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These can be categorized into three groups; cestodes, nematodes and trematodes. Examples include: Acanthocephala; Ascariasis (roundworms) Cestoda (tapeworms) including: Taenia saginata (human beef tapeworm), Taenia solium (human pork tapeworm), Diphyllobothrium latum (fish tapeworm) and Echinococcosis (hydatid tapeworm)
Cestodes are parasites of vertebrates, with each species infecting a single definitive host or group of closely related host species. All but amphilinids and gyrocotylids (which burrow through the gut or body wall to reach the coelom [ 6 ] ) are intestinal, though some life cycle stages rest in muscle or other tissues.
Adult trematodes lay smaller numbers of eggs compared to cestodes or nematodes. However, the egg develops into a miracidia from which thousands of cercariae, or swimming larvae, develop. This means that one egg may produce thousands of adult worms. [15]
Other internal parasites are found living inside fish gills, include encysted adult didymozoid trematodes, [15] a few trichosomoidid nematodes of the genus Huffmanela, including Huffmanela ossicola which lives within the gill bone, [16] and the encysted parasitic turbellarian Paravortex. [17]
Cestodes (tapeworms) and digeneans (flukes) cause diseases in humans and their livestock, whilst monogeneans can cause serious losses of stocks in fish farms. [44] Schistosomiasis , also known as bilharzia or snail fever, is the second-most devastating parasitic disease in tropical countries, behind malaria .
On average, a mammal species hosts four species of nematode, two of trematodes, and two of cestodes. [85] Humans have 342 species of helminth parasites, and 70 species of protozoan parasites. [ 86 ] Some three-quarters of the links in food webs include a parasite, important in regulating host numbers.
In 2014–15, the WHO estimated that approximately 2 billion people were infected with soil-transmitted helminthiases, [6] 249 million with schistosomiasis, [60] 56 million people with food-borne trematodiasis, [61] 120 million with lymphatic filariasis, [62] 37 million people with onchocerciasis, [63] and 1 million people with echinococcosis. [64]
With its draft genome sequence published in 2015, F. hepatica is known to have the largest nuclear genome size among trematodes so far sequenced. It is about 1.3 Gb, [30] which is two times that of Opisthorchis viverrini with 634.5 Mb, the second largest genome among trematodes. [31] The genome is contained in 10 pairs of chromosomes.