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Parasitic worms, also known as helminths, are a group of large macroparasites; adults can generally be seen with the naked eye. Many are that are soil-transmitted and infect the gastrointestinal tract. Other parasitic worms such as reside in blood vessels.

Some parasitic worms, including and , are thus, they are not classified as helminths, which are endoparasites.

Parasitic worms live in and feed in living hosts. They receive and protection while disrupting their hosts' ability to absorb . This can cause weakness and 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 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 , 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, is the name of a type of helminth, and is the name of the infection caused by that helminth.


Taxonomy
Helminths are a group of organisms which share a similar form but are not necessarily . The term "helminth" is an artificial term. There is no real consensus on the taxonomy (or groupings) of the helminths, particularly within the . The term "helminth" contains a number of , many of which are completely unrelated. However, for practical considerations the term is currently used to describe four phyla with superficial similarities: (ringed or segmented worms), (), (roundworms), and (thorny-headed worms). The phylum includes two classes of worms of particular medical significance: the (tapeworms) and the (flukes and ), depending on whether or not they have segmented bodies.

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 field are the human parasites, and are classified as (nematodes) and , depending on whether they possess a round or flattened body, respectively.

(dermatophytosis) is actually caused by various , and not by a parasitic worm.


Reproduction and life cycle
The 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.

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.


Eggs
Generally, thousands or even hundreds of thousands of eggs are produced each time the female worm deposits its eggs - a process called . There is a large variation in the number of eggs produced by different species of worm at one time; it varies in the range of 3,000 to 700,000. The frequency of egg deposition from an adult helminth is generally daily, and can occur up to six times per day for some Taenia species. Adult trematodes lay smaller numbers of eggs compared to cestodes or nematodes. However, the egg develops into a from which thousands of , or swimming larvae, develop. This means that one egg may produce thousands of adult worms. Helminth eggs remain viable for 1–2 months in crops and for many months in soil, fresh water, and , or even for several years in , (historically called ), and – a period that is much longer compared to other microorganisms.
(2025). 9789241546850, World Health Organization. .
Feachem, R., Bradley, D., Garelick, H., Mara, D. (1983). Sanitation and Disease: Health Aspects of Excreta and Wastewater Management. John Wiley and Sons, New York, NY.

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:


Larvae
Larvae hatch from eggs, either inside or outside the host, depending on the type of helminth. For eggs in moist soil at optimal temperature and oxygen levels, the embryo develops into an infective larva after 2 to 4 weeks, named "second-stage larva". Once ingested by a host, this larva has the ability to get out of the egg, hatch in the small intestine and migrate to different organs. These infective larvae (or "infective eggs") may remain viable in soil for two years or longer.
(2025). 9780124159150, Academic Press.

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, , Hymenolepis spp., Echinococcus granulosus, Echinococcus multilocularis, Multiceps multicepsSchistosoma mansoni, Schistosoma japonicum, Fasciola hepatica, Enterobius, , , Rhabditis spp., , Necator americanus, Ancylostoma spp.
Pathological conditions caused in humansTapeworm infection, , alveolar echinococcosis, swimmer's itch, enterobiasis (pinworm infection, oxyuriasis), , (guinea worm), elephantiasis, , , infection (includes and Ancylostoma duodenale infection), , ,
Cylindrical
Present
Ends in anus
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 > 9,000Estimated > 800,000 to 1,000,000 Registered > 25,000
Number of species known to infect humans4016> 12,000
Species /Fasciola hepaticaAscaris lumbricoidesHookwormTrichuris trichiura spp.
Timeline of lifecycle stagesLarval formation Some days (eggs can survive for months)9–15 days18 days to several weeks1–2 days15–30 days
Larval growth After hatching, the larvae develop into cysticercoid, which can survive for years in an animal5–7 weeks as in snails and longer periods in wet environments as encysted 10–14 days5–10 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 months2–3 months2–8 weeks (can become dormant for months)
Lifespan of adult worm4–6 weeksSeveral years8–10 years1–2 yearsSeveral years1 year
Eggs laid per day250,000 to 700,0003,000 to 25,0003,000 to 250,000
Egg depositionFrequency up to 6 times a daydailydaily

daily
Number of eggs per event 50,000-100,000200,000 to 250,000 or more5,000-10,0003,000-20,000
Larvae per egg11300 ( ), 250,000 ( )1111

Draft genomes for all categories of helminth have been sequenced in recent years and are available through the ParaSite sub-portal of .


Use in medicine
Parasitic worms have been used as a medical treatment for various diseases, particularly those involving an overactive . As have evolved with parasitic worms, proponents argue they are needed for a healthy immune system. Scientists are looking for a connection between the prevention and control of parasitic worms and the increase in such as hay-fever in developed countries. Removal of parasitic worms from areas is correlated with an increase in autoimmune disorders in those areas. Parasitic worms may be able to damp down the immune system of their host, making it easier for them to live in the intestine without coming under attack. This may be one mechanism for their proposed medicinal effect.

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 (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....


Human stool samples
For medical purposes, the exact number of helminth eggs is less important and therefore most are made simply by identifying the appearance of the or eggs in . Due to the large quantity of eggs laid, can using as few as one or two . The (also called the Kato-Katz technique) is a laboratory method for preparing human stool samples prior to searching for parasite eggs. Eggs per gram is a laboratory test that determines the number of eggs per gram of feces in patients suspected of having a parasitological infection, such as schistosomiasis.


Relevance for sanitation
eggs can reach the soil when polluted , or are used as . Such soil is often characterized by moist and warm conditions. Therefore, the risk of using contaminated wastewater and sludge in agricultural fields is a real problem, especially in poor countries, where this practice is prevalent. Keraita B., Jiménez B., Drechsel P. (2008). Extent and Implications of Agricultural Reuse of Untreated, partly Treated and Diluted Wastewater in Developing Countries. CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources, Vol 3, No 58, pp 1–15 Helminth eggs are regarded as the main biological health risk when applying sewage sludge, fecal sludge or fecal matter on agricultural soils. The eggs are the infective stage of the helminths' life cycle for causing the disease .

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 used for 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.


Removal versus inactivation
Processes that remove particles, such as sedimentation, filtration or physically remove helminth eggs from wastewater (but do not inactivate them).Jimenez B., Chavez-Mejia A. (1997). Treatment of Mexico City Wastewater for Irrigation Purposes. Environmental Technology, Vol 18, pp 721–730Jiménez B., Maya C., Salgado G. (2001). The Elimination of Helminth Ova, Fecal Coliforms, Salmonella and Protozoan Cysts by Various Physicochemical Processes in Wastewater and Sludge. Water Science and Technology, Vol 43, No 12, pp 179–182 (DOI= 10.2166/wst.2001.0733) Therefore, waste stabilization ponds (lagoons), storage basins, constructed wetlands, rapid filtration or upflow anaerobic sludge blanket (UASB) reactors can be used.

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.


Measurements

Indicator organism

Environmental samples
For the purpose of setting treatment standards and reuse legislation, it is important to be able to determine the amount of helminth eggs in an environmental sample with some accuracy. The detection of viable helminth eggs in samples of wastewater, sludge or fresh feces (as a diagnostic tool for the infection ) is not straight forward. In fact, many laboratories in developing countries lack the right equipment or skilled staff required to do so. An important step in the analytical methods is usually the concentration of the eggs in the sample, especially in the case of wastewater samples. A concentration step may not be required in samples of dried feces, e.g. samples collected from urine-diverting dry toilets.


See also


Further reading
  • Dickson Despommier, People, Parasites, and Plowshares: Learning from Our Body's Most Terrifying Invaders, Columbia University Press, 2016 ().


External links

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