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The flatworms, flat worms, Platyhelminthes, or platyhelminths are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Unlike other bilaterians, they are acoelomates, and have no specialised circulatory and respiratory organs, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both ingestion and egestion ; as a result, the food cannot be processed continuously.
Trematoda is a class of flatworms known as flukes or trematodes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is usually a snail. The definitive host, where the flukes sexually reproduce, is a vertebrate. Infection by trematodes can cause disease in all five traditional vertebrate classes: mammals, birds, amphibians, reptiles, and fish.
Schistosoma is a genus of trematodes, commonly known as blood flukes. They are parasitic flatworms responsible for a highly significant group of infections in humans termed schistosomiasis, which is considered by the World Health Organization as the second-most socioeconomically devastating parasitic disease, with hundreds of millions infected worldwide.
Clonorchis sinensis, the Chinese liver fluke, is a liver fluke belonging to the class Trematoda, phylum Platyhelminthes. It infects fish-eating mammals, including humans. In humans, it infects the common bile duct and gall bladder, feeding on bile. It was discovered by British physician James McConnell at the Medical College Hospital in Calcutta (Kolkata) in 1874. The first description was given by Thomas Spencer Cobbold, who named it Distoma sinense. The fluke passes its lifecycle in three different hosts, namely freshwater snail as first intermediate hosts, freshwater fish as second intermediate host, and mammals as definitive hosts.
Digenea is a class of trematodes in the Platyhelminthes phylum, consisting of parasitic flatworms with a syncytial tegument and, usually, two suckers, one ventral and one oral. Adults commonly live within the digestive tract, but occur throughout the organ systems of all classes of vertebrates. Once thought to be related to the Monogenea, it is now recognised that they are closest to the Aspidogastrea and that the Monogenea are more closely allied with the Cestoda. Around 6,000 species have been described to date.
Monogeneans, members of the class Monogenea, are a group of ectoparasitic flatworms commonly found on the skin, gills, or fins of fish. They have a direct lifecycle and do not require an intermediate host. Adults are hermaphrodites, meaning they have both male and female reproductive structures.
Metagonimiasis is a disease caused by an intestinal trematode, most commonly Metagonimus yokagawai, but sometimes by M. takashii or M. miyatai. The metagonimiasis-causing flukes are one of two minute flukes called the heterophyids. Metagonimiasis was described by Katsurasa in 1911–1913 when he first observed eggs of M. yokagawai in feces. M. takahashii was described later first by Suzuki in 1930 and then M. miyatai was described in 1984 by Saito.
The Aspidogastrea is a small group of flukes comprising about 80 species. It is a subclass of the trematoda, and sister group to the Digenea. Species range in length from approximately one millimeter to several centimeters. They are parasites of freshwater and marine molluscs and vertebrates. Maturation may occur in the mollusc or vertebrate host. None of the species has any economic importance, but the group is of very great interest to biologists because it has several characters which appear to be archaic.
Schistosomatidae is a family of digenetic trematodes with complex parasitic life cycles. Immature developmental stages of schistosomes are found in molluscs and adults occur in vertebrates. The best studied group, the blood flukes of the genus Schistosoma, infect and cause disease in humans. Other genera which are infective to non-human vertebrates can cause mild rashes in humans.
Griphobilharzia amoena is a significant trematode that infect crocodiles such as the Australian freshwater crocodile, Crocodylus johnstoni, located in Darwin, Australia with reported illness in Irian Jaya as well. They possess a distinctive tegument that is composed of two lipid bilayers instead of a single bilayer. The double bilayer may be an adaptation to survive the host's immune response.
Heterophyes heterophyes, or the intestinal fish fluke, was discovered by Theodor Maximaillian Bilharz in 1851. This parasite was found during an autopsy of an Egyptian mummy. H. heterophyes is found in the Middle East, West Europe and Africa. They use different species to complete their complex lifestyle. Humans and other mammals are the definitive host, first intermediate host are snails, and second intermediate are fish. Mammals that come in contact with the parasite are dogs, humans, and cats. Snails that are affected by this parasite are the Cerithideopsilla conica. Fish that come in contact with this parasite are Mugil cephalus, Tilapia milotica, Aphanius fasciatus, and Acanthgobius sp. Humans and mammals will come in contact with this parasite by the consumption of contaminated or raw fish. This parasite is one of the smallest endoparasite to infect humans. It can cause intestinal infection called heterophyiasis.
Spirorchiidae is a family of digenetic trematodes. Infestation by these trematodes leads to the disease spirorchiidiosis. Spirorchiids are mainly parasites of turtles. It has been synonymised with Proparorchiidae Ward, 1921, Spirorchidae Stunkard, 1921, and Spirorchiidae MacCallum, 1921.
Clinostomum marginatum is a species of parasitic fluke. It is commonly called the "yellow grub". It is found in many freshwater fish in North America, and no fish so far is immune to this parasite. It is also found in frogs. Clinostomum marginatum can also be found in the mouth of aquatic birds such as herons and egrets. They are commonly present in the esophagus of fish-eating birds and reptiles. Eggs of these trematodes are shed in the feces of aquatic birds and released into water. Aquatic birds become hosts of this parasite by ingesting infected freshwater fish. The metacercariae are found right beneath the skin or in the muscles of host fish.
Aspidogaster conchicola is a trematode parasite of the Aspidogastrea subclass that commonly infects freshwater clams. It has not been well studied since it is of little economic or medical importance, but A. conchicola and its fellow aspidogastreans are of significant biological importance since they may represent a step between free-living and parasitic organisms.
Bivitellobilharzia nairi is a species of trematodes, part of the family Schistosomatidae. This is a fairly new identified endoparasite that was found in 1955 by Mudaliar and Ramanujachari, who first recorded the parasite in India. Researchers collected fecal samples of the Indian rhinoceros and were startled to find B. nairi eggs.
Metorchis conjunctus, common name Canadian liver fluke, is a species of trematode parasite in the family Opisthorchiidae. It can infect mammals that eat raw fish in North America. The first intermediate host is a freshwater snail and the second is a freshwater fish.
Choanocotyle is a genus of flatworms in the family Choanocotylidae. Species infect freshwater turtles.
Electrovermis zappum is a species of fish blood fluke discovered in the Gulf of Mexico and described in 2019. It has been placed in a new genus, Electrovermis. This parasite infects the lesser electric ray. Once it successfully invades the host, it migrates to and dwells in the lumen of the heart of the ray. Like other blood flukes, it thrives by feeding off of the blood of its host. The adults are morphologically and physiologically similar to other blood flukes that infect rays such as Orchispirium heterovitellatum. Unlike many other blood flukes that infect molluscs as an intermediate host, E. zappum infects bivalves such as clams. Both the rays and clams densely populate warm, shallow intertidal marine waters, which provide the parasite an opportunistic environment to carry out its life cycle stages.
Gastropod-borne parasitic diseases (GPDs) are a group of infectious diseases that require a gastropod species to serve as an intermediate host for a parasitic organism that can infect humans upon ingesting the parasite or coming into contact with contaminated water sources. These diseases can cause a range of symptoms, from mild discomfort to severe, life-threatening conditions, with them being prevalent in many parts of the world, particularly in developing regions. Preventive measures such as proper sanitation and hygiene practices, avoiding contact with infected gastropods and cooking or boiling food properly can help to reduce the risk of these diseases.
References
- ↑ Sarah Kaplan (September 9, 2016). "Scientists like Barack Obama so much that they named a parasite after him". The Washington Post.
- ↑ Roberts, Jackson R.; Platt, Thomas R.; Orélis-Ribeiro, Raphael; Bullard, Stephen A. (2016). "New Genus of Blood Fluke (Digenea: Schistosomatoidea) from Malaysian Freshwater Turtles (Geoemydidae) and its Phylogenetic Position Within Schistosomatoidea". Journal of Parasitology. 102 (4): 451–462. doi:10.1645/15-893. PMID 27042972. S2CID 30035139.
