Control of Schistosoma mansoni transmission by provision of domestic water supplies

SCHISTOSOMIASIS Control of Schistosoma mansoni transmission by provision of domestic water supplies A preliminary report of a study in St Lucia * P. JORDAN,1 LILIAN WOODSTOCK,2 G. 0. UNRAU,3 & J. A. COOK 3 As an experimental control measure to reduce the transmission of S. mansoni, an individual household water supply was provided in 400 houses in 5 rural settlements of the Riche Fond Valley, St Lucia. This population ofabout 2 000 hadpreviously been dependent for water on infective streams and rivers. Six other settlements in the valley, all provided with limitedpiped waterfrom public standpipes, served as the comparison area. After 2 years the incidence, prevalence, and intensity ofinfection with S. mansoni were significantly lower in the household water supply area, whereas all these indices ofinfection had increased in the comparison area. An adequate, reliable, and convenient supply of water can reduce the transmission of S. mansoni and should be considered as a control measure in other endemic areas. The effect of a domestic water supply and health education on the transmission of S. mansoni has never been investigated fully, although results from South Africa (5, 6) and Brazil (1) suggest that some degree of reduction in transmission follows the installation of domestic piped water. Such an investigation is now in progress in St Lu- cia where, as part of a comparative evaluation of different methods of S. mansoni control, an individ- ual household water supply was provided in 5 settle- ments of the Riche Fond Valley during 1970-72. Six other settlements in the same valley served as the comparison area. This paper reports preliminary results of the effect of household water supplies on S. mansoni transmis- sion among children aged 0-14 years. The effect on transmission among adults will be the subject of a separate paper. THE PROJECT AREA The 5 settlements of the household water supply (HWS) area-Grande Ravine, Thomazo, Grande * From the Research and Control Department, Castries, St Lucia, West Indies. Reprints may be obtained from the authors at that address. 1 Director; member of External Staff, British Medical Research Council, seconded to the Rockefeller Foundation. 2 Formerly Laboratory Supervisor, recruited by the Overseas Development Administration, London, UK for the Government of St Lucia. 3Staff Member, the Rockefeller Foundation. Riviere, Morne Panache, and Debonnaire-have a combined population of about 2 000 (about 400 houses) and are situated in the southwestern part of the Riche Fond Valley on the Atlantic side of St Lucia. To the west and south, hills rising to over 350 m separate these settlements from other in- habited areas, but to the north and east there is ready access to the comparison settlements (see map in the companion paper by Unrau, p. 2 of this issue). Grande Ravine (population about 400) is situated on a small hill in the middle of the valley, and previously was dependent for water on a stream about 340 m from the centre of the settlement. Thomazo (population about 245) lies close to the headwaters of the valley's main river, the Mabouya; Grande Riviere (population 770) is situated on the valley floor very close to the same river, with houses extending up a nearby hillside; and Morne Panache (population 275) is located higher up this hill. De- bonnaire (population .300) is a short distance away and close to a tributary of the Mabouya river. The 6 settlements of the comparison area, which also have a combined population of about 2 000, are situated close to a main tributary of the Mabouya river on the northern side of the valley. Under a government water scheme introduced in this area in 1969, 16 public standpipes were constructed along the main road at about 350-m intervals. Some of the householders paid for connections to be made to their homes. 3316 - 9 - BULL. WORLD HEALTH ORGAN., Vol. 52, 1975 P. JORDAN ET AL. Before 1969 in the comparison area, and prior to the installation of household water supplies in the control area, valley residents obtained water from the nearest stream or river. Washing of clothes, bathing, and swimming were also common activities involving contact with these waters, which are be- lieved to be the main transmission sites of S. mansoni in the dry season when snail populations increase in the slow-flowing rivers (9). For agricultural workers in the valley there continues to be an occupational risk of infection since Biomphalaria glabrata is not uncommon in the drainage systems of the banana fields. It is believed, however, that transmission at these sites is sporadic and probably occurs in the wetter months of the year. Sanitation is poor, although the construction of pit atrines is being encouraged by the government. FIELD SURVEY TECHNIQUES Household census On the initial survey in 1968, all houses in the HWS and comparison areas were visited and num- bered and the name, age, and sex of every occupant were noted. On subsequent household visits, as the survey teams became more familiar with the areas, a few houses previously overlooked because of thick vegetation were found; these, as well as any newly constructed houses, were also visited and numbered. Although many families possessed birth certifi- cates, inspection of all these to verify the ages stated orally would have been time-consuming and trouble- some to the householders. Accordingly, the stated age was recorded in all instances, and verification was done for a sample of the population comprising 1 031 children and 290 adults. Of the 1 031 children, 75% of the stated ages exactly matched those on birth certificates and a further 21.5% differed by just 1 year; only 1.06% (11) differed by more than 5 years. Of the 290 adults, 45.8% of the stated ages exactly matched birth certificates and a further 28.2% differed by only 1 year; of the 10.6% (31) showing a discrepancy of more than 5 years, 1% (3) differed by 10 years. These levels of accuracy were considered sufficient to justify grouping all children by 5-year age groups, and grouping all adults by 10- year age groups. An attempt was also made on household visits to follow changes in the population caused by births, deaths, immigration, and emigration. This was com- plicated by frequent changes in the occupancy of houses available for rental; in addition, some per- sons moved into the valley only to move out after 1 or 2 years. The following 5-year (1968-73) figures on population movement for the HWS area, al- though not fully accurate, are nevertheless indicative of the general pattern and support the contention that the rural population of St Lucia is relatively static at present. Type of change No. ofpersons Movement within the HWS area 110 Emigration 382 To: Castries (island capital) 83 elsewhere in valley 59 outside valley 71 outside St. Lucia 127 Deaths 42 Immigration 381 From: Castries 24 elsewhere in valley 40 outside valley 43 outside St. Lucia 18 Births 256 These changes represent a turnover of approxi- mately 8% of the population per annum. As an overall result, the proportion of the population under 15 years of age increased from 50% in 1968 to 55% in 1972. Stool collections Although the prevalence of infection with S. man- soni was known to vary considerably in different parts of the valley, the extent of the variation was unknown and this, combined with unknown varia- tions in settlement size, made the design of a sam- pling technique difficult. It was further appreciated that, although the rate of response to the initial request for samples might be good, repeated requests for further samples, in order to determine changes in the parameters of infection, might prove less pro- ductive. Instead of examining a sample of the population, therefore, a total survey of the 0-14-year-old popula- tion was attempted in each of the 5 years, with specimens from adults (those over 14 years of age) being collected in alternate years. Stool containers, individually labelled, were left at each house with instructions that all occupants provide a specimen. Collection of the containers was made the following day; at many houses, however, 2 or 3 return visits 10 CONTROL OF SCHISTOSOMA MANSONI Table 1. Prevalence of S. mansoni infection in persons responsive and less responsive in providing stool specimens Responsive Less responsive Age group No. S. mansoni % No. S. mansoni % (years) positive/ preva- positive/ preva- No. examined lence No. examined lence 0-14 206/386 53.4 42/82 51.2 15-39 100/127 78.7 33/43 76.7 > 40 71/103 68.9 10/21 47.6 were necessary to retrieve containers, and even then success was not invariably complete. Although some containers probably held a stool specimen from another person than the one named on the label-containers may have been accidentally mixed or 2 persons may have shared a container-it is believed that few such cases occurred. Over 75% of the children provided stool speci- mens at each survey. Among adults, males were less responsive than females, particularly those between the ages of 15 and 50. In an attempt to determine how representative of the whole population were those who freely provided stools, a group of those freely giving stools was compared for prevalence with a group who gave specimens only after con- siderable persuasion. As shown in Table 1, only in those above the age of 40 years was there evidence (X2 significant at 5% level) that prevalence was greater among those freely offering stools than among those less responsive. LABORATORY TECHNIQUES Stools were examined qualitatively by a formalin- glycerine sedimentation method, up to 3 slide pre- parations being examined for S. mansoni ova. The presence of other helminth ova, hookworm, Ascaris sp., Trichuris sp., and larvae of Strongyloides sp. were noted. When sufficient material was available, stools positive for S. mansoni were also examined quantitatively by the filtration-staining method (2); 10 ml of stool was emulsified in 90 ml of formol- glycerol solution and 1 ml of suspension was filtered through each of 2 filter papers, of which 1 was examined and the other kept for any subsequent checking. (In one series of stools, the mean weight of 10 ml of stool was 11.5 g.) Maintenance of standard of stool examination Since the results of large-scale parasitological stu- dies, such as described here, depend on the com- petence of locally trained microscopists, a system of checking at least 10% of the negative slides was introduced. The percentage of negative slides found positive on checking is shown in Table 2, together with the calculated false negative rate (FNR; see Definitions, p. 12). The FNR is the more realistic measure of the two for the maintenance of examina- tion standards because it takes into account the prevalence of infection in the population being examined. This is illustrated in Table 3 by the use of hypothetical figures from a high-prevalence area (75.7%) before control effected a change to low prevalence (11.3%). Although 15.8% and 0.65% of the checked slides were found positive before and Table 2. Results of checking negative slides from household water supply (HWS) and comparison areas, 1971 and 1973 Area and Total sNo.(%) No. No. % Calculated False year examined Smansoni negative checked positive % positive negativepositive rate a HWS 1971 658 293 (44.5) 365 123 4.8 47.2 0.056 1973 598 221 (37.0) 377 74 2.7 38.6 0.043 Comparison 1971 815 298 (36.6) 517 66 1.5 37.5 0.026 1973 910 501 (55.1) 409 148 6.0 57.8 0.047 a No significant difference between the rates at 5 % probability level. 11 P. JORDAN ET AL. Table 3. Calculations of false negative rate in a hypo- thetical high-prevalence area before control and in the same area after control when prevalence has been reduced High Low prevalence prevalence (before control) (after control) No. of stools examined 346 319 No. S. mansoni-positive 262 36 prevalence (%) 75.7 11.3 No. S. mansoni-negative 84 283 No. re-examined 38 155 No. positive on reexamination 6 (15.8%) 1 (0.65%) calculated missed positives 15.8% of 84 0.65% of 283 = 13.3 (13) = 1.8 (2) calculated total No. of positives 262 + 13 = 275 36 + 2 = 38 false negative rate 13/275 = 0.047 2/38 = 0.053 after control, respectively, suggesting a change in the standard of microscopy, the FNR is the same and the standards of microscopic examination are therefore comparable. Accuracy of a single stool examination Apart from the possibility of microscopists missing S. mansoni ova, it was recognized that with the examination of a single stool specimen some infected persons excreting low numbers of eggs would be undiagnosed. Since this error is likely to be greater in low-prevalence than in high-prevalence areas, 3 stool specimens were examined from children in settle- ments with different prevalence rates. The difference between prevalence based on a single stool and the rate based on 3 stools was found to be greater in the low-prevalence settlements, as follows: 1-stool prevalence 8.1 %, 12.0%, 17.5 %, 23.0%, 25.0%, 49.1 %, 51.6%, 63.0% 3-stool prevalence 16.2 %, 24.0%, 37.5 %, 42.8 00, 41.6%, 70.1%, 74.1%, 79.0% The relationship between the single stool and the 3 stools examined could be expressed by the formula: y = 4.987 + 1.72754x-0.0077x2 where y = prevalence on 3 stools and x = preva- lence on a single stool. This is not unlike the correction factor worked out by Scott (7). Although the formula applies to children, it may not be applicable to older age groups. DEFINITIONS The following terms not in common use in the measurement of infection with and transmission of S. mansoni have been employed in tables and anal- ysis. Data from cross-sectional surveys Response rate: The number of persons in a speci- fied age or sex group who provided faecal samples for examination, expressed as a percentage of the total number of specimens requested. Point prevalence rate: The number of persons in a specified age or sex group found infected with S. mansoni, expressed as a percentage of the total number in the group who were examined. Intensity of infection: The geometric mean of the egg counts, expressed in terms of eggs per ml of faeces of infected individuals in a particular age or sex group. False negative rate: The calculated number of S. mansoni positive slides missed on examination, expressed as a proportion of the total number of positives in the sample examined (see Tables 2 and 3). Potential contamination factor: The sum of the product of the prevalence and the geometric mean of egg output for the different age groups. Data from longitudinal studies When surveys are repeated after 1, 2, or more years, some individuals who previously provided stool specimens will provide second specimens. From these paired results the following measurements of transmission can be obtained: Conversions: The number of persons in an age or sex group whose stools were negative for S. mansoni at the first examination but who on re-examination after 1 or 2 years were found to have become infected (i.e., a negative stool converting to a positive stool). Reversions: The number of persons whose stools were positive for S. mansoni at the first examination but who on re-examination after 1 or 2 years were found to have become negative (i.e., a positive stool reverting to a negative stool). Incidence rate (rate of new infections): The num- ber of conversions expressed as a percentage of the total number in a specified age or sex group who were negative for S. mansoni on first examination. 12 CONTROL OF SCHISTOSOMA MANSONI (Although the age group initially may be 0-4 years, if incidence data are collected 2 years later the children are 2-6 years old.) Rate of loss of infection: The number of reversions expressed as a percentage of the total number in a specified age or sex group who were infected (i.e., stool positive for S. mansoni on first examination subsequently becoming negative). First and second cohort prevalence rates: The num- bers found infected on first and second examinations expressed as percentages of those examined. (Num- ber positive on second examination equals number positive on first, plus number of conversions, minus number of reversions.) STATISTICAL METHODS Point prevalence and incidence For comparison of point prevalence or incidence data from different areas, between sexes, or at different times, Cochran's method for combining 2 x 2 tables was used (8). Cohort results Results of examining the same individual on differ- ent surveys to determine the significance of changes in the percentage infected were analyzed by compar- ing the difference between the numbers of conver- sions and reversions by the sign test. Quantitative data For comparison of the geometric means of egg counts of different age groups with data from differ- ent areas, between sexes, or at different times, the individual differences were weighted by the reci- procal of the variance. The weighted mean difference was regarded as normally distributed and having, under the null hypothesis, a mean of 0 and a variance of the reciprocal of the sum of the weights. Accord- ingly, to test whether the mean difference differed from 0, a normal approximation was taken of Z = d/S.E. d (method proposed by C. White, personal communication, 1973). PARASITOLOGICAL RESULTS Household water supplies were made available to the 5 settlements over a 2-year period: in Grande Ravine in 1970; in Thomazo, Grande Riviere, and Morne Panache in 1971; and in Debonnaire in 1972. Prevalence, incidence, and intensity of infection are Table 4. Point prevalence rates of S. mansoni infection in 1968 in HWS and comparison areas HWS Comparison Age group (years) No. % No. % examined positive examined positive 0-4 173 12.7 252 7.5 5-9 161 60.8 291 40.8 10-14 98 68.3 210 63.8 15-19 58 56.8 120 73.3 20-29 65 55.3 143 54.5 30-39 54 46.2 99 41.4 40-49 68 41.1 112 38.3 50-59 44 36.3 74 43.2 >60 40 27.5 104 29.8 therefore considered during 3 phases of the pro- gramme: 1968-70, before the supplies were installed; 1970-72, the period of installation; and 1973, when all settlements in the HWS area had had water for between 1 and 3 years. Changes in the status of S. mansoni infection in individuals between 1968 and 1970 are compared with similar changes between 1971 and 1973. Prevalence Point prevalence rates of S. mansoni infection in all age groups in the HWS and comparison areas in 1968 are shown in Table 4. The usual pattern is evident in both areas, with peak rates occurring in the second decade of life. Point prevalence rates in the younger age groups at different surveys are shown in Table 5. In 1968, 1969, and 1970-the precontrol phase-prevalence was significantly higher in the HWS area than in the comparison area. The pattern of change over this period was the same in the 2 areas: a decrease in 1969 followed by an increase in 1970 to a level higher than in 1968, although in neither area was the difference significant. In 1973 there was a slight increase in prevalence in the comparison area but a significant decrease (0.1 % level) in the HWS area, where water had been available in all 5 settlements during the year. When the 1973 results from both areas were corrected for a single stool examination by the formula given above, the corrected prevalence was still significantly lower in the HWS area. 2 13 P. JORDAN ET AL. Table 5. Point prevalence rates of S. mansoni infection among children in the HWS and comparison areas before and after installation of household water supplies Age group Before installation After % ChangeAge group ntlaoninstallation between(years) 1970 and 1968 1969 1970 a 1973 1973 HWS 0-2 2/88 (2.3%) 23/136 (16.9%) 15/107 (14.0%) 5/88 (5.7%) -59.3 3-5 38/124 (30.6 %) 49/181 (27.1 %) 51/153 (33.3 %) 18/126 (14.3 %) -57.1 6-8 62/97 (63.9%) 64/133 (48.1 %) 82/141 (58.2%) 60/166 (36.1 %) -38.0 9-11 54/77 (70.1 %) 83/127 (65.4 %) 88/119 (73.9 %) 67/120 (55.8 %) -24.5 12-14 31/46 (67.4 %) - 56/67 (83.6 %) 71/98 (72.4 %) -13.4 Comparison 0-2 3/136 (2.2 %) 5/155 (3.2 %) 12/161 (7.5 %) 25/145 (17.2 %) +129.3 3-5 42/228 (18.4%) 32/238 (13.4%) 41/239 (17.2%) 79/183 (43.2%) +151.2 6-8 69/183 (37.7%) 69/203 (34.0%) 95/251 (37.8%) 115/213(54.0%) +42.9 9-11 85/153 (55.6 %) 85/145 (58.6 %) 94/170 (55.3 %) 155/219 (70.8 %) +28.0 12-14 74/122 (60.7 %) - 95/127 (74.8%) 127/150 (84.7 %) +13.2 Higher prevalence: HWS HWS HWS comparison Significance level: 0.1 % 0.1 % 0.1 % 0.1 % a Data obtained before Grande Ravine was supplied in June 1970. In comparison with the highest precontrol preva- lence rates of 1970, the 1973 prevalence rate in the HWS area was significantly lower and that in the comparison area was significantly higher. Incidence The annual incidence of new infections during the 3 phases is shown in Table 6. During 1968/69 incidence was low and similar in the HWS and comparison areas. During 1969/70 it increased in both areas but was significantly higher in the HWS area. During 1970/71 it dropped in both areas and significantly so (0.1 % level) in the HWS area (water in Grande Ravine for 1 year). Incidence was, however, similar in the 2 areas, although slightly higher in the HWS area. During 1971/72 there was a small increase in incidence in the HWS area (water available during the year in 4 of the 5 settlements) but a marked increase in the comparison area, where incidence was significantly greater. During 1972/73 a marked drop in incidence occurred in the HWS area. Although a drop was also noted in the comparison area, it was not as great, and the statistical significance of the difference in incidence between the 2 areas increased from the 1 % level in 1971/72 to the 0.1 % level in 1972/73. Intensity of infection The intensity of infection (geometric mean of the egg counts of infected children) in the 2 areas is shown in Table 7. In the period 1968-70 intensity of infection was generally higher in the HWS area than in the comparison area (prevalence and incidence were also higher). Egg loads decreased in both areas during this period, which probably signifies that at some time prior to 1968 transmission had been at a higher level than it was between 1968 and 1970 (cf. the low incidence during 1968/69). During 1970-72 the in- tensity of infection showed little change but was slightly higher in the HWS area. In 1973, however, the intensity of infection was significantly lower in the HWS area than in 1972, 14 CONTROL OF SCHISTOSOMA MANSONI Table 6. Incidence of S. mansoni infection among children in the HWS and comparison areas before, during, and after installation of household water supplies Age group Before installation During installation After installation (years) 1968/1969 1969/1970 1970/1971 1971/1972 1972/1973 HWS 0-2 9/78 (11.5 %) 10/78 (12.8 %) 6/60 (10.0 %) 5/80 (6.3 %) 5/67 (7.5 %) 3-5 12/73 (16.4 %) 24/88 (27.3 %) 17/74 (23.0 %) 29/106 (27.4 %) 11/84 (13.1 %) 6-7 14/30 (46.7 %) 14/32 (43.8 %) 11/35 (31.4 %) 13/46 (28.3%) 7/50 (14.0 %) 8-10 18/34 (52.9 %) 27/47 (57.4 %) 10/25 (40.0 %) 21/40 (52.5%) 3/29 (10.3 %) [11-13] - - [1/6 (16.7 %)] [18/22 (81.8 %)] [1/7 (14.3 %)] 0-10 53/215 (15.6 %) a 75/245 (30.6 %) 44/194 (22.7 %) 68/272 (25.0 %) 26/230 (11.3 %) Change between surveys: increase decrease increase decrease Significance of change: 0.1 % 0.1 % N.S. 0.1 % Comparison 0-2 9/86 (10.5 %) 12/124 (9.7 %) 7/75 (9.3 %) 18/68 (26.5 %) 19/89 (21.3 %) 3-5 22/126 (17.5%) 35/161 (21.7%) 16/120 (13.3 %) 40/119 (33.6%) 20/79 (25.3%) 6-7 6/49 (12.2%) 25/79 (31.6%) 28/106 (26.4%) 44/87 (50.6%) 22/65 (33.8%) 8-10 22/56 (39.3 %) 31/67 (46.3 %) 27/54 (50.0 %) 34/58 (58.6 %) 22/55 (40.0 %) [11-13] - - [14/23 (60.9 %)] [6/17 (35.3 %)] [7/18 (38.9 %)] 0-10 59/317 (12.4%) a 103/431 (23.9%) 78/355 (22.0%) 136/332 (41.0%) 83/288 (28.8%) Change between surveys: increase decrease increase decrease Significance of change: 0.1 % N.S. 0.1 % 1 % Higher incidence: HWS HWS HWS comparison comparison Significance level: N.S. 5 % N.S. 1 % 0.1 % a Corrected for 12 months from 18-month interval between surveys. while in the comparison area it was significantly higher. For the first time, therefore, intensity of infection was greater in the comparison area than in the HWS area. Taking into consideration changes in prevalence and intensity of infection in the 2 areas, between 1970 and 1973 the potential contamination factor of 0-14- year-old children showed a decrease of 47% in the HWS area but an increase of 77% in the comparison area. Change in S. mansoni infection status over 2-year periods In each area the change in S. mansoni infection status in a cohort of 0-13-year-old children examined in 1968 and again in 1970 was compared with the change in a cohort of similar age examined in 1971 and again in 1973. The results are shown in Table 8. In the HWS area between 1968 and 1970, new infections (conversions) outnumbered apparent los- ses of infection (reversions) by 82 to 15, so that the cohort prevalence rose from 42% to 62% (the children were 0-13 years old in 1968 but 2-15 years old in 1970). A similar pattern of change took place in the comparison area, with conversions (103) out- numbering reversions (31) and the cohort prevalence increasing from 32% to 46%. In the comparison area between 1971 and 1973, conversions (183) again exceeded reversions (29) and the cohort prevalence increased from 36% to 64%. In the same years in the HWS area, however, conversions (43) were outnumbered by reversions (51) and the cohort prevalence showed little change (from 46% to 44%). 15 16 P. JORDAN ET AL. Table 7. Intensity of S. mansoni infection (geometric mean (GM) of egg output per ml faeces), as well as standard deviation (SD) of log values of individual counts and the number of counts, in the HWS and comparison areas Before installation During installation After installation group 1968 1969 1970 1971 1972 1973 (years) SDSSDSSDDGM SoDg No. GM SD No. GM SD No. GM SD No. GM SoD No. GM loD No. HWS 0-5 35 0.5118 38 31 0.5196 62 20 0.3578 51 22 0.4375 38 23 0.4075 57 25 0.5438 28 6-8 49 0.6057 41 50 0.5135 54 38 0.5469 70 28 0.4046 64 28 0.4198 72 19 0.3616 56 9-11 57 0.5904 48 57 0.5864 71 34 0.5360 74 38 0.5192 75 35 0.4859 88 32 0.4325 62 12-14 72 0.6456 28 - - - 60 0.5267 54 43 0.5840 72 46 0.5219 108 37 0.4875 61 Change between surveys: decrease decrease decrease decrease decrease Significance of change: N.S. 0.1 % N.S. N.S. 5 % Comparison 0-5 39 0.4858 41 15 0.2444 27 18 0.3894 44 28 0.3977 20 20 0.3569 73 32 0.5167 85 6-8 57 0.6366 61 42 0.5589 61 23 0.4164 74 23 0.4786 64 34 0.4622 117 37 0.4778 100 9-11 56 0.5889 78 39 0.5274 65 34 0.4645 91 20 0.5262 75 38 0.4791 129 58 0.5708 138 12-14 59 0.5428 68 - - - 51 0.4811 80 39 0.6241 63 38 0.4536 96 56 0.5082 108 Change between surveys: decrease decrease increase increase increase Significance of change: 0.1 % N.S. N.S. N.S. 0.1 % Higher intensity: HWS HWS HWS HWS HWS comparison Significance level: N.S. 0.1 % 5 % N.S. N.S. 0.1 % Table 8. Change in status of S. mansoni infection among cohorts of children from the HWS and comparison areas examined in 1968 and 1970 (before installation of household water supplies), and among similarly aged cohorts from the 2 areas examined in 1971 and 1973 (after installation) Before installation After installation Area age group (years) age group (years) year year 0-4 5-9 10-13 0-4 5-9 10-13 HWS No. examined 138 142 47 150 172 91 1 st cohort prevalence 1968 16 % 58 % 70 % 1971 19% 52 % 80 % conversions 40 (34%) 31 (52%) 11 (79%) 15 (12%) 18 (22%) 10 (56%) reversions 5 (23%) 8 (10%) 2 (6%) 17 (61 %) 24 (27%) 10 (14%) ratio of conversion to reversion 8.0 3.9 5.5 0.9 0.8 1.0 2nd cohort prevalence 1970 41 % 74% 89% 1973 17% 48% 80% Comparison No. examined 176 213 124 160 268 120 1st cohort prevalence 1968 6% 37 % 61 % 1971 9% 32% 78% conversions 31 (19 %) 50 (37 %) 22 (46 %) 60 (41 %) 106 (59 %) 17 (63 %) reversions 4 (40%) 17 (22%) 10 (13%) 7 (47%) 15 (17%) 7 (8%) ratio of conversion to reversion 7.8 2.9 2.2 8.6 7.1 2.4 2nd cohort prevalence 1970 21 % 52 % 71 % 1973 43 % 66 % 86 % CONTROL OF SCHISTOSOMA MANSONI DISCUSSION Comparability of the 2 areas In any region where schistosomiasis is endemic it is virtually impossible to find two localities with the same point prevalence rates, incidence, and intensity of infection, the one to be used as a comparison area against which to assess control measures adopted in the other. In the case of the present study transmission had been, and during the immediate precontrol period was, at a higher level in the HWS area than in the comparison area (consistently higher prevalence and incidence, Tables 5 and 6). In that period, however, the 2 areas showed the same pattern of change with both parameters of infection, and they are thus considered sufficiently comparable to demonstrate changes in the transmission of S. mansoni following the installation of household water supplies in the test area. Effect of household water supplies on transmission The best measure of a reduction in the transmis- sion of S. mansoni is incidence, i.e., the rate of new infections. In the reports from many studies of control methods attention has mainly been paid to the resultant fall in incidence among children under the age of 7 years, and little information has been presented on the effect of control on incidence among older children and adults, who may acquire infection in quite different situations. Although childien under the age of 7 years may be a suitable group to study in many areas, where transmission is very high a much younger group, e.g., 0-3-year-olds, may have to be taken. The decision made in this preliminary report to include data on older children of 7-14 years of age, as well as on the customary 0-6-year age group, admittedly involves several disadvantages. Firstly, older children are more mobile and are therefore more likely to acquire infection outside the control area if that is of limited extent. Secondly, among the 0-6-year age group only a few of those originally negative would actually have been passing schisto- some eggs; however, in older age groups the number of false negatives is likely to be greater because these children are more likely to have an easily missed low level of infection. Thirdly, in areas of high endem- icity only a few of the older children (11-13 years of age) may be negative and valid numbers for calculat- ing incidence may be difficult to obtain. On the other hand, the inclusion of older children offers a distinct advantage. With successful control, the rate of new infections in the 0-6-year age group may rapidly be lowered to a point beyond which further reduction cannot be measured with con- fidence but, among older children, who initially have a higher level of incidence, continued reduction in transmission from year to year can be assessed over a longer period of time. Nevertheless, the 0-6-year age group provides a useful group for a long-term cohort study for assessment of control. The yearly variations in incidence in the HWS and comparison areas may be considered remarkable, but snail populations and therefore transmission in St Lucia depend on climatic features that vary from year to year. (Such variations may be less marked in irrigation systems than in " natural habitats ".) Of these climatic features, rainfall probably exerts the greatest influence on snail populations. As shown in Fig. 1, rainfall in the drier months (January-June) during 1968-70 varied to some extent from the average, but in the 6 wetter months rainfall was near or above average and was therefore sufficient to wash out snail populations in the streams; in these years, therefore, transmission was probably dependent on snail colonies existing in streams in the drier months, with changes in incidence being inversely related to rainfall in these months. In 1971 and 1972, rainfall in the drier months was again about average, but that in the wetter months was well below average and was therefore probably insufficient to wash out snail Fig. 1. Rainfall during the dry months of January-June (hatched) and the wet months of July-December (unhatched). The 30-year averages for the dry and wet periods are shown by the broken and solid horizontal lines, respectively. The incidence of S. man- soni infection in children aged 0-1 0 years in the Riche Fond Valley comparison area is shown by 6 horizontal dots in each of the periods from July to June. 50 40 130 20 20° 10 _ ls'tG0 -.15f1 17 P. JORDAN ET AL. populations in the streams; presumably transmission continued during the " wet season " in these years, leading to a sharp rise in incidence between the surveys of 1971 and 1972. Although the increase in rainfall in the wetter months of 1972 was only slight, incidence in that year was lower than in 1971. Data from the different surveys conducted in the 2 areas show the increasing incidence with age (Table 6), but it is striking that during 1972-73 this pattern did not occur in the HWS area, incidence there being low and remarkably steady in all age groups except the 0-2-year-olds. Farooq & Hairston (3) have drawn attention to the rate of loss of infection, and have shown that in Egypt this decreased with age. In Table 8 it is seen that the apparent loss of infection (reversions) de- creased with age and between 1968 and 1970 was indirectly related, in each area, to the incidence rate and also to the percentage infected. The decreasing apparent loss of infection is prob- ably due to a number of interacting factors. Among the younger children incidence is low; there is consequently little chance of additional infection being acquired in a year, and light infections are more likely than heavier infections to be missed at a subsequent examination. Among the older children incidence is higher and there is consequently greater likelihood of an increase in worm burden, but also less chance both of an infection being missed and of a loss of infection. The combined effect of a population acquiring new infections and losing old infections determines the cohort prevalence and point prevalence rates and, with control preventing new infections, the rate of loss increases (Table 8, 1971-73 data). When the number of new infections equals that of lost infec- tions there is no change in the cohort prevalence rates, which leads to a reduction in point prevalence. As incidence continues to decline the rate of loss may increase, with further reduction in point prevalence. The reduced incidence and increased loss of infec- tion in the HWS area were associated with a reduc- tion in intensity of infection which, in conjunction with the lower prevalence, resulted in a reduction in the potential contamination factor. Preliminary data from sentinel snail exposures show fewer snails becoming infected in the HWS area but larger numbers becoming infected in the comparison area (E. S. Upatham, personal communication, 1974). Whether this reduction is due to reduced egg output or is a reflection of less defaecation on the river banks is debatable. The parasitological results, considering all the parameters of infection, indicate that the availability of household water supplies has brought about reduced transmission. Although the results might not have been so remarkable if an intensive health education programme had not also been instituted, it is nevertheless apparent that when adequate alterna- tive water sources are made available in St Lucia, customs can be changed and children can be taught to stay out of the rivers. Studies of water contact in the HWS area show a 95% reduction in the number of persons going into the rivers (P. Dalton, personal communication, 1973). Water supplies and other methods of control It is not expected that household water supplies alone will lead to a complete cessation of transmis- sion. As far as is known, however, no single method of control will do so. Snail control has in the past been considered the most effective means of reducing transmission of schistosomiasis (4), but this method would appear to require unending application of molluscicides and to provide no other benefits than the control of the snail intermediate host and reduced transmission. Other aquatic organisms are killed along with the snails, and the long-term ecological effects of mollusciciding are still unknown. Chemotherapy, whether used en masse or for the treatment of infected persons only, will probably reduce the risk of severe disease in many persons by reducing, if not eliminating, their worm load. Never- theless, this method requires continued vigilance for infected immigrants, and the long-term effect of treatment on transmission has yet to be demon- strated, although the immediate effects are, of course, reduced prevalence and intensity of infection. The provision of a good, reliable, and convenient source of water as a substitute for the infective rivers and streams not only reduces transmission of S. man- soni but also benefits the population in other ways. As stated by Horwitz (quoted by Wolman & Bosch, 10): " If a single program were chosen which would have the maximum health benefits, which would rapidly stimulate social and economic development, and which would materially improve the standard of living of people, that program would be water supply with provision for running water into or adjacent to the house." Although the capital cost of water supplies, and particularly of an individual household supply, may be considered high, such a system represents a capital 18 CONTROL OF SCHISTOSOMA MANSONI asset to any community-one that can be instru- mental in reducing gastroenteritis in infants, typhoid, skin diseases, etc., and generally leads to a cleaner and more healthy population. The recurrent costs of the household water sup- plies project in the Riche Fond Valley are lower than the annual costs of a mollusciciding operation in a nearby valley, and after a few more years the overall costs of the water supplies will probably be no greater than those of mollusciciding. Public health administrators in many areas where schistosomiasis is endemic have yet to be convinced of the public health importance of the disease and are therefore unenthusiastic about committing limited financial resources to a mollusciciding campaign. They may be more willing to provide the necessary finance for a control measure such as the provision of household water supplies, which reduces the prevalence not only of schistosomiasis but also of other diseases. Whether the water should be provided free is a matter for local decision, but a small charge may not be unacceptable and would help meet the recurrent costs. Emphasis appears to be placed on schistosomiasis in relation to irrigation schemes, where, in many instances, the disease can rightfully be termed " man made." In such schemes there is no shortage of water. Accordingly, consideration should be given to using some of the water for providing adequate public standpipes (if an individual household supply is thought too sophisticated a system) and adequate laundry and showering facilities. The St Lucia scheme provides 1 laundry tub and 1 shower per 12.5 houses; with a public standpipe system a ratio of 1 tub and 1 shower to 10 houses might be more appropriate. It should be noted, however, that in view of the increasing transmission in the com- parison area, the public'standpipe system installed there in 1969 can have had little, if any, effect on transmission. Household water supplies are not advocated as a control method in every endemic area. Nevertheless, in some areas the provision of adequate, safe, and convenient water should be considered as an alterna- tive or as a supplement to other forms of schisto- somiasis control. ACKNOWLEDGEMENTS Thanks are due to Dr C. White, Department of Epidemiology and Public Health of the School of Medicine, Yale University, who acted as statistical consultant and suggested many of the methods for analysis of the results reported here. The Research and Control Department is supported by the Government of St Lucia, the Rockefeller Foundation, and the Overseas Development Administration, London, UK, Scheme R.2108 A-C. RItSUME LUTTE CONTRE LA TRANSMISSION DE SCHISTOSOMA MANSONI PAR INSTALLATION DE L EAU COURANTE DANS LES HABITATIONS A Ste-Lucie, dans la vallee de Riche Fond oui les infections A Schistosoma mansoni sont endemiques, cinq villages d'une population totale d'environ 2 000 per- sonnes ont et dotes en 1970-72 d'installations d'eau courante pour chaque habitation ainsi que de buanderies- douches publiques et de piscines d'un modele simple (secteur EC). Six autres villages de la meme vallee, ou des postes d'eau publics avaient ete installes en 1969, ont servi de secteur temoin. Pour reunir des donnees de base sur la prevalence, l'incidence et l'intensite des infections A S. mansoni, des echantillons de selles avaient et6 recueillis dans les deux secteurs A l'occasion de tournees de porte-a-porte faites de 1968 A 1970; les premiers postes d'eau courante ont ete installes dans un des villages en 1970. L'etude s'est poursuivie en 1971 et 1972, periode pendant laquelle l'eau courante a et6 install6e dans les quatre autres vil- lages, ainsi qu'en 1973, annee ou les cinq villages du secteur EC etaient donc desservis depuis une A trois annees. Auparavant, tous les indices d'infection etaient legere- ment plus eleves dans le secteur EC, mais les variations d'une ann6e A l'autre y etaient a peu pres les memes que dans le secteur temoin. A la fin de l'ann&e 1973, tous ces indices avaient diminue dans le secteur EC alors qu'ils avaient augmente dans le secteur temoin, ou ils se situaient desormais a un niveau plus eleve que dans le secteur EC. Dans des cohortes d'enfants de 0 a 13 ans suivies pendant deux annees avant le debut du programme 19 20 P. JORDAN ET AL. (1968-70), la proportion de sujet infectes avait augmente sensiblement dans les deux secteurs. En suivant des cohortes de meme age, de 1971 A 1973, les auteurs ont constat6 une augmentation notable de la proportion de sujets infectes dans le secteur temoin alors que cette proportion n'augmentait pas dans le secteur EC. Un approvisionnement commode et suffisant en eau salubre, doubie d'une campagne d'6ducation sanitaire, a ainsi permis de r6duire considerablement la trans- mission de l'infection A S. mansoni parmi les enfants du secteur EC; d'autre part, l'installation de postes d'eau dublics n'a pas suffi A empecher une augmentation sen- sible de la transmission dans le secteur temoin. Les depenses d'equipement entrainees par l'installation de 1'eau courante dans chaque habitation sont sans doute elevees, mais les frais d'entretien du systeme n'ont pas depass6 le couit de la campagne de traitement des eaux par les molluscicides dans une vall&e voisine. II semble donc que I'amenagement d'un reseau de distribution d'eau, outre les nombreux autres avantages qu'il pr6sente, est une mesure plus indiqu6e en sant6 publique que les campagnes de traitement des eaux par les molluscicides, qui sont de duree indefinie et n'ont d'effets sp6cifiques que contre les agents de la schistosomiase. REFERENCES 1. BARBOSA, F. S. ET AL. Transactions of the Royal Society of Tropical Medicine and Hygiene, 65: 206- 213 (1971). 2. BELL, D. R. Bulletin of the World Health Organiza- tion, 29: 525-530 (1963). 3. FAROOQ, M. & HAIRSTON, N. G. Bulletin ofthe World Health Organization, 35: 331-338 (1966). 4. MCMULLEN, D. B. Biological and environmental control of snails. In: Ansari, N., ed. Epidemiology and control of schistosomiasis (bilharziasis). Balti- more, University Park Press, 1973, p. 533. 5. PITCHIoRD, R. J. South African medical journal, 40 (suppl. Oct.): 14 pp. (1966). 6. PITCHFORD, R. J. South African medical journal, 44: 475-477 (1970). 7. Scorr, J. A. American journal of tropical medicine, 22: 647 (1942). 8. SNEDECOR, G. W. & COCHRAN, W. G. Statistical methods, 6th ed. Ames, Iowa State University Press, 1967, p. 253. 9. STURROCK, R. F. International journal for parasitol- ogy, 3: 175-194 (1973). 10. WOLMAN, A. & BOSCH, H. US water supply lessons applicable to developing countries. In: White, G. F., ed. Water, health, and society. Bloomington, Indiana University Press, 1973, p. 230.