Seasonal fluctuation of Ascaris reinfection incidences in a rural Korean population

Abstract

This study was undertaken to estimate the reinfection incidence of Ascaris lumbricoides after biannual blanket mass chemotherapy and to determine the characteristic seasonal fluctuation pattern in a Korean rural village. 612 residents from 4 Ri's of Hwasung Gun, Kyunggi Do, Korea were selected and divided into 6 groups according to hamlets, each of which consisted of about 100 inhabitants. The previous infection rate with Ascaris as 42.3% (34.4~50.6% by group) as determined by the egg detection method and 46.2% (36.2~51.9% by group) by the worm detection method, on average. The number of worms per infected case was 2.7 (1.5~4.2 by group) on average. Reinfection rates as estimated by egg detection ranged from 8.0 to 31.5% (16.9% on average) in the first follow-up period and from 14.0 to 20.0% (17.5% in average) in the second follow-up. Seasonal fluctuation was marked and there were roughly two peaks, of which the larger appeared from February to March and the smaller from August to September. Reinfection rates as determined by the worm detection method ranged from 20.6 to 34.9% (mean 25.7%) and from 14.3 to 21.9% (mean 19.6%) in respective follow-up examinations. A seasonal fluctuation was also observed and the peak incidence appeared one month earlier than those made by egg-positive conversion. The number of worms per infected case decreased to 1.6 on average in both follow-ups. Young Ascaris detection rates ranged from 6.0 to 12.9% (mean 9.4%) and from 2.3 to 5.8% (mean 3.8%) in respective follow-ups. A seasonal fluctuation was also observed. From January to February, the positive rates were up to 12.9% and 12.7% respectively, which formed the higher peak. A lower peak was observed during July to August. From the above results, the actual time of Ascaris reinfection were November to December for the higher peak and May to June for the lower peak in this Korean population. Pickled vegetables were suggested as a very probable source of Ascaris reinfection late autumn to winter season.