[Study on the transmission threshold of filariasis. Collaborating Research Group on the Transmission Threshold of Filariasis].

Twenty-one villages from ten provinces and one autonomous region with different mf rates in population and different levels of mean mf density per 60 microliters blood in positive cases and representative species of filaria and vector after DEC treatment were selected as study sites. In these villages a longitudinal investigation for a period of 3-5 years was carried out without adopting any control measure. The main results were: 1. Among 279 residual microfilaremia cases, 235 (84.2%) turned negative spontaneously and successively. The negative conversion rates were 86.6% (46/53) in malayan cases, 60.8% (28/46) in bancroftian cases after 3 years' follow-up, and 100% (29/29) in malayan cases, 87.4% (132/151) in bancroftian cases after 4-5 year's follow-up. Among 110 bancroftian cases followed-up to seven years, 109 (99.1%) turned negative. 2. Twenty-two new microfilaremia cases were detected, of which, 18 might be missed in previous blood examination, 4 might be new infection. 3. Mf rate in the population in twenty villages has been trending down, whereas in one village fluctuated around the original level. Among the 20 villages, no microfilaremia case was found in 7 villages. 4. The natural infection rates of filarial larvae and infective rates in mosquito vectors were in descending trend in most of these villages, and no filarial larva was found in 12 villages.(ABSTRACT TRUNCATED AT 250 WORDS)