to study the influence of increased worldwide dengue activity on international travelers , 2,259 patients were studied retrospectively for dengue antibodies after returning from dengue - endemic countries . a 36-month period from january 1996 to december 1998 was compared with a 27-month period from january 2002 to march 2004 . we recruited travelers who came to the travel clinic of the berlin institute of tropical medicine , germany , with fever ( n = 1,091 ) or diarrhea without fever ( n = 1,168 ) and for whom serum samples were available . thus , 2,259 patients ' serum samples were tested for anti - dengue igm and igg by using an igm - capture elisa and an igg indirect elisa ( panbio pty ltd . , a probable acute infection was defined according to manufacturer 's instruction as having a sample : calibrator absorbance ratio of igm 1.0 . acute probable primary infection was characterized by the elevation of igm 1.0 with igg 4.0 , and acute secondary infection was characterized by the elevation of igg 4.0 ( 13 ) . for a more specific diagnosis , all serum samples from patients with probable dengue infections were then investigated by using e / m and nonstructural protein ns1 serotype - specific igm elisas and ns1 serotype - specific igg elisa as described previously ( 11,12 ) . furthermore , an additional confirmatory testing was performed by using immunofluorescence assays ( euroimmun ag , luebeck , germany ) , and if these results were contrary , sera collected during the acute phase of illness were processed by using polymerase chain reaction assays to detect viral nucleic acid . among the recruited patients , 1,163 ( 51.5% ) antibodies were detected by the screening test in 127 ( 5.6% ) serum samples , indicating probable acute dengue infection . the more specific analysis confirmed infection in 64 cases ( 2.8% prevalence ) , including 8 ( 12.5% ) patients with secondary immune response . one of these 8 secondary and none of the 56 primary infections led clinically to dhf . among 1,091 patients with fever and 1,168 diarrhea patients without fever , 51 ( 4.7% ) and 13 ( 1.1% ) , respectively , had an acute dengue infection . the highest prevalence of dengue antibodies ( 4.6% ) , indicating acute infection , was found in patients returning from asia ( n = 1,020 ) ( table 1 ) , including southeast asia ( 7.4% of 500 total travelers and 11% of 310 febrile travelers ) and the indian subcontinent ( 1.8% ) . traveling in southeast asia was associated with a significantly higher risk compared to other disease - endemic areas in africa and latin america ( odds ratio 5.3 , 95% confidence interval 3.29.0 ) . comparing patients with and without acute dengue infection , no significant difference was seen in the median length of travel ( 28 vs. 24 days , respectively , p = 0.083 , mann - whitney test ) or the median age of the patients ( 32 vs. 33 years , respectively , p = 0.58 , mann - whitney test ) . patients 3044 years of age had the highest antibody prevalence ( 37 [ 3.8% ] of 966 ) . * four patients are not included in this analysis since they traveled to > 1 continent . when patients from 1996 to 1998 ( n = 1,073 ) were compared with those from 2002 to 2004 ( n = 1,186 ) , a slight increase was seen in the overall prevalence , from 2.7% to 3.0% , although this finding was not significant ( p = 0.63 ) . the figure shows annual dengue prevalence among travelers to thailand and to the indian subcontinent , highlighting that infection rates fluctuate strongly between years and between quarters within years . in the last quarter of 1997 and 1998 , 64 travelers returned from thailand , and 14 ( 22% ) acquired an acute dengue infection . among those , 5 were infected by the serotype denv-1 , 3 by denv-2 , and 4 by denv-3 ( table 2 ) . in 2 cases lines indicate acute dengue infection after returning from thailand ( n = 223 ) or from the indian subcontinent ( n = 495 ) , both with and without fever , and in the total febrile population returning from all travel destinations ( n = 1,091 ) . in this study population , 4.7% of all febrile patients returning from different areas of the tropics had dengue antibodies that indicated acute infection . this number underlines the effect on international travelers to dengue - endemic areas . as long as no dengue vaccine is commercially available , among patients without fever , 13 ( 1.1% ) had detectable dengue antibodies compatible with acute dengue infection , which underscores that symptoms commonly associated with dengue , such as fever , myalgia , arthralgia , and exanthema , are helpful for diagnosis when present , but the absence of typical symptoms does not exclude infection . a study on 483 cases of imported dengue infections in europe showed that dhf developed in 2.7% of the patients ; immigrants from dengue - endemic countries returning to europe after visiting their home country were at higher risk for more severe disease than europeans ( 9 ) . however , immigrants from dengue - endemic countries have a higher prevalence of dengue antibodies from previous infections . in our population , all 54 patients with primary dengue infection had classical dengue fever , and 1 of 8 patients with secondary dengue infection had dhf . the patient with dhf was born in sri lanka and immigrated to germany 2 decades ago . these observations might be taken as more evidence for the importance of letting patients know that they have been infected with dengue and should , therefore , protect themselves from infection with subsequent serotypes . fluctuations in prevalence between years , especially the maximal prevalence in 1998 , correspond to similar observations on record . in a study conducted among israeli travelers to southeast asia from 1994 to 1998 , a sharp increase in incidence was noted in 1998 compared with previous years ( 14 ) . similar to the worldwide increase of cases reported to the world health organization , the number of swedish travelers returning from southeast asia with dengue fever was considerably higher in 1998 than during previous years ( 3 ) . decreasing resources for vectorborne infectious disease prevention and control ( 15 ) might have contributed to this epidemic in southeast asia , which followed the economic crisis in 1997 . the risk among a cohort of dutch short - term travelers to dengue - endemic areas in asia from 1991 to 1992 showed marked seasonal variation for the indian subcontinent ( 7 ) . in our population , such seasonal variations were not detectable . the infection rates were more influenced by major outbreaks , such as the one in india in 2003 or in southeast asia in 1997 to 1998 . similar findings have been described for israeli travelers during their trip to thailand in 1998 ( 14 ) and in german travelers to brazil and thailand in 2001 and 2002 , respectively ( 4 ) . to screen our samples , elisa - based tests for igm and igg igm elisas had a sensitivity of 100% in primary infections and 99% in secondary infections . the specificity was 100% in non - flavivirus infections and 80% in japanese encephalitis virus infections when an igg sample : calibrator absorbance ratio of 3.0 was used as a cutoff ( 13 ) . however , because our study was retrospective , only single serum samples were available from each traveler . to increase specificity , all serum samples that indicated probable infections were further investigated with more specific elisa techniques . by using virus - infected culture supernatants as the source of viral antigens , the e / m - specific capture igm has been found to differentiate reliably between japanese encephalitis , dengue , west nile virus , and yellow fever ( 12 ) . furthermore , an ns1 isotype- and serotype - specific elisa can reliably differentiate japanese encephalitis virus infection , japanese encephalitis virus vaccination , and primary and secondary dengue virus infection ( 11 ) . in primary infection , igm is detectable 38 days from the onset of symptoms ( 8) ; thus , some of our travelers might have had false - negative test results if samples were taken during the acute phase of illness . therefore , the true infection rates in our study might have been higher than the numbers indicated by single - sample serology . overall , we demonstrated an almost stable rate of dengue infections among berlin institute of tropical medicine patients returning from all tropical regions when recent years are compared with the mid-1990s . large outbreaks like those in 19971998 in southeast asia ( especially thailand ) and 2003 in major cities of india , all popular tourist destinations , contributed to the numbers . quarterly and annual fluctuations might lead to misinterpretation of probable trends if data are derived only from short - term observations . in addition , this variability underscores the importance of tourists ' seeking information before traveling to dengue - endemic areas .
we studied 2,259 german citizens after they returned from dengue - endemic countries from 1996 to 2004 . 
 serotype - specific dengue antibodies indicated acute infections in 51 ( 4.7% ) travelers with recent fever and 13 ( 1.1% ) travelers with no recent fever , depending largely on destination and epidemic activity in the countries visited .