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Parasitic worms, also known as helminths, are a polyphyletic group of 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 , 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 . This can cause weakness and disease in the host, and poses a global health and economic problem. Parasitic worms cannot reproduce entirely within their host's body; they have a life cycle that includes some stages that need to take place outside of the host. 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.Jirillo, E., Magrone, T., Miragliotta, G. (2014). "Immunomodulation by Parasitic Helminths and its Therapeutic Exploitation". In: Pineda, M.A., Harnett, W. (eds.), Immune Response to Parasitic Infections (Vol. 2, pp. 175–212), Bentham eBooks, , . All parasitic worms produce 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.
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 fungus, and not by a parasitic worm.
Helminths can be either hermaphroditic (having the sex organs of both sexes), like tapeworms and flukes (not including the blood fluke), or have their sexes differentiated, like the roundworms. All helminths produce eggs (also called ova) for reproduction.
Helminth eggs are resistant to various environmental conditions due to the composition of the egg shell. Each helminth egg species has 3 to 4 layers with different physical and chemical characteristics:
The process of larval maturation in the host can take from about two weeks up to four months, depending on the helminth species.
The following table shows the principal morphological and reproductive distinctions for three helminth groups:
| Examples | Taenia solium, Taenia saginata, Hymenolepis spp., Echinococcus granulosus, Echinococcus multilocularis, Multiceps multiceps | Schistosoma mansoni, Schistosoma japonicum, Fasciola hepatica | Ascaris, Enterobius, Filarioidea, Onchocerca, Rhabditis spp., Trichuris, Necator americanus, Ancylostoma spp. | |||||
| Pathological conditions caused in humans | Tapeworm infection, echinococcosis, alveolar echinococcosis | Schistosomiasis, swimmer's itch | Ascariasis, enterobiasis (pinworm infection, oxyuriasis), filariasis, dracunculiasis (guinea worm), elephantiasis, enterobiasis, filariasis, hookworm infection (includes Necatoriasis and Ancylostoma duodenale infection), onchocerciasis, trichinosis, trichuriasis | |||||
| Cylindrical | ||||||||
| Present | ||||||||
| Cuticle | ||||||||
| Ends in anus | ||||||||
| Dioecious | ||||||||
| Lips, teeth, filariform extremities, and dentary plates | ||||||||
| Number of species| colspan="2" 6000Lamonthe Argumedo, R., Garcia Prieto, L. (1988). Human helminthiasis in Mexico. Treatment and prophylaxis , A.G.T. Editor, S.A., 1st edition, Mexico. () | Estimated > 15,000Pumarola, A., Rodríguez-Torres, A., García, R.J.A., Piedrola, A.G. (1987). Medical Microbiology and Parasitology (in Spanish), Ediciones Científicas y Técnicas, S. A., Barcelona, Spain, pp 850 – 880 Registered > 9,000 | Estimated > 800,000 to 1,000,000 Registered > 25,000 | ||||||
| Number of species known to infect humans | 40 | 16 | > 12,000 | |||||
| Species | Hymenolepis nana | Taenia solium /Taenia saginata | Fasciola hepatica | Ascaris lumbricoides | Hookworm | Trichuris trichiura Toxocara spp. | ||
| Timeline of lifecycle stages | Larval formation | Some days (eggs can survive for months) | 9–15 days | 18 days to several weeks | 1–2 days | 15–30 days | ||
| Larval growth | After hatching, the larvae develop into cysticercoid, which can survive for years in an animal | 5–7 weeks as cercariae in snails and longer periods in wet environments as encysted metacercariae | 10–14 days | 5–10 days (after maturing can survive for weeks outside the host)| rowspan="2" | 60–70 days (from hatching to mature state) | 5–6 days | ||
| Maturation to adult | 2 months (from cysticercoid to adult) | 3–4 months | 2–3 months | 2–8 weeks (can become dormant for months) | ||||
| Lifespan of adult worm | 4–6 weeks | Several years | 8–10 years | 1–2 years | Several years | 1 year | ||
| Eggs laid per day | 250,000 to 700,000 | 3,000 to 25,000 | 3,000 to 250,000 | |||||
| Egg deposition | Frequency | up to 6 times a day | daily | daily | daily | |||
| Number of eggs per event | 50,000-100,000 | 200,000 to 250,000 or more | 5,000-10,000 | 3,000-20,000 | ||||
| Larvae per egg | 1 | 1 | 300 cercariae ( Schistosoma), 250,000 metacercariae ( Fasciola) | 1 | 1 | 1 | 1 | |
Draft genomes for all categories of helminth have been sequenced in recent years and are available through the ParaSite sub-portal of WormBase.
One study suggests a link between the rising rates of metabolic syndrome in the developed worlds and the largely successful efforts of Westerners to eliminate intestinal parasites. The work suggests Eosinophil (a type of white blood cell) in fat tissue play an important role in preventing insulin resistance by secreting interleukin 4, which in turn switches into "alternative activation". Alternatively-activated macrophages are important to maintaining glucose homeostasis (i.e., blood sugar regulation). Helminth infection causes an increase in eosinophils. In the study, the authors fed rodents a high-fat diet to induce metabolic syndrome, and then injected them with helminths. Helminth infestation improved the rodents' metabolism. The authors concluded:
Although 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....
Due to this strong shell, helminth eggs or ova remain viable in soil, fresh water and sewage for many months. In feces, fecal sludge and sewage sludge they can even remain viable for several years. Helminth eggs of concern in wastewater used for irrigation have a size between 20 and 90 μm and a relative density of 1.06–1.23. It is very difficult to inactivate helminth eggs, unless temperature is increased above 40 °C or moisture is reduced to less than 5%. Eggs that are no longer viable do not produce any larvae. In the case of Ascaris lumbricoides (giant roundworm), which has been considered the most resistant and common helminth type, fertilized eggs deposited in soil are resistant to desiccation but are, at this stage of development, very sensitive to environmental temperatures: The reproduction of a fertilized egg within the eggshell develops at an environmental soil temperature about 25 °C which is lower than the body temperature of the host (i.e., 37 °C for humans). However, development of the larvae in the egg stops at temperatures below 15.5 °C, and eggs cannot survive temperatures much above 38 °C. If the temperature is around 25 °C, the infectiousness occurs after nearly 10 days of incubation.
Helminth ova cannot be inactivated with chlorine, UV light or ozone (in the latter case at least not with economical doses because >36 mg/L ozone are needed with 1 hour contact time).
Helminth ova can be inactivated in sewage sludge treatment if the temperature is increased over 40 °C or moisture is reduced to less than 5%. Best results can be obtained when both of these conditions are met together for an extended period of time.Schmidt, G.D., Roberts, L.S. (1981). Foundations of Parasitology, second ed. C.V. Mosby Company, 795 pp Details about the contact time under these conditions and other related environmental factors are generally not well-defined for every type of helminth egg species. Helminth eggs are considered highly resistant biological structures.
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