View source for Parasitic worm

{{Short description|Large type of parasitic organism}}
{{About|the group of organisms|the infection caused by this group of organisms|Helminthiasis|other uses|Parasitic worm (disambiguation)}}
[[File:Collage of various helminth eggs.png|thumb|[[Egg]]s of different species of parasitic worm]]

'''Parasitic worms''', also known as '''helminths''',<ref name="CDC">{{cite web |title=CDC - Parasites - About Parasites |url=https://www.cdc.gov/parasites/about.html |website=www.cdc.gov |language=en-us |date=20 April 2018}}</ref> are a [[polyphyletic]] group of large [[Parasitism#Basic concepts|macroparasites]]; adults can generally be seen with the naked eye. Many are [[intestinal worms]] that are [[soil-transmitted helminth|soil-transmitted]] and [[intestinal parasite infection|infect]] the [[gastrointestinal tract]]. Other parasitic worms such as  [[schistosomes]] reside in blood vessels.

Some parasitic worms, including [[leeches]] and [[monogenea]]ns, are [[ectoparasites]]{{snd}} thus, they are not classified as helminths, which are [[Parasitism#Basic concepts|endoparasites]].

Parasitic worms live in and feed in living [[host (biology)|hosts]]. They receive [[nourishment]] and protection while disrupting their hosts' ability to absorb [[nutrient]]s. This can cause weakness and [[disease]] in the host, and poses a global health and economic problem.<ref>{{Cite journal |last1=Hildersley |first1=Katie A. |last2=McNeilly |first2=Tom N. |last3=Gillan |first3=Victoria |last4=Otto |first4=Thomas D. |last5=Löser |first5=Stephan |last6=Gerbe |first6=François |last7=Jay |first7=Philippe |last8=Maizels |first8=Rick M. |last9=Devaney |first9=Eileen |last10=Britton |first10=Collette |date=2021 |title=Tuft Cells Increase Following Ovine Intestinal Parasite Infections and Define Evolutionarily Conserved and Divergent Responses |journal=Frontiers in Immunology |volume=12 |pages=781108 |doi=10.3389/fimmu.2021.781108 |pmc=8646091 |pmid=34880874|doi-access=free }}
</ref> 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.<ref name=":10">{{Cite web |url=https://www.cdc.gov/parasites/about.html |title=CDC Centers for Disease Control and Prevention, about parasites |publisher=CDC |access-date=28 November 2014}}</ref> 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.<ref name=":4">Jirillo, E., Magrone, T.,  Miragliotta, G. (2014). [http://www.eurekaselect.com/125669/volume/2 "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, {{doi|10.2174/97816080598501140201}}, {{ISBN|978-1-60805-985-0}}.</ref> All parasitic worms produce [[egg]]s 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.{{Citation needed|date=September 2018}} For example, ''[[Ascaris]]'' is the name of a type of helminth, and [[ascariasis]] is the name of the infection caused by that helminth.

{{TOC limit|3}}

== Taxonomy ==
[[Image:Hookworms.JPG|thumb|right|[[Hookworm]]s attached to the intestinal [[mucous membrane|mucosa]]]]

[[Image:Threadworm.jpg|thumb|right|Two [[pinworm (parasite)|pinworm]]s]]

[[File:Ascaris lumbricoides life cycle.tif|thumb|Image showing life cycle inside and outside of the human body of one fairly typical and well described helminth: [[Ascaris]] lumbricoides]]

Helminths are a group of organisms which share a similar form but are not necessarily [[phylogenetics|evolutionarily related]]. The term "helminth" is an artificial term.<ref>{{Cite web|url = http://www.ucmp.berkeley.edu/help/navphylogeny.html|title = Navigating the Phylogeny Wing, University of Berkeley, USA|access-date = 19 December 2014}}</ref><ref>{{Cite web|url = http://www.tolweb.org/tree/phylogeny.html|title = Tree of Life web project|access-date = 19 December 2014}}</ref> There is no real consensus on the taxonomy (or groupings) of the helminths, particularly within the [[nematodes]].<ref name=":1">{{Cite web|url=http://www.path.cam.ac.uk/~schisto/helminth_taxonomy/index.html |title=Schistosomiasis Research Group, University of Cambridge, UK |access-date=19 December 2014 |url-status=dead |archive-url=https://web.archive.org/web/20141013012251/http://www.path.cam.ac.uk/~schisto/helminth_taxonomy/index.html |archive-date=13 October 2014 }}</ref> The term "helminth" contains a number of [[Phylum|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), [[Platyhelminths|Platyhelminthes]] ([[flatworm]]s), [[Nematoda]] (roundworms), and [[Acanthocephala]] (thorny-headed worms).<ref name=":1" /> The phylum [[Platyhelminthes]] includes two classes of worms of particular medical significance: the [[cestodes]] (tapeworms) and the [[trematodes]] (flukes and [[schistosoma|blood flukes]]), depending on whether or not they have segmented bodies.<ref name="CDC"/><ref name=":2" />

There may be as many as 300,000 species of parasites affecting vertebrates,<ref>{{Cite journal|last1=Dobson|first1=A.|last2=Lafferty|first2=K. D.|last3=Kuris|first3=A. M.|last4=Hechinger|first4=R. F.|last5=Jetz|first5=W.|year=2008|title=Homage to Linnaeus:How many parasites? How many hosts?|journal=PNAS|volume=105|issue=Suppl 1|pages=11482–11489|doi=10.1073/pnas.0803232105|pmc=2556407|pmid=18695218|bibcode=2008PNAS..10511482D|doi-access=free}}</ref> and as many as 300 affecting humans alone.<ref>{{Cite journal|last1=Cox|first1=F. E. G.|year=2002|title=History of Human Parasitology|journal=Clinical Microbiology Reviews|volume=15|issue=4|pages=595–612|doi=10.1128/CMR.15.4.595-612.2002|pmc=126866|pmid=12364371}}</ref>

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.<ref name=":2">{{cite journal | author = Maya C., Torner-Morales F.J., Lucario E.S., Hernández E., Jiménez B. | year = 2012 | title = Viability of six species of larval and non-larval helminth eggs for different conditions of temperature, pH and dryness | journal = Water Research | volume = 46 | issue = 15| pages = 4770–4782 | doi = 10.1016/j.watres.2012.06.014 | pmid = 22794801 | bibcode = 2012WatRe..46.4770M }}</ref>

[[Ringworm]] (dermatophytosis) is actually caused by various [[fungus|fungi]], and not by a parasitic worm.<ref>{{cite web |last1=Mayo Clinic Staff |title=Ringworm (body) - Symptoms and causes |url=https://www.mayoclinic.org/diseases-conditions/ringworm-body/symptoms-causes/syc-20353780 |website=Mayo Clinic |access-date=4 July 2018}}</ref><ref>{{cite web |title=Ringworm |url=https://www.aad.org/public/diseases/contagious-skin-diseases/ringworm |website=American Academy of Dermatology |access-date=4 July 2018}}</ref>

== Reproduction and life cycle ==
The [[life expectancy|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.<ref name=":4" />

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.<ref name="ReferenceA">{{Cite journal |last1=Salehi |first1=Alireza |last2=Razavi |first2=Mahsa |last3=Vahedi Nouri |first3=Nasrollah |date=2022-12-21 |title=Seasonal Prevalence of Helminthic Infections in the Gastrointestinal Tract of Sheep in Mazandaran Province, Northern Iran |journal=Journal of Parasitology Research |language=en |volume=2022 |pages=e7392801 |doi=10.1155/2022/7392801 |issn=2090-0023 |pmc=9797291 |pmid=36588778 |doi-access=free }}</ref> All helminths produce eggs (also called ova) for reproduction.<ref>{{Cite journal |last1=Nouri |first1=Nasrollah Vahedi |last2=Rahmatian |first2=Reza |last3=Salehi |first3=Alireza |date=2022-06-07 |title=Prevalence of Helminthic Infections in the Gastrointestinal Tract of Cattle in Mazandaran Province (Northern Iran) |journal=Journal of Parasitology Research |language=en |volume=2022 |pages=e7424647 |doi=10.1155/2022/7424647 |issn=2090-0023 |pmc=9197603 |pmid=35711670 |doi-access=free }}</ref>

===Eggs===
Generally, thousands or even hundreds of thousands of eggs are produced each time the female worm deposits its eggs - a process called [[oviposition]]. 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 [[miracidia]] from which thousands of ''[[cercariae]]'', or swimming larvae, develop. This means that one egg may produce thousands of adult worms.<ref name=":9">{{Cite web|url = https://www.cdc.gov/parasites/fasciola/biology.html|title = Centers for Disease Control and Prevention: Parasites - Fascioliasis (Fasciola Infection)|access-date =13 January 2015 }}</ref> Helminth eggs remain viable for 1–2 months in crops and for many months in soil, fresh water, and [[sewage]], or even for several years in [[Human feces|feces]], [[fecal sludge]] (historically called [[night soil]]), and [[sewage sludge]] – a period that is much longer compared to other microorganisms.<ref name="World Health Organization">{{cite book|last1=WHO|title=Guidelines for the Safe Use of Wastewater, Excreta and Greywater, Volume 4 Excreta and Greywater Use in Agriculture.|date=2006|publisher=World Health Organization|location=Geneva|isbn=978-9241546850|edition=third|url=http://www.susana.org/en/resources/library/details/1004}}</ref><ref name=":12">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.</ref>

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:<ref name="ReferenceA"/>{{ordered list
| type = a|the 1 to 2 outer layers are formed of [[mucopolysaccharide]]s and [[protein]]s,|the middle layers consist of [[chitin]]ous material and serve to give structure and mechanical resistance to the eggs, and|the inner layer is composed of [[lipid]]s and proteins and is useful to protect eggs from desiccation, strong acid and bases, oxidants and reductive agents as well as detergent and proteolytic compounds.<ref name = ":3"/><ref>{{cite journal | author = Fairweather I., Threadgold L.T. | year = 1981 | title = Hymenolepis nana: the fine structure of the embryonic envelopes | journal = Parasitology | volume = 82 | issue = 3| pages = 429–443 | doi=10.1017/s0031182000066968| pmid = 7243350 | s2cid = 222626 }}</ref><ref>{{cite journal | author = Lýsek H., Malínský J., Janisch R. | year = 1985 | title = Ultrastructure of eggs of Ascaris lumbricoides Linnaeus, 1758. I. Egg-shells | url= https://folia.paru.cas.cz/pdfs/fol/1985/04/15.pdf | journal = Folia Parasitologica | volume = 32 | issue= 4 | pages = 381–384 | pmid = 4085927 }} {{open access}}</ref><ref>{{cite journal | author = Quilès F., Balandier J.Y., Capizzi-Banas S. | s2cid = 39350893 | year = 2006 | title = In situ characterisation of a microorganism surface by Raman microspectroscopy: the shell of Ascaris eggs | journal = Analytical and Bioanalytical Chemistry | volume = 386 | issue = 2| pages = 249–255 | doi=10.1007/s00216-006-0638-4| pmid = 16900382 }}</ref>
}}

=== 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.<ref name=":13">{{Cite book |last1=Bogitsh |first1=Burton J. |title=Human Parasitology |last2=Carter |first2=Clint E. |last3=Oeltmann |first3=Thomas N. |publisher=Academic Press |year=2012 |isbn=978-0-12-415915-0 |location=UK |pages=269–345 |chapter=General Characteristics of the Nematoda (Chapter 15), Intestinal Nematodes (Chapter 16)}}</ref>

The process of larval maturation in the host can take from about two weeks up to four months, depending on the helminth species.{{citation needed|date=January 2021}}

The following table shows the principal morphological and reproductive distinctions for three helminth groups:

{| class="wikitable"
! colspan="2" | !! colspan="2" | Tapeworms<br>(Cestodes)!! colspan="1" |Flukes<br>(Trematodes)!! colspan="4" |Roundworms<br>(Nematodes)
|-
| colspan="2" |Examples
| colspan="2" |''[[Taenia solium]],'' ''[[Taenia saginata]],'' ''[[Hymenolepis (flatworm)|Hymenolepis]] spp.,'' ''[[Echinococcus granulosus]],'' ''[[Echinococcus multilocularis]]'', ''Multiceps multiceps''
|''[[Schistosoma mansoni]]'', ''[[Schistosoma japonicum]]'',
''[[Fasciola hepatica]]''
| colspan="4" |''[[Ascaris|Ascaris spp.]]'', [[Pinworm (parasite)|''Enterobius'']], ''[[Filarioidea]], [[Onchocerca|Onchocerca spp.]]'', ''Rhabditis spp.'', ''[[Trichuris|Trichuris spp.]],''  ''[[Necator americanus]]'', ''Ancylostoma spp.''
|-
| colspan="2" |Pathological conditions caused in humans
| colspan="2" |[[Tapeworm infection]], [[echinococcosis]], [[Alveolar hydatid disease|alveolar echinococcosis]]
|[[Schistosomiasis]], [[swimmer's itch]]
| colspan="4" |[[Ascariasis]], [[Pinworm infection|enterobiasis (pinworm infection, oxyuriasis)]], [[filariasis]], [[dracunculiasis]] (guinea worm), [[elephantiasis tropica|elephantiasis]], [[enterobiasis|enterobiasis (pinworm)]], [[filariasis]], [[hookworm]] infection (includes [[Necatoriasis]] and ''[[Ancylostoma duodenale]]'' infection), [[onchocerciasis]], [[trichinosis]], [[trichuriasis|trichuriasis (whipworm)]]
|-
| colspan="2" |Shape|| colspan="2" |Segmented plane||Unsegmented plane|| colspan="4" |Cylindrical
|-
| colspan="2" |[[Body cavity]]|| colspan="2" |None||None|| colspan="4" |Present
|-
| colspan="2" |Body covering|| colspan="2" |[[Tegument (Helminth)|Tegument]]||Tegument|| colspan="4" |[[Cuticle]]
|-
| colspan="2" |Digestive tube|| colspan="2" |None||Ends in cecum|| colspan="4" |Ends in anus
|-
| colspan="2" |Sex|| colspan="2" |[[Hermaphroditic]]||[[Hermaphroditic]], except [[schistosomes]] which are [[dioecious]]|| colspan="4" |[[Dioecious]]
|-
| colspan="2" |Attachment organs|| colspan="2" |[[Sucker (platyhelminth anatomy)|Sucker]] or [[bothridia]], and [[Rostellum (helminth)|rostellum]] with hooks||Oral sucker and ventral sucker or [[acetabulum (morphology)|acetabulum]]|| colspan="4" |Lips, teeth, filariform extremities, and dentary plates
|-
| colspan="2" |
Number of species
| colspan="2" |6000<ref name=":5">Lamonthe Argumedo, R., Garcia Prieto, L. (1988). [http://www.cabdirect.org/abstracts/19890856899.html;jsessionid=02B0A5F1A56ABCF2168CCE818117C655;jsessionid=73E2DEA523DD5F13598E546729323C4B Human helminthiasis in Mexico. Treatment and prophylaxis] {{Webarchive|url=https://web.archive.org/web/20141229132708/http://www.cabdirect.org/abstracts/19890856899.html;jsessionid=02B0A5F1A56ABCF2168CCE818117C655;jsessionid=73E2DEA523DD5F13598E546729323C4B |date=2014-12-29 }}, A.G.T. Editor, S.A., 1st edition, Mexico. ({{ISBN|9684630514}})</ref>
|Estimated > 15,000<ref name=":0">Pumarola, A., Rodríguez-Torres, A., García, R.J.A., Piedrola, A.G. (1987). [https://books.google.com/books?id=Nlego0fDRUQC&pg=PR18&lpg=PP1&focus=viewport&dq=microbiolog%C3%ADa+y+parasitolog%C3%ADa+m%C3%A9dica+pumarola&hl=de Medical Microbiology and Parasitology] (in Spanish), Ediciones Científicas y Técnicas, S. A., Barcelona, Spain, pp 850 – 880</ref> Registered > 9,000<ref>{{Cite web|url = http://animaldiversity.org/accounts/Trematoda/|title = Animal diversity web| website=[[Animal Diversity Web]] |date = September 2001|access-date = 17 December 2014}}</ref>
| colspan="4" |Estimated > 800,000 to 1,000,000
Registered > 25,000<ref name=":0" />
|-
| colspan="2" |Number of species known to infect humans
| colspan="2" |40<ref name=":5" />
|16<ref name=":0" />
| colspan="4" |> 12,000<ref name=":0" />
|-
| colspan="2" |Species
|
''[[Hymenolepis nana]]''
|
''[[Taenia solium]] /[[Taenia saginata]]''
|
''[[Fasciola hepatica]]''
|
''[[Ascaris lumbricoides]]''
|
Hookworm
|
''[[Trichuris trichiura]]''
|''[[Toxocara]] spp.''
|-
| rowspan="4" |Timeline of lifecycle stages
|Larval formation
|
|
Some days (eggs can survive for months)<ref name=":11">{{Cite web|url = https://www.cdc.gov/parasites/taeniasis/biology.html|title = Centers for Disease Control and Prevention|access-date = 22 January 2015|website = Parasites - Taeniasis (Biology)}}</ref>
|
9–15 days<ref name=":5" />
|
18 days to several weeks<ref name=":6">{{Cite web|url = https://www.cdc.gov/parasites/ascariasis/biology.html|title = Centers for Disease Control and Prevention: Parasites - Ascariasis|access-date =13 January 2015 }}</ref>
|
1–2 days<ref name=":7">{{Cite web|url = https://www.cdc.gov/parasites/hookworm/biology.html|title = Centers for Disease Control and Prevention: Parasites - Hookworm|access-date =13 January 2015 }}</ref>
|
15–30 days<ref name=":8">{{Cite web|url = https://www.cdc.gov/parasites/whipworm/biology.html|title = Centers for Disease Control and Prevention: Parasites - Trichuriasis (also known as Whipworm Infection)|access-date =13 January 2015 }}</ref>
|
|-
|Larval growth
|
|
After hatching, the larvae develop into cysticercoid, which can survive for years in an animal<ref name=":11" />
|
5–7 weeks as ''[[cercariae]]'' in snails and longer periods in wet environments as encysted ''[[metacercariae]]''<ref name=":9" />
|
10–14 days<ref name=":6" />
|
5–10 days (after maturing can survive for weeks outside the host)<ref name=":7" />
| rowspan="2" |
60–70 days (from hatching to mature state)<ref name=":8" />
|
5–6 days<ref name=":5" />
|-
|Maturation to adult
|
|
2 months (from cysticercoid to adult)<ref name=":11" />
|
3–4 months<ref name=":9" />
|
2–3 months<ref name=":6" />
|
2–8 weeks<ref name=":5" /> (can become dormant for months)
|
|-
|Lifespan of adult worm
|
4–6 weeks
|
Several years<ref name=":11" />
|
8–10 years<ref name=":5" />
|
1–2 years<ref name=":6" />
|
Several years<ref name=":7" />
|
1 year<ref name=":8" />
|
|-
| colspan="2" |Eggs laid per day
| colspan="2" |250,000<ref name=":10" /> to 700,000<ref name=":5" />
|3,000 to 25,000<ref name=":0" />
| colspan="4" |3,000<ref name=":10" /> to 250,000<ref name=":5" />
|-
| rowspan="2" |Egg deposition
|Frequency
|
|
up to 6 times a day<ref name=":11" />
|
|
daily<ref name=":6" />
|
daily<ref name=":7" />

|
daily<ref name=":8" />
|
|-
|Number of eggs per event
|
|
50,000-100,000<ref name=":11" />
|
|
200,000<ref name=":6" /><ref name=":13" /> to 250,000 or more<ref name=":5" />
|
5,000-10,000<ref name=":5" />
|
3,000-20,000<ref name=":8" />
|
|-
| colspan="2" |Larvae per egg
|1
|1
|300 ''[[cercariae]]'' (''[[Schistosoma]]''), 250,000 ''[[metacercariae]]'' (''[[Fasciola]]'')<ref name=":0" />
|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]].<ref>{{cite web|url=http://parasite.wormbase.org|title=WormBase ParaSite|access-date=15 April 2016}}</ref>

==Use in medicine==
{{Main|Helminthic therapy}}

Parasitic worms have been used as a medical treatment for various diseases, particularly those involving an overactive [[Immune system|immune response]].<ref name="BBC">{{cite web
  |title=Eat worms - feel better
  |work=[[BBC News]]
  |url=http://news.bbc.co.uk/1/hi/health/3287733.stm
  |date=3 December 2003
  |access-date=13 July 2011}}</ref> As [[human]]s have evolved with parasitic worms, proponents argue they are needed for a healthy immune system.<ref name="BBC" /> Scientists are looking for a connection between the prevention and control of parasitic worms and the increase in [[Allergy|allergies]] such as hay-fever in developed countries.<ref name="BBC" /> Removal of parasitic worms from areas is correlated with an increase in autoimmune disorders in those areas.<ref name="Weinstock">{{cite journal |last1=Weinstock |first1=Joel |title=Do We Need Worms to Promote Immune Health? |journal=Clinical Reviews in Allergy & Immunology |date=October 2015 |volume=49 |issue=2 |pages=227–231 |doi=10.1007/s12016-014-8458-3 |pmid=25326880 |s2cid=22950215 |url=http://dx.doi.org./10.1007/s12016-014-8458-3 |access-date=25 October 2020|url-access=subscription }}</ref> 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.<ref name="BBC" /> This may be one mechanism for their proposed medicinal effect.{{citation needed|date=January 2021}}

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 eosinophils (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 [[macrophage]]s 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.<ref name="science">{{cite journal |last=Wu |first=Davina |title=Eosinophils Sustain Adipose Alternatively Activated Macrophages Associated with Glucose Homeostasis |journal=[[Science (journal)|Science]] |date=8 April 2011 |volume=332 |pages=243–247 |url=http://sabre.ucsf.edu/docs/Science-2011-Wu-243-7.pdf |access-date=18 April 2011 |issue=6026 |doi=10.1126/science.1201475 |pmid=21436399 |pmc=3144160|display-authors=etal|bibcode=2011Sci...332..243W |archive-url=https://web.archive.org/web/20160418231924/http://sabre.ucsf.edu/docs/Science-2011-Wu-243-7.pdf |archive-date=2016-04-18 }}</ref>  The authors concluded:

<blockquote>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....<ref name="science" /></blockquote>

=== Human stool samples ===
For medical purposes, the exact number of helminth eggs is less important and therefore most [[diagnosis|diagnoses]] are made simply by identifying the appearance of the [[worm]] or eggs in [[feces]]. Due to the large quantity of eggs laid, [[physicians]] can [[diagnose]] using as few as one or two [[fecal smear]]s.{{citation needed|date=August 2015}} The [[Kato technique]] (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.{{citation needed|date=January 2021}}

== Relevance for sanitation ==
[[File:Processed helminth eggs samples and Mc Master slide (4369782501).jpg|thumb|Processed helminth eggs samples from a dry toilet in Kenya]]
[[File:Helminth Eggs determination (in faeces) (4369783299).jpg|thumb|Analysing for helminth eggs in samples of feces from a dry toilet in Kenya]]Helminth eggs can reach the soil when polluted [[wastewater]], [[sewage sludge]] or [[human waste]] are used as [[fertilizer]]. 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.<ref name=":3">{{cite journal |author=Jimenez B |year=2007 |title=Helminth ova removal from wastewater for agriculture and aquaculture reuse |journal=Water Science & Technology |volume=55 |issue=1–2 |pages=485–493 |doi=10.2166/wst.2007.046 |pmid=17305174|doi-access=free |bibcode=2007WSTec..55..485J }} {{free access}}</ref><ref>Keraita B., Jiménez B., Drechsel P. (2008). [https://www.researchgate.net/publication/248908925 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</ref> Helminth eggs are regarded as the main biological health risk when applying sewage sludge, fecal sludge or fecal matter on agricultural soils.<ref name="World Health Organization" /> The eggs are the infective stage of the helminths' life cycle for causing the disease [[helminthiasis]].{{citation needed|date=July 2020}}

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.<ref name="World Health Organization" /><ref name=":12" /> Helminth eggs of concern in [[wastewater]] used for [[irrigation]] have a size between 20 and 90&nbsp;μm and a relative density of 1.06–1.23.<ref name=":3" />  It is very difficult to inactivate helminth eggs, unless temperature is increased above 40&nbsp;°C or moisture is reduced to less than 5%.<ref name=":3" /> 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&nbsp;°C which is lower than the body temperature of the host (i.e., 37&nbsp;°C for humans).<ref name=":13" /> However, development of the larvae in the egg stops at temperatures below 15.5&nbsp;°C, and eggs cannot survive temperatures much above 38&nbsp;°C. If the temperature is around 25&nbsp;°C, the infectiousness occurs after nearly 10 days of incubation.<ref name=":2" /><ref>{{cite journal |author=Alouini Z., Jemli M. |year=2001 |title=Destruction of helminth eggs by photosensitized porphyrin |journal=Journal of Environmental Monitoring |volume=3 |issue=5 |pages=548–551 |doi=10.1039/b103471p |pmid=11695127}}</ref><ref>{{cite journal |author=Capizzi-Banas S., Deloge M., Remy M., Schwartzbrod J. |year=2004 |title=Liming as an advanced treatment for sludge sanitisation: helminth eggs elimination - Ascaris eggs as model |journal=Water Research |volume=38 |issue=14–15 |pages=3251–3258 |doi=10.1016/j.watres.2004.04.015 |pmid=15276741|bibcode=2004WatRe..38.3251C }}</ref>

=== Removal versus inactivation ===
Processes that remove particles, such as sedimentation, filtration or [[Flocculation|coagulation-flocculation]] physically remove helminth eggs from wastewater (but do not inactivate them).<ref>Jimenez B., Chavez-Mejia A. (1997). [http://www.tandfonline.com/doi/abs/10.1080/09593331808616590 Treatment of Mexico City Wastewater for Irrigation Purposes]. Environmental Technology, Vol 18, pp 721–730</ref><ref>Jiménez B., Maya C., Salgado G. (2001). [https://iwaponline.com/wst/article-pdf/43/12/179/429089/179.pdf 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) {{free access}}</ref> Therefore, [[stabilization pond|waste stabilization ponds]] (lagoons), storage basins, [[constructed wetlands]], rapid filtration or [[upflow anaerobic sludge blanket]] (UASB) reactors can be used.{{cn|date=April 2025}}

Helminth ova cannot be inactivated with chlorine, UV light or ozone (in the latter case at least not with economical doses because >36&nbsp;mg/L ozone are needed with 1 hour contact time).{{citation needed|date=January 2021}}

Helminth ova can be inactivated in [[sewage sludge treatment]] if the temperature is increased over 40&nbsp;°C or moisture is reduced to less than 5%.<ref name=":3" /> Best results can be obtained when both of these conditions are met together for an extended period of time.<ref>Schmidt, G.D., Roberts, L.S. (1981). Foundations of Parasitology, second ed. C.V. Mosby Company, 795 pp</ref> 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.<ref name=":2" /> Helminth eggs are considered highly resistant biological structures.<ref name=":3" />

=== Measurements ===

==== Indicator organism ====
[[File:Identification and quantification of helminth eggs.png|thumb|Identification and quantification of helminth eggs at UNAM university in Mexico City, Mexico]]
{{excerpt|Indicator organism#Indicator helminth eggs|file=no}}

==== 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 [[helminthiasis]]) 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 toilet]]s.{{citation needed|date=January 2021}}

== See also ==
{{Portal|Biology|Medicine}}
* [[Helminthology]]{{snd}} the study of parasitic worms and their effects on their [[host (biology)|host]]s
* [[Anthelminthic|Anthelmintic]]
* [[Parasitism]]
* [[Schistosomiasis]]{{snd}} a disease caused by parasitic [[flatworm]]s called [[schistosoma]]s
* [[Lymphatic filariasis]] caused by [[filarial worms]]
* [[Dracunculiasis]] caused by the [[roundworm]] ''[[Dracunculus medinensis]]'' (Guinea worm)

==References==
{{Reflist|2}}

== Further reading ==
* [[Dickson Despommier]], ''People, Parasites, and Plowshares: Learning from Our Body's Most Terrifying Invaders'', [[Columbia University Press]], 2016 ({{ISBN|978-0231161954}}).

==External links==
{{commons category|Helminths}}
* [https://web.archive.org/web/20071017030045/http://www.niaid.nih.gov/factsheets/roundwor.htm Parasitic Roundworm Diseases]
* [https://www.who.int/intestinal_worms/en/ World Health Organisation (WHO) topic page on Intestinal worms]

{{Helminthiases}}
{{Diseases of Poverty}}
{{Authority control}}

{{DEFAULTSORT:Parasitic Worm}}
[[Category:Worms (obsolete taxon)]]
[[Category:Conditions diagnosed by stool test]]
[[Category:Foodborne illnesses]]
[[Category:Helminthiases]]
[[Category:Intestinal infectious diseases]]
[[Category:Tropical diseases]]
[[Category:Parasitic protostomes]]
[[Category:Sanitation]]