| | | DEN 1 is antigenically closely related to other types of DEN virus. Currently, the dengue viruses are considered to be related more closely to each other than to other flaviviruses. The dengue viruses comprise a separate complex in Group B ( [24] , [25] ). The concept of a complex or subgroup also exists partially on a disease, vector and epidemiologic basis. All DEN viruses possess one or more antigens common to others in Group B. | | There appears to be considerable strain variation within types and a number of strains have been found difficult to type by standard methods ( [11] - [13] ). | | TH-Sman, previously listed in the Catalogue (No. 163) was suggested as a possible type 6 since it was isolated from one of the early Thai hemorrhagic fever cases in Bangkok in 1958 ( [11] , [12] , [14] , [15] ) and was found eventually to differ antigenically to a significant degree from type I Hawaiin. Differences have been shown by CF ( [11] , [12] ), plaque neutralization, [16] immunodiffusion and immunoelectrophoresis [18] , and by the soluble CF (SCF) antigen [19] but not by antibody absorption in the HI test [20] . It is generally accepted that strain differences among DEN viruses may be rather prominent. Their signifance is as yet unknown. | | |
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