An adult Japanese man who had just returned from Thailand developed dengue hemorrhagic fever (DHF). A primary infection of dengue virus (DENV) was confirmed, specifically DENV serotype 2 (DENV-2), on the basis of the detection of the virus genome, a significant increase in the neutralizing antibody and the isolation of DENV-2. DHF is often observed following a secondary infection from another serotype of dengue virus, particularly in children, but this case was a primary infection of DENV. Japan is a non-endemic country for dengue disease. In fact, only Japanese encephalitis (JE) is known to be a member of the endemic flavivirus family. In this study, IgG antibody against Japanese encephalitis virus (JEV) was detected. JEV belongs to the family of dengue virus and prevails in Japan, particularly Kyushu. Among many risk factors for the occurrence of DHF, a plausible candidate could be a cross-reactive antibody-dependent enhancement (ADE) mechanism caused by JEV antibody. This indicates that most Japanese travelers who living in dengue non-endemic areas, particularly Kyushu, should be aware of the occurrence of DHF.

A patient, an adultJapanese traveler who had just returned from Thailand, had developed denguehemorrhagic fever (DHF). A primary infection of dengue virus (DENV) wasconfirmed, in particular, DENV serotype 2 (DENV-2) via the detection of the virusgenome, a significant increase in its specific neutralizing antibody and the isolationof DENV-2. DHF is often observed following a secondary infection from another serotypeof dengue virus, particularly in children, but this case was a primaryinfection of DENV. Japan is a non-endemic country of dengue disease. Instead,only Japanese encephalitis (JE) is known to be an endemic flavivirus family. Inthis study, IgG antibody against Japanese encephalitis virus (JEV) was detected.JEV belongs to the family of dengue virus and prevails in Japan, particularly inKyushu. Among many risk factors for the occurrence of DHF, a plausiblecandidate could be a cross-reactive antibody-dependent enhancement (ADE)mechanism by JEV antibody. This indicates that most Japanese travelers, wholive in non-endemic areas of dengue, particularly in Kyushu, should payattention to the occurrence of DHF.