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 the Perspectives: Risks Travelers Face section later in this chapter.) A reasonably exact 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 realtime 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 more than 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 is frequently updated (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 less than 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 significant illness or 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.
Figure 1-01. Estimated incidence rate per month of infections and fatal accidents among travelers in developing countries, 2010
1Unpublished; used with permission of Robert Steffen, Zurich, Switzerland.
SURVEILLANCE NETWORKS AND TRACKING OF DISEASE PROFILES
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 ill travelers who have been exposed in many countries, and who are seen during and after their return home. One such network, GeoSentinel, a collaborative effort of the International Society of Travel Medicine and CDC, has developed a profile of the relative likelihood of travel-related disease stratified by region of travel in the developing world (Figure 1-02).
Based on 17,353 ill returned travelers seen at 31 clinical sites on 6 continents, the destination-specific differences in relative frequencies (proportionate morbidity) are apparent for most diseases. Figure 1-02 shows destination-specific proportions of ill returned travelers with each diagnosis, rather than incidence rates, which can be used to assist with risk-profiling of prospective travelers during the pre-travel medical consultation. When individual diagnoses were collected into syndrome groups and examined for all regions together, 226 of every 1,000 ill returned travelers seen by participating clinicians had a systemic febrile illness, 222 had acute diarrhea, 170 had a dermatologic disorder, 113 had chronic diarrhea, and 77 had a respiratory disorder. Region-specific disease occurrence data indicated the following:
- Febrile illness was most likely from Africa and Southeast Asia.
- Malaria was among the top 3 diagnoses from every region.
- Over the past decade, dengue became the most common febrile illness from every region outside sub-Saharan Africa.
- In sub-Saharan Africa, rickettsial disease was second only to malaria as a cause of fever.
- Respiratory disease was most likely in Southeast Asia.
- Acute diarrhea was disproportionately seen in travelers from south-central Asia.
A recent GeoSentinel study indicated that disease profiles in European travelers, when compared with travelers from North America and other continents, did not differ, further showing that disease risk is dependent on destination and place of exposure rather than on origin country of the traveler.
Figure 1-02. Proportionate morbidity among ill travelers returning from the developing world, according to region of travel
1From Freedman DO, Weld LH, Kozarsky PE, Fisk T, Robins R, von Sonnenburg F, et al. Spectrum of disease and relation to place of exposure among ill returned travelers. N Engl J Med. 2006 Jan 12;354(2):119–130. Copyright © 2006 Massachusetts Medical Society. All rights reserved.
2Proportionate morbidity, not incidence rate, is shown for each of the top 22 specific diagnoses for all ill returned travelers within each region. STD denotes sexually transmitted disease. Asterisks indicate syndromic diagnoses for which specific etiologic diagnoses could not be assigned.
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 is often not reflective of 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. 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 package travel vs. self-directed travel. Recent information on long-stay vs. short-stay travel has been published.
- Caumes E, Ehya N, Nguyen J, Bricaire F. Typhoid and paratyphoid fever: a 10-year retrospective study of 41 cases in a Parisian hospital. J Travel Med. 2001 Nov–Dec;8(6):293–7.
- Chen LH, Wilson ME, Davis X, Loutan L, Schwartz E, Keystone J, et al. Illness in long-term travelers visiting GeoSentinel clinics. Emerg Infect Dis. 2009 Nov;15(11):1773–82.
- Freedman DO, Weld LH, Kozarsky PE, Fisk T, Robins R, von Sonnenburg F, et al. Spectrum of disease and relation to place of exposure among ill returned travelers. N Engl J Med. 2006 Jan 12;354(2):119–30.
- Gautret P, Schlagenhauf P, Gaudart J, Castelli F, Brouqui P, von Sonnenburg F, et al. Multicenter EuroTravNet/GeoSentinel study of travel-related infectious diseases in Europe. Emerg Infect Dis. 2009 Nov;15(11):1783–90.
- Greenwood Z, Black J, Weld L, O’Brien D, Leder K, Von Sonnenburg F, et al. Gastrointestinal infection among international travelers globally. J Travel Med. 2008 Jul–Aug;15(4):221–8.
- Hill DR. Health problems in a large cohort of Americans traveling to developing countries. J Travel Med. 2000 Sep–Oct;7(5):259–66.
- Jensenius M, Davis X, von Sonnenburg F, Schwartz E, Keystone JS, Leder K, et al. Multicenter GeoSentinel analysis of rickettsial diseases in international travelers, 1996–2008. Emerg Infect Dis. 2009 Nov;15(11):1791–8.
- Leder K, Wilson ME, Freedman DO, Torresi J. A comparative analysis of methodological approaches used for estimating risk in travel medicine. J Travel Med. 2008 Jul–Aug;15(4):263–72.
- Lederman ER, Weld LH, Elyazar IR, von Sonnenburg F, Loutan L, Schwartz E, et al. Dermatologic conditions of the ill returned traveler: an analysis from the GeoSentinel Surveillance Network. Int J Infect Dis. 2008 Nov;12(6):593–602.
- Liese B, Mundt KA, Dell LD, Nagy L, Demure B. Medical insurance claims associated with international business travel. Occup Environ Med. 1997 Jul;54(7):499–503.
- Mutsch M, Spicher VM, Gut C, Steffen R. Hepatitis A virus infections in travelers, 1988–2004. Clin Infect Dis. 2006 Feb 15;42(4):490–7.
- Nicolls DJ, Weld LH, Schwartz E, Reed C, von Sonnenburg F, Freedman DO, et al. Characteristics of schistosomiasis in travelers reported to the GeoSentinel Surveillance Network 1997–2008. Am J Trop Med Hyg. 2008 Nov;79(5):729–34.
- Schwartz E, Weld LH, Wilder-Smith A, von Sonnenburg F, Keystone JS, Kain KC, et al. Seasonality, annual trends, and characteristics of dengue among ill returned travelers, 1997–2006. Emerg Infect Dis. 2008 Jul;14(7):1081–8.
- Steffen R, Amitirigala I, Mutsch M. Health risks among travelers—need for regular updates. J Travel Med. 2008 May–Jun;15(3):145–6.
- Steffen R, deBernardis C, Banos A. Travel epidemiology—a global perspective. Int J Antimicrob Agents. 2003 Feb;21(2):89–95.
- Steffen R, Rickenbach M, Wilhelm U, Helminger A, Schar M. Health problems after travel to developing countries. J Infect Dis. 1987 Jul;156(1):84–91.
- Torresi J, Leder K. Defining infections in international travellers through the GeoSentinel Surveillance Network. Nat Rev Microbiol. 2009 Dec;7(12):895–901.
- Whitty CJ, Mabey DC, Armstrong M, Wright SG, Chiodini PL. Presentation and outcome of 1107 cases of schistosomiasis from Africa diagnosed in a nonendemic country. Trans R Soc Trop Med Hyg. 2000 Sep–Oct;94(5):531–4.