Variation and stability in Schistosoma haematobium egg counts: a four-year study of Gambian children

397 TRANSACTIONS OF THE ROYAL SOCIETY OF TROPICAL MEDICINE AND HYGIENE, VOL. 72, No. 4, 1978

Variation

and stability in Schistosoma haematobium a four-year study of Gambian children

egg counts:

H. A. WILKINS AND A. SCOTT*

Division of Parasitology, National Institute for Medical Research, Mill Hill, London NW7 lAA, and Tropical Epidemiology Unit, London School of Hygiene and Tropical Medicine, Keppel Street, London WClE 7HT Summary Individual changes in egg counts of a group of Gambian children were observed over a four-year period by obtaining replicate specimens on several successive days. In a focus where transmission begins in late July and increases in intensity until it stops in October or November, the egg counts of children less than 10 years old rose between November/December and March/April, and fell during subsequent months. These apparent annual cycles of superinfection were less obvious in older children. Some children of all ages showed large changes in counts in a few months and 7% of observations over a 12-month period were of falls by over 90 %. Despite these fluctuations there was a significant degree of relative stability in subjects’ counts when compared with those of other members of the group. The observations suggest that the worm burdens of some children are in a dynamic but steady state. Among the factors regulating this may be the acquisition of a degree of immunity to superinfection. The results also prompt speculation whether man’s immune response may, in some circumstances, affect the egg-laying worms of the established infection. The role of protective immunity in the epidemiology of the infection appears complex and needs further study. While the findings are compatible with the hypothesis that concomitant immunity occurs in man, they suggest that it is unlikely to be solely responsible for the lower prevalence and intensity of infection in adults. Introduction Schistosoma haematobium characteristically shows a decreased prevalence and intensity of infection among older subjects in endemic areas which has been attributed to age-related changes in the pattern of water contact or to the gradual acquisition of protective immunity. The evidence has been reviewed at length (SMITHERS & TERRY, 1969; WARREN, 1973; WHO, 1974). Most investigations which have bearing on the issue are point prevalence studies. This approach prohibits the detailed analysis of changes in worm burden in individual subjects which is necessary for a complete understanding of the host-parasite relationship. However, MCCULLOUGH & BRADLEY (1973) have shown that individual changes in egg count can be studied by collecting daily sequential replicate specimens and have used this approach in a three-year cohort study of a group of Tanzanian They considered that their data were children.

explicable on an hypothesis of immunity to superinfection acquired bv the end of the first decade of life after which the worm burden was lost exponentially with a ‘half-life’ of several years. We report here a comparable study of a group of children in the Daru area of The Gambia The intensity and prevalence of infection is high in this focus (WILKINS, 1977a) and transmission of infection can only occur between July and November (SMITHERS, 1956). This led us to study seasonal changes in egg count. The relation of these to age, and our other findings, lead to conclusions which differ from those of MCCULLOUGH & BRADLEY (1973). Although our observations are compatible with the hypothesis that an immunity to superinfection occurs, they suggest that the dynamics of worm burden in individual subjects and the role of protective immunity in the epidemiology of S. haematobium are both more complex than the simple model described by BRADLEY & MCCULLOUGH (1973). Methods Subjects The subjects of the study were all initially resident in the villages of Daru. Abeokuta or Tandi. The age of the
398

S. haematobium

Table

I-The

number

EGG COUNTS

of sets of specimens

IN GAMBIAN

which were obtained

CHILDREN

with varying

numbers

of replicates

Number

of replicates in set

2

3

4

5

6

7

8

9

10

11

12

Number

of sets

4

1

5

12

27

43

78

206

72

37

12

-~

~

Table II-The number of subjects and their tions and the effect of observer variation

age and mean egg count in successive

Age in years No. of subjects

Range and mean

Egg count: log, ,, (ova/lOml+ Observer

April June Ott Dee April June Sept Dee

1971 1971 1971 1971 1972 1972 1972 1972

39 39 38 33 39 43 39 42

6-15 5-15 5-15 5-15 6-15 6-15 6-15 6-15

10.7 10.7 10.7 10.5 11.5 10.7 10.5 10.5

HAW X

April Nov April April

1973 1973 1974 1975

42 45

7-15 7-15 8-17 9-19

11.6 10.8 12.1 13.5

H:W HAW HAW HAW

tz

X Both HAW

Mean of all subjects

Age -~

5 6 i 9 10 11 12 13 14 15 16 17 18 19

No. of observations 230 29 2 50 57 74 2: 35 13 5 2 1

often made difficult by everyday activities which frequently involved agricultural work some distance from the village. It must be enphnsized that, as far as is known, no specific chemotherapy for schistosomiasis was received by any of the children during the course of the study. Among this community, symptoms of tract disease seldom lead to hospital urinary All the subjects strenuously denied attendance. seeking treatment which the conservative attitudes of the society make credible.

Egg count: log,, (ova/l0 Mean 2.496 2.856 2.874 2.790 2.726 2.618 2.817 2.700 2.493 2.719 2.760 2.826 2.549 2.517 3.209

1)

Within person variance

2.754 2.814 2.434 2.236 2.541 2.631 2.730 2.833 2.636 2.829 2.648 2.988 2.806

Table III-The mean egg count in subjects of different ages. All observations with the exception of those made in December 1972 by technician X (years)

sets of observa-

0.077 0.089 0.123 0.177 0.145 0.137 0.097 0.071 0.111 o-094 0.081 0.101 0.083

have been included ml + 1) S.D. 0.098 0.320 0.675 0.722 0.682 0.728 0.625 o-594 0.699 0.587 0.522 0.466 0.391 0.575 -

Collection and examination of specimens Subjects were given a wide-mouthed bottle between 12 and 2 p.m. and asked to pass a complete urine specimen into it. Although it was impracticable to obtain specimens passed under direct supervision, the issue and collection of containers was personally supervised throughout. A filtration technique was used to enumerate the ova in a 10 ml aliquot of urine as previously described (WILKINS, 1977a). Specimens of less than 10 ml were discarded.

H. A. WILKINS AND A. SCOTT

399

An attempt was made to collect nine or more replicate sp&imens on successive days during the course of each two-week visit to the area. The movement of subjects on occasion made this impossible, as Table I shows. Analysis of data The observations were transferred to punch cards before computer analysis. All egg counts were subjected to a log,, (N + 1) transformation before MCCULLOUGH & BRADLEY any further analysis. (1973) have shown that this stabilizes the variance of sets of daily replicates and it also facilitates the analysis of proportional change. Unless otherwise stated all egg count data are expressed here in this transformed form. Results Group mean egg count and individual day to day variation Table II shows the number and age of subjects from whom specimens were obtained at each visit and gives details of microscopy and its results. After the initial observations by HAW, the microscopy was delegated to an experienced and highly qualified technician who initially counted in a Subsequently, as shown comparable manner. below, it became apparent that this was no longer the case and thereafter all counts were made by the author. To determine the extent of day to day variation in the egg count of the subjects an analysis of variance was carried out which allowed for the The results are unequal number of replicates. given in Table II. The magnitude of within-person variance between days was similar to that found by MCCULLOUGH & BRADLEY (1973). Variation of egg count with age The mean egg count of the subjects of the study was similar at all ages as shown by Table III. This contrast with the results of the previous point prevalence studies (WILKINS, 1977a) is probably due to the youngest subjects only having been selected if known to be infected and also to the unusually high counts in the small sample of subjects in the oldest age groups. Variation in egg count with season Between December 1971 and Aoril 1972 the egfz counts of young children increaseh more than dyi those of older subjects. Observations over the same period in the two subsequent years showed the Since transmission only occurs same trend. between July and late October or November, with the greatest risk of infection at the end of the season (SMITHERS, 1956), this is approximately the time of year during which superinfections might become apparent. We shall term this period the incremental period and the remainder of the year During the latter, a the decremental period. converse tendency was apparent with the younger children showing a larger fall in count than the older. These effects are shown in Fig. 1 which is based on differences between the means of sets of replicates obtained from a subject at the beginning and

Fig. 1. The relationship between age and seasonal change in egg count. The difference between the mean count of the sets of replicates collected at the beginning and end of each season was found for all The mean change of the subjects, in subjects. three-year age bands, was calculated and is shown as a moving average.

end of each period. All such differences from the three years were pooled and the mean change in egg count of subjects in age bands of three years was calculated before plotting the results as a moving average. Differences over the decremental period in 1972 were calculated using the technician’s counts in December, while the author’s December counts were used for the subsequent incremental period. Fig. 1 implies that the mean egg count of a group of children under 10 years of age will fluctuate considerably during the year, while there will be much less change in that of older subiects. It should be emphasized- that this conclusiob concerns the central tendency of groups of subjects among which individuals may differ considerably as discussed below. The possible confounding effect of factors other than age in this relationship was examined using the technique of multiple regression analysis. The incremental and decremental periods were studied separately with age and age squared as predictor variables for change in egg count. A predictor term for the year of observation was included and also a term for the intensity of infection which was taken as the mean of observations at the beginning and end of the period to avoid spurious regression effects. Interactions between the terms were included. Since the precision with which the seasonal change in egg count was known varied between the subjects due to differing numbers of replicates, the observations from each subject were weighted inversely to the variance of the change of the subject. In the analysis, predictor terms were removed until only those with significant coefficients remained. These are shown in Table IV, which confirms that age is an important factor in determining the changes in egg count in both seasons. The effect is probably best considered as a non-linear

400

S. haematobium EGGCOUNTSIN GAMBIANCHILDREN

Table TV-The results of the multiple standard errors and significance level periods of the year in both models

Period Incremental

Decremental

Variable

Coefficient

regression analysis. The partial regression coefficients, of the significant predictor variables are shown for both

Linear model SE

Constant Age Age2 Year 2

0.673 -0.038 -0.224

0.159 0.015

Co;;ant

-0.437 0.040 -0.506

0.185 0.017

Age2 Year 1

0.075

0.095

P

Non-linear model Coefficient SE Significance

to*001
1.742 -0.261 0.111 -0.232

0.606 0.123 0.006 0.074

to.005 <0*05
to*05 <0.02
-2.130 0.382 -0.016 -0.513

0.692 0,136 0.006 o-092

<0*005
function but the evidence for this is uncertain in the incremental season. Table IV does not show an effect of intensity in the incremental season since none of the combinations of predictor variables showed it to be significant. Since the data was logarithmically transformed this suggeststhat the proportion by which a subject’s egg count increases is unlikely to be influenced by the initial intensity of infection, i.e. that, other things being equal, increases from l,OOO/lO ml to 2,000/10 ml are as frequent as those from lOO/lO ml to 200/10 ml or lO/lO ml to 20/10 ml.

due to changes in urine flow was also studied. We have described elsewhere seasonal changes in creatinine concentration and the relationship between day-to-day changes in this parameter and egg count (WILKINS, 1977b). Although this showed that there were difficulties in the use of the derived parameter ‘ova/mg creatinine’ we calculated this for specimens collected between December 1972 and April 1974. An analysis similar to that described above showed that the same trend in relation to age was present as with the original egg count data. The relationship therefore seems unlikely to be due to changes in urine flow.

Table V-Showing the partial regression coefficients, standard errors and significance level of predictor variables in the decremental period in a model which includes the effect of intensity of infection

Individual

Variable Constant Age Age2 Intensity Age x intensity A& ; intensity

Coefficient -12.686 2.379 -0.106 3.765 -0.715 0.032 -0.547

S.E. 4.840 0.932 0.044 1.710 0.331 0.016 0.097

P <0.02 <0*02 <0.02 <0*05 <0*05 <0*05
In the decremental season the possible effect of intensity is obscure since it was shown to be a significant predictor variable only if interaction terms were included as shown in Table V. This implies that older subjects will show greater falls if heavily infected, whereas among the younger subjects the falls will be greater in the lightly infected. The low level of significance and this unexpected interaction with age suggest that caution should be exercised before accepting this result as representative since it may reflect events in a small number of possibly atypical subjects at the extremes of the age distribution. The possibility that the relationship between ages and seasonal changes in egg count was an artefact

variation

with season in egg count

The above analyses have been concerned with the mean change in egg count of groups of subjects. Some subjects showed very large changes in a few months. Fig. 2 shows the distribution of the size of individual changes in egg count in the two seasons in two age groups. All the changes from each season were pooled after an adjustment so that they were

Fig. 2. The relation of individual changes in egg count to age and season. Individual changes were calculated and then an adjustment made so that they are shown as relative to the mean change of the group as a whole. The results from three years have been pooled.

H. A. WILKINS

401

AND A. SCOTT

relative to any change in the group as a whole during the season and year of observation. This eliminates any systematic factors which may make one year different from another, such as changes in urine flow or observer. In both age groups and both seasons some subjects showed large changes. Since the number of replicates taken and the day-to-day variation in egg count are such that most changes greater than & 0.350 are significant at the 5 % level, the figure shows that the seasonal changes varied among subjects more than would be expected if the random effects of day to day variation were the only factor responsible. Variation in egg count over twelve months The age effect appears to be of equal magnitude and opposite direction in the incremental and decremental periods which suggests that change over a 12-month interval will probably be much less influenced by the subject’s age. This was confirmed with a multiple regression analysis similar to that already described, with changes observed in each of the four successive 12-month periods beginning in March 1971 as the dependent variable. This showed the year term to be the only significant predictor variable. A relationship between age and change in egg count over a 12-month period is implicit in the results of the point prevalence studies (WILKINS, 1977a). It is likely that it was not demonstrated here due to the number of observations being inadequate to show an effect which is probably small in comparison with the differences between years and subjects. Individual falls in egg count during 12 months To enable some comparison with MCCULLOUGH & BRADLEY’S

(1973)

study,

which

showed

large

falls

in count in two of the 33 subjects, individual changes over 12 months were analysed. During the course of the study a total of 145 observations of change in an individual’s egg count over 12 months were made in successive periods beginning in March or April. Seven subjects showed one, and a further subject, two falls in log egg count by more than 1.0, i.e. to less than 10% of the initial level. Details of the changes in these subjects are shown in Table VI. Relative stability of egg count The extent to which there was stability of individual counts relative to those of other members of the group was examined using the observations made during successive March or April visits. Table VII shows there was a considerable degree of relative stability over 12 months which appears to decrease over longer periods of observation. The relative stability exists in the presence of the seasonal fluctuations as exemplified by the results shown in Fig. 3. This shows the changes in the egg counts of subjects who provided specimens on all occasions between December 1972 and April 1974. Since the microscopy was all done by the author, the data are relatively homogeneous. The synchronous fluctuations which it shows have some of the features of those observed by McCullough & BRADLEY

(1973).

402

s. huemutobh

IN GAMBIANCHILDREN

EGG COUNTS

Table VII-Showing the number of subjects followed over different periods of time and the relative stability of egg count amongst them as shown by Spearman’s rank correlation coefficient. Calculations were made with data from successive March or April specimen collections Number of years follow up Initial year of observation 1971 1972 1973 1974

n

1 R

P

37 31 36 4.4

0.717 0.452 O-692 0.636


n

;

29 0.553 31 0.318 37 0.764 -

Fig. 3. Individual fluctuations in egg output over 16 months. The means of successive sets of replicate counts are shown for subjects who provided specimens on all occasions between December 1972 and April 1974. Discussion Both the present study and that of MCCULLOUGH & BRADLEY (1973) were, of necessity, carried out with egg counts in random noon urine specimens. Changes in urine flow rates, which have been considered in studies in T anzania (MCCULLOUGH & BRADLEY, 1973) and The Gambia (WILKINS, 1977b), or in the time of peak egg excretion which has been shown to be variable (BELL, 1969), or in the technical factors, as here, make inferences about changes in total egg output from changes in the egg counts in such specimens subject to uncertainty. Speculative assumptions must be made but these can to some extent be minimised if, as below, changes are considered where possible, relative to the group as a whole rather than in individual absolute terms. Within both the Tanzanian and Gambian cohorts there was a degree of stability of individual egg counts relative to those of the group as a whole. This fInding suggested two alternative hypotheses to MCCUUOUGH & BRADLEY (1973); either that the worm burden of an infected subject is in a dynamic state of equilibrium with worms simultaneously being gained and lost, or that immunity to superinfection is established by the end of the first decade

P
n

ii

30 0.303 32 O-430 -

P NS <0*02 -

n

i

32 0.474 -

P
of life, after which the worm burden declines exponentially with a ‘half-life’ of several years. They considered the first possibility unlikely, finding it difficult to envisage the mechanism responsible for the persistent relative stability, and proposed that the second applied, thereby postulating that the experimentally derived concept of concomitant immunity was important in man. During the course of the Tanzanian study, the egg counts of the group as a whole fluctuated, being higher in the interim observations than at the beginning or end. The concomitant immunity hypothesis depends on the assumption that these fluctuations were due to changes in urine flow caused by environmental factors. We have suggested that changes of the implied magnitude are likely to be rare (WILKINS, 1977b) and others have questioned the validity of the suggested interpretation of the data for different reasons (JORDANet al., 1974). The present study supports these doubts. The younger Gambian children showed seasonal changes in egg count relative to the older children. If these reflected changes in total egg output their timing suggests the occurrence of repeated annual cycles of superinfection following which egg output falls appreciably within a few months. The statistical analysis suggests that these changes are of similar magnitude in both heavily and lightly infected subjects, if considered as a proportion of the initial value. Thus the lightly infected subjects probably acquire fewer worms during the transmission season than their heavily infected contemporaries, but during subsequent months the egg output falls by a similar proportion in many subjects. These suppositions suggest mechanisms which may account for the observed relative stability. There will be a tendency towards a persisting individual equilibrium worm burden and also towards within-group relative stability, if the number of worms acquired each year is a comparatively persistent individual characteristic and the proportion lost in a given time is similar in most subjects. In these circumstances the equilibrium worm burden will be determined by the rate of superinfection. It will be reached when the worm population is of such a size that the operation of the mortality rate among it leads to the loss of a number of worms similar to that which is gained during each transmission season. Random variation in both

403

H. A. WILKINS AND A. SCOTT

acquisition and loss will cause the real-life situation to show a variability not implied by this hypothetical model. However, in both studies the highest egg counts were over a hundred times greater than the lowest. Therefore the rank within the group of many subjects will be comparatively unaffected by changes in count by a fact& of the brder of two. Thus the observed decree of relative stabilitv is compatible with some iariation in worm burdens due to changes in individuals’ rates of acquisition and loss of parasites. The suggestion that some subjects persistently acquire more worms than others is reasonable. Clearly there may be persisting differences between individuals in water contact patterns. It is of interest that two of the persistently lightly infected subjects seemed much less keen on swimming in the transmission sites than their contemporaries. These were a deaf mute girl who seldom was seen far from home and a nervous apprehensive boy who did not like to swim with other children. It is also easy to envisage that there may be persistent differences in susceptibility to superinfection due to innate factors or to differences in the strength of any protective immunity which has been acquired. While the assessmentof the relative importance of each factor will demand further work, including very detailed individual water-contact studies, it is apparent they are not mutually exclusive possibilities. The interpretation of the findings in the older Gambian children is less clear since it is difficult to establish the extent to which they show seasonal changes in egg output. However, superinfection appears to be less frequent than in younger children. This may reflect the development of increasing resistance to superinfection and/or a decrease in water contact with increasing age. As discussed above, further studies will be necessary to investigate these alternatives in detail. Our initial impressrons are that egg counts may show little seasonal fluctuation or raoid falls in those of the older children who are’ still frequently exposed to infection. This would imply the existence of a degree of protective immunity. The older subjects as a group showed a smaller fall in count during the decremental season than the younger ones. If our suppositions about superinfection are correct this may reflect the existence of a lower mortality rate or of fall in egg output among worms which have been established in the host for more than one or two years, since such worms presumably constitute a greater proportion of the worm burden of older subjects. This, although the worm population of some heavilv infected small children mav ‘turnover’ with an appreciable proportion being -gained and lost every year, lightly infected adults in endemic areas may have a population of long-lived survivor worms which changes very slowly. The results of this study do not conclusively prove the importance of concomitant immunity in man but they are compatible with the hypothesis that a degree of resistance to superinfection is gradually acquired. As JORDAN et al. (1974) have pointed out, other evidence exists which strongly suggests that man can acquire protective immunity.

However, a major difference between the results of our studies and those of MCCTJLLOUGH & BRADLEY (1973) is the conclusion that the intensity of infection, or rate of egg production, can decline much more rapidly than they proposed. This suggests that some subjects who are in the second and third decade must still sometimes acquire further worms since, if they did not, the intensity of infection would fall with increasing age more rapidly than we have shown it does (WILKINS, 1977a). The results prompt speculation whether man’s immune response may on occasion affect the established infection. This possibility is suggested by the large falls in egg count in a few subjects and our demonstration (WILKINS & CAPRON,1977) that high levels of antibody are associated with a tendency towards a subsequent fall in egg count. Although the attentions of experimentalists have mainly been directed to the phenomenon of resistance to infection following cercarial challenge, the possibility merits consideration since there is good evidence that the adult worm can be affected by the immune response in some circumstances (SMITHERS 81 TERRY, 1969; 1970; WEBBE et al., 1976).

HOCKLEY & SMITHERS,

It is relevant to recall FISHER’S (1934) earlv consideration of the role of immunity. in the epidemioloev of S. intercdutum. He concluded. from the few animal experiments which had then been done, that “ The host is capable of an antibilharzial response which manifests itself firstly against the schistosomula of subsequent invasions and secondly against the mature parasite when there is hyperinfestation.” He distinguished between these two forms of immunity as a and ,6and anticipated much of BRADLEY & MCCULLOUGH’S(1973) model in pointing out that some of the features of schistosome epidemiology could be explained if a-immunity was strong and the worm long-lived. However, his age-prevalence studies led him to conclude that b-immunity also existed, being a closely allied phenomenon the occurrence of which was related to the intensity of exposure to infection. Further understanding of the role of protective immunity in the epidemiology of schistosome infections will need clearer evidence of the occurrence and effectiveness of a and p-immunity in relation to age and to the intensity both of existing infection and of exposure to further infection. Some clarification of the dynamics of worm burden may be obtained from the results of interrupting transmission with molluscicide. This should initially have little effect on the intensity of established infections if worms are long lived and concomitant immunity strong. On the other hand, if the situation is as complex as we suggest, egg output will fall rapidly in some of the younger age groups. There is some evidence that a rapid fall occurs. CLARK(19661 observed a fall in orevalence after two years of mollusciciding, ihich was greatest in older children in whom it was initially highest. Since control was partial, as shown by the incidence of infection in the younger children, he concluded that the older children ‘resisted’ infection and.‘overcame’ some of the existing infection, i.e. that both a and p-immunity existed. More recently, LYONS (1974) has reported a fall in egg output of I

I

404

S.haematobium

EGG COUNTS IN GAMBIAN CHILDREN

over 60% in boys aged five to 14 in the first year of intervention. We are currently making similar studies in The Gambia which we hope will test some of the speculative hypotheses discussed here. Acknowledgements We are grateful to Professor D. J. Bradley, Dr. S. R. Smithers and Dr. G. Webbe for advice and encouragement, and to Dr. I. A. McGregor and subsequently Dr. R. S. Bray for their support when Director of the MRC Laboratories at Fajara. We are indebted to the staff of the Laboratories for their help and, in particular, to Mr. Kebba Janneh and the late Mr. Lamin Manneh. References Bell, D. R. (1969). Clinical trials and diagnostic methods in schistosomiasis. Annals of th> New York Academv of Sciences. 160. 593-601. Bradley, D. J. & &Cullough, F’. S. (1973). Egg output stability and the epidemiology of Schistosoma haematobium. II. An analysis of the epidemiology and endemic S. haematobium. Transactions of the Royal Society of TroPical Medicine and Hygiene, 67, 491-500. Clarke. V. de V. (19661. The influence of acauired resistance in the epidemiology of bilhariiasis. Central African Journal of Medicine, 12 (Supplement), l-30. Fisher, A. C. (1934). A study of schistosomiasis in the Stanleyville district of the Belgian Congo. Transactions of the Royal Society of Tropical Medicine and Hygiene, 28, 277-306. Hockley, D. J. & Smithers, S. R. (1970). Damage to adult Schistosoma mansoni after transfer to a hyperimmune host. Parasitology, 61, 95-100. Jordan, P., Cook, J. A. & Davis, A. (1974). Schistosoma haematobium infection: immunity or concomitant immunity. Transactions of the Royal Society of Tropical Medicine and Hygiene, 68, 340-341. Lyons, G. R. L. (1974). Schistosomiasis in NorthWest Ghana. Bulletin of the World Health Organization, 51, 621-632.

McCullough, F. S. & Bradley, D. J. (1973). Egg output and stability and the epidemiology of Schistosome haematobium. I. Variation and stability in S. haematobium egg counts. Transactions of the Royal Society of Tropical Medicine and Hygiene, 67, 475-490. Smithers, S. R. (1956). On the ecology of schistosome vectors in The Gambia with evidence of their role in transmission. Transactions of the Royal Society of Tropical Medicine and Hygiene, 50, 354-365. Smithers, S. R. & Terry, R. J. (1969). The immunology of schistosomiasis. Advances in Parasitology, 7, 41-73. Warren, K. S. (1973). Regulation of the prevalence and intensity of schistosomiasis in man. Immunity or ecology? Journal of Infectious Diseases, 127, 596-609. Webbe, G., James, C., Nelson, G. S., Smithers, S. R. & Terry, R. J. (1976). Acquired resistance to Schistosome haematobium in the baboon (Papio anubis) after cercarial exposure and adult worm transplantation. Annals of Tropical Medicine and Parasitology, 70, 41 l-424. World Health Organization (1974). Immunology of schistosomiasis. Bulletin of the World Health Organization, 51, 533-595. Wilkins, H. A. (1977a). Schistosoma haematobium in I. The intensity and a Gambian community. prevalence of infection. Annals of Tropical Medicine and Parasitology, 71, 53-58. Wilkins, H. A. (1977b). Variation in urinary creatinine concentration and Schistosoma haematobium egg counts. Transactions of the Royal Society of Tropical Medicine and Hygiene, 71, 411-415.

Wilkins, H. A. & Capron, A. (1977). Schistosoma haematobium in a Gambian communitv. IV. Antibody levels and change in egg outpu< Annals of Tropical Medicine and Parasitology, 71,187-195.

Accepted for publication

20th February,

1978.