Serological cross-reactivity and identification of an acute Seoul orthohantavirus case in a dengue outbreak from Vietnam

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BACKGROUND: In dengue-endemic regions like Vietnam, viral haemorrhagic fevers (VHFs) with overlapping clinical presentations often lead to misdiagnoses. This study investigated alternative VHF pathogens in patients clinically suspected of dengue but negative by standard diagnostics during a 2016 outbreak in central Vietnam. METHODS: Among 198 hospitalized patients, 52 dengue virus (DENV)-negative cases were tested using enzyme-linked immunosorbent assays for immunoglobulin G (IgG)/IgM against seven VHFs and molecular assays for pan-flaviviruses and hantaviruses. Positive amplicons were sequenced and subjected to phylogenetic analysis. RESULTS: High flavivirus IgG seropositivity was observed, especially for Japanese encephalitis virus (96%), DENV, tick-borne encephalitis virus and West Nile virus (92% each), with notable cross-reactivity. IgM responses were specific, with 37% of patients positive for anti-JEV and 33% for anti-DENV. Pan-flavivirus reverse transcription polymerase chain reaction confirmed DENV-1 RNA in four patients. One patient tested positive for hantaviral RNA and seropositive for both IgG and IgM. Sequencing and phylogenetic analysis identified Orthohantavirus seoulense (SEOV) as the causative agent, clustering with SEOV strains from Vietnam and Indonesia. The detection of SEOV highlights co-circulation of rodent-borne viruses during arboviral outbreaks. CONCLUSIONS: This study highlights significant serological cross-reactivity among flaviviruses and underdiagnosed circulation of hantaviruses such as SEOV. Enhanced multimodal diagnostic surveillance of pathogens remains essential.

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