An appraisal of some recent diagnostic assays for Japanese encephalitis.

A study was undertaken in South Arcot district of Tamil Nadu, India to assess relative merits of selected diagnostic techniques for Japanese encephalitis. During the transmission seasons of 1993-1995, a total of 85 patients (mostly pediatric) clinically diagnosed as acute encephalitis or other related central nervous system (CNS) disorders were examined; in 53 (62.4%) a laboratory diagnosis of JE was established. In terms of diagnostic value, immunoglobulin M (IgM) antibody capture ELISA (MAC ELISA) on convalescent serum had the highest sensitivity (89%) and negative predictive value (NPV) (50%). This was followed by MAC ELISA on acute serum and CSF which had similar sensitivity (84%) and NPV (40%). The hemagglutination inhibition test and Toxorhynchites splendens inoculation technique for virus isolation were also similar in sensitivity (68%) and NPV (25%). The virus antigen detection technique by IFA in cells of cerebrospinal fluid (CSF) was the least sensitive (58%). The distinct advantage of the acute serum ELISA is that it can be carried out on a single finger-prick blood specimen. The IFA on CSF cells is the most rapid diagnostic test since it requires only 2-3 hours to complete. Therefore, both these tests also offer potential tools for JE surveillance programs.

Comparative evaluation of bioassay and ELISA for detection of Japanese encephalitis virus in field collected mosquitos.

Comparative evaluation of enzyme-linked immunosorbent assay (ELISA) and bioassay (virus isolation in Toxorhynchites splendens larvae and identification by immunofluorescence test using virus specific monoclonal antibody) was carried out in order to define a suitable strategy for monitoring Japanese encephalitis virus infection in field mosquitos. A total of 8,850 adult female mosquitos in 177 pools (Culex tritaeniorhynchus 91, Cx. vishnui 59 and Cx. fuscocephala 27) collected from an endemic area of Tamil Nadu were examined by both the techniques. In ELISA, 9 pools which had optical densities (OD) equal to the mean of normal infected pools plus > or = 4 standard deviations (SD) mean considered positive and all of them were virus positive by the bioassay also. Sixty-five pools had OD = Mean + 2-3 SD and 103 pools had OD = Mean + < 2 SD of normal pools. From these groups, 12 (18.5%) and 8 (7.8%) pools respectively were found to be virus positive by the bioassay. In total 29 (16%) pools were positive by the bioassay as against 9 (5%) by ELISA. This study demonstrated that the bioassay is sensitive for estimation of true positives and ELISA is a rapid screening system. A protocol has now been developed for surveillance in which field pools are first screened by ELISA and only those with OD = Mean + > or 2 SD are assayed in Toxorhynchites. By excluding a large majority of pools with low OD (Mean + < 2 SD), which are likely to yield to only a small percentage of true positives, the cost, time and labor involved are greatly reduced.(ABSTRACT TRUNCATED AT 250 WORDS)