Exposure of Gambian children to Anopheles gambiae malaria vectors in an irrigated rice production area

Variation in exposure of children to malaria vectors of the Anopheles gambiae complex was recorded in a Gambian village situated near an irrigated area of rice cultivation. Observations were made in 1987 and 1988 during two dry seasons, when pumped water was used to grow rice, and two rainy seasons, when rice was produced using a combination of irrigated and rainfed paddies. Routine collections of mosquitoes were made from under bednets. Most of these specimens were assumed to have fed on the occupants of the net and thus represented a crude measure of exposure to malaria. Most nets in the village were in good condition, but even these were a poor defence against blood-seeking mosquitoes. Two annual peaks in the numbers of An.gambiae s.l. corresponded with the irrigation of rice paddies in the dry and wet seasons. When there were few vectors in the village the frequency distribution of mosquitoes caught under nets was described best by a Poisson process. When high numbers were present the daily distributions were over-dispersed and fitted a negative binomial model. The spatial distribution of mosquitoes varied between dry and wet seasons and was related to the predominant wind direction at night, suggesting that wind assisted the dispersal of mosquitoes from their breeding sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Within-person variation in urinary sodium, potassium and creatinine concentrations, and their relationship to changes in the blood pressure of adult male Gambians

Variation in urinary electrolyte and creatinine concentrations was studied in untimed, casual specimens obtained from 65 adult Gambian males over a 15-month period during which up to nine specimens were obtained from each subject. Measurements of blood pressure were made at the times when specimens were collected. The within-person variation in urinary creatinine, sodium and potassium concentrations were all found to be larger than that between persons. Within individual subjects, the sodium concentration tended to be comparatively low, and the potassium concentration high, in specimens in which the creatinine concentration was high. This suggests that the excretion of sodium was reduced at times of relative dehydration while the excretion of potassium may have been unrelated to the urine flow. The data also suggest that changes in the state of hydration were associated with changes in blood pressure since an individual subject's pressure was inversely related to the creatinine concentration. The negative relationship of the sodium:potassium ratio with creatinine concentration casts doubt on the simple use of this ratio as an indicator of dietary intake. After adjusting for creatinine concentration by multiple regression, changes in systolic pressure were shown to be positively correlated with changes in sodium concentration. These observations show the importance of replicating measurements of urinary electrolyte concentrations. The demonstration of complex inter-relationships between blood pressure and urinary concentrations of creatinine, sodium and potassium emphasizes the need for care in the interpretation of findings from causal, untimed urine specimens.

Reproducibility of measurements of blood pressure in a tropical environment

The reproducibility of measurements of the blood pressure of 65 adult male Gambians was investigated to assess the role that replication of measurements may have in epidemiological studies in a tropical environment. Observations were made at approximately seven week intervals over a period of 15 months and measurements were repeated three times on each occasion a subject was seen. The magnitude of the variation of an individual subject's BP between occasions, and between readings on the same occasion, was comparable with those which have been observed in studies in developed countries in temperate climates. The variation between occasions was greater than that between readings on the same occasion. There was a significant fall in systolic pressure between the first and third occasions, and a more gradual fall in diastolic pressure. The observations suggest that an individual subject's BP level would be better characterized by making a second reading on another occasion than by making three or more readings on the same occasion. These results have implications for the design of epidemiological studies and for the diagnosis of hypertension in the tropics.

Time-motion analysis of and heart rate responses to amateur ice hockey officiating

The physical activity of referees (n = 4) and linesmen (n = 6) was analyzed from videotapes of four varsity hockey games to determine the frequency, average duration, and total time spent in each of six activity categories. The average duration of activity was low while frequency of occurrence was high. Low intensity activities occupied 96% of total time for referees and 94% of time for linesmen. Heart rate responses were recorded every 5 seconds and synchronized to the videotapes for all referees and 5 linesmen. Heart rates were above 70% of maximum for 70% of the total ice time, which was greater than expected for the observed physical activity. Psychological stress during the course of a varsity hockey game could contribute to the high heart rate responses. It is concluded that training to increase aerobic fitness is an important aspect of ice hockey officiating to increase resistance to fatigue and aid in stress management.

Ability of Anopheles gambiae mosquitoes to transmit malaria during the dry and wet seasons in an area of irrigated rice cultivation in The Gambia

The seasonality of malaria transmission was studied in a Gambian village situated in an area where rice was cultivated. Observations were made during two dry seasons, when pump-fed irrigation was used to grow rice, and in the intervening rainy season, when rice was cultivated using a combination of irrigated and rain-fed paddies. Clinical episodes of malaria were mainly confined to the months during and soon after the rainy season. In the wet season the prevalence of parasitaemia was higher in febrile subjects than in afebrile controls but the reverse applied during the dry seasons. However, the biting rates of Anopheles gambiae complex mosquitoes in the two dry seasons (2.5 and 0.8 bites/child/night respectively) were greater than or similar to that in the rainy season (0.6 bites/child/night). The proportion of human bloodmeals (0.53 vs 0.75) and the survival of mosquitoes (parity rates of 0.41 vs 0.58) were both lower in the dry seasons than in the rains. The low prevalence of morbidity due to malaria in the dry season and the observed fall in the sporozoite rate may therefore have been due to a reduction in the vectorial capacity of the An. gambiae population. However, reduced transmission in the dry season may also have been due to the direct effect of high temperatures on the parasite in the vector.

Human IgE, IgG4 and resistance to reinfection with Schistosoma haematobium

A well recognized feature of the immune response to parasitic helminth infections, including schistosomiasis, is the production of large amounts of specific and nonspecific IgE1,2. Immunological pathways involving IgE can lead to damage to the developing schistosomulum and it has been suggested that responses involving IgE could have evolved as protection against helminth infections. There has been no epidemiological evidence to support this idea and the only significant IgE responses known in man are those involved in the pathogenesis of allergic disease. Here we measure serological response during reinfection with S. haematobium and demonstrate that IgE antibodies in man can be beneficial. Our results support the hypothesis that the slow build-up of IgE to high levels and the early production of IgG4 antibodies, which may block IgE pathways are responsible for delaying the development of protective immunity to S. haematobium.

Knowledge of AIDS, use of condoms and results of counselling subjects with asymptomatic HIV2 infection in The Gambia

A questionnaire administered to subjects seen during a serological survey in The Gambia revealed that knowledge of AIDS and HIV infection was limited. Males, those with a secondary education and people who lived in urban areas had a better understanding but only 17% of women seen in rural areas had any knowledge of the condition. Only 8% of the subjects seen had used condoms in the preceding 12 months; during this time half of them had done so on less than five occasions. Subjects with a secondary education were more likely to have used condoms. A counsellor met 31 asymptomatic seropositive subjects identified during this survey on two occasions. In the majority, the information given caused anxiety rather than modification of behaviour and, at the time of the second interview, only one subject had discussed the situation with the partner and begun using condoms. Some of the cultural factors which may affect the outcome of counselling in an African society are discussed in the light of these findings.

Antibody to schistosomulum surface carbohydrate epitopes in subjects infected with Schistosoma haematobium

The binding of immunoglobulin to carbohydrate epitopes on the surface of Schistosoma mansoni schistosomula was measured with 125I-labelled protein A in sera from 67 Gambian subjects living in a focus of intense S. haematobium transmission. The levels of such antibody differed considerably between subjects and there was significant variation between age groups. The highest mean level, in subjects aged between 8 and 14 years, was significantly greater than that in older subjects. Previous studies have shown that resistance to post-treatment reinfection with S. haematobium is related to age in this focus, and is greatest in those aged 15 or more years. These differing relationships with age suggest that antibodies directed against carbohydrate epitopes on the surface of the schistosomulum do not have a major protective role in man.

Reinfection after treatment of schistosome infections

Today, chemotherapy has a central role in the control of schistosome infections. Although the costs involved may be high in relation to local expenditures on health, externally funded mass treatment programmes can lead to large reductions in the prevalence and intensity of schistosome infections. But the benefits of treatment to a community that has been involved in a mass chemotherapy programme, or to an individual patient seen in a health centre, will be limited if reinfection after treatment is rapid and intense. Despite the efficacy of the available drugs few, if any, control programmes based on mass chemotherapy have interrupted transmission and come anywhere near to eradicating schistosome infection.

Surface and species-specific antigens of Schistosoma haematobium

Of the surface antigens identified by radio-iodination, two-dimensional gel analyses showed no similarities between those of Schistosoma haematobium and Schistosoma mansoni, thus providing a basis for the species specificity of these antigens described previously (Simpson, Knight, Hagan, Hodgson, Wilkins & Smithers (1985) Parasitology 90, 499-508). The surface antigens of S. haematobium were glycosylated and comprised an acidic polypeptide of Mr 17,000 as well as a complex set of polypeptides of approximate pI 6-7, which resolved in the Mr range 20,000-30,000. At least one of the lower Mr forms of this complex is also present in the adult worm. Limited cross-reaction was observed with S. mansoni infection sera and this may be due to a shared carbohydrate epitope. In contrast, extensive cross-reaction was observed using sera from mice immunized with S. bovis. This pattern parallels the species-specificity of vaccine-induced immunity. Extensive cross-reaction was also observed within cell-free translation products of m-RNA from adult worms of S. haematobium and S. mansoni by use of heterologous human infection sera. The few antigens which were species-specific may represent surface antigens.

Comparative trials of regimes for the treatment of urinary schistosomiasis in The Gambia

Alternative regimes for the treatment of Schistosoma haematobium infection were compared in two trials. Praziquantel at a dose of 40 mg kg-1 appeared to cure 63% of a random sample of heavily infected subjects; significantly more than the 18% cured by three fortnightly doses of metrifonate at 10 mg kg-1. However, praziquantel led to a greater incidence of mild, transient side-effects. A single dose of metrifonate was found to be an inadequate treatment in the same group of subjects as it left 53% with an egg count of at least 100 ova/10 ml. A combination of 10 mg kg-1 of metrifonate and 25 mg kg-1 of niridazole had a similar effect to that of a single dose of metrifonate alone and it had more side-effects. Reduced doses of praziquantel had less effect on egg counts than the standard regime, but the difference was not significant in the case of 20 mg kg-1. Although a combination of metrifonate and praziquantel, each at 10 mg kg-1, had a greater effect than either constituent alone, the difference was not significant. Factors affecting the choice of drug for use in mass treatment of urinary schistosomiasis in The Gambia are discussed. The present findings suggest that the standard regime of praziquantel should be used or, if this is not possible, a three-dose metrifonate regime.

Resistance to reinfection after treatment of urinary schistosomiasis

The process of reinfection after treatment was studied in a cohort of subjects in a focus of intense Schistosoma haematobium infection. Detailed observations were made at water contact sites of cercarial densities and of water contact by members of the cohort. Individual values of a cumulative index of exposure to infection were calculated using these observations and assumptions which were made about the effect of different water contact activities on the entry of cercariae into the skin. Among groups of subjects with an apparently similar intensity of exposure to infection, reinfection tended to be much heavier in children under 10 years of age than in 10 to 14-year-olds, while only light infections were found in the few adults who became reinfected. This trend for reinfection to decrease with increasing age, after an allowance for variation in exposure, was highly significant (p less than 0.001). These observations suggest that subjects in this area slowly acquire an increasing degree of immunity to the acquisition of S. haematobium infection which is effective in the absence of a mature egg laying infection.

In-vitro antibody-dependent killing of schistosomula of Schistosoma haematobium by human eosinophils

Schistosomula of S. haematobium have been shown to be susceptible to in vitro killing by eosinophils in the presence of serum from an infected individual. The highest level of killing was found after 48 h in culture. Killing was related to the eosinophil to schistosomula ratio, being highest at 5000: 1. Killing was also related to serum concentration, being highest at a 1/10 final dilution, falling to background levels at a 1/120 final dilution. At a cell: target ratio of 2000: 1 and at a serum dilution of 1/10 eosinophils from subjects with high peripheral blood eosinophil counts were, cell for cell, more active in killing S. haematobium schistosomula than were eosinophils from subjects with lower counts. Sera taken from adults resident in an endemic area gave higher levels of killing in the presence of eosinophils than did sera taken from adults with no history of exposure.

Eosinophilia and resistance to Schistosoma haematobium in man

We have measured the levels of infection with Schistosoma haematobium in children resident in an endemic area of The Gambia before and 3 months after successful chemotherapy and following reinfection. An exposure index was calculated from data collected on water contact, cercarial densities and infected snail densities at water contact sites. Peripheral blood eosinophil levels were recorded and the ability of serum (heat inactivated) from the children to allow killing of schistosomula of S. haematobium was examined. Of 50 children with a post-treatment egg count of less than 1 ovum/10 ml urine, 26 were classified as reinfected, acquiring greater than 1 ovum/10 ml urine over the transmission season. Twenty-four were classified as not reinfected, acquiring less than 1 ovum/10 ml of urine over the same period. These two groups did not differ with respect to their estimated age, weight or pretreatment egg counts. Children who were reinfected had significantly higher levels of exposure and significantly lower peripheral blood eosinophil counts than children who were not reinfected. At all levels of exposure children with high eosinophil counts were less likely to be reinfected than those with lower counts. But antibody-dependent, complement-independent killing of schistosomula of S. haematobium by eosinophils was barely detectable and did not differ between reinfected and non reinfected groups. These observations suggest that subjects with elevated counts are less susceptible to reinfection but the mechanisms involved are not apparent.

The schistosomulum surface antigens of Schistosoma haematobium

Surface antigens of Schistosoma haematobium were identified by 125I-surface labelling of schistosomula followed by immunoprecipitation of the solubilized, labelled surfaces. The major antigens, after electrophoresis, formed a continuous smear corresponding to a molecular weight in the range 35-24 000; in addition, a 17 000 antigen was also identified. These surface antigens, in contrast to somatic antigens, were species-specific, as judged by immunoprecipitation with human anti-S. mansoni serum and serum from mice vaccinated with highly irradiated S. mansoni cercariae. S. haematobium surface antigens, however, were recognized to some extent by serum from mice chronically infected with S. mansoni. It is suggested that this cross-reactivity may reflect the heterologous immunity demonstrated experimentally between these two species, whilst the species-specificity of vaccine sera to surface antigens may mirror the highly specific immunity induced by vaccination.

Schistosoma haematobium infection and haemoglobin concentrations in a Gambian community

Specimens of urine and blood were collected from as many members as possible of a community living in an area of urinary schistosomiasis in The Gambia. The light infections found in many of the subjects appeared to have little or no effect of haemoglobin levels, but some of the intense infections were associated with reduced haemoglobin levels. Significant differences in haemoglobin levels between ova-positive and ova-negative subjects were apparent only in males aged from 15 to 44 years, and significant differences in the prevalence of anaemia between groups with different egg counts were apparent only in children aged from five to seven years and from eight to 14 years.

Isotope renography and urinary schistosomiasis: a study in a Gambian community

A transportable apparatus for isotope renography, which allowed deconvolution analysis, was used to study the prevalence and prognosis of abnormalities associated with urinary schistosomiasis. Before carrying out studies in a heavily infected community, observations were made in a non-endemic area to allow derivation of criteria for abnormality. Comparison of the findings in the two areas showed that changes suggesting urinary tract obstruction were more common in the endemic area in subjects between nine and 45 years but not in older subjects. Measurements of effective renal plasma flow showed renal function was impaired in the endemic area in subjects older than 17 years but not in younger subjects. In the endemic area the results of renography were unrelated to the urinary egg count of the subjects examined, but there was an improvement in the abnormal renograms in a group of subjects aged between nine and 20 years who were re-examined a year after treatment with metrifonate. Follow-up data about 316 subjects was obtained two years after renography. Nine subjects had died, including four of the five subjects with abnormalities suggesting both obstruction and over-all loss of renal function. These findings, which are comparable to the results of similar studies using radiological techniques, suggest urinary schistosomiasis may be a significant cause of mortality in adults in intensely infected communities.

Field studies on Bulinus senegalensis Muller and the transmission of Schistosoma haematobium infection in a Gambian community

Seasonal changes in populations of Bulinus senegalensis were studied in temporary pools on the Senegambian plateau. The first snails to appear had survived aestivation and had a mean height of 3 mm. Thereafter two patterns of change in snail numbers and size were observed probably related to differences between pools in snail mortality rates. The prevalence of patent schistosome infections appeared to be related to the inferred differences in snail mortality rates. The prevalence and intensity of human Schistosoma haematobium infection was greatest in villages near large pools where snail populations had relatively low schistosome infection rates compared with that in a small, shallow pool. Differences in patterns of water use are an important determinant of the intensity of human infection as well as differences in the infection rate of snails in the nearby pool. Differences between years in the rate of acquisition of S. haematobium by children were observed, with little transmission occurring in a year when mid-season rainfall was deficient. There were significant differences between villages in the annual variations of transmission.

Dynamics of Schistosoma haematobium infection in a Gambian community. I. The pattern of human infection in the study area

The roles of some of the factors thought to be responsible for the characteristic relationship between age and the intensity and prevalence of Schistosoma haematobium infection have been investigated. In this initial report a study population in an area of intense infection is described, as are the methods used. Subsequent papers report the effect of interrupting transmission with molluscicide in part of the area and compare changes in egg count in this treated area with changes in an area where no intervention took place. These allow a consideration of age-specific rates of loss and acquisition of infection to be made. The intensity and prevalence of infection varied between villages both in the treated and untreated areas, but the relation of age to the pattern of infection was regardless of the level of infection in the villages. Over-all, the number of subjects and their pattern of infection was similar in both areas. The prevalence of infection tended to be higher in adult males than females but preliminary water contact observations suggest males are less exposed to infection. Observations over the three-year study period emphasize the extent of population movement in the study area and point to its importance in the planning of control measures.

Dynamics of schistosoma haematobium infection in a Gambian community. II. The effect on transmission of the control of Bulinus senegalensis by the use of niclosamide

Niclosamide was used to interrupt transmission of Schistosoma haematobium by Bulinus senegalensis in seasonal rainwater pools for a period of three years. Snail populations were progressively reduced to approximately 1% of the numbers in untreated pools. There was little or no evidence of acquisition of new infection by children in the area during the period of intervention. The intensity of infection in a cohort of children initially under 10 years old followed for three years fell by more than 50%, while there was a ten-fold increase in a similar group in a nearby untreated area. The mean annual cost (1982) of control per head of the population protected was pound 0.50 (US $0.89).

Dynamics of Schistosoma haematobium infection in a Gambian community. III. Acquisition and loss of infection

During a three-year period of effective control of Schistosoma haematobium transmission by molluscicide application the mean number of S. haematobium ova passed by subjects in the treated area fell in an exponential manner which suggested that the mean life span of the worm was 3.4 years. Parallel observations were made in a similar but untreated area. A comparison of the observations in these two areas suggested that in the untreated area subjects of all ages acquired infection during the course of the study. At the end of the study over 50% of the egg output in most age groups in the untreated area appeared to come from worms acquired during the preceding three years. There were substantial differences between age groups in the amount of infection acquired. Children, aged between eight and ten years at the end of the study, appeared to be passing perhaps a thousand times more ova from worms acquired during the preceding three years than were middle-aged subjects. Preliminary observations suggest that age- and sex-related differences in the pattern of water contact may not fully account for age- and sex-related differences in the rate of acquisition of infection and its prevalence. The probable significance of protective immunity in the epidemiology of schistosome infections is discussed.

The significance of proteinuria and haematuria in Schistosoma haematobium infection

The intensity and prevalence of proteinuria and haematuria were studied with urinanalysis reagent strips in a Gambian community in which the intensity of infection with Schistosoma haematobium was high. The level of proteinuria present was shown to be related to the intensity of infection. Follow-up for 12 months showed that infected subjects with heavy proteinuria had a good prognosis. These findings suggest that the urinary protein in such subjects is likely to originate from lesions in the bladder and ureters and that advanced glomerular pathology is probably rare. The relationship of the levels of proteinuria and haematuria to the egg count suggests they may be parameters which could have value as indications for chemotherapy. Detailed study showed that the effect of treating all the subjects who had both 30 mg/100 ml or more of protein and at least a trace of haematuria would have been very similar to treating all those with an egg count of 200 ova/10 ml or more. Since urinalysis with reagent strips is very simple and rapid it could have a role in mass chemotherapy campaigns, particularly those aimed at the identification and treatment of heavily infected subjects.

Schistosoma haematobium in a Gambian community. IV. Antibody levels and change in egg output

The indirect fluorescent antibody (FAT) and indirect haemagglutination (IHA) tests were used to study the serological response to Schistosoma haematobium in a Gambian community. The FAT was frequently positive in very young children and also in adults, in whom urine examination did not show ova. This was much less often the case with the IHA although this test sometimes gave negative results in heavily infected children. Examination of paired sera taken at six month intervals apart showed that titres changed in some subjects and that overall there was a slight change in antibody level. Correlation of antibody level with subsequent changes in egg output showed that high titres were associated with a tendency for the egg output to fall. This effect was most obvious over a period of 16 months' follow up and was not explicable solely in terms of a coincidental relationship with age or intensity of infection. It is suggested that serological parameters may have some relation to protective immunity and that the immune response must be considered as a factor in the epidemiology of the infection.

Schistosoma haematobium in a Gambian community. III. The prevalence of bacteriuria and of hypertension

A field study in an area of intense infection with Schistosoma haematobium showed a 6-6% prevalence of bacteriuria in males under 25 which was significantly greater than in a non-endemic area where no cases were found. In older subjects the prevalence in both areas was less than 1%. Similarly designed studies showed a slight increase in mean blood pressure levels and in the prevalence of hypertension in the endemic area. This could not with certainty be attributed to the effects of S. haematobium and the levels recorded were unlikely to be associated with a poor prognosis. The findings suggest that bacteriuria but not hypertension, may sometimes determine the outcome of S. haematobium infection in this community.

Schistosoma haematobium in a Gambian community. II. Impaired cell-mediated immunity and other immunological abnormalities

A decreased response rate in delayed hypersensitivity reactions was found in a population heavily infected with Schistosoma haematobium. Laboratory studies showed a depressed response of lymphocytes to PHA in infected subjects. Together these findings suggest the presence of a defect in cell-mediated immunity in some infected subjects. IgG and IgM levels were increased and the prevalence of rheumatoid factor was greater in the infected population. The possibility of non-specific changes in the control of the immune system in schistosome infections and the relation of the present findings to the increased prevalence of lymphoproliferative tumours in infected subjects are discussed.

Schistosoma haematobium in a Gambian community. I. The intensity and prevalence of infection

The intensity and prevalence of Schistosoma haematobium infection has been measured in a community in the laterite plateau area of the McCarthy Island Division, The Gambia. The prevalence reaches 100% by the age of 10 years and counts of over 1000 ova in 10 ml of noon urine are common. During the second and third decade prevalence and intensity of infection decline and the mean egg output of a cohort falls by approximately 50% every three years. In subjects aged over 30 sex differences in exposure to infection and egg output suggest the possible effect of protective immunity.

Variation in urinary creatinine concentration and Schistosoma haematobium egg count

Variation in urinary creatinine concentration was studied in a Gambian community. Whilst there was appreciable day to day variation there were also differences between subjects and between different times of year. Day to day changes in creatinine concentration correlated with changes in egg count, which were, in proportion, smaller. While there are difficulties in the use of creatinine as a reference index, it is suggested that knowledge of creatinine concentrations may eliminate some of the ambiguities arising in the interpretation of Schistosoma haematobium egg counts in random, untimed urine specimens. The relation between the two variables and the extent of seasonal changes in creatinine concentration suggest that changes in the mean egg count of a group of subjects due to changes in urine flow are usually small and seldom of the magnitude which some have assumed.

Schistosoma haematobium egg counts in a Nile delta community

A study of Schistosoma haematobium egg counts was made in a community near Alexandria. This showed that the intensity of infection was similar to that which has been reported in some of the comparable surveys which have been made in sub-Saharan Africa. This finding highlights the need for further studies in defined communities of the pathogenicity of the infection. The egg counts of subjects who recalled treatment with tartar emetic within the last two years were similar to those who said they had never been treated. This suggests that the cost effectiveness of chemotherapeutic methods needs further evaluation.