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[Density fluctuation of microfilariae and the role of residual infection source in filariasis transmission after its interruption].

OBJECTIVE
To investigate the density fluctuation of microfilariae, persistence of microfilaremia and possible new infection due to residual microfilaremia in areas with filariasis transmission interrupted.


METHODS
The observation site was made in a village of Jishou City, Hunan Province. Inhabitants were regularly examined by thick blood smear and the density fluctuation of residual microfilaremia in known and newly-found cases were followed up. With a consent from the cases with residual microfilaremia, no treatment was given until they naturally turned negative. Antifilarial antibody level was detected by IFAT and a test kit for filariasis-special IgG4. Culex quinquefociatus was dissected to determine the natural infection rate and density of III stage filarial larvae in transmission season. The identified cases were followed-up by interviews and physical examinations to see if clinical manifestations appeared.


RESULTS
Blood examination was carried out for all inhabitants for 10 times, 4 cases with microfilaremia, including 3 cases found at the beginning of the project and one newly infected case, were discovered after the interruption of filariasis transmission in the 19-year period. Among the 4 cases followed up, one case naturally turned negative within 7 years, one case became negative in the 9th year but returned positive in the 12th year, and then naturally turned negative in the 13th year. The 3rd case turned negative in the 14th year and was again positive in the 19th and the 20th years, and became negative through diethylcarbamazine (DEC) treatment in the 21st year. The new case was found to have microfilaremia in the 16th year and kept positive for 5 years until DEC treatment. Serological tests (IFAT and special IgG4) revealed no new positive cases. The natural infection rate and larvae density in Culex quinquefasciatus decreased annually. Conclusion The persistence period of residual microfilaremia in individual cases might last for more than 20 years after filariasis transmission has been interrupted.