Estimating the prevalence and intensity of Schistosoma mansoni infection among rural communities in Western Tanzania: The influence of sampling strategy and statistical approach

Abstract

BackgroundSchistosoma mansoni is a parasite of major public health importance in developing countries, where it causes a neglected tropical disease known as intestinal schistosomiasis. However, the distribution of the parasite within many endemic regions is currently unknown, which hinders effective control. The purpose of this study was to characterize the prevalence and intensity of infection of S. mansoni in a remote area of western Tanzania.Methodology/Principal findingsStool samples were collected from 192 children and 147 adults residing in Gombe National Park and four nearby villages. Children were actively sampled in local schools, and adults were sampled passively by voluntary presentation at the local health clinics. The two datasets were therefore analysed separately. Faecal worm egg count (FWEC) data were analysed using negative binomial and zero-inflated negative binomial (ZINB) models with explanatory variables of site, sex, and age. The ZINB models indicated that a substantial proportion of the observed zero FWEC reflected a failure to detect eggs in truly infected individuals, meaning that the estimated true prevalence was much higher than the apparent prevalence as calculated based on the simple proportion of non-zero FWEC. For the passively sampled data from adults, the data were consistent with close to 100% true prevalence of infection. Both the prevalence and intensity of infection differed significantly between sites, but there were no significant associations with sex or age.Conclusions/SignificanceOverall, our data suggest a more widespread distribution of S. mansoni in this part of Tanzania than was previously thought. The apparent prevalence estimates substantially under-estimated the true prevalence as determined by the ZINB models, and the two types of sampling strategies also resulted in differing conclusions regarding prevalence of infection. We therefore recommend that future surveillance programmes designed to assess risk factors should use active sampling whenever possible, in order to avoid the self-selection bias associated with passive sampling.

Epidemiology of Schistosoma mansoni infection in Ituri Province, North-eastern Democratic Republic of the Congo

“…Indeed, in both the 2016 and 2017 studies, we observed a positive association between infection prevalence and intensity at the village level. Similar observations have been made in other endemic settings for S. mansoni [58] and other Schistosoma species [67].…”

“…Schistosomiasis particularly affects poor communities [1,[55][56][57] without the means to protect themselves from risky water contacts. Hence, rural communities are especially affected [58][59][60][61][62], but suburban and urban poor communities should also be considered [63][64][65]. Thus, it is not surprising that the participants living in Ngezi village in Bunia city had a very high S. mansoni infection burden, comparable to villages on the shore of Lake Albert.…”

The prevalence of schistosomiasis in Uganda: A nationally representative population estimate to inform control programs and water and sanitation interventions

“…This is in line with a similar finding from western Tanzania where distribution of S . mansoni was more widespread than was previously understood [42]. This was the first national survey in Uganda to investigate schistosomiasis beyond known endemic areas and adds to work on the epidemiology of S .…”

Prevalence, infection intensity and geographical distribution of schistosomiasis among pre-school and school aged children in villages surrounding Lake Nyasa, Tanzania

“…The north-western region located along the Lake Victoria basin and the southern part of the Lake Victoria are known to be highly endemic to both S. mansoni and S. haematobium infections 4 . However, there is limited information on the geographical distribution of schistosomiasis in the two other large water bodies in Tanzania, Lake Tanganyika in the western region 4 , 20 , 21 and Lake Nyasa in the southern highlands 4 , 22 . We focused on the Lake Nyasa, in the southern part of the country, on the international border between Malawi, Mozambique and Tanzania.…”