Additionally, approaches to quantify to an organismal level remain in their infancy for eukaryotic targets. Some questions still remain regarding the accuracy and reproducibility of using qPCR to measure trematodes (or for that matter, eukaryotic organisms) in water. As qPCR cercariometry enables the quantification of cercariae in the water and at recreation sites, it measures the potential exposure of bathers to cercariae. Cercariae concentration and distribution in the water body may be influenced by environmental factors such as wind and water movement, or predation, inactivation due to UV light, or death (Haas 1994 McCarthy 1999). qPCR cercariometry offers a notable advantage to traditional snail infection prevalence methodologies when the end goal is assessment of swimmer’s itch risk. This assay can detect species of the family Schistosomatidae, including Trichobilharzia spp., Schistosomatium douthitti, Allobilharzia spp., Gigantobilharzia, spp., and Dendritobilharzia spp. A quantitative polymerase chain reaction assay for the detection of avian schistosomes, or swimmer’s itch-causing cercariae, in recreational water was validated and published in 2015 (Narayanan et al. Furthermore, as cercariae are essential both for transmission of the parasite, and cause swimmer’s itch, quantifying their abundance in the water is key to understanding both the ecological and public health aspects of this parasite. 2016), making detection difficult, and collecting and screening snails is time-consuming. Notably, avian schistosomes typically show a low prevalence in snail intermediate hosts (Crews and Esch 1986 Brown et al. emarginata, it has a number of downsides. While this method assesses the success of a specific parasite species in infecting a snail species such as S. Shed cercariae are then morphologically identified using a stereomicroscope. This method requires collecting a large number of snails from multiple sites around a lake and allowing the snails to shed parasites. Monitoring efforts for swimmer’s itch typically rely on assessments of infection prevalence of relevant parasites in snails. Swimmer’s itch is a prominent problem in recreational lakes in northern Michigan, where the predominant species responsible for swimmer’s itch is Trichobilharzia stagnicolae, hosted by the common merganser ( Mergus merganser) and the snail Stagnicola emarginata (Blankespoor and Reimink 1991 Keas and Blankespoor 1997). The resultant immune response leaves itchy papules that can last for weeks (Baird and Wear 1987 Verbrugge et al. Swimmer’s itch, or cercarial dermatitis, is caused by epidermal penetration of humans by larval cercariae in surface waters (Brant and Loker 2009). While the cercariae of some species of schistosome (those of the genus Schistosoma) are known to infect humans, species of the genera Trichobilharzia, Gigantobilharzia, Dendritobilharzia, Anserobilharzia, and Schistosomatium cannot, but are able to penetrate human skin and lead to a condition known as swimmer’s itch (Cort 1928 Batten 1956 Kolářová et al. 1980 McCarthy 1999 Zbikowska 2004 Coady et al. The number of cercariae released each day can vary significantly between hundreds to tens of thousands of larval parasites and can be influenced by factors such as temperature, duration of infection, and immunogenetic determinants of snail-schistosome compatibility (Sluiters et al. Cercariae then seek out their definitive host to complete their life cycle. Sporocysts produce cercariae asexually, which are then shed from infected snails (Cort 1928 Blair and Ottesen 1979 Blair and Islam 1983). Miracidia penetrate the snail host and develop into sporocysts. Adult worms reside in the avian or mammalian hosts and produce embryonated eggs that typically exit the definitive host via feces or urine into the aquatic environment, where they hatch into a free-living larval stage (miracidium). Parasites of the family Schistosomatidae utilize avian or mammalian definitive hosts and snail intermediate hosts to complete their life cycles.
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