Helminths

Parasitic worms, also known as helminths, are large macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted and infect the gastrointestinal tract. Other parasitic worms such as schistosomes reside in blood vessels. Fasciola hepaticaNumber of speciesRegistered > 25,000Hymenolepis nanaTaenia solium /Taenia saginataFasciola hepaticaAscaris lumbricoidesHookwormTrichuris trichiuraSome days (eggs can survive for months)9–15 days18 days to several weeks1–2 days15–30 daysAfter hatching, the larvae develop into cysticercoid, which can survive for years in an animal5–7 weeks as cercariae in snails and longer periods in wet environments as encysted metacercariae10–14 days5–10 days (after maturing can survive for weeks outside the host)60–70 days (from hatching to mature state)5–6 days2 months (from cysticercoid to adult)3–4 months2–3 months2–8 weeks (can become dormant for months)4–6 weeksSeveral years8–10 years1–2 yearsSeveral years1 yearup to 6 times a daydailydailydaily50,000-100,000200,000 to 250,000 or more5,000-10,0003,000-20,000Although sparse in blood of persons in developed countries, eosinophils are often elevated in individuals in rural developing countries where intestinal parasitism is prevalent and metabolic syndrome rare. We speculate that eosinophils may have evolved to optimize metabolic homeostasis during chronic infections by ubiquitous intestinal parasites…. Parasitic worms, also known as helminths, are large macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted and infect the gastrointestinal tract. Other parasitic worms such as schistosomes reside in blood vessels. Some parasitic worms, including leeches and monogeneans, are ectoparasites - thus, they are not classified as helminths, which are endoparasites. Parasitic worms live in and feed in living hosts. They receive nourishment and protection while disrupting their hosts' ability to absorb nutrients. This can cause weakness and disease in the host. In their adult forms, parasitic worms cannot multiply in humans. Helminths are able to survive in their mammalian hosts for many years due to their ability to manipulate the host's immune response by secreting immunomodulatory products. All parasitic worms produce eggs during reproduction. These eggs have a strong shell that protects them against a range of environmental conditions. The eggs can therefore survive in the environment for many months or years. Many of the worms referred to as helminths are intestinal parasites. An infection by a helminth is known as helminthiasis, helminth infection, or intestinal worm infection. There is a naming convention which applies to all helminths: the ending '-asis' (or in veterinary science: '-osis') is added at the end of the name of the worm to denote the infection with that particular worm. For example, Ascaris is the name of a type of helminth, and ascariasis is the name of the infection caused by that helminth. Helminths are a group of organisms which share a similar form but are not necessarily related as part of evolution. The term 'helminth' is an artificial term. There is no real consensus on the taxonomy (or groupings) of the helminths, particularly within the nematodes. The term 'helminth' contains a number of phyla, many of which are completely unrelated. However, for practical considerations the term is currently used to describe four phyla with superficial similarities: Annelida (ringed or segmented worms), Platyhelminthes (flatworms), Nematoda (roundworms), and Acanthocephala (thorny-headed worms). The phylum Platyhelminthes includes two classes of worms of particular medical significance: the cestodes (tapeworms) and the trematodes (flukes and blood flukes), depending on whether or not they have segmented bodies. The number of different helminth species is vast: it is estimated to be around one million species. The nematodes are the most diverse of all the helminths with the highest number of species. There may be as many as 300,000 species of parasites affecting vertebrates, and as many as 300 affecting humans alone. Helminths of importance in the sanitation field are the human parasites, and are classified as Nemathelminthes (nematodes) and Platyhelminthes, depending on whether they possess a round or flattened body, respectively. Ringworm (dermatophytosis) is actually caused by various fungi and not by a parasitic worm. The lifetime of adult worms varies tremendously from one species to another but is generally in the range of 1 to 8 years (see following table). This lifetime of several years is a result of their ability to manipulate the immune response of their hosts by secreting immunomodulatory products.