Assessment of Giardia and Cryptosporidium spp. as a microbial source tracking tool for surface water: application in a mixed-use watershed

Abstract

Knowledge of host specificity, combined with genomic sequencing of Giardia and Cryptosporidium spp., has demonstrated a microbial source tracking (MST) utility for these common waterborne microbes. To explore the source attribution potential of these pathogens, water samples were collected in a mixed rural-urban watershed in the Township of Langley, in southwestern British Columbia (BC), Canada, over a 2-year period. Cryptosporidium was detected in 63% of surface water samples at concentrations ranging from no positive detection (NPD) to 20,600 oocysts per 100 liters. Giardia was detected in 86% of surface water samples at concentrations ranging from NPD to 3,800 cysts per 100 liters of water. Sequencing at the 18S rRNA locus revealed that 50% of Cryptosporidium samples and 98% of Giardia samples contained species/genotypes (Cryptosporidium) or assemblages (Giardia) that are capable of infecting humans, based on current knowledge of host specificity and taxonomy. Cryptosporidium genotyping data were more promising for source tracking potential, due to the greater number of host-adapted (i.e., narrow-host-range) species/genotypes compared to Giardia, since 98% of Giardia isolates were zoonotic and the potential host could not be predicted. This report highlights the benefits of parasite genomic sequencing to complement Method 1623 (U.S. Environmental Protection Agency) and shows that Cryptosporidium subtyping for MST purposes is superior to the use of Giardia subtyping, based on better detection limits for Cryptosporidium-positive samples than for Giardia-positive samples and on greater host specificity among Cryptosporidium species. These additional tools could be used for risk assessment in public health and watershed management decisions.

FIG 1

Sampling sites within the Salmon River watershed used in this study for genotyping of Giardia and Cryptosporidium isolates. A, B, and C refer to Hopington Aquifer A, Aquifer B, and Aquifer C, respectively. The map was generated using an open-source orthophoto (taken in 1995) and ArcView GIS.

FIG 2

Prevalence of Cryptosporidium detected at two sampling sites (Coghlan Creek and Salmon River) in the Township of Langley, British Columbia, using Method 1623. Sep, September; Dec, December; Mar, March; Jun, June; Oct, October; Jan, January; Apr, April; Aug, August; Nov, November.

FIG 3

Prevalence of Giardia detected at two sampling sites (Coghlan Creek and Salmon River) in the Township of Langley, British Columbia, using Method 1623.