This study was undertaken to estimate the reinfection incidence of Ascaris lumbricoides after biannual blanket mass chemotherapy and to figure out the characteristic seasonal fluctuation pattern in a Korean rural village.

A total of 612 residents from 4 Ri's of Hwasung Gun, Kyunggi Do, Korea was selected and divided into 6 groups according to hamlets, each of which consisted of about 100 inhabitants.

Each group received blanket biannual mass chemotherapy with 10 mg/kg of pyrantel pamoate respectively in each month, during the period from April 1977 to September 1978 (Group A-April and October 1977 and April 1978, Group B-May and November 1977 and May l978, and so on). Beforehand Ascaris infection status was evaluated by examining eggs and worms expelled exactly after the first mass chemotherapy checked during the first 6 months and the reinfection incidences were estimated in follow-up examinations during next l2 months by the above two kinds of methods and by detection of young Ascaris positive cases. Young Ascaris was defined as worm shorter than l3 cm and lighter than 500 mg and considered to have introduced to human host during the period from the previous 2 months to 2 weeks.

The results were summarized as follows:

1. Beforehand infection rate with Ascaris were 42.3% (34.4~50.6% by group) by egg detection and 46.2% (36.2-51.9% by group) by worm detection methods in average. Number of worms per infected case was 2.7 (1.5-4.2 by group) in average.

2. Reinfection rates estimated by egg detection ranged from 8.0 to 3l.5% (l6.9% in average) in the first follow-up period and from 14.0 to 20.0% (l7.5% in average) in the second follow-up. Seasonal fluctuation was found marked and there were roughly two peaks, of which the larger appeared from February to March and the smaller from August to September.

3. Reinfection rates by worm detection method ranged from 20.6 to 34.9% (25.7% in average) and from 14.3 to 21.9% (19.6% in average) in respective follow-up examinations. Seasonal fluctuation was also observed and the peak incidence appeared one month earlier than those made by egg positive conversion. Number of worms per infected case decreased to 1.6 in average in both follow-up.

4. Young Ascaris detection rates ranged from 6.0 to 12.9% (9.4% in average) and from 2.3 to 5.8% (3.8% in average) in respective follow-up. 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. Lower one 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 one in Korean population. Pickled vegetable was suggested as a most probable source of Ascaris reinfection during late autumn to winter season.