B. Gryseels

Underestimation of Schistosoma mansoni prevalences

Field methods used for detecting Schistosoma mansoni infection miss a certain proportion of the infections. Prevalences of infection appear to be far under-estimated by faecal screening, with important consequences for control and research. Sake de Vlos and Bruno Gryseels investigate how the number of undetected infections can be statistically inferred from population surveys.

Detection of circulating anodic antigen by ELISA for seroepidemiology of schistosomiasis mansoni

Sera of individuals from Burundi excreting eggs of Schistosoma mansoni (prevalence 35%; 178 subjects) and of similar individuals from Maniema, Zaire (prevalence 95%; 99 subjects), and of 159 Dutch and 81 Zairean non-infected controls, were screened by enzyme-linked immunosorbent assay for the presence of schistosome circulating anodic antigen (CAA). No false positive results were obtained. The sensitivity of the test was 75% in Burundi and 93% in Zaire, a significant difference (P less than 0.05). However, in matched egg output classes the test results did not differ significantly; 60% and 67%, respectively, of those excreting 1-100 eggs per gram of faeces (epg), 86% and 100% of those excreting 101-400 epg, and 100% of those excreting over 400 epg were detected. The efficiency of the assay was 91% in Burundi and 93% in Zaire. The Spearman rank coefficient of correlation between antigen titre and egg output (determined by 3 consecutive Kato egg counts) was 0.61 in Burundi and 0.82 in Zaire. The sensitivity of the test compared well with a single egg count. In addition, preliminary data showed that occasionally CAA was detectable in serum of individuals not excreting schistosome eggs. As CAA is found only in the presence of living worms, such cases reflect active infections.

Oxamniquine Cures Schistosoma mansoni Infection in a Focus in Which Cure Rates with Praziquantel Are Unusually Low

An outbreak of Schistosoma mansoni in northern Senegal was observed in 1988, and chemotherapy with praziquantel in this recently established focus resulted in very low parasitologic cure rates. Among other explanations, the emergence of a praziquantel-tolerant parasite strain was feared. To study this hypothesis further, 138 persons with endemic S. mansoni infection were randomly allocated to treatment with either 20 mg/kg oxamniquine or 40 mg/kg praziquantel. Parasitologic cure rates at 6 weeks were significantly higher in the oxamniquine group (79%) compared with those in the praziquantel group (36%; P = .0043). The reduction in egg counts was generally good, but 12% less reduced in the praziquantel group. These results confirm that cure rates with praziquantel were abnormally low, whereas oxamniquine performed satisfactorily, as in other areas in which S. mansoni is endemic. The possibility of a praziquantel-tolerant S. mansoni strain must therefore be studied carefully.

A model for variations in single and repeated egg counts in Schistosoma mansoni infections

Faecal egg counts are often used to measure Schistosoma mansoni infection, but the considerable variation between successive counts complicates their interpretation. The stochastic model described in this paper gives a description of observed egg counts in a population and can be used as a tool to gain an insight into the underlying worm load distribution. The model distinguishes between two sources of variation in egg counts: (1) variation caused by the difference in worm load between individuals, and (2) the variability of egg counts for an individual with a given worm load. Empirical data, single and repeated measurements, from surveys in five villages in Burundi and Zaire have been used to fit and validate the model. We have discussed possible mechanisms that explain the differences in estimated values between the villages. The model indicates that the expected number of eggs in a stool sample per S. mansoni worm pair is lower than suggested by autopsy data and that, possibly as a consequence of immunity, the inter-individual variation in worm loads decreases with age.

Worm burdens in schistosome infections

Schistosomiasis, caused by fluke worms of Schistosoma spp, is one of the most common tropical diseases. Despite decades of research and progress towards the control of the disease, many aspects of the dynamics of infection and immunity remain unresolved. There is, in fact, not even an approximate measure of how many worms are harboured by infected humans. Epidemiological, mathematical and biomedical arguments indicate that individual worm burdens in endemic areas number hundreds to thousands of adult schistosomes, instead of the few to dozens generally assumed on the basis of available autopsy data. As Bruno Gryseels and Sake de Vlas here discuss, this hypothesis has important consequences for research and control, as many constants in schistosomiasis research have to be reconsidered.

Morbidity, due to schistosomiasis mansoni, and its control in Subsaharan Africa

The measuring and monitoring of morbidity are essential components of schistosomiasis control programmes. Bruno Gryseels and Anton Polderman assess the objectives, difficulties and importance of morbidity control of schistosomiasis mansoni in subsaharan Africa.

Side effects of praziquantel in the treatment of Schistosoma mansoni in Maniema, Zaire

In a small community in Maniema, Zaire, heavily infected with Schistosoma mansoni, direct observations were made of the side effects of praziquantel in the hours immediately after treatment. Intense abdominal discomfort and the production of bloody diarrhoea were observed in more than half of the treated population. These effects were seen both in children and in adults and the onset of the symptoms was registered within 30 min of treatment. The frequency of the side effects was correlated with the intensity of the infection. No satisfactory explanation could be given why the side effects in Maniema appear to be so intense compared with the general experience elsewhere.

Preliminary study of urinary schistosomiasis in a village in the delta of the Senegal river basin, Senegal

Three years after the first cases of urinary schistosomiasis infection were reported in the village of Mbodiene, Senegal, Schistosoma haematobium eggs were found in 87% of the inhabitants of this village; 30% were heavily infected (> 50 eggs per 10 mL urine). The prevalence of infection was very high in all age groups, but children showed more intense infections. No difference between sexes was found. In the special situation of a very high prevalence, test strips for proteinuria and haematuria are not very useful for the individual diagnosis of S. haematobium infection. Six and 12 weeks after treatment with a single dose of praziquantel (40 mg/kg), S. haematobium eggs were found in 25% and 30% of the treated subjects, respectively. Bulinus globosus was identified as intermediate host, but other snail vectors may also play a role. S. mansoni eggs were found in 1% of the population. Both S. haematobium and S. mansoni are spreading in the delta of the Senegal river.

Schistosoma haematobium in Morocco: Moving From Control to Elimination

In this article, Hammou Laamrani and colleagues summarize the Moroccan schistosomiasis control programme and discuss the challenges ahead for schistosomiasis elimination. In 1994, a programme was initiated by the Moroccan Ministry of Health to eliminate schistosomiasis from Morocco by the year 2004. In 1997, this objective had been achieved in three out of 20 affected provinces. This article discusses the background and strategies of this programme, as well as the achievements, the problems encountered and the challenges ahead, along with suggestions as to how to reach the goal of elimination of urinary schistosomiasis in Morocco, and possibly elsewhere in Africa.

The epidemiology of schistosomiasis in Burundi and its consequences for control

This paper summarizes the results of a series of studies on the epidemiology, morbidity and transmission of Schistosoma mansoni in Burundi, and discusses their consequences for control. The main endemic area is the Imbo lowland, consisting of the Rusizi plain, the urban focus of Bujumbura, and the shores of lake Tanganyika; a small, new focus was discovered in the highlands, around lake Cohoha. Distribution studies on 5-10% population samples with duplicate 28 mg Kato smears in these 4 foci showed prevalences of 33%, 26%, 17%, 19% and mean (positive) egg loads of 110, 105, 92, 144 eggs/g, respectively. The combined population at risk was estimated to be 400,000 people, the total number of detectable cases 90,000. Prevalences and intensities varied greatly at the subregional, local and even sublocal level. The age- and sex-related prevalences and intensities of infection showed typical peaks in children and adolescents, but remained relatively high in adults in many areas; these patterns varied from one area to another and could be related to ecology and water contact. Morbidity studies showed that, in children as well as in adults, schistosomiasis-related morbidity such as (bloody) diarrhoea, hepatomealy and splenomegaly was apparent mainly in areas with prevalences over 30-40%. The intermediate hosts were Biomphalaria pfeifferi (Imbo), B. sudanica (Tanganyika marshes) and B. stanleyi (Cohoha). Population dynamic studies showed strong seasonal variations, the patterns of which were focal and even erratic in space and time. Snail densities and cercarial infection rates (0.85% overall in B. pfeifferi) were low.(ABSTRACT TRUNCATED AT 250 WORDS)