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

Abstract

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.

Fig. 1

Map of Africa showing: those SCI-assisted countries that provided datasets included in this study, species of schistosome found in each, cohort sizes, and year of commencement of control programme

Fig. 2

Map of Uganda showing results from baseline prevalence mapping of intestinal schistosomiasis in the country. The three main areas of schistosomiasis transmission are situated along the shores of Lake Victoria, Lake Albert, and Albert Nile. The three levels of Schistosoma mansoni endemicity at baseline are represented by closed circles: high (≥400 epg, violet); medium (100–399 epg, purple), and low (1–99 epg, pale pink) transmission. Figure reproduced with permission from Zhang et al. (2007)

Fig. 3

Differences in infection markers between those individuals followed up in the longitudinal cohort for all years (red bars), and those lost to follow-up (blue bars): a Mean infection intensity of Schistosoma mansoni in Uganda at baseline; b Prevalence of heavy intensity (≥400 epg) of S. mansoni in Uganda at baseline; c Mean infection intensity of S. haematobium in Burkina Faso at baseline; d Prevalence of heavy intensity (≥50 eggs/10 ml urine) of S. haematobium in Burkina Faso at baseline. P-Values are stated where differences are statistically significant (P ≤ 0.05), otherwise they are omitted

Fig. 4

Changes in egg count for each of the countries and endemicity areas under praziquantel treatment in: a and b Schistosoma mansoni endemic areas (intensity measured as eggs per gram of faeces), and in c and d S. haematobium endemic areas (intensity measured as eggs per 10 ml urine). Data were collected annually. The lines linking the data points are included for illustrative purposes are not verified parasitologically. These lines assume a constant force of infection and assume 95 % efficacy of treatment for S. mansoni and 99 % efficacy of treatment for S. haematobium

Fig. 5

Changes in the model-derived estimate of prevalence of heavy infection (S. mansoni ≥400 epg; S. haematobium ≥50e/10 ml) with treatment with praziquantel. a and b S. mansoni areas; c and d S. haematobium areas. Note difference in y-axis scales. Data were collected annually. The lines linking the data points are included for illustrative purposes are not verified parasitologically. These lines assume a constant force of infection and assume 95 % efficacy of treatment for S. mansoni and 99 % efficacy of treatment for S. haematobium