current data are sparse, and incidence in local populations often may not reflect travelers’ risk because of different risk behaviors, previous infection, or preexisting vaccination campaigns.
  • Developing better surrogate markers for malaria exposure during travel to facilitate interventional studies for novel malaria chemoprophylaxis drugs or alternative dosing regimens. Such information is difficult to obtain because of the inability to perform placebo drug studies, given the life-threatening nature of the infection.
  • Studying the effect of high-risk medical conditions or immunocompromising medications on travel outcomes.
  • Improving understanding of the impact of host behavior related to different types of travel, such as tourism, business travel, travel to visit friends and relatives, missionary travel, and volunteer travel.
  • Conducting research to gain more insight on exposure-related factors, such as urban vs rural travel, luxury vs rough travel, season of travel, and organized vs self-directed travel. Recent information on long-stay vs short-stay travel has been published.

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