Changes in human schistosomiasis levels after the construction of two large hydroelectric dams in central Côte d'Ivoire.

Changes in human schistosomiasis levels after the construction of two large hydroelectric dams in central Cote d'lvoire E.K. N'Goran,1 S. Diabate,2 J. Utzinger,3 & B. Sellin4 The construction of large dams has been shown to increase the prevalence and intensity of human schistosomiasis. However, until now no study had been carried out to assess the impact of such a project in C6te d'lvoire. For Kossou and Taabo, two large dams which became operational in the 1970s, baseline data are available on schistosomiasis prevalence in the surrounding area before dam construction, so that the changes in schistosomiasis levels can be assessed. We re-evaluated the prevalence of Schistosoma haematobium and S. mansoni in November 1992, by analysing 548 urine and 255 stool samples, respec- tively, from schoolchildren from five villages around each lake. A marked increase in the overall prevalence of S. haematobium was observed, from 14% to 53% around Lake Kossou and from 0 to 73% around Lake Taabo. Baseline data for S. mansoni are only available for Lake Taabo, where a prevalence of3% was found in 1979 and where the prevalence in 1992 was still low at 2%. The construction of these two large dams therefore led to little change in S. mansoni prevalence but to a significant increase in that ofS. haematobium. Introduction It has been emphasized that the construction of large dams may result in adverse health effects, of which one of the most frequently observed is a significant increase in the prevalence of human schistosomiasis (1). Schistosomiasis is considered to be a sensitive indicator disease since infection rates often change soon after the completion of dams (2). In relation to the construction of the Aswan Dam in Egypt (3) and of two other dams in sub-Saharan Africa (4), a changing pattern of schistosomiasis was observed with a shift in predominance from Schistosoma haematobium to S. mansoni. The most recent exam- ple is from Senegal, where an outbreak of S. mansoni was observed only 3 years after the Diama Dam became operational (5-7). In COte d'Ivoire, a total of 22 dams over lOm high were built, mainly in the 1960s and 1970s (1). Despite the great environmental changes and ex- ' Maitre-Assistant, Faculte des Sciences et Techniques, Laboratoire de Biologie Animale, Centre Universitaire de Cocody, 22 BP 582, Abidjan 22, C6te d'Jvoire. Correspondence should be sent to this author. 2 Technicien, Centre Universitaire de Formation en Entomologie Medicale et Veterinaire de Bouake, Universite de C6te d'lvoire, Bouake, CMte d'lvoire. 3 Ph.D. student, Department of Public Health and Epidemiology, Swiss Tropical Institute, 4002 Basel, Switzerland. 4 Directeur de Recherche ORSTOM, ORSTOM/Institute Pasteur, Ahtananarivo, Madagascar. Reprint No. 5810 tensive population movements that have resulted, the impact on public health in general and on schistosomiasis in particular has not been investi- gated so far. A rise in the prevalence of both S. haematobium and S. mansoni was expected in the villages located around Lake Kossou (1), but no sur- vey data have been published. For the two artificial lakes of Kossou and Taabo, parasitological surveys on human schistosomiasis were undertaken before the dams became operational to investigate whether a change in schistosomiasis levels occurred. The overall prevalence of infection with S. haematobium was 14% for Lake Kossou (8) and zero for Lake Taabo (9). Baseline data for S. mansoni are only available for Lake Taabo, where the prevalence of infection was 3% (9). WHO initiated the present study, which was undertaken in November 1992, and had two main aims. First, to re-evaluate the prevalence and inten- sity of schistosomiasis and to assess the changes that had occurred since the dam constructions. Second, to investigate whether the construction was followed by a shift in predominance from S. haematobium to S. mansoni. Materials and methods Study area Kossou and Taabo Dams are both located in central Cote d'Ivoire on the Bandama River, the longest Bulletin of the World Health Organization, 1997, 75 (6): 541-545 © World Health Organization 1997 541 E.K. N'Goran et al. river in the country (Fig. 1). They became opera- tional in 1972 and 1979, respectively. Kossou Dam is located at the southern point of the "V Baoule", where the savanna meets the forest (10), while Taabo Dam is located 95 km downstream of Kossou with the water regime of Lake Taabo being deter- mined by that of Lake Kossou. Description of the Kossou and Taabo Dams With a maximum height of 58m, Kossou Dam is the highest in Cote d'Ivoire and has a total length of 1800m. Lake Kossou extends approximately 130km along the Bandama Blanc Valley and covers an area of 1780 km2. The reservoir has an estimated size of 27675 X 106m3 (11, 12). Taabo Dam has a maximum height of 34m and is 7500m long. Lake Taabo has a surface of 69km2 and an estimated volume of 69 x 106m3 (12, 13). Both dams are clas- sified as embankment dams and are built with earth and rockfill. They are used for hydroelectric power production. Fig. 1. Map of C6te d'lvoire showing the artificial lakes of Kossou and Taabo in the centre of the country and the five study villages around each lake. V. R41§ 1 T eho Environmental alterations Vast environmental alterations followed the con- struction of Kossou and Taabo Dams. The banks of the lakes are steep and the water covers decaying pre-reservoir vegetation, which provides an ideal food base for the snails that are the intermediate hosts for schistosomiasis. In addition, Kossou Dam leaks at its base, which has also created favourable snail habitats (10). Further details of the changes that occurred due to the construction of Kossou Dam have been reported elsewhere (14,15). Study villages and subjects Data were collected from five villages around Lake Kossou (Alaou Bassi, Angosse, Bocabo, Kossou town and Suibonou); baseline data for S. haematobium were available for four of these villages (8). Around Lake Taabo, data were collected from five villages (Ahondo, Bonikro, Taabo town, Taabo village and Tokohiri); baseline data were available for Ahondo before dam construction (9) and for Ahondo and Taabo village shortly after completion of the dam. Schoolchildren were selected as the age group of interest (16), but in Suibonou where no school exists, a total of 54 children and young people aged 4-24 years were recruited. Diagnostic techniques S. haematobium infections were diagnosed by the results of a single urine filtration, as described by Plouvier et al. (17). More than 50 eggs per lOml of urine was classified as a heavy infection (18). S. mansoni infections were diagnosed by a single stool examination, applying the Kato-Katz thick-smear method (19). In the village of Suibonou, only S. haematobium infections were assessed. Results Kossou Dam The prevalence and intensity of schistosomiasis in five villages around Lake Kossou are given in Table 1. A total of 290 schoolchildren were examined for S. haematobium. Eggs of S. haematobium were found in 154 subjects, giving an overall prevalence of 53%. Heavy infections were recorded in 34% of all posi- tive subjects (53/154). A considerable range in both prevalence and intensity of infection was observed for the five villages, which emphasizes that the dis- ease is focally distributed. The highest prevalence and intensity were observed in the village of Bocoba, where eggs of S. haematobium were excreted by 79% WHO Bulletin OMS. Vol 75 1997542 Large hydroelectric dams and schistosomiasis in C6te d'lvoire Table 1: Prevalence and intensity of human schistosomiasis in five villages located around Lake Kossou, C6te d'lvoire, 1992 Schistosoma haematobium Schistosoma mansoni Eggs/10ml urine: No. subjects No. No. subjects No. Village examined positive <50 ¢50 examined positive Bocabo 53 42 (79)a 18 (34) 24 (45) 30 0 (0) Angoss6 42 10 (24) 7 (17) 3 (7) 42 0 (0) Suibonou 54 34 (63) 28 (52) 6 (11) NDb - Kossou town 67 24 (36) 17 (25) 7 (10) 21 4 (19) Alaou Bassi 74 44 (59) 31 (42) 13 (18) 28 2 (7) Total 290 154 (53) 101 (35) 53 (18) 121 6 (5) a Figures in parentheses are percentages. b ND = no children diagnosed. of the children, and 45% of the children had more than 50 eggs per 10ml of urine. Examinations for S. mansoni were carried out on 121 schoolchildren in four villages. Eggs were found only in six children, two from Alaou Bassi and four from the village of Kossou town, resulting in a low overall prevalence of 5%. Before Kossou Dam became operational, 1031 subjects were examined for S. haematobium and an overall prevalence of infection of 13.7% was found (8, 20). Therefore, our data showed a highly signifi- cant increase in overall prevalence (X12 test, ldf = 110.7; P < 0.0001) that was paralleled by significant increases in the villages of Bocabo (from 9% to 79%; Xi2 = 40.9, P < 0.0001), Suibonou (from 9% to 63%; Xi2 = 39.2, P < 0.0001) and Alaou (from 4% to 59%; Xi2 = 40.3, P < 0.0001) but by only a slight, statisti- cally insignificant increase in Angossd (from 13% to 24%; X12 = 2.2, P = 0.14). Taabo Dam The occurrence of schistosomiasis in the villages se- lected around Taabo Dam is presented in Table 2. A total of 258 schoolchildren were diagnosed for S. haematobium and the overall prevalence was 73%. Very high prevalences of 95% and 93% were re- corded in Tokohiri and Taabo town, respectively. Overall, 44% of the infected children excreted more than 50 eggs per 10ml of urine. A total of 134 schoolchildren were examined for S. mansoni in four villages; only three children were found positive, resulting in a low overall prevalence of 2%. Before Taabo Dam became operational, 120 children were examined in the village of Ahondo. No eggs of S. haematobium were detected, but four chil- dren were found positive for S. mansoni (9). Two years after the dam became operational, 50 chil- Table 2: Prevalence and intensity of human schistosomiasis in five villages located around Lake Taabo, C6te d'lvoire, 1992 Schistosoma haematobium Schistosoma mansoni Eggs/10ml urine: No. subjects No. No. subjects No. Village examined positive <50 ¢50 examined positive Tokohiri 41 39 (95)a 5 (12) 34 (83) 18 1 (6) Ahondo 31 21 (68) 9 (29) 12 (39) 31 0 (0) Bonikro 43 27 (63) 19 (44) 8 (19) 43 1 (2) Taabo village 89 52 (58) 45 (51) 7 (8) 42 1 (2) Taabo town 54 50 (93) 28 (52) 22 (41) NDb - Total 258 189 (73) 106 (41) 83 (32) 134 3 (2) a Figures in parentheses are percentages. b ND = no children diagnosed. WHO Bulletin OMS. Vol 75 1997 543 E.K. N'Goran et al. dren were examined in Ahondo and another 50 chil- dren in Taabo village. Two children in each village were found with S. haematobium eggs but only two with S. mansoni eggs in Ahondo (13). Comparison of the baseline data before and shortly after the dam became operational with our data revealed a highly significant increase in the prevalence of S. haematobium infection (P < 0.001) but no change in that of S. mansoni infection, which remained low. Discussion There is a general agreement that the rapid environ- mental alterations caused by the construction of large dams often lead to a negative impact on human health. Ecological changes may not only modify the incidence, prevalence and intensity of diseases al- ready present in a certain region, but may also result in the introduction and spread of new diseases (1). Changes in schistosomiasis prevalences frequently follow dam constructions and have been shown to be sensitive indicators (2). A common problem in analysing possible changes is that suitable baseline data for the period preceding dam constructions are often not avail- able. In water development projects that were im- plemented one or more decades previously, it frequently happens that no parasitological baseline data were collected or that the results were not pub- lished or are not accessible (2). This is confirmed by a large-scale study of water resource development schemes from a neighbouring country, Mali (21). A difficulty in making comparisons of pre- valences over time is that the methods used may have changed. In the villages around Lake Taabo the baseline data had been obtained for schoolchildren using urine filtration and the Kato-Katz method for detection of S. haematobium and S. mansoni infec- tions, respectively (9,13). These same methods were used in our study of schoolchildren, hence, our data allow direct comparison with the baseline data for the Taabo Dam. The change observed was highly significant. In the case of Lake Kossou care is needed in comparing baseline data with our results because of differences in the age groups of subjects examined and in the diagnostic methods used. In the baseline study, schoolchildren and adults were examined by means of urine sedimentation for S. haematobium infections (8). In the present study, only schoolchil- dren were recruited and a single urine filtration test was used for detection of S. haematobium eggs (17). Schoolchildren are at highest risk for schistosomiasis infection and the prevalence of infection in this age group is usually higher than that in older age groups (16). Furthermore, it is estimated that urine sedi- mentation is about 20% less sensitive than urine fil- tration; in areas of relatively low prevalence, urine sedimentation may even be 35% less sensitive (22). As a result, the earlier prevalence data may be an underestimate and hence have produced an over- estimate of the impact of the construction of Kossou Dam. However, even taking these limitations for Lake Kossou into consideration, the increase in prevalence of S. haematobium infection was still marked. Our findings for villages around both Lakes Kossou and Taabo thus show that the construction of the two large hydroelectric dams resulted in a marked increase in the prevalence of S. haematobium. The increase in S. haematobium was not paral- leled by a similar increase in S. mansoni. This result is in full agreement with the studies reported by Brinkmann et al. (21) for Mali and Tayo & Jewsbury (23) for Nigeria, who also found that S. mansoni remained stable while S. haematobium increased. It is interesting that in other places in Africa, S. mansoni has become the predominant type of schistosomiasis infection after dam construction (3, 4, 7). This difference could be due to the absence of the intermediate host snail. However, Biomphalaria pfeifferi, the intermediate host of S. mansoni, was reported to be very abundant in Lake Taabo already in the early 1980s (13). Acknowledgements We thank the doctors and other staff in the basic rural health centres of Yamoussoukro, Bouafl6 and Tiassald. WHO is thanked for financial support. Special thanks are due to Dr K.E. Mott, for constant encouragement and advice, and to J.M. Jenkins for critically reading and supplying much appreciated comments on the manu- script, as well as improving the English. Resume Modification du niveau des schistosomiases humaines aprbs la construction de deux grands barrages hydro6lectriques en Cote d'lvoire centrale 11 a ete montre que la construction de grands bar- rages de retenue d'eau entrainait une recrudes- cence des schistosomiases (bilharzioses) humaines, se traduisant par une augmentation de la pr6valence et de l'intensite de l'infection. Cependant, aucune etude veritable n'a W conduite pour evaluer l'impact des grands barrages r6alis6s en Cote d'lvoire. Pour deux d'entre eux, on dispose de donnees sur les prevalences avant la mise en eau, ce qui autorise une appreciation de leur impact WHO Bulletin OMS. Vol 75 1997544 Large hydroelectric dams and schistosomiasis in C6te d'lvoire sur la sant6 des populations riveraines. Les bar- rages hydroelectriques de Kossou et de Taabo ont ete mis en service dans les annees 1970 et les prevalences de Schistosoma haematobium avant la mise en eau 6taient de 14% pour Kossou et de 0% pour Taabo. Les donnees relatives a S. mansoni, disponibles seulement a Taabo, font 6tat d'une pre- valence de 3% chez les enfants scolarises. Nous avons reevalue les prevalences de S. haematobium et de S. mansoni en 1992 par analyse des urines et des selles de 548 et 255 enfants d'age scolaire, provenant respectivement de 5 villages situes autour de chacun des lacs de retenue. 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