Spatial distribution of risk factors for Cryptosporidium spp. transport in an Irish catchment.

Cryptosporidium spp. has become a major public health concern in many parts of the globe, including Ireland, as a result of recent reported waterborne outbreaks of Cryptosporidiosis. Continuous monitoring of Cryptosporidium spp. in water supplies is not feasible, so a risk-forecasting approach is required. This study reports a globally applicable approach for evaluating the spatial variation in relative risk of contaminating surface water by Cryptosporidium spp. based on a risk potential index (RPI) as an indicator of the potential pollution of surface water. The RPI is predicted by readily available data on land use, rainfall, soil type, slope, soil moisture deficit, and distance from water course. A small catchment in County Meath, Ireland, was chosen to illustrate the analysis of the approach. Data for the study area were digitized and rectified using surveyed ground control points to capture each of the RPI factors, field boundaries, and land use. The six parameters were classified and assigned a relative risk score out of 5. A Geographic Information Systems overlay analysis then was used to calculate a cumulative relative risk score for each month of the year. The analysis indicated that April and June experienced a relatively low risk of Cryptosporidium spp. transport compared with other months of the year. June had the least risk, because more than 98% of the catchment was estimated to be of low or moderate risk (RPI ranges = 0 to 2). December had the highest risk of Cryptosporidium spp. transport, because approximately 20% of the catchment area had a moderately high to very high risk (RPI ranges = 2 to 5). The study also made an attempt to reduce the risk of contaminating surface water by alternative land-use practice and relocating the field boundaries. The study demonstrated a semi-quantitative and readily implemented method for using spatial risk assessment for planning land management to reduce the risk of surface water contamination by Cryptosporidium spp. from agricultural sources. The approach could be used in any catchment with similar hydrological characteristics subject to a surface water pathogen contamination risk from grazing livestock.