STUDY ON THE TRANSMISSION THRESHOLD VALUE OF BANCROFTIAN FILARIASIS

Objective To elucidate the transmission dynamic and epidemic trend of bancroftian filariasis occurred under the condition with no control me asure taken 5 years after elimination of filariasis. Methods A 10-year longitudinal observation (from 1984 to 1994) was made in Huayuan Vill age in Shengli Township of Tancheng County, which used to be a high ba ncroftian filariasis-endemic area in southern part of Shandong Provinc e. Results The microfilarial rate decreased from 0.56% before the stud y to 0.12% after the study and 8 out of the 9 previous microfilaria-po sitive cases became negative gradually. During the study period, 6 new microfilaremia cases were detected, 5 of which became negative natura lly within 3 to 4 years. Eighty-eight point eight nine per cent of the detected patients with microfilaremia converted into IgG4-negative af ter 10 years. The natural infective rate of vectors decreased year by year and became zero by the tenth year of the study, the annual transm ission potency decreased also from 3.47 to zero by the tenth year. Con clusions It showed that under the local natural environment the biting rate representing the vector density which was obtained by capture me thod was from 24.1 to 52.5 person/night among the residents who did no t use mosquito nets, and 13.5 to 21 person/night among the residents w ho used mosquito nets. The microfilarial rate of 0.56% in population w ith the average microfilarial density of 6.6 to 20.7 capita/60 mu l ea r blood of residual microfilaria-positive patients might be considered as the terminal threshold of transmission.