Schistosomiasis in north-western Ghana

SCHISTOSOMIASIS Schistosomiasis in north-western Ghana G. R. L. LYONS' A survey of8 274 people in the Ghana-2101 project area showed that 12% were passing ova ofSchistosoma haematobium in the urine, the infection rate rising to a peak of34% in males 15-19 years of age. S. mansoni, despite the wide distribution of its potential intermediate host, was not encountered in 1 698 boys examinedfor it. Urinary schistosomia- sis in northern Ghana isfocal in character and is usually contracted in standing water during the dry season. A method of control was developed that depends on the identification of localities subject to relatively intense and prolonged transmission, followed by dry season mollusciciding of the water sources in each locality infested with the snail hosts. Two such control cycles were carried out in 30 localities. The results suggest that selective, dry season, focal control of schistosomiasis can be effective in reducing transmission. The schistosomiasis pilot control project known as Ghana-5 (later Ghana-2101) was a joint Ghana Government-World Health Organization exercise that included, as a specific objective, the determina- tion of the most effective and economical means of controlling schistosomiasis under Ghanaian conditions. The project was established at Wa in north-western Ghana in 1962, and its devel- opment up to 1965 has been summarized by McCullough.a Before the pilot control scheme was set up an epidemiological survey of the human host was car- ried out in 1967-68 to determine the prevalence and distribution of urinary and intestinal schistosomiasis in the project area. Part I of this report describes the survey and Part II the control trial. I. EPIDEMIOLOGICAL SURVEY The project area as described by McCullough a was modified in 1966 by the elimination of about 40% of its surface area; Wa town was also excluded but remained the headquarters of the project. It is estimated that the revised project area covers some 1166 km2 and at the time of the survey it contained approximately 39 500 people living in 118 villages (34 per km2). In its revised form it lies between 9048' and 10°7' N and 2026' and 2°45' W. There are only 4 towns with more than 1000 inhabitants; most of the population lives in villages of 100-300 inhabi- tants. It is a purely agricultural area, the crops being those usually associated with the Guinea savannah of West Africa of which it is typical. The climate is characterized by a long dry season from November to March or April. The average annual rainfall is 110.5 cm, most of which occurs in August and September. After reviewing existing data on the prevalence of urinary schistosomiasis in children in this area, and taking into account the time and staff available, it was decided to examine a 25% single-stage sample of the population of the project area. The sample was obtained by random selection of villages, their popu- lations being accumulated until the number of in- dividuals just exceeded 25% of the estimated total population of the project area. In the case of villages with more than 500 inhabitants only 50% were accumulated and every second house was visited; otherwise all the occupants of every house were examined. In all, 43 of the 118 localities were represented in the survey. METHODS Urine examination A preliminary study had confirmed that, as else- where, a slightly higher yield of urine samples posi- 1 Formerly Team Leader, Ghana-5 Project, Wa, Upper Region, Ghana. Present address: c/o Barclay's Bank, Ltd, 153, Sloane Street, London, S.W.I., England. a McCullough, F. S. (1965). Unpublished assignment report AFRO/BILHARZ/12. 3308 - 621 BULL. WORLD HEALTH ORGAN., Vol. 51, 1974 G. R. L. LYONS tive for S. haematobium could be expected if the examinations were carried out in the early afternoon. However, since the survey coincided with farming activity there was no alternative but to collect samples as early in the day as possible; most of the samples were obtained between 06 h 00 and 08 h 00. Qualitative examinations only were made: a single sample of 10 ml of urine was centrifuged on a hand centrifuge for 5 min and the terminal drop, after removal of the supernatant, was examined for ova. Note was taken of the presence or absence of macroscopic blood and, in the final preparation, of red blood cells. Repeated daily urine examinations of a group of children by means of the method described above but with the addition of egg-counts have shown that on average 78% of infected (i.e. ova-positive) persons are detected by a single urine examination, and that the probability of being detected increases with the intensity of infection (G. R. L. Lyons, unpublished data, 1969). Stool examination Initially faecal specimens were taken from all boys up to 14 years of age, but owing to lack of time this was later restricted to every second boy in this age- group included in the urine survey; in all, 1 698 stools were examined. The specimens were obtained under strict supervision and, like the urine samples, were examined in the field. Approximately 1 g of stool was suspended in 10 ml of normal saline. After centrifugation for 2 min the supernatant was replaced with fresh saline and the sediment was again suspended by stirring and then recentrifuged. The process was repeated until the supernatant had become clear; a drop of the final sediment was then examined. This method, although it is not outstandingly sensitive, was selected on its reputation for being simple, reliable and rapid (1). RESULTS Urine examinations Representativeness of the sample. Urine samples were obtained from 8 304 people in the course of the survey; for 30 of these the information entered on the record forms was deficient in some respect and they were rejected. The remaining 8 274 individuals repre- sented a 20.9% sample of the estimated population of the project area. The age and sex composition of the sample is shown in Table 1 in the same age-groups as those of the national census, together with the 1960 census figures for the Wala Local Council Area to which the project area belongs. Not surprisingly, since it is easier to obtain census data in respect of small children than it is to obtain urine samples from them in the field, both males and females less than 1 year old were underrepresented; a number of older people were also apparently omitted. The disparity between the sample examined and the population as a whole was, however, most marked at ages 1-4 and 5-9, and it would appear that the ages of some children under 4 years may have been over-estimated. Table 1. Age and sex distribution of the sample examined compared with that of the Wala Local Council Area (1960 census) Males Females (years) sample examined Wala Local Council sample examined Wala Local Council No. % No. % No. % No. % <1 85 2.0 2 607 4.0 92 2.3 2 722 4.1 1-4 419 9.8 10900 16.9 422 10.6 11 136 16.8 5-9 1 180 27.5 11 425 17.7 818 20.5 9740 14.7 10-14 606 14.1 6 927 10.7 341 8.6 5 014 7.5 15-24 465 10.8 7 869 12.2 517 13.0 9 796 14.7 25-44 936 21.8 15815 24.5 1 385 34.8 21 622 32.5 45-64 515 12.0 6 450 10.0 382 9.6 4 828 7.3 >65 85 2.0 2525 3.9 26 0.7 1 597 2.4 total 4 291 100 64 518 100 3 983 100 66 455 100 622 SCHISTOSOMIASIS Table 2. Age and sex distribution of infections with S. haematobium Males Females Total (yeas No: No. No.(years) examin- positive % examin- positive % examin- positive % ed ed ed >4 504 10 2.0 514 8 1.6 1 018 18 1.8 5-9 1 180 106 9.0 818 81 9.9 1 998 187 9.4 10-14 606 182 30.0 341 56 16.4 947 238 25.1 15-19 284 97 34.2 235 63 26.8 519 160 30.8 20-24 181 54 29.8 282 61 21.6 463 115 24.8 25-29 233 59 25.3 474 71 15.0 707 130 18.4 30-34 231 33 14.3 375 25 6.7 606 58 9.6 35-39 259 25 9.7 278 21 7.6 537 46 8.6 40-44 213 13 6.1 258 14 5.4 471 27 5.7 45-64 515 27 5.2 382 16 4.2 897 43 4.8 >65 85 3 3.5 26 1 3.8 i11 4 3.6 total 4291 609 14.2 3983 417 10.5 8274 1 026 12.4 Age and sex distribution of infected persons. In all, 4291 males were examined of whom 609 (14.2%) were found to be infected with S. haematobium; of 3 983 females 417 (10.5 %) were infected. The preva- lence in the entire sample amounted to 12.4%. In Table 2 the age distribution of infection with S. haematobium is shown for each sex in 5-year groups to age 44, and for those aged 45-64 and > 65 years. The curve of infection with age resembles in its general form those obtained in other surveys of S. haematobium. Blood visible to the naked eye was recorded in only 17 urine samples (14 of them in boys 8-15 years of age) and each case was associated with the presence of ova of S. haematobium; this represents 1.7% of all positive urine samples. Red blood cells were present in 2 368 (28.6%) of the 8 274 urine samples. In only 1 026 (43.3%) of these were ova of S. haemnatobium also seen. This finding reflects the presence of the numerous light infections which are typical of much of the project area, as well as the error involved in diagnosis by the examination of a single urine sample. Variability of prevalence rates between villages. Great variation in prevalence was observed between the villages studied. When classified by the per- centage of autochthonous infections, no transmis- sion was apparently taking place in 7 of the 43 villages and in 21 others the autochthonous preva- lence was less than 10%. Prevalence as related to source of water. The diversity of water sources used at different times is characteristic of the area and obscures the risk of infection associated with each. Of the 1 277 in- dividuals using only rivers, streams and pools 218 (17.6%) were infected; of those using dams only 51 out of 627 (8.1 %) were infected; and of the 161 people who relied on ponds, shallow wells or bor- row-pits for all purposes 14 (8.7 %) were infected. Of 1 506 individuals with access to bore-holes or deep wells only 5 relied on them exclusively; consequently the prevalence of S. haematobium in those who used a bore-hole or deep well was not significantly lower than in those without such an amenity. The reservoirs, most of which had been built within the previous 15 years, were present in 11 of the localities. Most of these appeared to offer un- stable conditions for the snail hosts and, although several had become infested with Bulinus (Bulinus) truncatus rohlfsi, the snail had disappeared from others. There is no doubt that at the present time the great majority of infections with S. haematobium are contracted through contact with rivers and streams at points other than where they have been impounded. As the dry season progresses the larger seasonal rivers contract into long pools, some of which 623 624 G. R. L. LYONS contain water until the beginning of the following rainy season; B. (B.) truncatus is collected in the pools from about November to May. B. (Physopsis) globosus is typically associated with the smaller seasonal streams, many of which have a steep, step- like profile; they flow rapidly when heavy rain falls but soon revert to a chain of narrow pools. B. (P.) globosus is responsible for most of the active trans- mission in the area between the end of the rains and the drying up of these smaller pools. The data from snail sampling, as well as the present survey, have continued to support the opinion of McCullough a that in this part of its course the Black Volta, which forms the western boundary of the project area and is the only perennially flowing river, is not associated with schistosomiasis transmission. Prevalence as related to activities involving water contact. In order to facilitate the comparison of prevalence rates in those who were exposed to infection in the course of each activity with those who were not, the findings which follow are based on the 6 341 people who had contact for any purpose with the water of rivers, streams, or pools. Almost all the people who use the rivers obtain water from them for washing as well as for drinking, so that it has not been possible to assess the relative risk associated with these two activities. The overall infection rate was significantly higher in those who swam (or bathed) in streams, rivers, or pools than in non-swimmers of both sexes. The different age-composition of the 2 groups invalidates the overall comparison, however, and when the data are restricted to those aged 10-29 years there is no significant difference for either sex. This is partly due to the scattered distribution of the snail hosts which allowed many of the waters to be used for swimming without risk of infection, and partly to the fact that in many villages swimming in the pools was pro- hibited during the dry season. There was some indication that fishing exposed adult males to an increased risk of infection; the dry- season use of the purse-seine net and the practice of suspending nets between stakes sunk in the mud of the larger pools involved the greatest exposure to infected water. Significantly higher rates of infection were recorded among those involved in the dry- a McCullough, F. S. (1965). Unpublished assignment report AFRO/BILHARZ 12. season activity of fetching water and mixing clay for building blocks and earthenware utensils. Prevalence as related to size of community. A significant inverse relationship was found between the village size (i.e., population) and prevalence of infection with S. haematobium, the infection rates in the villages of < 100, 100-249 and > 250 inhabitants being, respectively, 20.7%, 16.0%, and 8.9% (X2 = 128, P< 0.001; 2 d.f.). It was suspected that this finding might be related to the fact that reservoirs, bore-holes and deep wells were more readily avail- able in the larger communities. However, examina- tion of the data shows that the relatively low pre- valence in localities having > 250 inhabitants existed whether safe water was available or not, and in the smaller villages the prevalence was actually higher where there was such a facility. Prevalence as related to ethnic group. Of the principal ethnic groups represented in the area much lower infection rates (5.5 %) were found in Walas than in Lobis (14.7%) or Dagartis (13.9%). This finding is undoubtedly related to the fact that 85% of the Walas live in the larger villages and towns with which lower infection rates are associated, compared with 41 % of the Lobis and 38% of the Dagartis. Prevalence as related to school attendance. There were no significant differences in infection rates between children who attended school and those who did not. Stool examinations None of the 1 698 stools examined contained ova of S. mansoni. The absence of S. mansoni infections in the 43 villages studied is noteworthy in view of the fact that the potential vector, Biomphalaria pfeiffieri gaudi, is present in large numbers in some of the larger seasonal rivers, attaining its maximum density in pools during the late dry season. The reason for the failure of S. mansoni transmission to become established was not fully investigated but it may be noted that, since compounds are not built on the flood-plains of these rivers, the dwellings are gen- erally at a considerable distance from the infested habitats during the dry season and the likelihood of faecal contamination of the water is correspondingly reduced. Furthermore, even were defaecation to occur on the banks of the river the absence of rain during the period at which Biomphalaria is most abundant would reduce the probability of viable eggs being washed into the water. The transfer of ova of SCHISTOSOMIASIS S. mansoni to water may take place in several other ways, however, and since scattered foci of this parasite are already present in northern Ghana it is probable that at least localized transmission will occur in the future in an area where the snail hosts are so widely distributed. II. CONTROL TRIAL McCullough (2, 3) and Odei (4, 5) noted that the local snail hosts of S. haematobium were most abundant at the end of the rains or during the early dry season (October-December); Odei (4) considered that the risk of human infection was greatest in the dry season. More recently monthly observations on cercarial infection rates in the snails have confirmed that transmission takes place mainly at this time, but that in suitable habitats it may commence as early as September. Since most infections are contracted in standing water during the dry months it was con- sidered that the application of molluscicide at this time would be the logical and most economical means of interrupting transmission in the selected localities. The project area originally comprised the 4 sectors shown in Fig. 1. In 1966 two of these were discarded because of a lack of schistosomiasis cases. Of the 2 remaining, sector II, consisting of 84 villages and representing about two-thirds of the surface area and total population, was used for the control trial; the adjacent sector IV, with 34 villages, was used for comparison. In 1968 and 1969 urine surveys were carried out in all the villages of both sectors, only boys 5-14 years of age being examined. In these and all subsequent surveys egg-counting was included, the method being essentially that of Jordan (6). The place of birth of each child was recorded, as well as his movements prior to the survey. With this information it was possible to establish with reasonable accuracy the prevalence of autochthonous infections, an autoch- thonous case being defined as one in a child born in the locality concerned who had not lived elsewhere for more than one week. In general, it is true of the project area that those who leave it do so for long periods; those who move within the area normally return within a week. It was decided that, to qualify to be controlled, a locality should have a minimum of 5% of autochtho- nous cases among the boys aged 5-14 years, together with a minimum arithmetic mean egg-count in these cases of 40 eggs of S. haematobium per 10 ml urine. The data from the urine surveys indicated that at these levels a number of villages with insignificant Fig. 1. Boundaries of original Ghana-5 project area and water courses and dams in the two sectors which comprise the revised area. degrees of transmission could be excluded from the control programme, as well as those where there was apparently none. Of the 84 villages of sector II 30 qualified to be treated; 16 of the 34 villages in sector IV fulfilled the same criteria and were there- fore regarded as comparable. METHODS The identification of water sources In 1967 sector II was photographed by an aerial survey team then working in northern Ghana. Two _i, Seasonal rivers with permanent pools \- Temporary streams . Dams ,,,,, Marsh 0 5 10 WIO 75105 625 G. R. L. LYONS sets of black-and-white photographs were supplied to the project (scale 1: 10 000), one of which was issued to the field teams in the form of locality mosaics. Having identified the available sources of water in each of the localities to be treated it was then necessary to confirm by questioning and observation which sources were actually in use at the time of the teams' visits. The identification of snail infested habitats Since infection rates in snails are generally low the presence of either of the local intermediate hosts (B. (P.) globosus or B. (B.) truncatus) rather than the finding of infected snails was the indication for initiating control in the 30 selected localities. All static bodies of water in use in each of the 30 villages were sampled monthly. The sampling implement was a long-handled sieve. Restricted habitats (less than about 200 m in circum- ference) were sampled in their entirety at the rate of 3 dips per m of circumference; in dams and large riverine pools only the water contact points were sampled and treated. In this case the bank was sampled for a distance of 20 m on either side of the contact point, the rate of dipping being increased to 10 per m to compensate for the generally lower snail densities. The collectors sampled as far from the bank as the depth of the water permitted. The total habitats/contact points sampled monthly ranged from 174 to 375. Application of the molluscicide The molluscicide used was niclosamide 70% wet- table powder. It was applied with Hudson X-pert pressure sprayers in the nozzles of which were inserted metal disks with a central aperture of approximately 1.5 mm. Ponds and smaller riverine pools (up to ± 200m in circumference) were treated with sufficient chemical to provide a concentration of 0.5 mg/litre. In the large habitats the concentration was increased to 1 mg/litre and the chemical was applied for a distance of 40 m on either side of the contact point, the treatment being repeated if and when sampling showed that the snails had reappeared within 20m of the contact point. The large habitats were sprayed to a distance of 10m from the bank or to the outer edge of the emergent vegetation, whichever was the greater distance; inflatable rubber boats were avail- able for measurement of water volume and mol- luscicide application when necessary. Only when the emergent vegetation was so thick as to interfere seriously with the contact of the mol- luscicide with the water was it cut off to surface level before spraying. Public acceptance of the control measures was generally good, and only towards the end of the dry season was it necessary on a few occasions to suspend sampling and treatment of water in villages relying on a single source. EVALUATION Operational evaluation Two cycles of control were carried out, from October to April 1969-70 and from October to May 1970-71. The follow-up data for the treated habitats are summarized for both cycles of control in Table 3. Of the 41 habitats requiring retreatment in either cycle, reinfestation was due to flooding from up- stream at the onset of the rains (and possibly the emergence of some aestivating snails) in 31; in 3 rain fell within a few hours of molluscicide application and flooded the habitat. Of the remaining 7, 5 had extremely dense vegetation which prevented ade- quate penetration by the molluscicide (in 3 of these the snails were nevertheless absent from the contact points for 2 months), and in 2 it is thought that subsurface movement of the water caused loss of the chemical downstream. In general, the results indicate a high level of operational efficiency and justify the decision to restrict mollusciciding of the larger habitats to those points at which the population had contact with the water. Epidemiological evaluation Evaluation was based on the examination of boys 5-14 years of age in 3 groups of localities, as follows: a. The 30 treated villages of Sector II. b. The 16 untreated villages of Sector IV. c. 27 (i.e., 50%) of the villages of sector II which did not qualify to be treated; these last, which were selected at random, were included with the object of observing any change in their epidemi- ological status over the years. The age distribution of the boys examined in 1969, 1970, and 1971 is shown in Table 4. Pre- and post- control data in respect of prevalence (total and autochthonous), egg-counts of the autochthonous cases, the annual incidence of new cases, and the rate of reversion from positive to negative are shown in Table 5. The following points should be noted: 626 SCHISTOSOMIASIS Table 3. Follow-up data for the treated habitats; volume of water treated; and consumption of niclosamide in each cycle Control cycle Type of habitat 1969-1970 1970-1971 No. habitats No. requiring No. habitats No. requiring treated 2nd application treated 2nd application large riverine pools (contact points) 93 17 73 3 a dams (contact points) 1 0 7 0 small riverine pools (less than approx. 200 m. in circumference) 61 10 74 a ponds b 6 0 4 0 " pockets c 1 0 0 - total 162 27 (16.7 %) 158 14 (8.9 %) total molluscicide applications 189 174 total volume of water treated (m') 43 120 20 112 total consumption of niclosamide (kg) 53 26 a One required 3 applications. b Natural or man-made depressions unconnected with watercourses. c Stagnant inlets at margin of flowing water. 1. The annual incidence of new cases has been calculated from the formula: Incidence = 1-2/y), where x = the proportion of boys negative at the beginning of the year who were still negative at the end of it, and y = the average interval in months between the examinations (7, 8). 2. In calculating incidence and reversion rates no account was taken of movements during the previous year since a negligible number of boys had stayed outside their villages and then returned in time for reexamination. 3. The egg-counts were based on the number of eggs seen in a 50-mm3 sub-sample taken after centrifuging 10 ml of urine and removing the super- natant fluid to 0.2 ml (or, if the deposit was copious, to 0.3 or 0.4 ml). The arithmetic mean count was calculated on confirmed cases only, the geometric mean count was calculated on all boys examined, a x + 1 transformation being applied to accom- modate zero counts. 4. Of necessity all urine samples were collected between 06 h 00 and 08 h 00 and were examined in the field. The pre- and post-control counts were unavoidably performed by different personnel who were, however, trained and supervized in the same manner. The routine microscopists, on whose results prevalence and incidence were calculated, remained unchanged throughout. The data from the three groups of villages are considered separately below. Data from the treated villages of sector IL An attempt was made to maintain the size and age composition of the groups. Boys who had reached the upper age limit were replaced with the new 5- year-olds. However, boys of other ages dropped out for various reasons and it was impossible to keep the age composition constant. Considering separately the boys 10-14 years of age in the treated villages, among whom there was no signi- ficant change in age structure, a decline in prevalence occurred from 48.5% in 1969 to 41.8% in 1971, a reduction just short of significance at the 95% level of probability (X2 = 2.8, 0.05 <P < 0.10). A very satisfactory fall in the incidence of new infections followed the first cycle; although this was consolidated it was not much improved upon by the second cycle. The residual incidence in 1970-71 was attributable to: (i) false negatives in 1970-8 of the 30 " new " cases in 1971 already presented red blood cells without eggs in the urine in the 1970 examina- tion; (ii) early cessation of the rains in 1970 which 627 628 G. R. L. LYONS i O | x0) 0| z b ss i "It O oq C -|0 c0 0 0 00 00 C0 0)x C'N N 0 CIo 0 0 ( N o0 E! O Z E O0 CX~~~P 9) CO CO CO t .1 C4 0 Etx_Xa D DO = 0Eo Nx 10w o 00(O 10DO oN o Z U O CD(0 10 N o. 0 10 ) 0 0 O0 P.: N4 O COO o O 0'> in L0 _ n 0 D O CD X en 0 co10_ , c0 m-1 NO -E CD o0(on wN _cO N (01 C')r ) 00_ - oo 0_0 CO N '- 0 N C4 0o) .R X 0) C N - N N N -- N 00 N 00 NOl) N 1-N %N D C10 C'O -Z co 10 0 o o0 1.0 N ~10 )N 1 0 C') ( 000)0 N6 00i N CY) u >. 0EW LD q I)N (0 N N '-1000 0 0) 10 ED 0 100 N -C') 00( N6 0(0 o O~~ (0 ~ C') (01010 C) co)C) ( 0.0 coN .0 0 10C) N N (0~ 0 C' m C co o' ( '- C '- ' -4 (00(06 o 'r N 1)0100 N 0)-0 - 05 00001m N N 00100 qt 10 C - Zc (0c 0) 0 _ N C' Nt N CO 10N 10 0 O~~~~ (D O C') ~~~~~~ 0100) ~~~ N C' co00 E %-~~~~~~ CL Z NrO0 Nr- ~~~~0 0) '~~~~~~~~~~~~~~~~~(C~~~~~~~~~~~~~~~~~ SCHISTOSOMLASIS 629 Table 5. Pre- and post-treatment examinations of the indicator-group: summary of results in the treated villages of sector 11, the untreated comparative villages of sector IV, and the untreated villages of sector 11 Treated villages Untreated villages Untreated villages of sector II of sector IV of sector I1 1969 1970 1971 1969 1970 1971 1969 1970 1971 Prevalence total No. examined 716 674 651 508 461 402 596 582 555 No. positive 274 214 189 165 166 129 56 49 38 percentage 38.3 31.8 29.0 32.5 36.0 32.1 9.4 8.4 6.8 autochthonous No. examined 606 609 576 464 428 368 465 488 478 No. positive 235 192 173 153 155 119 38 31 24 percentage 38.8 31.5 30.0 33.0 36.2 32.3 8.2 6.4 5.0 Egg-counts, autochthonous No. of counts 197 190 172 133 152 119 34 31 24 arithmetic mean count 228 75 134 134 80 83 16 33 19 geometric mean count 1.892 1.026 1.119 1.221 1.292 1.199 0.090 0.101 0.075 Incidence rate a No. negative in year indicated 315 346 380 278 232 239 461 455 452 No. negative in previous year 397 380 410 349 286 255 482 478 460 average interval in months 11.3 12.1 12.0 11.8 11.7 12.1 11.4 12.3 12.0 incidence (% per year) 21.8 8.9 7.3 20.7 19.3 6.2 4.6 4.7 1.7 Reversion rate a No. positive in year indicated 194 193 172 94 122 129 37 30 29 No. positive in previous year 221 233 199 112 143 147 54 45 37 average interval in months 11.2 12.1 12.1 11.7 12.3 12.2 11.4 12.5 12.1 reversion (% per year) 13.1 17.0 13.5 16.5 14.4 12.1 32.9 32.3 21.5 a Since a negligible number of children had lived elsewhere in the interval between examinations no account has been taken of move- ments in the calculation of incidence and reversion rates. allowed transmission to become established briefly Table 6. Comparison of the variances of the geometric before the second cycle commenced; (iii) a few cases distributions of egg-counts of the boys in the treated possibly becoming infected at isolated foci remote villages in 1969 and 1970 from the village. None of the new cases in 1971 had lived elsewhere during the previous year. Year of Sum Degrees Variance F The geometric mean egg-count fell sharply after observation of squares of freedom the first control cycle. A comparison of the within- sample variances showed that the distributions in 1969 429.6938 567 0.5447 1969 and 1970 were not homogeneous, so that the 1.86 significance of the fall in the mean count cannot be 1970 234.1456 606 0.2922 assessed (Table 6). Nevertheless, the change in the 4 G. R. L. LYONS distributions was due to a reduction in the higher counts and this in itself is suggestive of a successful first cycle of control. A slight but not statistically significant rise in the geometric mean egg-count was recorded in 1971. Owing to the marked fall in the geometric mean in 1970, and also to the need to exclude the possible influence of changed age composition and the de- parture from the area ofsome boys with high counts, the data of 1969 and 1970 have been reworked using data only from those individuals who were present in both years. -Despite the upward shift in the average age of the group in the interval, the mean arithmetic and geometric counts fell from 216 to 88 and from 1.710 to 1.252, respectively; once again the geometric distributions were heterogeneous. Data from the untreated comparative villages of sector IV. The indicator group in these villages underwent a similar change in age composition. The prevalence and geometric mean egg-counts of the autochthonous cases remained virtually unchanged after 2 years, but there was a dramatic drop in the incidence of new cases. Data from the untreated villages of sector II. In these villages, in which transmission was believed to be minimal or absent, prevalence, egg-counts, and the incidence of new cases all tended to decline. Also, total prevalence always exceeded the preva- lence of autochtonous cases. The decline in the rate of reversion from positive to negative in 1970-71 seems to be inconsistent with the rest of the findings but may be due to the relatively small number of positives available for reversion. DISCUSSION For a control scheme such as that described above three procedures are necessary, namely: Location of the water-bodies in use in each locality; confirmation of the presence of the snail species locally responsible for transmission; and effective application of the molluscicide to the infested habitats. For the last of these the follow-up snail surveys provided evidence of a satisfactory degree of operational efficiency; for the first two evidence of success depends on the demonstration of a reduction in the transmission of the disease. Taken by themselves the results from the treated villages suggest that, in general, the infested bodies of water in use were correctly identified and that local transmission was effectively curtailed or terminated. However, in 1970-71 a fall in incidence also occurred in the comparative localities. It is believed that the untreated villages used for com- parison were not in fact comparable for the following reasons: a. The three or four comparative villages most productive of cases relied on shallow temporary pools during the early dry season and later resorted to wells. At such sites transmission is prolonged by heavy and prolonged rains and ceases early-or may not be possible at all-following an early ending of the rains. The incidence of new cases in the com- parative villages in 1968-1969 was much inflated as a result of the heavy rains of 1968 (151 cm, the second highest rainfall since records began in Wa in 1917), whereas 1970 produced the lowest rainfall on record in Wa (75 cm) and several known transmission sites were already dry by October. b. By setting the criteria at a minimum level of prevalence and egg-counts it was intended to separ- ate the villages in both sectors that had a schisto- somiasis problem from those in which little or no transmission was taking place. While this objective was satisfactorily achieved the criteria which were selected did not distinguish sufficiently clearly the variations in intensity of transmission which existed among the more highly infected villages. On average, the minimum values were exceeded by a wider margin in the treated than in the comparative vil- lages. Had the criteria been set at a higher level, particularly with regard to the egg-counts, the com- parative villages with unstable transmission depen- dent upon exceptional rainfall would have been eliminated and the remainder would have been more genuinely comparable to those remaining in the sector to be treated. c. In several of the comparative localities it became evident that transmission had ceased before the trial began with the result that, although the overall autochthonous prevalence and mean egg- counts qualified them as comparative villages, new infections were not appearing among the boys up to 8 or even 10 years of age. A number of pools in these villages which formerly yielded the intermediate hosts (F. S. McCullough, unpublished data) do so no longer, for reasons which are not known. Although it is accepted that the results from the comparative localities failed to supply statistical evidence of the success of the control measures in the treated villages, this fact should be kept in perspec- tive. An analysis of the results of 2 cycles of control 630 SCHISTOSOMIASIS showed that control of the snail hosts had been highly successful; an immediate reduction in the incidence of new infections was recorded in the treated villages, which was sustained over 2 years and for which there appears to be no other explanation than that it resulted from the use of molluscicides. It is considered that selective, dry season, focal control of infested habitats is the logical means of controlling schistosomiasis under the conditions of the project area, and that it deserves to be tested in other areas of the interior savannah of West Africa. The measures applied are intended to reduce the rate of superinfection, and hence morbidity, in those communities which are subject to exceptionally heavy transmission. Depending on the nature of the transmission site control would no doubt have to be applied annually in some cases, whereas in others it is probable that the effects would be more lasting and that the egg-counts would decline to a level which would justify control on a more intermittent basis. In either case chemical control is regarded as a useful palliative until more permanent measures can be introduced. The urine surveys to select the treated and com- parative villages cost approximately USS 0.30-0.40 per head of population. The cost of the control operation alone, per water treatment (USS 124 averaged over both cycles) or per rn3 of water treated (US$ 0.71), is extremely high owing to the expense of locating and sampling the water sources in use. The most useful unit cost is the cost per head of the population of the treated villages per annual cycle; this is estimated at USS 2.34 per person per annum. By substituting ground reconnaissance for the aerial photographs, by placing snail sampling on a qualita- tive rather than a quantitative basis, and by wider spacing of the visits to each locality the annual per capita cost of control might be reduced to about USS 1.50. There would, however, be a clear advantage in giving priority to the treatment of the larger com- munities, since the expense involved in mapping and sampling the water sources in use in small villages, and the time spent in travelling between them, is often disproportionately great in relation to the total number of people benefiting from a control scheme of this kind. ACKNOWLEDGEMENTS It is a pleasure to record my appreciation of the staff of the Bilharziasis Control Unit who carried out the work described in this paper, often under difficult working conditions, and particularly the technical officers in charge of the field teams, Mr G. B. A. Takyi, Mr J. Appiah, Mr A. B. Madah, and Mr D. Y. Kofi. I also wish to acknowledge the contributions of Mr S. Q. Moini, WHO Sanitary Engineer, and Mr E. A. Ashitey, Senior Technical Officer, National Malaria Services, Ghana. I am especially grateful for the encouragement and constructive criticism of Professor N. G. Hairston. RtSUM1t LA SCHISTOSOMIASE AU NORD-OUEST DU GHANA II est ressorti d'une enquete portant sur 8274 habitants de la zone du projet Ghana-2101 que 12% d'entre eux 6liminaient des ceufs de Schistosoma haematobium dans leurs urines; le taux d'infection s'elevait jusqu'A 34% parmi les hommes de 15-19 ans. Malgr6 l'abondance de son hote interm6diaire potentiel, on n'a trouv6 S. mansoni chez aucun des 1698 garcons examin6s. Au nord du Ghana, la schistosomiase urinaire revet un caractere focal et se contracte dans les eaux stagnantes, durant la saison seche. Etant donne la dispersion des mollusques h6tes et la nature transitoire de nombre des collections d'eau, les risques d'infection varient beaucoup d'un endroit a l'autre. Pour faire face a cette situation, on a mis au point une forme de lutte focale et selective. LA oiu l'on a trouv6 un minimum de 5% de cas autochtones parmi les gargons de 5-14 ans et oA la moyenne arithme- tique des nombres d'aeufs chez ces sujets n'6tait pas inf6rieure a 40 acufs de S. haematobium par 10 ml d'urine, on a pratiqu6 deux cycles d'application de molluscicide pendant la saison seche. Trente villages repondaient A ces criteres; dans une zone voisine, 16 villages y repon- daient egalement et ont ete pris comme temoins. Dans les villages soumis aux mesures de lutte, des pr6levements mensuels ont et6 effectu6s dans toutes les collections d'eau couramment utilis&es par la population et toutes celles que l'on a trouv6es infest6es par l'une des especes locales de mollusque h6te ont ete traitees au niclosamide en poudre A 70% dispersable dans l'eau. Les petits habitats ont ete trait6s sur toute leur etendue; dans les plus grands, le molluscicide n'a ete applique, A la dose de lmg/litre, qu'aux points de contact de la population avec l'eau. Les resultats obtenus sur le plan operationnel ont 6te satisfaisants. Sur les 320 habitats traites, 41 seulement 631 632 G. R. L. LYONS ont diu recevoir une seconde application de molluscicide durant la meme saison seche. D'apres la prevalence apres traitement, les num6rations d'ceufs et la frequence des cas nouveaux, on a tout lieu de croire que la trans- mission a pu etre interrompue dans les villages traites. Dans les villages temoins, la frequence des cas nouveaux a aussi diminu6 durant la seconde ann6e, pour des raisons qui sont considerees comme etrangeres a l'ex- p6rience. REFERENCES 1. WATSON, J. M. Medical helminthology. London, Bailliere, Tindall & Cox, 1960, pp. 373-375. 2. MCCULLOUGH, F. S. Annals of tropical medicine and parasitology, 51: 235-248 (1957). 3. MCCULLOUGH, F. S. Bulletin of the World Health Organization, 27: 161-170 (1962). 4. ODEI, M. A. West African medical journal, 13: 60-70 (1964). 5. ODEI, M. A. Ghana medicaljournal, 6: 120-125 (1967). 6. JORDAN, P. Periodicity of ova output and intensity of infection (S. haematobium). In: Annual report 1959-1960, East African Institute for Medical Re- search. Nairobi, Government Printer, 1960, p. 25. 7. PESIGAN, T. P. ET AL. Bulletin of the World Health Organization, 18: 345-455 (1958). 8. FAROOQ, M. & HAIRSTON, N. G. Bulletin of the World Health Organization, 35: 331-338 (1966).