Severity-related molecular differences among nineteen strains of dengue type 2 viruses

Comparative nucleotide sequencing was carried out on dengue type 2 virus (D EN-2) strains isolated from patients in Northeast Thailand during the epide mic season in 1993, The patients exhibited different clinical manifestation s ranging from dengue fever (DF) to dengue haemorrhagic fever (DHF)/dengue shock syndrome (DSS), The results classified 19 DEN-2 strains into 3 subtyp es according to nonsynonymous amino acid replacements. The strain isolated from a DSS patient eliciting secondary serological response belonged to sub type I, whereas 13 strains isolated from DHF patients with secondary respon se and 2 strains from DF patients with primary response belonged to subtype II. On the other hand, 3 strains isolated from DF cases evoking either pri mary or secondary response belonged to subtype III. These results suggest t hat subtype III virus infection could result in clinically milder manifesta tion irrespective of the serological response compared with subtype I or II viruses. The RNA secondary structure predicted for the 3' noncoding region showed 4 different structures (A, B, C, and D). The result also indicates that different subtypes of DEN-2 serotypes are circulating in a single epid emic in Thailand.