Simplifying Schistosome Surveillance: Using Molecular Cercariometry to Detect and Quantify Cercariae in Water

Abstract

Avian schistosomes are considered a public health nuisance due to their ability to cause swimmer’s itch when accidentally encountering humans rather than their intended avian hosts. Researchers have been monitoring their presence and abundance through snail collections and cercariometry. Cercariometry methods have evolved over the last several decades to detect individual schistosome species from a single water sample, simplifying the monitoring of these parasites. This methodological evolution coincides with the development of the field of environmental DNA (eDNA) where genetic material is extracted from environmental samples, rather than individual organisms. While there are some limitations with using molecular cercariometry, notably the cost and its inability to differentiate between life cycle stages, it substantially reduces the labor required to study trematode populations. It also can be used in complement with snail collections to understand the composition of avian schistosomes in an environment.

Large-scale spatial drivers of avian schistosomes in Northern Michigan inland lakes

“…Prior studies found evidence of resource limitation in some northern Lymnaea populations (Cuker, 1983;O'Brien et al, 2005), suggesting that growth of attached algae (and by extension water clarity and/or nutrients) might sometimes be important drivers of these snail populations. Other studies have found an important role for top-down regulation of Lymnaea populations by fish and invertebrate predators (Hershey, 1990;Merrick et al, 1991), which we were unable to assess in this study. Future studies are needed to expand the geographical scope of our knowledge about SI drivers in temperate inland lakes and use manipulative experiments to test whether the relationships reported here are likely to be causal.…”

“…Briefly, we used a recently detailed protocol and custom primers that target a highly conserved region of 18S ribosomal RNA gene sequences specific to schistosome parasites (Jothikumar et al ., 2015 ). This assay only differentiates schistosomes vs non-schistosomes; we assumed that any amplified DNA largely belonged to the genus most associated with swimmer's itch in this geographic region, Trichobilharzia , though other SI-causing schistosomes from the genera Schistosomatium and Gigantobilharzia have also been observed in Michigan (Muzzall et al ., 2003 ; McPhail et al ., 2022 ; Rudko et al ., 2022 , Soper et al ., 2023 ). We decided to divide each field sample in half prior to extraction, which we hoped would allow us to assess potential sources of error in the assay (i.e.…”

“…Avian schistosomes cannot complete their life cycles in humans, but their cercariae sometimes mistake humans for birds and penetrate our skin resulting in the distinctive rash (Verbrugge et al ., 2004 a ; Horák et al ., 2015 ). Anecdotal evidence suggests that SI is increasing in Michigan inland lakes, where it discourages swimming and other recreational activities impacting the local economy during the summer (Muzzall et al ., 2003 ; Verbrugge et al ., 2004 b ; McPhail et al ., 2022 ; Soper et al ., 2023 ). The drivers for distribution of human schistosomes have been widely studied at multiple spatial scales (Yang et al ., 2006 ; Brooker, 2007 ; Zhou et al ., 2011 ); however, ecological drivers of among-lake variation in avian schistosomes have been relatively understudied (McMullen and Brackett, 1948 ; Rudko et al ., 2018 ; Soper et al ., 2023 ).…”

Swimmer’s itch control: Timely waterfowl brood relocation significantly reduces an avian schistosome population and human cases on recreational lakes

“…In contrast, Rudko et al [ 29 ] utilized qPCR to estimate the number of cercariae in 25 L surface water samples taken weekly from multiple sites on each lake and utilized a citizen science network for sample collection. Advantages of this approach include the ability to sample more sites more frequently and citizen scientists can be trained to properly collect samples [ 12 , 60 , 61 ]. However, this measure is multiple steps removed from the number of snails infected and the amount of merganser to snail transmission since each snail infected does not produce the same number of cercariae.…”

Zoonotic Threats: The (Re)emergence of Cercarial Dermatitis, Its Dynamics, and Impact in Europe

“…Countries outside Europe may exhibit similar CD agents and challenges but also differences, as follows. Both the USA and Canada use the cercariometry approach for detection of species involved in CD [ 113 , 114 ], which is not a tool commonly used in Europe. These countries also feature the presence of Lymnaea sp., but, in contrast, Physa sp.…”