Acta Tropica 75 (2000) 279 – 289 www.elsevier.com/locate/actatropica
Measuring exposure to Schistosoma japonicum in China. III. Activity diaries, snail and human infection, transmission ecology and options for control Yuesheng Li a,b,*, Adrian C. Sleigh a, Gail M. Williams a, Allen G.P. Ross a, Y. Li b, Simon J. Forsyth a, Marcel Tanner c, Donald P. McManus a a Tropical Health Program, Australian Centre for International and Tropical Health and Nutrition, The Uni6ersity of Queensland and the Queensland Institute of Medical Research, 300 Herston Road, Herston, Brisbane, Qld 4029, Australia b Hunan Institute of Parasitic Diseases, Huabanqiao Road, Yueyang, Hunan 414000, The People’s Republic of China c Swiss Tropical Institute, Socinstrasse 57, PO Box Ch-4002, Basel, Switzerland
Received 31 August 1999; received in revised form 13 December 1999; accepted 13 December 1999
Abstract We used activity diaries and snail detection to relate water contact and Schistosoma japonicum infection among a cohort of 178 residents on two islands in the Dongting Lake, China. Water exposure to each of 12 mapped water zones around the islands was calculated (m2 min/day) for each subject. Infected Oncomelania hupensis hupensis snails in this area are focal and were found in only five of the 12 zones, with the highest rate being 5.7%. Thirty-one subjects (17%) were re-infected with a mean intensity of 63.2 epg. Mean water contact was 7.9 m2 min/day; 98% of water exposure was due to economic activity and only 2% due to swimming or bathing, washing and other necessities of daily life. Males had more exposure and infection than females (PB0.05). Infected subjects had more exposure (10.2 m2 min/day) than those not infected (7.44 m2 min/day) (PB 0.05). Compared with uninfected subjects, those infected had 2.9 times more exposure in infected-snail zones (PB 0.01). Also, human infection intensity (epg) correlated well with exposure to infected snail zones (r=0.552, PB0.01). People B 20 years old had the highest re-infection (21.4%) and intensity (3.77 epg). Median exposure for 20–49-year-olds (9.00 m2 min/day) was nearly double that of those aged B 20 or \50 years old (5.5 m2 min/day). We conclude that map-referenced water contact and snail evaluation boosts accuracy of activity-diary measurements in large transmission foci for the Asian schistosome. Protecting against faecal contamination of snail inhabited sites, and against occupational exposure for island residents, should be a priority of future research. Potential strategies for migrating buffaloes and families living on visiting fishing boats are explored. © 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Schistosoma japonicum; Water exposure; Snails; Oncomelania; Activity diaries; Schistosomiasis control; China
* Corresponding author. Tel.: + 61-7-33620401; fax: +61-7-33620104. E-mail address:
[email protected] (Y. Li) 0001-706X/00/$ - see front matter © 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 0 1 - 7 0 6 X ( 0 0 ) 0 0 0 5 6 - 5
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1. Introduction Human schistosomiasis japonica is a behavioral and occupational disease associated with poor human hygiene, insanitary animal husbandry and economic activities. It results from the fecal contamination of snail-infested surface waters, and is exemplary of the intimate relationship between social and economic infrastructure, behavior, belief systems and culture, and transmission of parasitic infection (Huang and Manderson, 1992). Overall several million people are infected with Schistosoma japonicum in China, Indonesia and the Philippines. The remaining endemic areas in mainland China are mainly along the middle reaches of the Yangtze river, especially around southern tributaries, lakes and marshlands. Although effective and non-toxic praziquantel treatment has been available for more than one decade in China, prevalence of this parasite remains moderately high in lake and marshland areas where snail control is difficult and occupational exposure is unavoidable (Li et al., 1999). Over the last three decades, many water contact studies have been carried out to explore transmission determinants and quantify the relationship between water exposure and schistosome infection (Farooq and Mallah, 1966; Dalton and Dole, 1978; Tayo, 1980; Kloos et al., 1983; Jordan et al., 1993; Akogun and Akogun, 1996). However, quantification of actual cercarial exposure is as yet impossible, and indirect assessment is also difficult because of the complex interplay of human behavior, variable immune responses to repeated infection and great focal variability of the local ecology. Water contact studies generally estimate cercarial exposure indirectly by observation or questionnaire. When transmission areas are well-defined, observation is an ideal way to record individual water contact. However, when transmission areas are dispersed over a large area or are poorly defined, this approach is expensive, time-consuming and inaccurate (Ross et al., 1998a). Questionnaires are a simple and cheap alternative and are now widely used to measure water exposure, as well as symptoms of infection (Mota and Sleigh, 1987; Lengeler et al., 1991; Wu et al., 1993; He et al., 1997; Li et al., 1997; Zhou
et al., 1998). However, questionnaires depend on individual recall and for water contact this may be biased. Several indices have been developed for calculating exposure to infection or risk of infection (Jordan et al., 1993; Wu et al., 1993; He et al., 1997; Ross et al., 1998a). These generally reflect water-related activities duration and frequency of water contact, and symptoms that could relate to schistosomiasis. In a previous study we designed, validated and used a new and simple instrument (personal activity diaries, daily self-report) to measure water contact and developed a model for calculating water exposure expressed as square metre minutes of body surface area per day, adjusted for body size (Ross et al., 1998a,b). Then we developed a statistical approach for normalising water exposure data, deriving individual z-scores after fourth root transformation of square metre minutes/day (Li et al., 1999) to identify those with heavy exposure. These data were related to longitudinal re-infection studies indicating the epidemiologic susceptibility to endemic infection for a group of Chinese individuals living on two islands in the Dongting Lake region of China. Here, we further improve the activity diary approach by measuring contact with map-referenced snail-surveyed water contact sites. This refines our assessment of exposure to infection and related immunity to re-infection for a cohort of individuals known to be at risk in a typical endemic area in the Lake District of China. We report the utility of the map-referenced snail-related water contact measurement system and consider the overall implications for schistosomiasis control in the lake and marshland areas of China.
2. Materials and methods
2.1. Study area and population The study was conducted on two islands in Dongting Lake, Hunan, China. The intermediate host snail for S. japonicum in this area is the Oncomelania hupensis hupensis subspecies (Davis et al., 1992) breeding in marshland and beach areas of the lake. The local population is sus-
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tained by lake fishing, aquaculture, vegetable farming, reed harvesting. The local residents, grazing migratory buffalos and fisherman on visiting boats are the main transmission source for schistosomiasis. Dongting Lake changes its water level by 15 m annually according to water flows in the Yangtze River with low water from November to April and floods from May to October. Accordingly schistosome transmission in the lake is seasonal confined to the periods of rising or receding water in spring (May – July) and autumn (September–November) (Jordan et al., 1993; Yuan, 1993). This reflects variation in temperature, water level, bovine and human water contact, snail reproduction and snail abundance. In this report, we analyse the water exposure and infection outcomes for 178 study subjects with complete longitudinal data for map-referenced snail-related water contact (see below) in 1997. In 1996, these individuals were included in an at-risk cohort (n = 250) of residents of five villages on the two study islands (Qingshan and Niangashan) in the south eastern sector of the Dongting Lake of Hunan Province, China (Ross et al., 1998a,b; Li et al., 1999). All 178 had been treated with praziquantel and then were prove to be free of infection 1 year before (1996) and all were still negative by stool examinations (two stools, six smears) in March 1997. Snail occurrence and infection data complemented the water-contact observations in the second year of cohort observation (1997) and are the basis for the exposure study reported here. We excluded those (20 subjects) who had been re-infected in the first year (1996) and focus our analysis on the remaining 178 cohort members who were still free of infection at the beginning of the second year of observation (April 1997).
2.2. Snail sur6ey Initial snail surveys to identify potential transmission sites were done in April 1996. Then, in March 1997, we used the standard Chinese survey method that covers each 1 km2 of potential snail habitats with a 50 × 50 sampling grid, with sampled quadrats (0.11 m2 ‘spots’) 20 m apart (total 2500 spots per km2). We placed the sampling
281
frame around the two islands, excluding areas that could not harbor snails. The geographic features and local custom divide the sampled area into a total of 12 water zones (Fig. 1). On average, each water zone (0.048 km2) was sampled at 120 spots. This yielded 1440 inspected surface spots (total area 158.4m2) on the beaches around the two islands, sampled from a total potential snail habitat area of 0.58 km2 (i.e. zones). The snails collected from each spot (0.11 m2) were recorded separately and then checked under a microscope for schistosome infection. The snail examinations revealed which water contact zones contained schistosome cercariae during the transmission season.
2.3. Map-referenced snail-related water contact measurement A map showing and naming the 12 water contact zones described above and the activity diary daily water-contact recording system described in our previous reports (Ross et al., 1998a,b), were distributed to the 178 subjects in early May 1997. The water contact study began as the lake water level rose around the two islands, which coincided with the first transmission season for that year. Local health staff visited subjects and checked the records once per week during the assessment period (two months). Variables recorded in the diaries included type of water contact activity, map-referenced location of surface water contacted, body part in contact with water and duration of water contact. After two months, at the end of July, all personal activity diaries prepared by the 178 subjects were collected and information processed by computer. Mean square metre minutes per day (m2 min/day) of body surface water exposure, with surface areas adjusted for body size (Ross et al., 1998a; Li et al., 1999), were calculated for specific water zones for each individual.
2.4. Parasitological examinations Kato–Katz thick-smear stool examinations (41.7 mg/smear, Katz et al., 1972) were performed to detect and grade S. japonicum infections (two
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stools and six smears), as described (Li et al., 1999). The stool examinations were done before (March 1997) and after (April 1998) the map-referenced snail-related water-contact assessment of May –July 1997.
group means. Kruskall–Wallis test was employed for statistical inferences if Bartlett’s test showed the variances in the samples to differ. Pearson’s correlation was used to assess association between water exposure and re-infection intensity. The minimum level considered for statistical significance was set at PB 0.05.
2.5. Statistical analysis EpiInfo version 6.01 (Centre for Diseases Control and Prevention, Atlanta, GA and WHO, Geneva, Switzerland) was used for basic statistical analysis and SPSS for further processing. The intensity of infection of infected groups was expressed as geometric mean egg counts per gram stool. For whole populations the geometric means were calculated using a log (n +1) transformation. Chi-square was used to evaluate differences between relative frequencies. Student’s t-test or ANOVA were used to detect differences between
3. Results A total of 1440 spots (0.11 m2) from 12 water zones (Fig. 1) around the beaches of the two islands were surveyed for snails in 1997. Overall, 415 (28.8%) spots had snails, 2106 snails were collected and examined for schistosome infection, and 39 snails (1.9%) from five of the water zones were found to be infected. The highest snail infection rate (5.7%, 12/210) was identified in the L
Fig. 1. Water zones and map of the two study islands in the Dongting Lake, China.
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Table 1 Snail presence and infection in water zones around study island’s human water contact, and correlation of human re-infection intensity, Dongting lake, China 1997–1998 Water zonesa
Snail spot (%) (n= 120) Collected snails
A B C D E F G H I J K L Others
28.3 0 0 48.3 10.8 13.3 34.2 91.7 42.5 33.3 25.8 17.5 NA
Total
28.8
102 0 0 316 41 84 180 896 137 120 110 120 NA 2106
Snail infected (%)
Water exposureb
Correlationc
Mean
9S.E.
r
P
2.94 0 0 0.63 4.78 0 0 2.23 0 0 0 5.71 NA
0.29 0.25 0.76 0.65 0.25 2.07 1.33 0.68 0.30 0.13 0.21 0.16 0.81
0.07 0.05 0.12 0.08 0.07 0.19 0.18 0.10 0.08 0.03 0.06 0.04 0.21
0.55 0.08 0.28 −0.10 0.45 −0.14 −0.06 0.17 −0.02 −0.09 −0.01 0.27 0.03
0.001 0.280 0.001 0.168 0.001 0.070 0.400 0.025 0.82 0.228 0.873 0.001 0.713
1.90
7.90
0.45
0.21
0.003
a
Each mapped zone was surveyed at 120×0.11m2 spots. Human water exposure in mean square metre minutes per day (see Section 2). c Pearson correlation to relate zone-specific water exposure with infection intensity (log10 egg+1) for 178 cohort subjects. b
zone (Table 1). The highest water contact (2.07 m2 min/day) in this cohort was recorded in the F zone (Fig. 1). Water exposure correlated significantly with re-infection intensity in five water zones, and four of these zones harboured infected snails. One infected snail zone (D) did not correlate to human infection, and one uninfected zone (C) did correlate. A total of seven water contact activities were documented: fishing, cutting grass, moving reeds (loading harvested reeds onto boats), dam building, washing, crossing-water and swimming. Most (86%) subjects were involved with fishing activities and a few (7.8%) had swimming activity during the observed period (Fig. 2). 98% of total water exposure (m2 min/day) was due to productive economic activity; only 2% of water contact was due to non-economic activities of daily life (washing, crossing water and swimming). Mean water contact for each individual in this cohort was 7.9 m2 min/day during the first transmission season in 1997. Re-infection was noted among 17.4% (31/178). On average, males had a significantly higher water contact (9.70 m2 min/
day) and re-infection intensity (73 epg) than females (2.21 m2 min/day; 20 epg) (PB 0.05). Males also had a 20% re-infection compared to 9% for females, but there were few females and the sex difference for re-infection was not statistically significant (P\ 0.05). Males had threefold greater contact with water in infected snail zones than females (2.42 versus 0.83 m2 min/day; PB 0.01) (Table 2). Overall, infected subjects showed a higher level of mean water contact (10.2 m2 min/day) than those not infected (7.44 m2 min/day) (PB 0.05). Infected persons had 2.9 times more water exposure in infected water zones (4.52 versus 1.54 m2 min/day) (PB 0.01); in contrast, there was no significant difference of water exposure by re-infection status in non-infected snail water zones (P\ 0.05). Re-infection intensity (eggs/g) weakly correlated with overall mean water exposure (r= 0.23, PB 0.01), but correlation was much stronger for exposure to water zones in the infected snail areas (r=0.55, PB0.01). There was no correlation at all between re-infection intensity and exposure to uninfected snail areas (r= 0.013, P\ 0.05) (Table 3).
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People younger than 20 years old had the highest re-infection rate (21.4%) and overall geometric mean intensity of infection (3.77 epg), but there were no statistically significant differences for incidence (P \0.05) or intensity (P \ 0.05) of re-infection among age groups. There were also no significant difference (P \0.05) of mean water exposure recorded by age although median water exposure for age 20 – 49 (9.00 m2 min/day) was much higher than for those B 20 or ]50 years old (5.5 m2 min/day) (Table 4).
4. Discussion The endemicity of schistosomiasis in an area is determined by geographical, meteorological, social, cultural and economic factors. Transmission occurs in a variety of ecological situations but essential requirements are human contact with water that is colonised by snail hosts and contaminated by faeces of infected mammals. In this Dongting Lake locality previous control programs included health education, safe-water supply systems and household sanitation along with chemotherapy (Li et al., 1997). These interventions have succeeded as confirmed by our obser-
vation that only a few subjects (7.8%) had swimming records; water exposure due to daily life, including recreation, made up only 2% of total exposure in this cohort. However, re-infection remains common and is almost entirely attributable to unavoidable occupational exposure due to economic production. Snail presence shows the potential risk for an area as a whole and for individual sites. Collection and examination of snails is laborious but it does identify high-risk areas and active transmission. Several other methods have been used to identify water bodies that are transmission sites for schistosomiasis (Jordan et al., 1993; Muhoho et al., 1997). Cercariometry involves direct evaluation of cercarial density but data depend on the volume of filtered water and the technique is not sensitive in large-sized habitats with low cercarial densities (Sturrock, 1986). Use of sentinel mice to detect transmission is logistically difficult, expensive and slow. Chinese scientists have developed a new method to detect cercariae using a special membrane (C-6) suspended in water at potential transmission sites (Cai et al., 1994). However, the C-6 membrane needs further development before being useful in lake areas where wave action tends to wash away adherent cercariae. Therefore, a
Fig. 2. Water contact activities recorded from May to July, 1997 in 178 subjects living on two islands, the Dongting Lake of China.
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Table 2 Water contact (m2 min/day) and S. japonicum re-infection by sex in 178 subjects living on two islands, Dongting Lake, 1997–1998 Sex
Reinfection %
Males (n = 135) Females (n = 43) P valuec
20.0 9.3 0.10
epga
72.9 19.9 0.04
Water exposure (m2 min/day) Ab zones
Bb zones
Total
Mean
9S.D.
Median
Mean
9S.D.
Median
Mean
9S.D.
Median
2.42 0.83 0.001
2.40 1.60
1.90 0.3
7.28 1.38 0.001
4.74 2.64
7.3 0.70
9.70 2.21 0.001
5.41 3.88
9.00 1.00
a
A geometric mean epg among the re-infected cases. A, infected-snail water zones; B, no-infected snail water zones. c Kruskall–Wallis test, comparing males and females. b
cheap and rapid method to evaluate focal risks in lake and marshland areas remains elusive. In the Philippines, field studies on S. japonicum have revealed no definite pattern in snail distribution for an entire waterway and mice could be infected even if there was no evidence of snail ‘colonies’ (Pesigan et al., 1958). Upatham (1974) reported that S. mansoni cercariae (in St. Lucia) were carried downstream as far as 195 m in running water habitats. Radke et al. (1961) showed that mice could be infected 600 m downstream from the point of S. mansoni cercarial released under field conditions. These data showed that geographic characteristics such as water flow along rivers and streams might be important to permit infection at a contact site distant from infected snail colonies. However our results, involving accurate measurement of water exposure and infection in Dongting Lake, show that contact with a few infected snail zones related well to human re-infection. This reflects the local ecology of beaches and snail habitats around the two study islands. Water on the beaches during the transmission seasons is relatively stable and flows very slowly and only five of the 12 water contact zones harboured any infected snails. Infected snails tended to occur in places that people or buffaloes visit frequently. The implications are that in similar areas, which include a large portion of the residual problem with schistosomiasis japonica in China, infected snails and schistosomiasis transmission are quite focal even though the lakes and marshlands, when first appraised, appear to be vast homogeneous areas.
Our water exposure measurement was confined to the first transmission period (2 months) in 1997. In our original plan, we aimed to get exposure data across both seasons. Unfortunately, a big summer flood destroyed houses on the islands and many subjects became homeless and were too busy re-building to record water contact in autumn when floods receded. Although water contact and schistosomiasis transmission in the spring is generally higher than in the autumn, there may have been some unmeasured variance in water exposure between the spring and autumn in some subjects. Despite this limitation, our analysis shows that use of map-referenced snail-evaluation boosts the accuracy of activity-diary water contact exposure studies in large transmission foci. Also, our data indicate the importance of infected snail presence at local foci of transmission in Dongting Lake. Schistosomiasis control in swamp and lake regions is still a long-term task faced in China. We know that chemotherapy reduces schistosome-induced morbidity at the community level in China and the Philippines (Wiest et al., 1994; Olveda et al., 1996; Ross et al., 1998c; Li et al., 2000), but rapid re-infection and falling community compliance blunts the long-term usefulness of this approach. Oncomelania snails are the necessary intermediate hosts for S. japonicum in China. When these snails are eradicated S. japonicum all transmission ceases, as shown by successful control activities for Pearl River foci in China (Sleigh et al., 1998a). However, eradication of snails in
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ing and other farming tasks elsewhere. Buffalos are hardy animals and are not likely to die from schistosomiasis. Cattle raising is sometimes tried on lake islands but is usually constrained by morbidity and mortality caused by schistosomiasis; for example, on our study islands an experimental herd of 60 cattle succumbed quickly to schistosome infection between 1994 and 1996. So, buffalos are the main animal reservoir in island areas and also move the infection from one site to another. The other major source of island infection is also mobile and arises from fishing families that moor houseboats along the beaches from May to November. Buffalo faecal control or restriction of buffalo grazing has not been attempted in island areas, but faecal control using a bucket system for houseboat families was tried in the 1950s without success. In our study area we found that transmission was confined to a few sites around the islands and related almost entirely to occupational exposure, visiting fisherman and migratory buffalos. These facts and experience call for a new approach to ecological control. Consideration of the foregoing suggests that remaining options to prevent residual schistosome transmission in Dongting Lake would be to immunise vertebrate hosts, to use drugs that prevent infection, or to prevent faecal contamination of potential transmission sites. Vaccine development for humans and bovines is under way but even if successful it will be many years before S. japonicum vaccines are available for routine use in humans. Vaccines for cattle and buffalos may be
extensive marshland and lake areas around the Yangtze river is presently impossible although snail eradication has been a priority of national schistosomiasis control programs for over four decades. Large-scale community participation for environmental management to snail eradication in lake and marshland areas has been tried with focal success in the 1970s. However, it did not eradicate the snails from the huge lake area around the Yangtze river and it cannot be repeated due to socio-economic reforms that have changed the working system throughout China (Sleigh et al., 1998b). The use of molluscicides has been successfully used in integrated schistosomiasis control. This approach to snail control is limited because of the recurring expense, and the environmental damage in many settings, especially the killing of fish. Biological control (snail predators, pathogens and competitors) is not yet useful in practice. An alternative approach to reduce transmission is to develop better faecal control by improving human sanitation and animal husbandry. In marshland and lake regions, cattle and buffaloes are the most important reservoirs of S. japonicum infection and may contribute 70 – 90% of the total egg excretion (Chen and Feng, 1999). In our study area, few buffalos are required for local agriculture. Most buffalos present on islands in the lake are brought there each January to graze unrestrained along the beaches. A herdsman guards each group of 100–200 animals. In May the owners return the animals (by boat) to begin plough-
Table 3 Water contact (m2 min/day) by intensity of reinfection in 178 subjects, the Dongting Lake, China, 1997–1998 Re-infection status (1998)
Water exposure (m2 min/day) Aa zones Mean
Not re-infected (n = 147) Re-infected epg]4–100 (n= 22) epg\100 (n = 9) P valueb a b
9S.D.
1.54 1.76 3.54 2.33 6.90 3.57 0.001 (r= 0.552)
Bazones
Total 9S.D.
Median
Mean
0.9 3.15 5.5
5.90 5.18 5.13 4.08 7.10 4.70 0.86 (r =0.013)
A, infected snail water zones; B, no-infected snail water zones. Based on Pearson correlation coefficient (shown in parentheses).
9S.D.
Median
Mean
5.15 4.85 6.10
7.44 5.86 8.64 5.20 14.0 7.30 0.003 (r =0.21)
Median 8.00 8.00 13.0
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287
Table 4 Water contact and geometric mean intensity of re-infection by age in 178 subjects from the Dongting lake region, 1997–1998 Age group Re-infection (years)
Water exposure (m2 min/day) Ab zones
B20 (n=14) 20–35 (n=54) 36–49 (n=68) 50+ (n= 42) P valuec
Bb zones
Total
(%)
(epg)a
Mean
9S.D.
Median
Mean
9S.D.
Median
Mean
9S.D.
Median
21.4
3.77
2.85
4.66
0.60
5.14
4.65
4.45
7.99
8.18
5.50
16.6
1.84
1.48
1.82
0.85
6.42
5.47
7.15
7.90
6.00
9.00
19.1
2.22
2.39
2.34
1.65
6.61
5.38
6.15
9.00
6.30
9.00
14.3
1.78
1.93
1.57
1.70
4.18
3.37
3.75
6.11
4.20
5.50
P = 0.54
P= 0.43
0.89 (r= 0.01)
0.18 (r= 0.11)
0.28 (r=−0.08)
a
A geometric mean epg among all the subjects. A, infected snail water zones; B, no-infected snail water zones. c Based on Pearson correlation coefficient (shown in parentheses). b
available sooner. Artesunate, a Qinghaosu derivative, has been shown to prevent infection by killing schistosomula (Chen and Feng, 1999; Xiao et al., 2000), but it requires fortnightly doses throughout the transmission season. Although it may be very useful for special high-risk group (army and other flood control workers), it may not be a feasible approach for large scale population-based application. So, control of residual transmission over the next few years will require development of a satisfactory method of sanitation for fishing families to use when they moor their boats inshore, and restriction of buffalo grazing near snail-infected water-contact sites. The former will involve research on appropriate technology with local co-operation. The latter would be feasible if the grass was cut and made available to the buffalos when they are excluded from snail-inhabited grazing areas. This could be a task for local government or for a private initiative related to animal feeding in the area. As well as these faecal control measures, local people may be able to reduce their water contact in high-risk foci if they know where these are located. Based on these data and the review of the experience made in these socio-ecological settings so far, we suggest that protecting against faecal
contamination of snail inhabited sites, and against occupational exposure to those sites, should be a priority of future research to control schistosomiasis in the lake and marshland region of China.
Acknowledgements We would like to thank the community members of Qingshan and Niangashan islands for participating in this study. This study was supported by the National Health and Medical Research Council of Australia, the UNDP/World Bank/WHO Special Program for Research and Training in Tropical Diseases (TDR) and the Swiss Tropical Institute.
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