While many potential threats posed by emerging infections continue to make headlines in the 21st century, limited attention is often paid to well demonstrated and defined infectious disease risks. As eloquently noted recently in TIME Magazine, humanity tends to worry about possibilities, while paying less attention to probabilities.1 This analogy helps to explain the travel-related concerns regarding H5N1 avian influenza that, to date, has not been acquired by a single traveler in any of the 12 countries that have reported human cases.2 At the same time, many travelers undertake journeys without interventions for infections that routinely cause illnesses and sometimes death. In spite of the periodic undulating concerns regarding avian influenza, we must keep in mind that according to the World Health Organization definition, the world has remained in a static phase 3 influenza “pandemic alert” for 3 full years now.3 Highlighted exotic infections, while often associated with increased concern, have rarely been observed in travelers. For example, similar to most previous outbreaks of Marburg fever, there was no traveler among the 252 cases with 227 deaths reported from Angola in 2005.4
In contrast, the severe acute respiratory syndrome (SARS) epidemic, now faded from travel medicine discussion, has been very relevant in the context of international travel and health. During the 2003 episode, 8,096 cases with 774 deaths were recorded and 142 were imported.5 Transmission occurred in hotels6 and during flights.7 The five cases imported to Canada resulted in 251 cases with an almost paralyzing impact on travel to a major metropolitan center, Toronto. The experience clearly illustrates that some global connections, in this case what apparently happened in restaurants and on markets in Southeastern China, has the potential to become very relevant to all of us. But SARS is now history, it has disappeared for the time being—thus is the current traveler at present no longer threatened by emerging infections?
As described in the above noted Time article on how we perceive risk, attention both by the media and public seem far less interested in infections with a low case fatality rate even if over time they are responsible for significant morbidity and mortality. Both dengue and chikungunya fever, for example, rarely made it to page 1 in industrialized countries, although they are far more relevant to the customers in travel clinics compared to “killer diseases.” Recent epidemics of chikungunya affected not only the French territory of Reunion—where 218,000 cases in a population of 777,000 were estimated to have contracted the infection in the past year—but also the neighboring islands of Mauritius, the Seychelles, Comores, Madagascar, and coastal regions of East Africa. On the other side of the Indian Ocean, the epidemic is still ongoing in several Indian States. Reports indicate that in the past months, many tourists imported chikungunya to European countries, Hong Kong, Canada, and to the United States.8 A total of 808 cases were imported in France between April 1, 2005, and August 31, 2006.
Dengue remains on a continuous rise. The number of endemic countries has increased from 9 in the 1970s to 60 in 1995 and to over 100 now 10 years later.9 Which destinations are still safe today for a divemaster who acquired dengue in the Maldives in the past, who is concerned about getting dengue hemorrhagic fever and who dislikes cold water at his work place? The majority of tropical beaches are at risk nowadays. Data from the GeoSentinel program demonstrate that dengue is now more frequently diagnosed than malaria in travelers having returned from the South America and Asia.10 In Australia, dengue is diagnosed 10 times more often than malaria among travelers who returned from Asia.11 On the other hand, the brief reappearance of malaria in Kingston, Jamaica, and Goa, India, demonstrate that travel health professionals need to remain vigilant for news, both for new and for the disappearance of outbreaks.12
What about changes in risk for “traditional travel infections?” Compared to the initial epidemiological surveys conducted more than 20 years ago, the incidence rate of travelers’ diarrhea has markedly decreased in all Southern European countries and is now below 8% except for Portugal.13 Rates have also decreased in Jamaica by 72%,14 and they have dropped now to 10 and 20%, respectively, in Chiangmai and Phuket (Thailand).15 The pattern of decrease, however, has not been globally uniform. In contrast to the aforementioned decrease, the incidence rate still exceeded 60% for a 2-week stay in India (Goa) and Kenya (Mombasa) in the late 1990s.16,17
The risk of hepatitis A infection has decreased by a factor of 10 among travelers to developing countries and the much-quoted historical monthly incidence rate of 1 per 300 nonimmunes is obsolete; 1 per 3,000 is more representative for a current travel risk.18,19 However, we have to maintain the recommendation strategy according to which all travelers to developing countries should be immune against hepatitis A.20 Among nine cases of hepatitis A included in a recent survey, three “had sought pretravel advice;”11 if the vaccine had not been recommended to these travelers, this is worse than bad luck. Why postpone hepatitis A vaccination in a business traveler who will stay in Africa for few days only—we must be aware that he is likely to get exposed during later travels again. Universal immunization against hepatitis A has even been rated to be justified in high-income countries with substantial risk such as the United States.21,22 Immunization programs are also important for nations where the second generation and therefore not naturally immunized members of traveling migrant populations (visiting friends and relatives [VFRs]) are at risk of hepatitis A acquisition during travel.23 Similar issues should also be considered in countries like Switzerland, where by the age of 30, almost 70% of the population will have visited a low-income country (M. Mohler-Kuo, personal communication, April 2007).
Typhoid fever no longer is a substantial risk for visitors to Southern Europe and to destinations in East Africa, Latin America, and most of Southeast Asia. The attack rate is clearly below 2 per 100,000.24 In contrast, the risk exceeds 10 to 50 per 100,000 in all countries in South Asia. It is not only a risk for VFRs but for backpackers and long-term residents as well.
Additionally, human immunodeficiency virus/acquired immunodeficiency syndrome and other sexually transmitted infections remain a constant threat to those who engage in behaviors associated with increased risk.
Not only are the epidemiological features of travel-related disease evolving, so are the characteristics of traveling itself. During the past 25 years, there has been a profound increase of global international travel. Much of it has taken place in Asia, the Middle East, Latin America, and Africa. In India alone, the volume of travelers passing through Indian airports quadrupled between 1981 and 2003 from 11 million to 44 million.25 Changes in transportation technology have also increased the ability of travelers to access previously isolated or hard to reach destinations. The first jet aircraft service to Bali began only in 1967 and similar access to Katmandu began in 1968.26
The collapse of the former Soviet Union and the end of the Cold War at the end of the 1980s, for example, opened opportunities for traveling in large areas of the world that were until then difficult to visit. Also, there is rapidly increasing outbound travel from this area, which so far has limited experience with the concept of protecting travelers’ health. Increased and easier access has also been associated with changes in traveler demographics. Easier access to travel now facilitates travel by the elderly, those with chronic illnesses, and other vulnerable groups. Additionally, refugees and others displaced by conflict and disaster continue to make up a rising component of vulnerable international travelers.27 Managing travel-related health risks in this growing population of nontraditional travelers is an area of increasing interest. Global migration has expanded, and it is now estimated that 191 million individuals live or work in countries other than where they were born. Since 1990, the greatest growth of this migrant population has occurred in the high-income world where some 41 million of these individuals now live.28 Migrant populations are traveling more frequently, and their travel is associated with the acquisition and importation of several travel-related diseases such as malaria, dengue, chikungunya, typhoid, cholera, and others.29
The evolution in the demographics and figures of international travel in the past 30 to 40 years is raising the importance of movements from areas of low disease prevalence to high incidence destinations and vice versa. This expands the practice of travel medicine beyond the traditional protection of travelers from low-incidence regions to areas of increased risk. As noted, during the SARS situation, these events can have significant impact.
To conclude—the targets we deal with are continuously moving: some risks are increasing, some decreasing, varying with the destination, the season, the traveler, the possible climate changes, and also the political situation. There are some targets, not necessarily those with the greatest headlines, being most relevant. The world’s population is more and more on the move. In spite of global political and health challenges, international travel has continued to show sustained growth over time, and it is expected to continue.30 Much of this growth will occur in populations of travelers with different characteristics from those of 30 years ago. Preventing and managing the travel-related health risks of these individuals will be increasingly challenging.
To meet these developing challenges, in this century, we have not yet been showered with new tools for travelers’ health diagnostics, vaccines, prophylactic, or therapeutic medications. Vaccines against dengue, for example, would immediately be broadly used, those against norovirus would be a relief at least to cruise passengers. Progress in diarrheal vaccines seems slow, and silence prevails with respect to new antimalarials that could be on the horizon.