Seroprevalence of Toxoplasmosis in the Residents of Cheorwon-gun, Gangwon-do, Korea

Toxoplasma gondii, an apicomplexan protozoa, can infect a broad range of host animals, including humans. This infection passes by asymptomatically and opportunistically in most infected hosts, but can cause toxoplasmosis in some individuals with such symptoms as encephalitis, chorioretinitis, and lymphadenitis in acquired infections and abortion and neonatal mortality in congenital infections [1-3].

In the Republic of Korea, seroprevalence of toxoplasmosis has been reported sporadically among various groups of outpatients in hospitals. It has been recognized generally that the positive rates of Koreans are in the range from 1.9 to 7.7% [4-6], except for patient groups or residents in Jeju island of 12.9% [6] or more recently 13.2% [7], which is maintained higher than the other regions of Korea. However, all positive rates in Korea are still significantly lower than, approximately 1/10 of, those of other endemic countries of 30-70% [1]. Here, we present a survey which has screened the seroprevalence of toxoplasmosis among the residents in several villages of Cheorwon-gun, Gangwon-do, which are localized near the demilitarized zone (DMZ) facing the North Korea and resulted in a 17.0% of the positive rate, the highest among the surveys ever done in Korea.

A total of 1,661 sera were collected from residents (866 males and 795 females) of villages scattered over the 6 administrative regions of Cheorwon-gun, including urban and rural environments, from November to December 2010 and the same 2 months of 2011, under the regulation of the IRB Committee of Chung-Ang University (No. 2010-06-03) held on June 18, 2010. The sera were checked for IgG antibody titers by ELISA using a crude extract RH strain antigen of T. gondii according to the method of [4], which resulted in a 17.0% positive rate (282 sera) (Table 1). The positive rate was significantly different (P<0.05) between the sex, 20.6% for males and 13.1% for females, as analyzed by the Student's t-test. Regionally, the residents of Geunnam-myeon showed the highest positive rate of 26.3%, with a similar differential partition of the sexes. The residents of Gimhwa-eup, which neighbors with Geunnam-myeon, showed a lower prevalence of 13.3%, with an equivalent partition between sexes. The positive rates by age did not follow the general tendency but increased gradually according to the ages such that the highest in males of fifties (28.7%) and in females of forties (20.0%) (Fig. 1). Furthermore, the positive optical density (OD) values of ELISA were distributed more widely in females than in males as shown in Fig. 2. The median OD values were 0.41 in males and 0.47 in females, and the mean OD was 0.49 and 0.55, respectively, both were higher in females.

Table 1.

Positive rates of toxoplasmosis in the residents of Cheorwon-gun, Gangwon-do according to the region and sex

Fig. 1

Comparison of the positive rates of anti-Toxoplasma antibody by sex and age.

Fig. 2

Comparison of the distribution of positive anti-Toxoplasma antibody titers in the residents of Cheorwon-gun, Gangwon-do. Solid line indicates the median value and dotted line the mean value.

The positive rate of 17.0% marks the highest score among the Korean subjects tested, which reflects some significant changes in the toxoplasmic environment, such as eating behavior, pet-loving preference, and residential mode, that have been regarded as the reason for relatively low prevalence among Koreans compared to those of highly endemic countries [6]. Besides the congenital infection by tachyzoites, T. gondii is commonly acquired by ingesting tissue cysts containing bradyzoites in undercooked pork or mutton [1] and by ingesting sporozoite-containing oocysts directly in water contaminated with cat feces [8]. Here, we postulate that the reasons for the high seroprevalence among the residents of Cheorwon-gun may include the location of the villages near the DMZ and popular consumption of the imported pork from countries with high toxoplasmic endemicity.

DMZ is now known to be a most well-preserved wildlife area, in which ocular toxoplasmosis occurred among people who had experienced eating wild boar and deer captured from this region [3]. The seroprevalence of residents in Geunnam-myeon was the highest, whereas that of residents in the neighboring Gimhwa-eup showed the lowest, which could not reflect the possibility of T. gondii infection just by the distance from DMZ. It was interesting that most of women infections were coupled with infections of their husbands simultaneously (i.e., familial infection, presumed by the same address), and also the infections were common among relatives (presumed by the name), which suggests share of live special viscera for traditional misbelieves of stamina or of undercooked meats captured by a snare. Men in their fifties and women of forties were more highly infected, which may suggest more opportunities of dining out to enjoy boned rip or porkchop of imported pork in addition to the above-mentioned reason regardless of their residence in urban or rural localizations. Recently, pork is imported from countries of high T. gondii endemicity, and has been consumed nation-widely with a competitive price without any inspection of T. gondii. Of course, the route of infection by ingesting oocysts cannot be ruled out because of water supply problems in those regions.

Altogether, the seroprevalence of toxoplasmosis has been screened among the residents of Cheorwon-gun which resulted in the highest positive rate among surveys done in Korea. The epidemiology of T. gondii infection should be further investigated in this region. Both of the proposed reasons for high prevalence in this study may well be applicable to all over the country nowadays, and thus it is necessary to screen the national toxoplasmic seroprevalence to find out symptomatic patients and to correlate it with other debilitating diseases.