Stability ofSt.LouisEncephalitis Viral Antigen Detected by EnzymeImmunoassay inInfected Mosquitoes

Theuseofenzyme immunoassay todetect St.Louisencephalitis (SLE)viral antigen invectormosquitoes enhances theeffectiveness ofsurveillance because infected mosquitoes canbeidentified more rapidly thanwith conventional virus isolation systems andbecause itisa simple andaccessible procedure. Infectivity among mosquitoes experimentally infected withSLEvirus was lost within 24hafter themosquitoes were stored at 27°Cand80%relative humidity; however, viral antigen remained stable underthese conditions andcouldbe detected byenzyme immunoassay 2weekslater. Desiccation further extended theperiod during whichantigen could bedetected to6weeks. Absorbances werehigher ininfected mosquitoes stored at27°Cthaninmosquitoes frozen continuously. Absorbances ininfected mosquitoes alsoincreased after repeated freezing andthawing andsonication. Bothphenomena may berelated totherelease ofantigen fromdecaying ordisrupted cells. The relative stability ofSLEviral antigen atambient temperatures lends flexibility toschemes whichuse direct antigen detection toidentify infected vectors. Surveillance systems canbedesigned without regard tocollecting living mosquitoes, andacoldchain isunnecessarytopreservespecimens, thusreducing thecostofsurveillance andexpanding thegeographic areastowhichitisaccessible.