Historic water quality patterns in endemic schistosomiasis regions of southern Africa: Implications for disease vector distribution

Abstract

Alterations in atmospheric temperature and rainfall have a major influence on the distribution patterns of vector-borne diseases, including schistosomiasis, by changing the optimum habitat of the intermediate hosts (vectors) that transmit these diseases. Water quality degradation and the creation of artificial freshwater habitats may widen or restrict the distribution of disease vectors. Understanding the influence of water quality on ecosystems is essential in determining the environments that vectors thrive in and where potential outbreaks of the disease they carry will occur. This study aims to analyse the changes in various water quality variables between 1977 and 2009 in the Mbombela river systems and how this may have altered the presence of schistosomiasis snail vectors in these areas. Historical water quality data for the Sabie, Kaap, Komati and Crocodile rivers were obtained from the Department of Water and Sanitation monitoring stations and separated into three decades (1977-1987; 1988-1998 and 1999-2009). The Physicochemical Driver Assessment Index (PAI) was used to determine the ecological category of the rivers in each decade. A Principal Component Analysis was also created for the different river systems to identify possible changes in water quality over the 3 decades. According to the PAI, the Sabie River remained in a relatively good state while the Crocodile, Komati and Kaap rivers had a low water quality to begin with and became further degraded over the three decades. The PCAs indicated the Sabie and Crocodile rivers had increased levels of salts, nutrients and pH between 1988-1998 and 1999-2009 while the Kaap River had high salt content and pH between 1977 and 1998 and high nutrients during the decade of 1999-2009. These results indicate that there has been a change in the water quality of Mbombela river systems with notable increases in salt, pH and nutrient levels. As a result, the aquatic biota have likely been negatively affected and this poses severe impacts on the presence and abundance of schistosomiasis vectors as the snails are sensitive to changes in salinity, nutrients and pH.