Serological and virological studies on patients with dengue hemorrhagic fever (DHF) in Chanthaburi Province, Thailand. II. Serological characteristics of viruses isolated from DHF patients using a clone of Singh's Aedes albopictus cells.

Four strains of dengue (DEN) virus were isolated using C6/36 cells, a clone of Singh's Aedes albopictus cells, from acute phase sera of DHF patients in Thailand in 1978. The isolates grown and passaged in C6/36 cells were not neutralized appreciably by standard rabbit antisera against each of the 4 types of DEN virus, but their serotypes could be identified by complement fixation (CF) tests. After the 4th or 10th passage in suckling mouse brain (SMB), however, three of the four strains showed type-specific reactions in neutralization (N) tests against the standard antisera. In cases of secondary infection, the highest N titer in the convalescent phase sera was demonstrated not against the isolated serotype, but against the serotype considered to have caused primary infection. Moreover in cases of secondary infection, the convalescent phase sera showed a significantly lower N titer against the newly isolated strain derived from the same patient than against the corresponding prototype standard virus. This phenomenon was not observed in a single case of primary infection.