Reflections on travel-associated infections in Europe

In The Lancet Infectious Diseases, Patricia Schlagenhauf and colleagues¹ analyse travel-associated morbidity in 32 136 travellers presenting to European sites of the GeoSentinel Network with a post-travel illness over the period 2008–12. Knowledge about illness in travellers contributes to the understanding of disease epidemiology in our interconnected world. In addition to their own ill health, travellers serve as sentinels of infectious diseases; moreover, they can also transmit and disseminate infections.2, 3, 4 Travellers have shown these important roles in emerging infections such as SARS (severe acute respiratory syndrome), dengue virus infection, chikungunya infection, and currently Ebola virus disease, and also in antibiotic resistance.3, 4, 5, 6 The GeoSentinel Surveillance Network, a collaboration between the International Society of Travel Medicine and the US Centers for Disease Control and Prevention, has described significant disease epidemiology in travellers, and has also identified emerging infections through travellers such as sarcocystis on Tioman Island and the ongoing dengue outbreak in Luanda, Angola, in 2013.7, 8, 9, 10

The strength of the GeoSentinel data, including the EuroTravNet data analysed by Schlagenhauf and colleagues, is the large number of reported cases from different parts of the world. The weakness, however, is that they are mainly descriptive and variations in disease patterns over the years might be attributed to evolving travel patterns including changes in destinations and itineraries. Thus, shifts in disease trends should be interpreted with caution because they might simply be related to changing exposure. Moreover, some diseases (eg, dengue fever) also occur in countries not reporting to GeoSentinel, simply because they are not tourist destinations.

Even with these inherent shortcomings in the dataset, the findings that tuberculosis transmission within Europe remains a problem and that dengue transmission has occurred in Europe provide new and very valuable insights into the transmission dynamics of these infections. The analysis of subgroups is an additional strength of the study showing reduced rates of malaria, HIV, and hepatitis type A in travellers who received pre-travel advice.

The authors identified travel to sub-Saharan Africa and Asia to be significantly associated with acquisition of travel-related illnesses; data for immigration travel also showed increased acquisition in travellers from Africa.¹ Interestingly, the trends over time showed an increased proportionate morbidity of vector-borne diseases, particularly malaria and dengue, from 2008 to 2012. Further, the authors show the association between dengue virus and certain destinations and countries of origin, highlighting the popular destinations for various nationalities. Previous analysis of dengue from the GeoSentinel Network detected an increasing trend in travellers before national outbreak trends came to public recognition.¹¹ The detailed information provided by the authors might help national health authorities to formulate optimum advice for their travellers.

The patterns of disease shown by the European sites relate to global disease outbreaks—eg, the predominance of influenza H1N1 pandemic among respiratory infections in 2009.1, 12 Another relevant and timely example is chikungunya, which emerged in 2004 in the island countries in the Indian Ocean including Comoros and Reunion, and subsequently affected India, Sri Lanka, Maldives, southeast Asia, and more recently the Americas and Pacific islands.¹⁰ The EuroTravNet data parallel such developments in the localities affected, again showing the utility of travellers as sentinels of emerging infections. Because chikungunya was first documented in the Caribbean in December, 2013, travellers with exposure in the Americas would not have been included in this report by Schlagenhauf and colleagues.

Importantly, the association of pre-travel consultation with significantly reduced proportionate morbidity for Plasmodium falciparum, HIV, and AIDS validates the goals of travel medicine practice. The finding that smaller proportions of travellers visiting friends or relatives obtained pre-travel advice compared with tourist travellers supports policies aimed at improving the provision of pre-travel interventions for travellers visiting friends or relatives.

Despite all its merits, a final challenge remains. That is, the analysis cannot derive actual risk of acquiring each travel-related infection in view of its method of sampling through a consortium of travel and tropical medicine centres, which lacks a denominator—that is, the population at risk. Nevertheless, this report on European travellers provides valuable information: an association of illness when travelling to sub-Saharan Africa and Asia; the potential of acquiring endemic infections even when travelling to other parts of Europe; an increased reporting of some vector-borne infections over time; the role of travellers as sentinels and disseminators of infections; and finally, the benefits of pre-travel health preparation. These findings contribute to the creation of targeted pre-travel advice for particular diseases, populations, and destinations.

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