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KMID : 0311219740050010003
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Yonsei Reports on Tropical Medicine
1974 Volume.5 No. 1 p.3 ~ p.44
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Ecological Studies of Clonochis sinensis
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Kim Dong-Chan
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Abstract
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An extensive study was conducted from 1967 to 1973 both in high and low endemic localities to understand better the ecology of Clonorchis sinensis, particularly the conditio -s and propagation of the infection. The following aspects were clarified.
The prevalence rate of clonc:-chiasis among the human population of Gimhae Gun, a high endemic locality where a total of 1, 809 persons were examined wa! 61. 2%. The prevalence rate for the population as a whole and for the group of schoolchildren studied separately was 68. 8% and 56.4%, respectively. The prevalence rate among the inhabitants in Goyang Gun, a low endemic locality, was 15. 2% with a total of 1, 314 persons examined. The rate for the population was 22.5%, and 9. 5% of the schoolchildren were found infected.
Comparing the riverside-delta and the inland areas in each locality, the prevalence rate among the inhabitants in the riverside areas was significantly higher than that found in inland areas.
By sex, for areas in each locality, the rate for males was relatively higher than that for-females. However, this difference was statistically significant only in the low endemic locality (Goyang Gun) both in its riverside and inland areas, but not in the high endemic locality (Gimhae Gun), even in its inland area.
By age group, in the high endemic locality, a sharp increase in infection was noted, particularly between the age group 0~4 and 5~9 where the rate jumped from 18. 9% to 61.7%. Then the rate increased generally by age groups up to the middle age groups. In the low endemic locality a low rate of infection was seen in the 5~9 year age group with the rate increasing less sharply to 40% for the 15~19 year group, then remaining at about that level up to the middle age groups. In the older age groups, the rate showed a tendency to slow down in both localities.
By age group and sex, in the high endemic locality the prevalence rate of both sexes was almost parallel up to the middle age groups with narrow difference between them, particularly in the young age groups. Particularly in the river-side areas, this sex difference was only slight, at times almost non-existent. In the low endemic locality, the rate in the male was significantly higher than in the female throughout the adult age groups.
Intensity of Clonorchis infection among the infected inhabitants, was in association with the prevalence of infection. Significant difference was seen statistically between the high and low endemic localities, and also between the riverside and inland areas in respect to intensity of infection. By sex, the difference was not consistent among the groups surveyed. From the total number of cases examined, the higher intensity of the infection in the male than the female was statistically significant in the high endemic locality but not in the low endemic locality.
Consumption of raw fresh-water fish was found to prevail among the inhabitants in these localities and this was the main source of infection. The risk of contamination with metacercariae may occur indirectly through other means than raw fish consumption, i.e. in the process of handling or preparing fish, etc.
As reservoir hosts, hogs, dogs and house rats were found infected with Clonorchis in both endemic localities with low to moderate infections. These animals particularly the rats were considered to contribute significantly to the propagation of the infection in the endemic areas in Korea.
Clonorchis infection in fish was closely related to the occurrence and distribution of Parafossarulus manchouricus?the only species of snail known to be a first intermediate host of Cloniorchis infection in Korea. Relatively low prevalence of the infection was found in this snail in both high and low endemic localities, with no significant difference between them.
Transmission of the Clonorchis cercariae from the snail to susceptible fresh-water fish hosts takes place in the mild to warm season from about May to October in the endemic areas in Korea. Regardless of the low rate infection among the snails, the high intensity of the infection in those snails found to be infected as well as the high population density of the snails in the endemic foci was considered sufficient to account for the high intensity of infection as seen in Pseudorasbora parva in these endemic localities.
In the resumption of the cercarial emergence from the infected snails in late spring, the cercariae were considered to be produced from new infection with miracidium after the snails have resumed their activities following their hibernation in early spring.
In many bodie3 of water at both localities, Clonorchis infection was found among fresh-water fish. The prevalence and intensity of the infection among fresh-water fish host varied significantly depending on the species, locality and ecological conditions. An examination of 440 specimens representing 20 species revealed 9 species harboring Clonorchis metacercariae. Among the 9 infected fresh-water fish species, 3 were found to have very low infection. The susceptible fish host carried Clonorchis metacercariae through-out the year with no significant variation in intensity of infection.
A major factor of the difference in endemicity between the two localities was the presence, size and distribution of bodies of water which accomodate large numbers of intermediate host snails and fish.
Based on findings stated above, it is concluded that the factors which determine the endemicity of the Clonorchis infection in a locality were the occurrence, size and distribution of the bodies of water accomodating the intermediate hosts, number of the intermediate host snails and fish, consumption rate of raw fresh-water fish among the inhabitants, opportunities of inhabitants to ingest the metacercariae by contamination through handling and preparing the fresh-water fish, and distribution of reservoir hosts. These factors control the process of Clonorchis life cycle, allowing the passage of the source of infection among the hosts, depending on the state of the infection in the population of final hosts: reservoir animals Ind man.
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KEYWORD
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