A travel medicine view of dengue and dengue hemorrhagic fever

Travel Medicine and Infectious Disease (2009) 7, 278e283

available at www.sciencedirect.com

journal homepage: www.elsevierhealth.com/journals/tmid

A travel medicine view of dengue and dengue hemorrhagic fever Eyal Meltzer a,b,**, Eli Schwartz a,b,* a

The Center for Geographic Medicine and Department of Medicine C, The Chaim Sheba Medical Center, Tel Hashomer 52621 Israel b Sackler School of Medicine, Tel Aviv University, Tel Aviv, Israel Received 19 April 2009; accepted 6 May 2009 Available online 22 July 2009

KEYWORDS Dengue; Dengue hemorrhagic fever; Travel; Antibody-dependent enhancement

Summary Dengue fever is the most prevalent arboviral infection worldwide, with up to 40% of the world population living in endemic regions. Among travelers to tropical countries, dengue infection is increasingly reported, and it is now a leading cause of post-travel fever. Outbreaks of dengue-like illness were already described since the 18th century, but it is only in the last half century that a severe form of the disease e dengue hemorrhagic fever (DHF) has been described. Although the cause of DHF is not established, the prevailing theory attributes the disease to antibody-dependent enhancement of viral replication, in the presence of a secondary dengue infection. Comparative studies of primary vs. secondary infection are difficult to perform in endemic countries because of the rarity of primary infection except during early infancy. Travelers on the other hand are usually diagnosed with primary infection and are therefore a better study population. Data on dengue and DHF among travelers appears to suggest that severe dengue and DHF occur in similar rates among cases with primary and with secondary infections. Epidemiological and physiological data from travelers suggest that the prevailing theory on the causes of DHF needs to be reconsidered. ª 2009 Elsevier Ltd. All rights reserved.

Introduction * Correspondence to: Eli Schwartz, The Center for Geographic Medicine and Department of Medicine C, The Chaim Sheba Medical Center, Tel Hashomer 52621 Israel. Tel.: þ972 52 6666132; fax: þ972 3 5302011. ** Correspondence to: Eyal Meltzer, The Center for Geographic Medicine and Department of Medicine C, The Chaim Sheba Medical Center, Tel Hashomer 52621 Israel. Tel.: þ972 54 4728552; fax: þ972 3 5303501. E-mail addresses: [email protected] (E. Meltzer), [email protected] (E. Schwartz).

Dengue fever (DF) is an acute febrile illness caused by four closely related arboviruses (designated DEN-V 1e4). The virus is transmitted to humans after a bite from an infected Aedes mosquito, with Aedes Aegypti being the most important vector. Infection with each DEN-V confers lifelong immunity against the specific serotype, but only temporary immunity against other three species, usually lasting only few months. Clinical DF occurs during either the first (i.e. primary) or the second (i.e. secondary)

1477-8939/$ - see front matter ª 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tmaid.2009.05.002

Dengue hemorrhagic fever in travelers infectious episode. Although in endemic areas many people are seropositive against all four DEN-V, 3rd and 4th infections are rarely recognized clinically.1e3 DEN-Vs are quite recent infectious agents of humans; genetic analysis suggests that they probably arose from closely related simian viruses within the last millennium.4 It is interesting to note that the first reports of DF in Western medicine originated not in the tropics but in quite temperate regions, when Benjamin Rush provided the first description of ‘‘Break-bone fever’’ during an outbreak in Philadelphia.5 However, by the early 20th century DF was only rarely encountered in industrialized countries (e.g. in Athens, in 19276), but was commonplace in most tropical regions. Currently, DF is one of the most frequent infectious diseases encountered in developing countries, and it is not rare to find seropositivity rates of 50% or more in endemic countries.7 During the last half-century, cases of severe DF have been increasingly recognized. These have been defined as dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS), requiring the presence of hemorrhagic phenomena and capillary leakage for DHF and circulatory collapse for the diagnosis of DSS (Table 1 e WHO definition of DHF). The pathogenesis of DHF has been a source of controversy since its recognition. The prevailing theory suggests a central role for secondary dengue infection in causing DHF8; however, other factors (e.g. pathogen virulence or the host’s genetic makeup) may in fact underlie DHF.9 The resolution of this controversy has been hampered by the absence of animal models, and especially by conflicting epidemiological data from studies done on endemic populations. Travelers from non-endemic countries can serve as ideal study populations to answer these questions, since travelers usually suffer from primary infection. The aim of this article is to review aspects of DHF epidemiology, clinical and physiological data, from a travel oriented perspective.

279 Table 1 WHO definitions of dengue, dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS). Dengue fever Probable  Acute febrile illness with two or more of the following:  Headache  Retro-orbital pain  Myalgia  Arthralgia  Rash  Hemorrhagic manifestations  Leukopenia  Supportive serology

Dengue hemorrhagic fever The following must all be present: 1. Fever or history of fever lasting 2e7 days 2. Hemorrhagic tendencies evidenced by at least one of the following  A positive tourniquet test  Bleeding from the mucosa, gastrointestinal tract, injection sites, or other  Hematemisis or melena 3. Thrombocytopenia (<100,000/mm) 4. Evidence of plasma leakage due to increased vascular permeability, manifest by at least one of the following  Rise in hematocrit 20% above the average for sex, age, and population  Drop in hematocrit after volume replacement 20% of baseline  Signs of plasma leakage (i.e. pleural effusions, ascites, and hypoproteinemia)

Dengue shock syndrome All of the above four criteria for dengue hemorrhagic fever must be present, plus evidence of circulatory failure manifested by  Rapid and weak pulse  Narrow pulse pressure (<20 mm Hg)  Hypotension for age  Cold clammy restless

DHF epidemiology It has been repeatedly noted in the past, that although the symptoms of DF were debilitating, mortality from the disease was very rare. For example, during the 1897 DF outbreak in Charleston South Carolina, when three quarters of a population of 65,000 were affected, only seven fatalities were noted.10 Similarly, in the 1927 epidemic that engulfed most of Egypt, mortality was reported as ‘‘virtually nil’’.11 However, it is important to note that the virus was only isolated in the 1940s, and up until the 1950s, the diagnosis of DF was based on clinical parameters and not on laboratory studies. The possibility that some of these early dengue-like outbreaks were not caused by DEN-V but by other arboviruses cannot be ruled out. During the 1950se1960s a major change in the perceived incidence and severity of DF occurred. From the mid 1950s, severe outbreaks of ‘‘viral hemorrhagic fever’’ (a concept that was first widely circulated only during the Korean War) were observed in South East Asia (SEA). These outbreaks were noted for severe cases that developed bleeding phenomena and capillary leak, leading occasionally to

severe shock and death. These severe cases, defined as DHF, were perceived to be a new disease. At first some confusion as to the etiology existed, but DEN-V was found to be the causative agent.12 Initially DHF was reported in SE Asia e from the Philippines to Indonesia. However, during the 1980s DF outbreaks increased both in number and in severity, with DHF first reported in India, the Caribbean and South America.13 A clear picture of DHF epidemiology is difficult to achieve, since the accuracy of DF/DHF diagnosis and reporting are suboptimal. Furthermore, DF outbreaks may lead to different DHF rates in neighboring countries, and even in the same country with different outbreaks. In some countries these changes follow a cyclical pattern. Overall, in endemic countries, DHF usually forms 1e3% of all DF cases, with fatality rates today of approximately 1%. It is estimated that currently, each year 50e100,000,000 cases of DF occur, with 250e500,000 cases of DHF and an annual mortality rate of 25,000. Approximately 2.5 billion people e 40% of the world’s population are now living in areas where dengue is endemic.14

280

What is DHF? After its recognition, DHF came to be regarded as a distinct disease state with a different pathogenesis from ‘‘classical’’ DF. Two clinical features have typified the outbreaks of severe DF that emerged in South East Asia in the 1950se1960s e the first was an unusual frequency of bleeding with severe thrombocytopenia, and the second was the propensity to develop hypovolemic shock, with concomitant evidence of serosal fluid accumulation, i.e. a capillary leak syndrome. These features were formalized in the WHO’s DHF definition, where DHF is defined as a combination of fever, severe thrombocytopenia, bleeding phenomena, and the demonstration of capillary leakage, by laboratory parameters or the presence of effusions (Table 1). However, fever and thrombocytopenia are almost universal in all dengue infections. Bleeding e both minor and major is encountered in ‘‘classical DF’’ as well. It is the demonstration of capillary leakage that in fact differentiates DHF from DF. This view was summarized in a recent review: ‘‘It is important to recognize that DHF is not merely a febrile disease with hemorrhagic manifestations, however severe, but is a qualitatively distinct, potentially lifethreatening leaky capillary syndrome’’.15 It should be noted that severe and even fatal dengue may occur outside the ‘‘pale’’ of DHF. Occasionally, major bleeding and other severe manifestations including encephalopathy, myocardial, hepatic and ocular damage, occur without evidence of capillary leakage. All of these have drawn attention to the inadequacy of the current definition of DHF.16

DHF: pathogenesis The prevailing theory as to the cause of DHF contends that DHF is an immune response to secondary dengue infection, caused by the phenomenon of antibody-dependent enhancement (ADE) of viral replication leading to excessive inflammatory reaction.8 ADE refers to a phenomenon where non-inhibitory, non-serotype-specific anti dengue antibodies form complexes with DEN-V. These complexes are taken up by Fc receptor bearing mononuclear cells, where the active virus replicates. This increase in viral replication leads to excessive cytokine release which eventually results in endothelial damage and capillary leakage. The evidence supporting the role of ADE in DHF stems from in vitro studies, with some in vivo studies on animal models.8 Data from human studies is sparse, but has suggested a role for ADE in at least some scenarios.17 The best support for the immune enhancement theory was found in epidemiological studies from South East Asia and also from Cuba. These have shown DHF to be largely associated with secondary infection.13,14,18,19 However, the validity of these associations was very early questioned, especially in the writing of the late Leon Rosen.9 In other areas of the world, different results were reported. In pacific islands (Niue and Fiji) no difference in DHF cases was noted between primary and secondary dengue20,21; similar results were reported from DF outbreaks in South America, where again DHF was more associated with virus

E. Meltzer, E. Schwartz type than with secondary infection.22e24 Furthermore, recent studies in Thailand have shown an increase in DHF among adults. Wichmann et al. have demonstrated that while DHF among children was indeed associated with secondary infection, this association was lost among adults.25 Finally, in a thorough study in Haiti, Halstead et al. have documented hyperendemic dengue transmission, highly similar to that encountered in Thailand, but a near absence of DHF e this was explained by a possible ‘‘racial resistance’’ of people of African origin that moderates dengue infection and prevents the emergence of DHF.26 Although it appears that the association of secondary infection with DHF may be occasional rather than the rule, the immune enhancement theory of DHF has performed an unusual feat for theories in infectious disease medicine: it has been regarded as an established fact. Despite conflicting sets of evidence, it became part of textbook medicine, whereas opposing views were sidelined.27e29

Dengue and DHF among travelers In endemic countries it is quite difficult to perform a comparative study of primary and secondary infections among adults, because of the rarity of primary infections in adults. The study of DHF in endemic population has additional limitations. The population studied is usually pediatric, and the effects of conditions other than DEN-V infection in this age group such as malnutrition, immune modifying helminthic infections may be important. It may be wrong therefore to generalize from findings in populations from endemic countries, to the general pathophysiology of DF/DHF. Travelers from non-endemic countries are an ideal study population, since they come from a variety of ethnic backgrounds, and are generally healthy, well nourished young adults. Indeed, data accumulated on DF in travelers questions secondary infection e immune enhancement theory of DHF. The history of DF among travelers returning ill after a sojourn in endemic regions, shows that DF was rarely reported until the last two decades. As Manson had pointed out a hundred years ago, the short incubation period of DF (5e8 days) and the long duration of sea travel from the tropics excluded DF as a cause of fever in returning travelers.30 The advent of the era of jet travel and mass tourism coincided with the emergence of DHF in Asia, and the subsequent massive increase in dengue in the Americas. Since the early 1980s31 DF and DHF were also been increasingly reported in returning travelers. Large registries of post-travel illness have shown DF is one of the most frequently reported diseases. In the GeoSentinel database, DF accounted for 2.1% of all reported travel related illnesses.32 Retrospective studies on dengue seroconversion in travelers have found posttravel dengue seroprevalence to range from 3.4% to 7.4%.33,34 Prospective studies have demonstrated similar seroconversion rates, ranging from 3%35,36 in short term European travelers, to 6.4% in longer staying Israeli travelers.37 Among febrile returning travelers, DF has consistently been found to be the leading cause of fever outside

Dengue hemorrhagic fever in travelers Table 2 Setting

281

Travel related dengue and dengue hemorrhagic fever.

(Ref)

Geosentinel international case registry32 TropNet Europe case registry51 European travelers43 Austrian travelers40 Japanese travelers44 Spanish travelers52 Dutch travelers53 British travelers54

Number of travelers

Hospitalization (%)

DHF/DSS (%)

Complicated dengue (%)a

522

24

2.3

NR

250

24

2.0

1.2

219 93 62 61 46 16

23 68 100b 100b 24 100b

0.9 7.3 3.2 0 4.8 18.9

11 11.8 20 8.2 8.7 NR

NR e not reported. a Complications other than DHF: such as bleeding phenomena without criteria for DHF, ocular damage, encephalopathy, etc. (definitions vary between publications). b Hospital based series.

Africa.38e41 It is also one of the leading causes of hospitalization of ill travelers.39 The greatest difference between native and non-native (traveler) cases of DF is in their likelihood of having had a previous exposure to dengue. In endemic conditions, repeated exposure makes primary DF a disease of infancy, with secondary attacks occurring during childhood, and little disease among adults. Most travelers present with primary infections and therefore, as would be expected according to the immune enhancement theory should present with little if any cases of DHF. However, as can be seen in Table 2, the reported incidence of DHF among travelers is in fact no less than the 1e3% usually seen in endemic populations. Thus, from the epidemiological point of view, travelers’ DHF morbidity does not fit the secondary infection theory. Only scarce serological data exists on of travelers with DHF. In a series of dengue infected Israeli travelers we diagnosed two cases of DHF (Schwartz e unpublished data). A few other reports have included data on primary and secondary dengue in travelers with DHF.42e45 Together with our cases, 8/9(88.9%) travelers with DHF had a primary infection. In addition, a few fatal cases of dengue infection have been reported in travelers and

involved primary infections that manifested as DHF with intracerebral bleeding as the cause of death.46 These data, although from a small number of cases also undermine the role of secondary dengue infection as a cause of DHF in travelers. Studies of capillary leakage in travelers with DF are lacking. The WHO definition for capillary leakage may not be suitable for travelers hospitalized in Western facilities. The criteria used to diagnose capillary leak are insensitive: a 20% rise in the hematocrit is hardly ever seen among travelers. Similarly, routine chest radiography requires the presence of several hundred cc’s for an effusion to be detected. Sonographic evaluation of fluid has been shown to be a much more sensitive tool, able to diagnose as little as 10e20 mL of effusion.47 Recently, we have analyzed data from a series of Israeli travelers, most of whom were diagnosed with ‘‘classical’’ DF. We found that the incidence of sonographic capillary leak (i.e. pleural effusion or ascites) was 32.3%. Capillary leak was seen with the same magnitude regardless of previous infection status (Schwartz E e unpublished data). It is interesting to note in this respect a recent volunteer trial, conducted as part of the dengue vaccine development program. Sonographic capillary leak was demonstrated in 7/12 volunteers. Not only were none of these cases clinically defined as DHF, but several were not even symptomatic!48 These data lead to the conclusion, that capillary leakage is probably part of the disease process of dengue infection per se. ‘‘DHF’’ probably represents a clinically more severe manifestation of the same underlying disease process as ‘‘classical’’ DF, rather than a distinct nosological entity (Figure 1).

Conclusions

Figure 1 Clinical presentation and capillary leakage during dengue infection.

In conclusion, DHF has long been attributed to secondary DEN-V infection, caused by immune enhancing antibodies gained during primary infection. Studies on travelers with DF raise questions as to the validity of this theory.

282 The debate on the causes of DHF has had implications far more serious than an academic debate. The general acceptance of the immune enhancing theory has had a significant impact on delaying the introduction of DEN-V vaccines due to the fear of increasing the risk of DHF in case the vaccine will not confer 100% efficacy against all four dengue serotypes.49,50 Abandoning the secondary infection theory as the as the most prominent factor of DHF pathogenesis may promote a more rapid introduction of Dengue vaccines. Only the introduction of such a vaccine can really improve the massive burden of DF related morbidity in endemic countries.

Conflict of interest The authors have no conflict of interest.

References 1. Gibbons RV, Kalanarooj S, Jarman RG, Nisalak A, Vaughn DW, Endy TP, et al. Analysis of repeat hospital admissions for dengue to estimate the frequency of third or fourth dengue infections resulting in admissions and dengue hemorrhagic fever, and serotype sequences. Am J Trop Med Hyg 2007;77: 910e3. 2. Anantapreecha S, Chanama S, A-nuegoonpipat A, Naemkhunthot S, Sa-Ngasang A, Sawanpanyalert P, et al. Serological and virological features of dengue fever and dengue haemorrhagic fever in Thailand from 1999 to 2002. Epidemiol Infect 2005;133:503e7. 3. Alvarez M, Rodriguez-Roche R, Bernardo L, Va ´zquez S, Morier L, Gonzalez D. Dengue hemorrhagic Fever caused by sequential dengue 1-3 virus infections over a long time interval: Havana epidemic, 2001-2002. Am J Trop Med Hyg 2006;75:1113e7. 4. Dunham EJ, Holmes EC. Inferring the timescale of dengue virus evolution under realistic models of DNA substitution. J Mol Evol. 2007;64:656e61. 5. Rush B. Medical inquiries and observations. By Benjamin Rush, M.D. professor of the institutes of medicine, and of clinical practice in the University of Pennsylvania, Volume I. Philadelphia: Thomas Dobson; 1794. 104e117. 6. Rosen L. Dengue in Greece in 1927 and 1928 and the pathogenesis of dengue hemorrhagic fever: new data and a different conclusion. Am J Trop Med Hyg 1986;35:642e53. 7. Thai KT, Binh TQ, Giao PT, et al. Seroprevalence of dengue antibodies, annual incidence and risk factors among children in southern Vietnam. Trop Med Int Health 2005;10:379e86. 8. Halstead SB. Observations related to pathogensis of dengue hemorrhagic fever. VI. Hypotheses and discussion. Yale J Biol Med 1970;42:350e62. 9. Rosen L. The Emperor’s New Clothes revisited, or reflections on the pathogenesis of dengue hemorrhagic fever. Am J Trop Med Hyg 1977;26:337e43. 10. Horlbeck HB. Dengue. Public Health Pap Rep 1896;22:191e6. 11. Kamal H. The 1927 epidemic of dengue in Egypt. Br Med J 1928; 1(3521):1104e6. 12. Halstead SB, Yamarat C. Recent epidemics of hemorrhagic fever in Thailand. Observations related to pathogenesis of a "New" dengue illness. Am J Public Health Nations Health 1965;55:1386e95. 13. Guzma ´n MG, Kouri GP, Bravo J, Soler M, Vazquez S, Morier L. Dengue hemorrhagic fever in Cuba, 1981: a retrospective seroepidemiologic study. Am J Trop Med Hyg 1990;42: 179e84.

E. Meltzer, E. Schwartz 14. Guzma ´n MG, Kourı´ G. Dengue: an update. Lancet Infect Dis 2002;2:33e42. 15. Cardosa MJ. Dengue haemorrhagic fever: questions of pathogenesis. Curr Opin Infect Dis 2000;13:471e5. 16. Rigau-Pe ´rez JG. Severe dengue: the need for new case definitions. Lancet Infect Dis 2006;6:297e302. 17. Endy TP, Nisalak A, Chunsuttitwat S, Vaughn DW, Green S, Ennis FA, et al. Relationship of preexisting dengue virus (DV) neutralizing antibody levels to viremia and severity of disease in a prospective cohort study of DV infection in Thailand. J Infect Dis 2004;189:990e1000. 18. Thomas L, Verlaeten O, Cabie ´ A, Kaidomar S, Moravie V, Martial J. Influence of the dengue serotype, previous dengue infection, and plasma viral load on clinical presentation and outcome during a dengue-2 and dengue-4 co-epidemic. Am J Trop Med Hyg 2008;78:990e8. 19. Kouri GP, Guzma ´n MG, Bravo JR, Triana C. Dengue haemorrhagic fever/dengue shock syndrome: lessons from the Cuban epidemic, 1981. Bull World Health Organ 1989;67(4):375e80. 20. Barnes WJ, Rosen L. Fatal hemorrhagic disease and shock associated with primary dengue infection on a Pacific island. Am J Trop Med Hyg 1974;23:495e506. 21. Kuberski T, Rosen L, Reed D, Mataika J. Clinical and laboratory observations on patients with primary and secondary dengue type 1 infections with hemorrhagic manifestations in Fiji. Am J Trop Med Hyg 1977;26:775e83. 22. Ocazionez RE, Corte ´s FM, Villar LA, Go ´mez SY. Temporal distribution of dengue virus serotypes in Colombian endemic area and dengue incidence. Re-introduction of dengue-3 associated to mild febrile illness and primary infection. Mem Inst Oswaldo Cruz 2006;101:725e31. 23. Cordeiro MT, Silva AM, Brito CA, Nascimento EJ, Magalha ˜es MC, Guimara ˜es GF, et al. Characterization of a dengue patient cohort in Recife, Brazil. Am J Trop Med Hyg 2007;77:1128e34. 24. Guilarde AO, Turchi MD, Siqueira Jr JB, Feres VC, Rocha B, Levi JE, et al. Dengue and dengue hemorrhagic fever among adults: clinical outcomes related to viremia, serotypes, and antibody response. J Infect Dis 2008;197:817e24. 25. Wichmann O, Hongsiriwon S, Bowonwatanuwong C, Chotivanich K, Sukthana Y, Pukrittayakamee S. Risk factors and clinical features associated with severe dengue infection in adults and children during the 2001 epidemic in Chonburi, Thailand. Trop Med Int Health 2004;9:1022e9. 26. Halstead SB, Streit TG, Lafontant JG, Putvatana R, Russell K, Sun W, et al. Haiti: absence of dengue hemorrhagic fever despite hyperendemic dengue virus transmission. Am J Trop Med Hyg 2001;65:180e3. 27. Peters CJ. Chapter 189. Infections Caused by Arthropod- and Rodent-Borne Viruses. In: Fauc AS, Braunwald E, Kasper DL, Hauser SL, Jameson L, Loscalzo J, editors. Harrison’s Principles of Internal Medicine. 17th Edition. McGraw-Hill; 2008. 28. Nimmannitya S. Chapter 41: Dengue and Dengue Hemorrhagic fever pp. 750e761. In: Cook GC, Zumla AI, editors. Manson’s Tropical Diseases. 22nd ed. Elsevier Health Sciences; 2008. 29. Tsai TF, Vaughn DW, Solomon T. Chapter 149 e Flaviviruses (Yellow Fever, Dengue, Dengue Hemorrhagic Fever, Japanese Encephalitis, West Nile Encephalitis, St. Louis Encephalitis, Tick-Borne Encephalitis). In: Mandell GL, Bennett JE, Dolin R, editors. Mandell, Bennett, & Dolin: Principles and Practice of Infectious Diseases. 6th ed. Churchill Livingstone; 2005. 30. Manson P. An Address on the diagnosis of fever in patients from the tropics. Br Med J 1909;1(2516):704e6. 31. Bastin R, Rodhain F, Frottier J, Vilde JL, Dournon E, Bricaire F, et al. Asiatic dengue in France: 9 cases, of which one of the thrombopenic type. Ann Med Interne (Paris) 1981;132(2): 106e8. 32. Schwartz E, Weld LH, Wilder-Smith A, et al. Seasonality, annual trends, and characteristics of dengue among ill

Dengue hemorrhagic fever in travelers

33. 34.

35.

36.

37.

38.

39. 40.

41.

42. 43.

returned travelers, 1997-2006. Emerg Infect Dis 2008;14: 1081e8. Allwinn R, Hofknecht N, Doerr HW. Dengue in travellers is still underestimated. Intervirology 2008;51(2):96e100. Ja ¨nisch T, Preiser W, Berger A, Niedrig M, Mikulicz U, Thoma B, Doerr HW. Emerging viral pathogens in long-term expatriates (II): Dengue virus. Trop Med Int Health 1997;2:934e40. Wichmann O, Lauschke A, Frank C, Shu PY, Niedrig M, Huang JH, et al. Dengue antibody prevalence in German travelers. Emerg Infect Dis 2005;11:762e5. Cobelens FG, Groen J, Osterhaus AD, Leentvaar-Kuipers A, Wertheim-van Dillen PM, Kager PA. Incidence and risk factors of probable dengue virus infection among Dutch travellers to Asia. Trop Med Int Health 2002;7:331e8. Potasman I, Srugo I, Schwartz E. Dengue seroconversion among Israeli travelers to tropical countries. Emerg Infect Dis 1999;5: 824e7. 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; 12(354):119e30. Stienlauf S, Segal G, Sidi Y, Schwartz E. Epidemiology of travelrelated hospitalization. J Travel Med 2005;12:136e41. Laferl H, Szell M, Bischof E, Wenisch C. Imported dengue fever in Austria 1990-2005. Travel Med Infect Dis 2006;4: 319e23. Bottieau E, Florence E, Clerinx J, Vlieghe E, Vekemans M, Moerman F, et al. Etiology and outcome of fever after a stay in the tropics. Arch Intern Med 2006;166:1642e8. Wittesjo ¨ B, Eitrem R, Niklasson B. Dengue fever among Swedish tourists. Scand J Infect Dis 1993;25:699e704. Wichmann O, Gascon J, Schunk M, Puente S, Siikamaki H, Gjørup I, et al. Severe dengue virus infection in travelers: risk factors and laboratory indicators. J Infect Dis 2007;195:1089e96.

283 44. Itoda I, Masuda G, Suganuma A, Imamura A, Ajisawa A, Yamada K, et al. Clinical features of 62 imported cases of dengue fever in Japan. Am J Trop Med Hyg 2006;75:470e4. 45. Huhtamo E, Vuorinen S, Uzca ´tegui NY, Vapalahti O, Haapasalo H, Lumio J. Fatal dengue virus infection in a Finnish traveler. J Clin Virol 2006;37:323e6. 46. Jensenius M, Berild D, Ormaasen V, Maehlen J, Lindegren G, Falk KI. Fatal subarachnoidal haemorrhage in a Norwegian traveller with dengue virus infection. Scand J Infect Dis 2007; 39:272e4. 47. Kocijancic I, Vidmar K, Ivanovi-Herceg Z. Chest sonography versus lateral decubitus radiography in the diagnosis of small pleural effusions. J Clin Ultrasound 2003;31:69e74. 48. Statler J, Mammen M, Lyons A, Sun W. Sonographic findings of healthy volunteers infected with dengue virus. J Clin Ultrasound 2008;36:413e7. 49. Edelman R. Dengue vaccines approach the finish line. Clin Infect Dis 2007;45(Suppl. 1):S56e60. 50. Stephenson JR. Understanding dengue pathogenesis: implications for vaccine design. Bull World Health Organ 2005;83: 308e14. 51. Jelinek T, Mu ´n M, Grobusch MP, ¨hlberger N, Harms G, Coracha Knobloch J, et al. Epidemiology and clinical features of imported dengue fever in Europe: sentinel surveillance data from TropNetEurop. Clin Infect Dis 2002;1(35):1047e52. 52. Mun ˜oz J, Puente S, Lo ´pez-Ve ´lez R. Clinical and epidemiological features of imported dengue in Spain. Med Clin (Barc) 2008; 131:18e21. 53. Bakker RC, Veenstra J, Dingemans-Dumas AM, Wetsteyn JCFM, Kager PA. Imported Dengue in The Netherlands. J Travel Med 1996;3(4):204e8. 54. Stephenson I, Roper J, Fraser M, Nicholson K, Wiselka M. Dengue fever in febrile returning travellers to a UK regional infectious diseases unit. Travel Med Infect Dis 2003;1:89e93.