Comparative Sensitivity of Focus Counting Method in the Infectivity Titration of Dengue Viruses and in the Neutralization Test

Abstract

Abstract:Infectivity titrations were performed for each of the 4 prototypes of dengue (DEN) viruses by focus counting on BHK21 cells (FFU), by plaque formation on LLC-MK2 cells (PFU), and by intracerebral inoculation into suckling mice (LD50) The FFU titers were almost similar as LD50 for all the 4 types of DEN and these titers were also similar to PFU except type 2 DEN (DEN-2), which showed lower PFU than FFU or LD50. Neutralization (N) tests were performed for standard anti-DEN sera by 50 per cent focus reduction (FR50) and by 50 per cent plaque reduction (PR50) methods. Both values were almost the same except anti-DEN-2, whcih showed higher PR50 than FR50. The principle of these methods were also applied to Japanese encephalitis and chikungunya viruses. Hemagglutination-inhibition (HI) and FR50-N tests on several human sera indicated that FR50 is more specific and sensitive than HI to detect typespecific antibodies.