We visited 67 nurseries and 25 kindergartens from January 1999 to April 1999 in Chunchon, Korea. Children in 22 institutions were visited twice for the examination. Pre-school children were examined for the presence of E. vermicularis egg by perianal swab method.

Of the 4,711 children examined, 434 (9.2%) were found to be positive. The egg positive rates of boys and girls were 10.1% and 8.1%, respectively. The rates were 7.8% in kindergartens and 9.7% in nurseries (Table 1). There was no correlation between the number of children in the institution and the positive rate (p>0.05). There were no significant differences in the rate not only between boys and girls (p>0.05), but also between kindergartens and the nurseries (p>0.05). The egg positive rate was less than 10% from 50 institutions stuided. The egg positive rates in 35 institutions were between 10 to 20%, and. On the other hand, 7 institutions showed the egg positive rate of equal to or over 20% (Fig. 1). Out of 1,113 children examined twice, 28 (2.5%) children were positive consecutively, 53 (4.8%) were positive with negative conversion, 47 (4.2%) were negative with positive conversion, and 985 (88.5%) were consecutively negative.

This survey was a whole number examination, meaning that every institution for pre-school children was included. The rate 9.2% may represent the E. vermicularis egg positive rate in a given age group in small cities all over Korea, like Chunchon with 250,000 inhabitants. Since the positive rate of pre-school children in Chunchon were 1.85% (total of 3,674) in 1997, and 3.0% (total of 4,005) in 1998 (Unpublished data), it can be said that there was a increase of the positive rate in Chunchon, Korea. The children who were egg positive were prescribed with Mebendazole (Vermax®) 100 mg single p.o. three times at 3 weeks interval. Prescription of the whole members in an institution may be better. However, it was known that treating only egg positive children could reduce the postitive rate of the entire group that were below 30% (Yang et al., 1997). In this survey, the positive rates of 90 out of 92 institutions were below 30%. It is very difficult to eradicate enterobiasis in a group due to newcomers who are infected. The continuous attention and screening on the enterobiasis can reduce the egg positive rate. A quarter of children was examined twice and the positive rate increased more than 4.2% in comparison with a single survey data. Egg positive rate at the first visit was 6.7%, and that at second visit was 7.3%. And the cumulative egg positive rate at two visits was 11.5%. Therefore, if every subjected children were examined twice, then the positive rate in this survey would increase from 9.2% to more than 12.4% when it was calculated from survey results. For more sensitive detection of egg and also worm infection, the repeated examination is recommended. The possibilities of detecting the heavy infection cases were found in consecutively positive categories of anal swab since the processes of infection loss in E. vermicularis were taken place continuously, allowing daily small allotments of discharge of fully gravid worms in heavy infection cases and massive discharge in a very short period of time is practically rare event (Cho et al., 1976). Practically, it is difficult to examine twice in the mass survey. When single examination was done, and the result was around 10%, it may be deduced that the rate by two times examination is about 4% higher than the single examination. Not only Lognormal but also negative binomial distribution of worms per each child was known to be found to fit the data with rarity of heavy infection cases and high frequency of noninfected cases (Chai et al., 1976). Therefore, in the positive children examined, the heavy worm burden (more than 40 worms) cases may be very rare, since the consecutive positive cases are rare (Cho et al., 1976).