cercaria

{{Short description|Larval form of parasite}}

{{About|the flatworm life stage|the genus of flatworms|Cercaria (genus)|the genus of single-celled organisms|Euglena}}

A cercaria (plural cercariae) is a larval form of the trematode class of parasites. It develops within the germinal cells of the sporocyst or redia.{{cite web|url=http://cal.vet.upenn.edu/projects/parasit06/website/glossary.htm|title=Glossary|work=VPTH 603 Veterinary Parasitology|publisher=University of Pennsylvania School of Veterinary Medicine|access-date=2019-05-22|archive-url=https://web.archive.org/web/20110718210627/http://cal.vet.upenn.edu/projects/parasit06/website/glossary.htm|archive-date=2011-07-18|url-status=dead}} A cercaria has a tapering head with large penetration glands.{{cite web|title=Schistosoma|url=http://parasite.org.au/para-site/text/schistosoma-text.html|publisher=Australian Society for Parasitology|accessdate=19 February 2011}} It may or may not have a long swimming "tail", depending on the species. The motile cercaria finds and settles in a host where it will become either an adult, or a mesocercaria, or a metacercaria, according to species.

Cercarial infection in water environments by non-human schistosome species causes dermatologic burden to nearby swimmers, fishermen and farmers. The cycle as mentioned above, starts with egg distribution whether fecal in route or from the nostril of a duck or goose.{{cite journal|last1=Horák|first1=P.|last2=Kolářová|first2=L.|last3=Adema|first3=C.M.|title=Biology of the Schistosome Genus Trichobilharzia|journal=Advances in Parasitology|volume=52|issn=0065-308X|year=2002|pages=155-233|doi=10.1016/S0065-308X(02)52012-1}}{{rp|157-159}} Miracidia infect snail reservoirs and form successive sporocysts. Released cercariae travel and infect nearby non-human mammals or birds, depending on species, or accidental hosts such as humans. This dead-end infection and host allergic reaction has been referred to as cercarial dermatitis or "swimmer's itch".{{cite web|url=https://www.cdc.gov/dpdx/cercarialdermatitis/index.html|title=Cercarial Dermatitis|website=DPDx|publisher=U.S. Centers for Disease Control and Prevention|access-date=21 December 2024|date=10 May 2019|author=((National Center for Emerging and Zoonotic Infectious Diseases, Division of Parasitic Diseases and Malaria))}}{{cite web|url=https://www.cdc.gov/swimmers-itch/about/index.html|title=About Swimmer's Itch|website=Swimmer's Itch (Cercarial Dermatitis)|publisher=U.S. Centers for Disease Control and Prevention|access-date=21 December 2024|date=10 September 2024}} Dermatitis can also be seen following exposure to the cercariae of schistosome species that cause patent infections in humans (schistosomiasis) in endemic areas.{{cite journal|last1=Kolářová|first1=L.|last2=Horák|first2=P.|last3=Skírnisson|first3=K.|last4=Marečková|first4=H.|last5=Doenhoff|first5=M.|year=2012|title=Cercarial Dermatitis, a Neglected Allergic Disease|journal=Clinical Reviews in Allergy & Immunology|volume=45|issue=1|pages=63-74|doi=10.1007/s12016-012-8334-y}}

Among fish, infection beneath the scale bedding by cercariae of other trematode species can result in black spot disease.{{cite web|url=https://www.outdoorcanada.ca/black-spot-disease/|title=The Real Truth About Black Spot Disease, and How It Affects Fish|date=8 January 2018|website=Outdoor Canada|access-date=21 December 2024|last=Pyzer|first=Gord}}{{cite book|last1=Meyers|first1=Theodore|last2=Burton|first2=Tamara|last3=Bentz|first3=Collette|last4=Ferguson|first4=Jayde|last5=Stewart|first5=Davis|last6=Starkey|first6=Norman|date=July 2019|publisher=Fish Pathology Laboratories, Alaska Department of Fish and Game|orig-date=Originally published in 2007|oclc=1104329201|chapter-url=https://www.adfg.alaska.gov/static/species/disease/pdfs/fishdiseases/black_spot_disease.pdf|chapter=Black Spot Disease (Neascus and Heterphyids)|access-date=21 December 2024|title=DISEASES OF WILD AND CULTURED FISHES IN ALASKA|pages=78-79}} This is an example of an encysted form, or metacercaria. For some trematode species, cercarial encystment takes place on aquatic plants (e.g. in the liver fluke Fasciola gigantica{{cite web|url=https://animaldiversity.org/accounts/Fasciola_gigantica/|title=Fasciola gigantica|website=Animal Diversity Web|last=Carmona|first=Marcus|editor-last=Mulcrone|editor-first=Renee|year=2013|access-date=21 December 2024}} and the intestinal fluke Fasciolopsis buski{{cite web|url=https://animaldiversity.org/accounts/Fasciolopsis_buski/|title=Fasciolopsis buski|last=Greiner|first=Lindsey|editor-last1=Liere|editor-first1=Heidi|editor-last2=Marino|editor-first2=John|editor-last3=OConnor|editor-first3=Barry|editor-last4=Mulcrone|editor-first4=Renee|website=Animal Diversity Web|year=2011|access-date=21 December 2024}}).

The term Cercaria is also used as a genus name in descriptions of species when only the larval form is known.{{Cite WoRMS| year = 2023| title = Cercaria Müller, 1773| id = 728666| access-date = 2023-02-26 }}

Rotifers (Rotaria rotatoria) produce a chemical, Schistosome Paralysis Factor, that suppresses cercaria swimming and reduces infections.{{cite journal |editor1-last=Khosla |editor1-first=Chaitan |last1=Gao|first1=Jiarong|last2=Yang|first2=Ning|last3=Lewis|first3=Fred A.|last4=Yau|first4=Peter|last5=Collins|first5=James J.|last6=Sweedler|first6=Jonathan V.|last7=Newmark|first7=Phillip A.|title=A rotifer-derived paralytic compound prevents transmission of schistosomiasis to a mammalian host|journal=PLOS Biology|volume=17|issue=10|year=2019|pages=e3000485|issn=1545-7885|doi=10.1371/journal.pbio.3000485|pmid=31622335 |pmc=6797223 |doi-access=free}}