In the past, parasitic infections were regarded as a major health problem in Korea. Before the 1970s, human parasitic infections, particularly soil-transmitted helminthiases, such as ascariasis, trichuriasis, and hookworm infections, were highly prevalent in the Korean population (1, 2). During the last three decades, prevalence of parasitic infections in Korea has decreased dramatically, largely due to nationwide control activities involving mass stool examination and selective mass treatment schemes for students (3).

However, in Korea, tissue invading parasitic infections remain relatively high compared to soil-transmitted parasitic infections. The infection rate of Clonorchis sinensis remains high along riverside areas, egg positive rate in Hamyang-gun was 16% and in Okcheon-gun was 14.2% (4). As ranges of freshwater crabs and crayfish have been rapidly reduced due to pollution of water bodies with environmental wastes, sources of human paragonimiasis infection have also diminished. In recent years, cases of clinical paragonimiasis have been found in a few patients after eating homemade "kejang" (5, 6). As for Taenia solium metacestode infection, the IgG positive rate was 4.0% in randomly-selected epilepsy patients. The positive rate (8.4%) was highest in patients living in Jeju-do (7).

Because the parasite-specific IgG antibody test using the ELISA method has been developed at our university since 1982, patients from hospitals nationwide are referred our hospital for diagnosis and monitoring of four tissue invading parasitic infections. The present study was conducted to determine the seroprevalence of IgG antibody against C. sinensis, Paragonimus westermani, T. solium metacestode, and sparganum by ELISA according to region, age, and clinical aspects in patients referred to Chung-Ang University Hospital from 1993 to 2006. We examined changes and trends associated with tissue invading parasitic infections in Korea.

Due to successful control of soil-transmitted helminthiasis, both lay people and medical professionals in Korea often ignore the presence of parasitic agents of disease. A nationwide stool examination survey conducted in 2004 reported a total helminth egg positive rate of 3.7%, while egg positive rate for C. sinensis was 2.4% (11). Overall IgG antibody positive rates of clonorchiasis and paragonimiasis, 5.0% and 1.4% in the present study, were higher than the egg positive rates of the nationwide stool examination survey in 2004.

This difference could be explained by the following reasons. First, nationwide egg positive rates were obtained by examination of stools collected from randomly selected and representative Korean residents, whereas the present data was collected from patients referred by physicians due to clinically suspicious parasitic infections. Second, examination methods differed. Stool microscopy for eggs was a standard technique, not only in clinical diagnosis but also in epidemiological studies. However antibody tests, such as ELISA, were developed because egg examinations showed low sensitivity. For this reason our study showed higher positive rates than the national stool examination survey in 2004.

Cross reactions observed included double positive (13%), triple positive (6%), and quadruple positive (1%). After review of clinical findings from multiple positive cases, we decided which parasitic infection was the most probable. When radiological images, biopsy interpretation, and parasite collection for the cross-reacting case showed evidence of infection with a parasite, the case was diagnosed as a respective parasitic infection. However, it is obvious that serological cross reactions often cause serious problems with regard to the specificity of serological tests. This study was limited by its retrospective nature, and the inability to confirm cross reactive cases by other methods.

In the present study, the seropositive rate of clonorchiasis was 5.0% (1.6-9.0%) in total subjects, and markedly decreased during the years of observation. The most practical method of clonorchiasis prevention is avoidance of eating raw, freshly pickled, or undercooked freshwater fish. Due to the habit of eating raw fish, enforcement of this guideline for prevention is difficult. However, following continuous education and mass treatment, the status of clonorchiasis has shown significant improvement (12).

A previous serological survey reported that the positive rate of cysticercosis was 2.1% in normal subjects and 4% in patients with chronic epilepsy (7). In another serological study in Seoul, the positive rate of cysticercosis was reported as 3.0% (13). However, in the present study, the positive rate was found to be as high as 4.2%. In our study, the major cause for referral was neurologic problems, which may have been a reason for the high positive rate. Most neurocysticercosis could be diagnosed by computerized tomography (CT). Even by CT, however, many cases escaped recognition or findings were too ambiguous for a definitive diagnosis. For this reason, serological tests may provide valuable information. Therefore, it is already well known that criteria for diagnosis of cysticercosis should be established based on a combination of neuroimaging studies, serological tests, clinical presentation, and exposure history.

For the years 1993 and 1994, the positive rate for cysticercosis was higher than for other parasites, due to an especially high proportion of referrals for neurologic problems (47.7% in 1993 and 1994 vs. 35.4% in 1995-2006). Neurocysticercosis without extracranial infection may reveal a negative reaction when only serum is tested. In other words, T. solium metacestode specific IgG antibody did not cross over the blood-CSF barrier in a significant amount, even when the level was sufficiently high in blood. The ELISA test for detection of the specific IgG antibody using paired samples of serum and CSF was very helpful for clinical differentiation of neurocysticercosis from neurologic diseases of other causes (8).

Incidence of paragonimiasis has been significantly reduced, but new cases continue to occur sporadically. The present study revealed that the seropositive rate for paragonimiasis was 1.6%. The positive rate rose in 2003-2005, and subsided in 2006. During 2003-2005, referred cases with eosinophlia increased proportionally. Therefore, paragonimiasis should be included in differential diagnosis of parasitic disease in patients with lung lesions associated with eosinophilia and/or elevated IgE level.

Interestingly, seropositive rates for all 4 parasites were highest in Seoul area, even the rate for clonorchiasis was higher than that of the known endemic areas. Korea has recently become the entire nation into an one-day life zone, and has experienced changes in life style, advances in transportation, and people frequently move between the provinces and Seoul. Using this transportation system, many patients who lived in rural areas visited university hospitals and large scale hospitals locating in large cities such as Seoul and Busan. By this behavior pattern, a large proportion of referred patients were from the metropolitan Seoul and Gyeonggi-do, and pushed up IgG seropositive rates for all 4 parasites. Some differences were observed in the positive rates of the 4 parasitoses among the referred areas. The seropositive rate of cysticercosis was higher in Chungcheongnam-do, and that of sparganosis was higher in Gangwon-do. Positive rates of the other parasitoses were similar to those reported previously (14). However, our positive rates were not the exact meaning of regional positive rates because of our positive rates of referred areas.

In conclusion, our results suggest new cases have occurred sporadically and nationwidely. Therefore, tissue invading parasitic infections should always be included in the differential diagnosis for patients with eosinophilia associated lesions of the CNS, liver, and lung.