Epidemiology and control of intestinal schistosomiasis on the Sesse Islands, Uganda: integrating malacology and parasitology to tailor local treatment recommendations

Abstract

Background: Intestinal schistosomiasis is often widespread among the populations living around Lake Victoria and on its islands. The Sesse Island group (containing some 84 islands), however, is typically assumed to be a low prevalence zone, with limited transmission, but has never been surveyed in detail. Here, we present a rapid mapping assessment, bringing together snail and parasite information, at 23 sites for the presence of intermediate host snails and at 61 sites for the prevalence of intestinal schistosomiasis in school-aged children (N = 905). Two different diagnostic tools were used and compared at 45 of these sites: Kato-Katz thick faecal smears and circulating cathodic antigen (CCA) urine dipsticks.

Results: Biomphalaria snails were found at 11 sites but in low numbers; none was found shedding schistosome cercariae. At 22 out of the 45 sites, local prevalence by urine and/or stool diagnostics was in excess of 50%, although mean prevalence of intestinal schistosomiasis overall was 34.6% (95% confidence intervals (CI) = 31.0-38.3%) by Kato-Katz and 46.5% (95% CI = 42.7-50.4%) by CCA if 'trace' reactions were considered infection-positive (if considered infection-negative, mean prevalence was 28.1% (95% CI = 24.7-31.7%)). Diagnostic congruence between CCA and Kato-Katz was poor and significant discordance in estimated prevalence by location was found, with each often inferring different mass drug administration regimes.

Conclusions: Accurate estimation of schistosome prevalence is important for determining present and future treatment needs with praziquantel; the wide range of schistosome prevalence across the Sesse Island group requires a treatment regime largely tailored to each island. In high prevalence locations, further malacological sampling is required to confirm the extent of local transmission, especially on the northern islands within the group. The observation that different diagnostic tests can provide varying results in terms of estimating prevalence by location, and hence change treatment recommendations, suggests that care must be taken in interpreting raw prevalence data. In particular, further research into the reasons for the differences in the poorer performance of the CCA test should be pursued.

Figure 1

Results of rapid malacological and parasitological mapping assessment of the Sesse Islands, Uganda, in January 2010. (a) 23 sites surveyed for Biomphalaria snails, with spots coloured by abundance. Site 38 had two habitat types: 'a' denotes a marsh habitat, where B. sudanica snails were found, and 'b' denotes a lake habitat, where B. choanomphala were found. All other sites were lake habitats. (b) A total of 61 sites where school children were surveyed for intestinal schistosomiasis using Kato-Katz thick smears. Of these, 45 were investigated using both Kato-Katz and CCA urine dipsticks. The remaining sites, where only Kato-Katz thick smears were used, are marked with '*' and the results by location will be presented elsewhere (here the data were only used for overall prevalence estimates). CCA 'trace' results were considered infection-positive here. The location name, geographical coordinates and prevalence of schistosomiasis for each of the 45 sites is reported in Table 1, along with prevalence if CCA 'trace' was considered infection-negative.

Figure 2

Comparison of egg per gram (EPG) scores and corresponding CCA test result as 'positive', 'trace' or negative. (a) Number of CCA tests for a particular EPG score, and the breakdown of the tests into 'positive', 'trace' and 'negative' results. (b) Breakdown of CCA test results for schoolchildren who had zero S. mansoni eggs detected in their Kato-Katz thick smears.

Figure 3

Box plot of CCA test band strength against S. mansoni egg per gram count. Egg count based on duplicate Kato-Katz thick smear; indents in the boxes denote 95% CI.

Figure 4

Scatter plot of prevalence as determined by Kato-Katz against that determined by CCA test. 'Trace' result as negative is compared to 'trace' result as positive. The graph area is shaded based on the WHO thresholds for different treatment recommendations. 'A' denotes a site where Kato-Katz and CCA (when 'trace' results are called negative) both show prevalence under 50%, but which rises to over 50% if 'trace' CCA calls are considered positive. 'B' denotes a site where Kato-Katz and CCA results agree on a prevalence but only when CCA 'trace' results are considered positive; if considered negative, CCA result drops below 50%.