8/11/2023 0 Comments Swimmers itch dogs![]() Often overlooked is the fact that these six human-infective Schistosoma species comprise only a small minority of species in the Schistosomatidae. japonicum) are primarily responsible for causing the 220 million cases of schistosomiasis, with a few additional species ( S. Schistosomes achieve their greatest notoriety for their role in causing the neglected tropical disease of human schistosomiasis. People in contact with the water are also at risk of skin penetration by the cercariae, which typically incite a strong inflammatory reaction, swimmer’s itch, and usually, but not always, die in the skin. The cercariae exit the snail, swim, and are carried by currents or wave action, and once they have located a merganser will penetrate the skin and continue the life cycle. A miracidium transforms into a mother sporocyst that produces multiple daughter sporocysts that migrate to the snail’s digestive gland where they produce numerous cercariae. ![]() Eggs hatch and release swimming, ciliated miracidia that locate and penetrate the freshwater snail host Stagnicola emarginata. Note the involvement of an avian definitive host such as the common merganser ( Mergus merganser) in which adult worms mate and reproduce, resulting in discharge of schistosome eggs into the water. In this case, Trichobilharzia stagnicolae is commonly implicated in swimmer’s itch outbreaks in oligotrophic lakes in Michigan, in the northern USA. Typical life cycle of an avian schistosome. Insofar as schistosome species that normally develop in birds and non-human mammals can penetrate human skin and cause varying degrees of pathology, the phenomenon of swimmer’s itch exemplifies a zoonosis, i.e., a disease transmissible from animals to humans. Once in their normal definitive host, they undertake a complex migration route to the intravascular system before they attain sexual maturity and produce eggs, which are passed in the feces, or occasionally by other routes. Cercariae are highly adapted for their role in locating hosts and then penetrating the skin barrier. Rather, schistosome cercariae penetrate directly into the skin or other epithelial surfaces, such as in the mouth or throat, to gain entrance into their definitive hosts. One distinctive aspect of the schistosome life cycle germane to this volume on avian schistosomes is that the free-swimming cercariae that are produced in large numbers in their snail hosts, once released into water do not encyst in or on other hosts and await ingestion by the definitive host, as is typical of many trematode life cycles. Their generalized life cycle ( Figure 1) is typical of trematodes in that they must undergo an obligatory period of larval development in molluscs, in this case particular species of freshwater or marine gastropods. Members of the digenetic trematode family Schistosomatidae are unusual trematodes in having separate adult male and female worms that inhabit the vascular systems of their avian or mammalian definitive hosts. Given the progress made in revealing their diversity and biology, and the wealth of questions posed by itch-causing schistosomes, they provide excellent models for implementation of long-term interdisciplinary studies focused on issues pertinent to disease ecology, the One Health paradigm, and the impacts of climate change, biological invasions and other environmental perturbations. Swimmer’s itch also exemplifies the value of adopting the One Health perspective in understanding disease transmission and abundance because the schistosomes involved have complex life cycles that interface with numerous species and abiotic components of their aquatic environments. ![]() Swimmer’s itch is a complex zoonotic disease manifested through several different routes of transmission involving a diversity of different host species. ![]() It seems increasingly likely that schistosomes have on two separate occasions colonized mammals. Basal lineages today are found in marine gastropods and birds, but subsequent diversification has largely taken place in freshwater, with some reversions to marine habitats. Collectively, schistosomes exploit 16 families of caenogastropod or heterobranch gastropod intermediate hosts. Recent years have witnessed a dramatic increase in our understanding of schistosome diversity, now encompassing 17 genera with eight more lineages awaiting description. The cercariae of many of these species can cause swimmer’s itch when they penetrate human skin. Although most studies of digenetic trematodes of the family Schistosomatidae dwell on representatives causing human schistosomiasis, the majority of the 130 identified species of schistosomes infect birds or non-human mammals.
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