Schistosomiasis

Schistosomiasis is a tropical parasitic disease, caused by blood-dwelling worms of the genus Schistosoma. The main human species are S mansoni (occurring in Africa and South America) and S japonicum (South and East Asia) causing intestinal and hepatosplenic schistosomiasis, and S haematobium (Africa) causing urinary schistosomiasis. Severe symptoms develop in predilected people with heavy and long-standing infections. Acute schistosomiasis, a flulike syndrome, is a regular finding in travel clinics. Although prevalences can be high, most infected people show limited, intermittent, or aspecific symptoms. The diagnosis of schistosomiasis relies on microscopic examination of stools or urine, serologic tests, and imaging. Praziquantel is the drug of choice, active against all species in a single or a few oral doses. Current control strategies consist mainly of preventive therapy in communities or groups at risk.     

Human water contacts patterns in Schistosoma mansoni epidemic foci in northern Senegal change according to age,sex and place of residence,but are not related to intensity of infection

In an epidemic focus in northern Senegal, adults had lower intensities of infection than adolescents, a phenomenon that could not be attributed to immunity acquired over the previous 10-15 years of exposure to the parasite because all age groups had had the same number of years' experience of the worm. This article considers whether this pattern could have been because of higher levels of exposure to the parasite in younger age groups. Personal contact with infected water was recorded using a questionnaire in Schistosoma mansoni foci not more than 3 years old and in another, 10-year-old focus. Many aspects of contact (e.g. frequency, duration or time of day of contact) may contribute to the number of encounters with infective cercariae (true exposure), so various assumptions regarding the relationship between water contact and true exposure were tested resulting in a range of exposure indices. People reported a mean of 4.4 separate contacts, and spent a median of 57 min per day in water. Patterns of water contact differed depending on the exposure index used, e.g. considering duration, males spent a longer time in water than females (P < 0.001). But using frequency, females had more contacts with water than males in most villages (P < 0.001). Generally, exposure levels dropped as people become aged (P < 0.001) and residents of the older focus were more exposed than residents of other foci (P < 0.002). Intensity of (re)infection was not related to exposure either alone or in models incorporating age, sex and/or village irrespective of the index used. There is therefore evidence that age, sex and place of residence determine exposure but none to suggest that exposure had an influence on the relationship between these factors and intensity of infection. We propose therefore that in this population other factors have principal importance in determining intensity of infection.     

Specificity of circulating antigen detection for schistosomiasis mansoni in Senegal and Burundi

The specificity of schistosome circulating antigen detection was determined in negative individuals from two S. mansoni- endemic countries, Senegal and Burundi, and compared with results from Dutch control individuals. A nearly absolute specificity was achieved for circulating anodic antigen (CAA) detection in serum, irrespective of the target population or sample pretreatment method. Circulating cathodic antigen (CCA) detection in serum and urine resulted in a lower specificity than serum CAA detection. Apparent large differences in specificity of CCA detection between countries were mainly due to pretreatment methods. Apparently, the alkaline/heating pretreatment method is not as effective as trichloroacetic acid (TCA)-pretreatment in removing (certain) interfering components, which may vary between populations. In view of the development of the urine CCA assay into a noninvasive screening test, a slightly lower specificity may still be acceptable. For precise epidemiological analyses the highly specific serum CAA assay remains the method of choice.     

The distribution of Schistosoma mansoni in the Rusizi plain (Burundi)

A regional survey of Schistosoma mansoni infection has been performed in the Rusizi Plain (Burundi), on a geographically evenly distributed 5% population sample (6203 subjects) in 41 localities. The overall prevalence was 33%, with 58% of the cases excreting less than 100 eggs per gram of faeces and a mean egg load of positives of 98 eggs per gram. The geographical distribution is very focal; local prevalences range from 3% to 63%, but vary also considerably on a sublocal level. Eight eco-epidemiological zones can be recognized. The infection rates are over 40% in the marshy cotton paysannats near Lake Tanganyika, the irrigated cotton paysannats of the northern plain and a deteriorated rice cultivation area in the south; 30-40% in the other cotton and rice areas of the south; 20-30% in the dry cotton paysannats of the mid-plain and the hors-paysannats at the foot of the mountains. Natural foci certainly occur, but most often man-made hydraulic systems are the basis of high prevalences. Overall the infection rates are maximal in individuals aged 15 to 20, but they remain relatively high in adults. The age-prevalence curves vary from one area to another, however. In rice culture areas the prevalences show an unexpectedly strong decline in adults, indicating that the impact of irrigation on schistosomiasis endemicity is not a problem of occupational exposure.     

Repeated community-based chemotherapy for the control of Schistosoma mansoni: effect of screening and selective treatment on prevalences and intensities of infection

The impact of repeated selective chemotherapy on prevalences and intensities of infection with Schistosoma mansoni was evaluated in Gihungwe Transversals 1 and 2 (initial prevalence 60%) and Buhandagaza/Kizina (initial prevalence 35%), two village clusters in Burundi. Surveys were carried out at months -6, -3, 0, 3, 6, 9, 12, 24, and 36, with reference to the first intervention; treatment with praziquantel (40 mg/kg) was given at months 0, 12, 24, and 36 to subjects showing parasite eggs on a single 28-mg Kato slide. A second slide was examined for monitoring purposes only. Over the pre-intervention period, the overall prevalences and intensities remained relatively stable, but important increases were observed in specific groups. The cure rate three months after the first treatment in those treated was 73% (Gihungwe) and 83% (Buhandagaza/Kizina), but the prevalence at the community level was reduced only by 50% and 46%, respectively. Fifty-six percent and 79%, respectively, of the remaining positive cases had not been treated, largely because they were (falsely) negative at the screening. Reinfection occurred mainly in Gihungwe and in younger age groups. One year after the second treatment, prevalences and intensities were further reduced in Gihungwe only; one year after the third treatment prevalences were not reduced further in either village group. The final prevalence of infection was approximately 25%, with infections with an intensity of over 100 eggs per gram of feces approximately 5%, in all four villages. Over 80% of the remaining cases had not been treated at the previous intervention; the sensitivity of the screening method appears to be a major determinant of the outcome of repeated selective treatment.     

On the calculation of intestinal schistosome infection intensity

The effects of using different methods to calculate individual infection intensities on the age-infection distribution of Schistosoma mansoni field data are demonstrated. Methods are tested on a maximum of three stool samples per person collected on three consecutive days; the methods considered for the calculation of individual infection intensities are the geometric mean (GM), arithmetic mean (AM) and pseudo geometric mean (GM of stool samples instead of replicates). In addition, the effects of calculating the infection intensity for each age group using either AMs or GMs are compared. Differences occur in the shape of the age-infection profiles obtained by using either the arithmetic or geometric group mean. When using the AM, peak infection intensity occurs in a younger age group compared to using the GM, and all three methods of calculating individual infection intensity give the first peak of infection in the same age group. However, differences occur in the position of the second peak which occurs earlier with the two GMs than with the AM. Bootstrapping procedures show that the individual AM, gives a different age group for the first peak of infection at least 25% of the time when compared to either of the GMs, and 31% of the time for the second peak, while the two GMs give the same peak age groups around 90-92% of the time for both peaks. When using the GM, to calculate infection intensity for each age group, there are no differences between the three methods used to calculate individual infection intensity. This is confirmed by bootstrapping procedures. The results are discussed in relation to the distribution of parasites and levels of parasite aggregation. The implications of the results for field studies are also discussed.     

Field trials of praziquantel and oxamniquine for the treatment of schistosomiasis mansoni in Burundi

Praziquantel and oxamniquine were evaluated under operational conditions for use in mass-treatment campaigns in the Rusizi Plain, Burundi. After 6 weeks, the cure rates for oxamniquine at 20, 30 and 40 mg/kg in children (less than 20 years) were respectively 47%, 67% and 86%; in adults they were 86%, 97% and 97%. The egg reduction rates were over 98% in all groups. For praziquantel at 20, 30 and 40 mg/kg the cure rates in children were respectively 58%, 63% and 78%; in adults, 55%, 87% and 91%. The egg reduction rates were respectively 92%, 96%, 98% and 91%, 98%, 98%. These results were largely confirmed by a follow-up 3 months after treatment. Oxamniquine frequently caused important dizziness and drowsiness, and in 2 cases epileptiform seizures. The side effects of praziquantel were mainly mild transient colics and diarrhoea. The cost of oxamniquine (in Burundi) was twice to three times the cost of praziquantel. Because of its better acceptability and its lower cost, with only slightly less good parasitological results, praziquantel, at 40 mg/kg in a single dose, has been selected as the drug of choice for mass-treatment campaigns in Burundi.     

Use of praziquantel against schistosomiasis: a review of current status

Praziquantel therapy has become an important component of any programme directed towards schistosomiasis control. This disease being prevalent in many disadvantaged countries, where the means and resources are limited, the strategies for its control on a large scale are also beset with various other constraints. In this review, the potentials of praziquantel therapy in containing the intensities, prevalences and schistosomiasis-associated morbidities in endemic areas of different geographical and epidemiological settings are examined. By and large, regular community-based treatment produces a longer term favourable impact on infection levels and morbidity, but the impact on the disease transmission appears limited. Children constitute a high risk group in schistosomiasis. They are usually subjected to more rapid and intense reinfection, but they also appear more responsive to praziquantel therapy in preventing or reversing the effects of the disease. Modern techniques used in the assessment of impact of praziquantel treatment in schistosomiasis control are outlined.     

Are poor responses to praziquantel for the treatment of Schistosoma mansoni infections in Senegal due to resistance? An overview of the evidence

This paper summarizes and concludes in-depth field investigations on suspected resistance of Schistosoma mansoni to praziquantel in northern Senegal. Praziquantel at 40 mg/kg usually cures 70-90% of S. mansoni infections. In an initial trial in an epidemic S. mansoni focus in northern Senegal, only 18% of the cases became parasitologically negative 12 weeks after treatment, although the reduction in mean egg counts was within normal ranges (86%). Among other hypotheses to explain the observed low cure rate in this focus, the possibility of drug resistance or tolerance had to be considered. Subsequent field trials with a shorter follow-up period (6-8 weeks) yielded cure rates of 31-36%. Increasing the dose to 2 x 30 mg/kg did not significantly improve cure rates, whereas treatment with oxamniquine at 20 mg/kg resulted in a normal cure rate of 79%. The efficacy of praziquantel in this focus could be related to age and pre-treatment intensity but not to other host factors, including immune profiles and water contact patterns. Treatment with praziquantel of individuals from the area residing temporarily in an urban region with no transmission, and re-treatment after 3 weeks of non-cured individuals within the area resulted in normal cure rates (78-88%). The application of an epidemiological model taking into account the relation between egg counts and actual worm numbers indicated that the low cure rates in this Senegalese focus could be explained by assuming a 90% worm reduction after treatment with praziquantel; in average endemic situations, such a drug efficacy would result in normal cure rates. Laboratory studies by others on the presence or absence of praziquantel resistance in Senegalese schistosome strains have so far been inconclusive. We conclude that there is no convincing evidence for praziquantel-resistant S. mansoni in Senegal, and that the low cure rates can be attributed to high initial worm loads and intense transmission in this area.     

Social research as an intervention tool in tuberculosis control.

Our multidisciplinary project on TB control in the Free State, South Africa, is targeting two dimensions for intervention: firstly, patients, to facilitate compliance and improve quality of care; secondly, the health care system, to identify weaknesses that require remedying and best practices to promote better TB control. This communication illustrates how social scientists can contribute towards the implementation of interventions related to their research, thus influencing TB policy, programme planning and practice more directly.