Estimation of changes in the force of infection for intestinal and urogenital schistosomiasis in countries with schistosomiasis control initiative-assisted programmes

Background

The last decade has seen an expansion of national schistosomiasis control programmes in Africa based on large-scale preventative chemotherapy. In many areas this has resulted in considerable reductions in infection and morbidity levels in treated individuals. In this paper, we quantify changes in the force of infection (FOI), defined here as the per (human) host parasite establishment rate, to ascertain the impact on transmission of some of these programmes under the umbrella of the Schistosomiasis Control Initiative (SCI).

Methods

A previous model for the transmission dynamics of Schistosoma mansoni was adapted here to S. haematobium. These models were fitted to longitudinal cohort (infection intensity) monitoring and evaluation data. Changes in the FOI following up to three annual rounds of praziquantel were estimated for Burkina Faso, Mali, Niger, Tanzania, Uganda, and Zambia in sub-Saharan Africa (SSA) according to country, baseline endemicity and schistosome species. Since schistosomiasis transmission is known to be highly focal, changes in the FOI at a finer geographical scale (that of sentinel site) were also estimated for S. mansoni in Uganda.

Results

Substantial and statistically significant reductions in the FOI relative to baseline were recorded in the majority of, but not all, combinations of country, parasite species, and endemicity areas. At the finer geographical scale assessed within Uganda, marked heterogeneity in the magnitude and direction of the relative changes in FOI was observed that would not have been appreciated by a coarser-scale analysis.

Conclusions

Reductions in the rate at which humans acquire schistosomes have been achieved in many areas of SSA countries assisted by the SCI, while challenges in effectively reducing transmission persist in others. Understanding the underlying heterogeneity in the impact and performance of the control intervention at the level of the transmission site will become increasingly important for programmes transitioning from morbidity reduction to elimination of infection. Such analyses will require a fine-scale approach. The lack of association found between programmatic variables, such as therapeutic treatment coverage (recorded at district level) and changes in FOI (at sentinel site level) is discussed and recommendations are made.

Integrated monitoring and evaluation and environmental risk factors for urogenital schistosomiasis and active trachoma in Burkina Faso before preventative chemotherapy using sentinel sites

Background

Over 1 billion of the world's poorest inhabitants are afflicted by neglected tropical diseases (NTDs). Integrated control programmes aimed at tackling these debilitating NTDs have been recently initiated, mainly using preventative chemotherapy. Monitoring and evaluation (M&E) of these integrated programs presents particular challenges over and above those required for single disease vertical programmes. We used baseline data from the National NTD Control Programme in Burkina Faso in order to assess the feasibility of an integrated survey design, as well as to elucidate the contribution of environmental variables to the risk of either Schistosoma haematobium, trachoma, or both among school-aged children.

Methods

S. haematobium infection was diagnosed by detecting eggs in urine. A trachoma case was defined by the presence of Trachomatous inflammation-Follicular (TF) and/or Trachomatous inflammation-Intense (TI) in either eye. Baseline data collected from 3,324 children aged 7-11 years in 21 sentinel sites across 11 regions of Burkina Faso were analyzed using simple and multivariable hierarchical binomial logistic regression models fitted by Markov Chain Monte Carlo estimation methods. Probabilities of the risk of belonging to each infection/disease category were estimated as a function of age, gender (individual level), and environmental variables (at sentinel site level, interpolated from national meteorological stations).

Results

Overall prevalence at the sentinel sites was 11.79% (95% CI: 10.70-12.89) for S. haematobium; 13.30% (12.14-14.45) for trachoma and 0.84% (0.53-1.15) for co-infections. The only significant predictor of S. haematobium infection was altitude. There were significant negative associations between the prevalence of active trachoma signs and minimum temperature, and air pressure. Conditional upon these predictors, these data are consistent with the two pathogens being independent.

Conclusions

Urogenital schistosomiasis and trachoma constitute public health problems in Burkina Faso. Sentinel site (at school level) surveys for these two NTDs can be implemented simultaneously. However, to support MDA treatment decisions in Burkina Faso, the protocol used in this study would only be applicable to hypoendemic trachoma areas. More research is needed to confirm if these findings can be generalized to West Africa and beyond.

The impact of single versus mixed schistosome species infections on liver, spleen and bladder morbidity within Malian children pre- and post-praziquantel treatment

Background

In the developing world co-infections and polyparasitism within humans appear to be the rule rather than the exception, be it any combination of inter-specific and/or inter- and intra-Genera mixed infections. Mixed infections might generate synergistic or antagonistic interactions and thereby clinically affect individuals and/or impact parasite epidemiology.

Methods

The current study uniquely assesses both Schistosoma mansoni- and Schistosoma haematobium-related morbidity of the liver and the bladder as assessed by ultrasound as well as spleen and liver morbidity through clinical exams. The impact of praziquantel (PZQ) treatment on such potential inter-specific schistosome interactions and resulting morbidity using uniquely detailed longitudinal data (pre- and one year post-PZQ treatment) arising from the National Schistosomiasis Control Program in three areas of Mali: Ségou, Koulikoro and Bamako, is also evaluated. At baseline, data were collected from up to 2196 children (aged 7-14 years), 844 of which were infected with S. haematobium only, 124 with S. mansoni only and 477 with both. Follow-up data were collected from up to 1265 children.

Results

Results suggested lower liver morbidity in mixed compared to single S. mansoni infections and higher bladder morbidity in mixed compared to single S. haematobium infections. Single S. haematobium or S. mansoni infections were also associated with liver and spleen morbidity whilst only single S. haematobium infections were associated with bladder morbidity in these children (light S. haematobium infection OR: 4.3, p < 0.001 and heavy S. haematobium infection OR: 19, p < 0.001). PZQ treatment contributed to the regression of some of the forms of such morbidities.

Conclusions

Whilst the precise biological mechanisms for these observations remain to be ascertained, the results illustrate the importance of considering mixed species infections in any analyses of parasite-induced morbidity, including that for the proposed Disability Adjusted Life Years (DALYs) revised estimates of schistosomiasis morbidity.

Use of Bayesian geostatistical prediction to estimate local variations in Schistosoma haematobium infection in western Africa

OBJECTIVE

To predict the subnational spatial variation in the number of people infected with Schistosoma haematobium in Burkina Faso, Mali and the Niger prior to national control programmes.

METHODS

We used field survey data sets covering a contiguous area 2750 x 850 km and including 26,790 school-age children (5-14 years old) in 418 schools. The prevalence of high- and low-intensity infection and associated 95% credible intervals (CrIs) were predicted using Bayesian geostatistical models. The number infected was determined from the predicted prevalence and the number of school-age children in each km(2).

FINDINGS

The predicted number of school-age children with a low-intensity infection was 433,268 in Burkina Faso, 872,328 in Mali and 580 286 in the Niger. The number with a high-intensity infection was 416,009, 511,845 and 254,150 in each country, respectively. The 95% CrIs were wide: e.g. the mean number of boys aged 10-14 years infected in Mali was 140,200 (95% CrI: 6200-512,100).

CONCLUSION

National aggregate estimates of infection mask important local variations:: e.g. most S. haematobium infections in the Niger occur in the Niger River valley. High-intensity infection was strongly clustered in western and central Mali, north-eastern and northwestern Burkina Faso and the Niger River valley in the Niger. Populations in these foci will carry the bulk of the urinary schistosomiasis burden and should be prioritized for schistosomiasis control. Uncertainties in the predicted prevalence and the numbers infected should be acknowledged by control programme planners.

A comparative study of the spatial distribution of schistosomiasis in Mali in 1984-1989 and 2004-2006

Background

We investigated changes in the spatial distribution of schistosomiasis in Mali following a decade of donor-funded control and a further 12 years without control.

Methodology/Principal Findings

National pre-intervention cross-sectional schistosomiasis surveys were conducted in Mali in 1984–1989 (in communities) and again in 2004–2006 (in schools). Bayesian geostatistical models were built separately for each time period and on the datasets combined across time periods. In the former, data from one period were used to predict prevalence of schistosome infections for the other period, and in the latter, the models were used to determine whether spatial autocorrelation and covariate effects were consistent across periods. Schistosoma haematobium prevalence was 25.7% in 1984–1989 and 38.3% in 2004–2006; S. mansoni prevalence was 7.4% in 1984–1989 and 6.7% in 2004–2006 (note the models showed no significant difference in mean prevalence of either infection between time periods). Prevalence of both infections showed a focal spatial pattern and negative associations with distance from perennial waterbodies, which was consistent across time periods. Spatial models developed using 1984–1989 data were able to predict the distributions of both schistosome species in 2004–2006 (area under the receiver operating characteristic curve was typically >0.7) and vice versa.

Conclusions/Significance

A decade after the apparently successful conclusion of a donor-funded schistosomiasis control programme from 1982–1992, national prevalence of schistosomiasis had rebounded to pre-intervention levels. Clusters of schistosome infections occurred in generally the same areas accross time periods, although the precise locations varied. To achieve long-term control, it is essential to plan for sustainability of ongoing interventions, including stengthening endemic country health systems.

Geostatistical maps are increasingly being used to plan neglected tropical disease control programmes. We investigated the spatial distribution of schistosomiasis in Mali prior to implementation of national donor-funded mass chemotherapy programmes using data from 1984–1989 and 2004–2006. The 2004–2006 dataset was collected after 10 years of schistosomiasis control followed by 12 years of no control. We found that national prevalence of Schistosoma haematobium and S. mansoni was not significantly different in 2004–2006 compared to 1984–1989 and that the spatial distribution of both infections was similar in both time periods, to the extent that models built on data from one time period could accurately predict the spatial distribution of prevalence of infection in the other time period. This has two main implications: that historic data can be used, in the first instance, to plan contemporary control programmes due to the stability of the spatial distribution of schistosomiasis; and that a decade of donor-funded mass distribution of praziquantel has had no discernable impact on the burden of schistosomiasis in subsequent generations of Malians, probably due to rapid reinfection.

Two-year impact of single praziquantel treatment on infection in the national control programme on schistosomiasis in Burkina Faso

OBJECTIVE

To evaluate the impact on schistosomiasis of biennial treatment with praziquantel (PZQ) among school-age children in Burkina Faso, the first country that achieved full national coverage with treatment of more than 90% of the school-age population.

METHODS

A cohort of 1727 schoolchildren (6-14 years old) was monitored at yearly intervals through a longitudinal survey. Additional groups of schoolchildren were monitored in cross-sectional surveys. Parasitological examinations for Schistosoma haematobium and Schistosoma mansoni were performed, and prevalence and intensity of infection before and after treatment were analysed.

FINDINGS

Data from the longitudinal cohort show that a single round of PZQ treatment significantly reduced prevalence of S. haematobium infection by 87% (from 59.6% to 7.7%) and intensity of infection by 92.8% (from 94.2 to 6.8 eggs/10 ml of urine) 2 years post-treatment. The impact on infection was also confirmed by a cross-sectional survey 2 years post-treatment. Importantly, the proportion of school-age children with heavy S. haematobium infection decreased from around 25% before treatment to around 2-3% 2 years post-treatment. Cross-sectional comparison of S. haematobium infection in 7-year-old children in their first year at school, who received treatment through community-based drug delivery, also showed significant reduction in both prevalence (65.9%) and intensity of S. haematobium infection (78.4%) 2 years after single treatment. A significant reduction in S. mansoni infection was also achieved.

CONCLUSION

Significant and sustained reduction in S. haematobium infection was achieved by biennial treatment in school-age children in Burkina Faso. This may provide a cost-effective treatment strategy for similar national schistosomiasis control programmes in sub-Saharan Africa.