In the 1920s, Faust reported that cats and dogs in North China were highly infected by CS. For instance, in Peking 37% of cats and 25% of dogs were infected, but there was no human infection because people tended to eat fully cooked fish.178
174 Li Juan-you et al., 1998.
175 Zeng Ming-an et al., 1995.
176 Lan Xiao-hui et al., 2002; Wang Cheng-ke, 1994; Zheng De-fu et al., 1994.
177 Wu Nian-biao et al., 2007.
178 Faust, 1921; 1925a; 1925b.
The infection rates of CS in North China in 1960-2004 are illustrated in Map 8.
Map 8: The Infection Rate of Clonorchiasis in North China, 1960-2004 (1) Shandong
In Shandong, it was found in 1962 that the infection of CS was distributed in 99 counties (cities, districts) located at low-lying area in the province’s middle, southern, and coastal regions, and the average infection rate was 2.3%, but the highest rate, 44.4%, was found among elementary school pupils.179 Consecutive investigations at 107 of the 139 counties (cities, districts) found the infection rates of residents at rural area varied from 1% to 30% and at some schools it was more than 40%. After many years of investigation and control the situation became stabilized in 1987. Available data of the infection rates are as follows: In 1960-1970, the infection rate of 96 counties was 1.5% (62646/4166180); in 1980, the rate of 75 counties was 1.2%
(7855/683936); and in 1990, the rate of 55 counties was 0.3% (158/53474). There was no significant difference between the male and female rates. Of the 158 cases found in 1990, there were 97 cases (61.4%) distributed in the southwest (Heze荷澤, Jining濟 寧, Tsaozhuang棗莊, Linyi臨沂), 20 cases (12.7%) in the north (Dezhou德州, Binzhou濱洲, Dongying東營), and 12 cases (7.6%) in the east (Qingdao青島, Yentai
179 Zhu Yu-guang, 1983.
煙台). These results were identical to the fact that there were more ponds containing water all year round in these areas. It was reported that in recent years, many rivers in Shandong were polluted by industrial waste water, but a certain amount of intermediate hosts of CS still survived in some ponds and rivers. In 1990, investigations in 48 counties each with samples from three villages found the total infection rate was 0.33% (151/46323); and there was no significant difference between male and female.180
In Linyi area, investigations during 1989-1991 at 13,511 villages found that 5,399 villages of these villages had ponds containing water all year round; residents of 519 villages received examinations and the infection rate was 0.7% (550/79938).
There were 21 villages where the infection rate was more than 3% and consequent investigations at these villages found the infection rates ranged from 0.02% to 6.9%, and the total infection rate was 2.2% (423/19209). After the investigation, there was followed-up curing for one month and then 19 villages were investigated again; as a result the total infection rate was reduced to 0.2% (5/2496).181
In November 1991, sample surveys in Junan莒南 County found the infection rate at Gaoyu Village高愉村was 0.56% (1/179) and zero (0/96) at Wanbianshan Village萬扁山村. Surveys in 1992 at four villages found the infection rates as follows:
Wanbianshan 0.68% (1/147), Gaoyu zero (0/171), Tsuitang崔堂 2.13% (4/188), and Liuhang柳行 0.99% (2/210). In 1993, it was found that both Wanbianshan (0/120) and Gaoyu (0/119) had no infection, while Tsuitang had 1.17% (2/171) and Liuhang 1.68% (3/178). No increasing trend was observable because public health conditions had been improved and the people had changed their daily habits.182
In Juye 巨 野 County, consecutive investigations at five villages during 1987-1994 found the infection rate was 5% (150/3000).183
In Jinxiang金鄉 County, investigations in 1979-1980 found the infection rate was 2.2% (729/33721). In 1987-1988, investigations and measures of control were adopted at villages where the infection rate was above 1% and the ponds kept water all year round. The result of investigations showed that the infection rate was 1.2%
(359/29640). In 1993-1995, sample surveys at villages with high, medium and low infection rates found that the total infection rate was 0.09% (7/8136).184 It was notable that the infection rate of Jinxiang was 1.02% (4835/478118) in 1989 and a comprehensive program of curing was carried out by given medicine to all sufferers.
In 2000, the effect of curing was examined at 183 villages and found that the infection
180 Wan Gong-qun et al., 2000; 2002.
181 Li Deng-jun et al., 1994b; 2007.
182 Li Deng-jun et al., 1994a; 1996.
183 Jiang Yu-lan et al., 1997.
184 Jin Yu-zhu et al., 1998.
rate was reduced to 0.63% (135/21364). Compared with the infection rate of 1.45% in 1998, the reduction was significant. In 2002, however, 49 new cases were found at places where high infection rates were observed before.185
In Heze City, during 1977-1990, residents above three years old at villages located closely to the Zhuzhaoxin洙趙新 River received examinations and the infection rate was 2.6% (139/5446). It was pointed out that the infection of CS prevailed in this area because of the following reasons: (1) The fish culture was developing and feces of chikens, ducks, and pigs were utilized to feed fish and shrimp;
(2) The people liked to keep small fish in mouth when fishing and to eat raw or half-baked fish; (3) CS produced a large amount of eggs and had endurable adaptability, while snails, the first intermediate hosts, existed widely in the water.186 In November 1991, another investigation at three villages found that the infection rate was 0.19% (2/1047).187
In Sishui泗水 County, the CS infection was first confirmed in 1962. In 1963, surveys at four villages along the Si泗 River found the infection rate was 1.55%. In 1980, infection rate of residents at Sishui Town was 0.96%. In April 1996, surveys at three villages along the river found the total infection rate was 0.18% (2/1128); the two sufferers were females living at Houzhai Village后寨村 where the infection rate was 0.37% (2/546). It was reported that these two sufferers had experiences of eating raw fish and moved into the village respectively from towns of Miaoguan苗館 and Xingcun星村 where no investigation had been conducted. Moreover, in 1980, the infection rate of Dabao Village大鮑村 was 0.28% and that of Hsujialou Village徐家 樓村 0.57%. Re-investigations at the two villages in 1996 found no infection, indicating the effect of curing.188
In Tsaozhuang City, investigation at Yuelou Village岳樓村 in 1975 found the infection rate was 7.0% (48/687) with no difference between male and female. In 2000, retrospective investigation found that of the 48 cases, four had died and 12 moved out, and re-examinations of the other 32 cases did not found any egg. At the same time, samples of 310 persons (162 male and 148 female) showed no infection, indicating the effect of curing. It was pointed out that previously, environment and health conditions were rather poor at Yuelou Village where there were lots of ponds and excrements from latrines and waste water of the village all flowed into ponds. All sufferers had the habit of eating fish raw or half-baked. After investigations, certain measures were taken to improve the living conditions and health education of the residents. In 2000, the village had become an industrial district with the environment
185 Chen Ji-feng et al., 2002.
186 Sun Yuan-chao et al., 1999.
187 Wang Su-zhen et al., 2002.
188 Tao Shi-kun, 2003.
and public health largely improved and the residents had changed their old habit.189 In 1994, examinations for students of elementary and middle schools at Taierzhuang台兒莊 district in Tsaozhuang found the average infection rate was 7.2%
and the highest rate was 23.5%. During 1985-1995, investigations and measures of control were widely conducted for five times and the infection rate was reduced to 1%.
In 2000, sample surveys found the infection rate was 0.3% (5/1550).190 In Boxing博 興 County, surveys at Dongjia Village董家村 in 1992 found the infection rate was 4.8% (104/2164) for residents and 6.6% (28/426) for students.191
A review article in 2007 contended that the infection of CS in Shandong had been brought under control because of the following reasons: (1) A combination of investigation, control, and health education had produced significant effects; (2) The people had changed their daily habits along with the improvement of economic conditions and popularity of health knowledge; (3) As the climate in Shandong continued to be dry, many ponds had dry up and wild fishes and snails lost the environment for propagation. It was suggested that the experience of control and health education in Shandong could provide a good example for other provinces.192 (2) Hebei
In Hebei, there were surveys at Tangshan唐山 and Funing撫寧 in 1980 and found the infection rates of CS among residents were respectively 3.6% (3/83) and 4.9% (46/936), and that of elementary school pupils was 1.5% (4/266).193 Available data are mostly related to Tianjin. From September 2003 to May 2004, sample surveys at 30 locations of Tianjin area found the infection rate was 0.16% (19/12000);
among the locations investigated, only Jixian薊縣 had 0.51% and Wuqing武清 had 0.06% while others had no infection. The higher infection rate at Jixian was possibly due to the habit of eating raw fish.194
4. Concluding Remark
The infection of Clonorchis sinensis forms a cycle among the parasite, the intermediate hosts, and the final hosts. Rivers, lakes and ponds provide suitable environment for intermediate hosts to propagate but it is human actions that make this cycle to run over and over again. Human practices in fish culture implied an idea of utilizing waste but the danger of parasitic infection was ignored. Moreover, the habit
189 Li Ping-luan et al., 2001; Fu Mei-rong, 2003.
190 Xue Ying-hui et al., 2001.
191 Li Guo-qian et al., 1998.
192 Li Deng-jun et al., 2007.
193 Huang su-fang et al., 1983; Gao Yong, 1983.
194 Zhu Chuan-fang et al., 2006.