Parasitic worms, also known ashelminths,[1] are apolyphyletic group of largemacroparasites; adults can generally be seen with the naked eye. Many areintestinal worms that aresoil-transmitted andinfect thegastrointestinal tract. Other parasitic worms such asschistosomes reside in blood vessels.
Some parasitic worms, includingleeches andmonogeneans, areectoparasites – thus, they are not classified as helminths, which areendoparasites.
Parasitic worms live in and feed in livinghosts. They receivenourishment and protection while disrupting their hosts' ability to absorbnutrients. This can cause weakness anddisease in the host, and poses a global health and economic problem.[2] 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.[3] Helminths are able to survive in their mammalian hosts for many years due to their ability to manipulate the host'simmune response by secreting immunomodulatory products.[4] All parasitic worms produceeggs 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 ashelminthiasis, 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] For example,Ascaris is the name of a type of helminth, andascariasis is the name of the infection caused by that helminth.
Helminths are a group of organisms which share a similar form but are not necessarilyevolutionarily related. The term "helminth" is an artificial term.[5][6] There is no real consensus on the taxonomy (or groupings) of the helminths, particularly within thenematodes.[7] The term "helminth" contains a number ofphyla, 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), andAcanthocephala (thorny-headed worms).[7] The phylumPlatyhelminthes includes two classes of worms of particular medical significance: thecestodes (tapeworms) and thetrematodes (flukes andblood flukes), depending on whether or not they have segmented bodies.[1][8]
There may be as many as 300,000 species of parasites affecting vertebrates,[9] and as many as 300 affecting humans alone.[10]
Helminths of importance in thesanitation field are the human parasites, and are classified asNemathelminthes (nematodes) andPlatyhelminthes, depending on whether they possess a round or flattened body, respectively.[8]
Ringworm (dermatophytosis) is actually caused by variousfungi, and not by a parasitic worm.[11][12]
Thelifetime 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.[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.[13] All helminths produce eggs (also called ova) for reproduction.[14]
Generally, thousands or even hundreds of thousands of eggs are produced each time the female worm deposits its eggs - a process calledoviposition. 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 someTaenia species. Adult trematodes lay smaller numbers of eggs compared to cestodes or nematodes. However, the egg develops into amiracidia from which thousands ofcercariae, or swimming larvae, develop. This means that one egg may produce thousands of adult worms.[15] Helminth eggs remain viable for 1–2 months in crops and for many months in soil, fresh water, andsewage, or even for several years infeces,fecal sludge (historically callednight soil), andsewage sludge – a period that is much longer compared to other microorganisms.[16][17]
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:[13]
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.[22]
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]
The following table shows the principal morphological and reproductive distinctions for three helminth groups:
| Tapeworms (Cestodes) | Flukes (Trematodes) | Roundworms (Nematodes) | ||||||
|---|---|---|---|---|---|---|---|---|
| Examples | Taenia solium,Taenia saginata,Hymenolepis spp.,Echinococcus granulosus,Echinococcus multilocularis,Multiceps multiceps | Schistosoma mansoni,Schistosoma japonicum, | Ascaris spp.,Enterobius,Filarioidea,Onchocerca spp.,Rhabditis spp.,Trichuris spp.,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 (pinworm),filariasis,hookworm infection (includesNecatoriasis andAncylostoma duodenale infection),onchocerciasis,trichinosis,trichuriasis (whipworm) | |||||
| Shape | Segmented plane | Unsegmented plane | Cylindrical | |||||
| Body cavity | None | None | Present | |||||
| Body covering | Tegument | Tegument | Cuticle | |||||
| Digestive tube | None | Ends in cecum | Ends in anus | |||||
| Sex | Hermaphroditic | Hermaphroditic, exceptschistosomes which aredioecious | Dioecious | |||||
| Attachment organs | Sucker orbothridia, androstellum with hooks | Oral sucker and ventral sucker oracetabulum | Lips, teeth, filariform extremities, and dentary plates | |||||
Number of species | 6000[23] | Estimated > 15,000[24] Registered > 9,000[25] | Estimated > 800,000 to 1,000,000 Registered > 25,000[24] | |||||
| Number of species known to infect humans | 40[23] | 16[24] | > 12,000[24] | |||||
| Species | Hookworm | Toxocara spp. | ||||||
| Timeline of lifecycle stages | Larval formation | Some days (eggs can survive for months)[26] | 9–15 days[23] | 18 days to several weeks[27] | 1–2 days[28] | 15–30 days[29] | ||
| Larval growth | After hatching, the larvae develop into cysticercoid, which can survive for years in an animal[26] | 5–7 weeks ascercariae in snails and longer periods in wet environments as encystedmetacercariae[15] | 10–14 days[27] | 5–10 days (after maturing can survive for weeks outside the host)[28] | 60–70 days (from hatching to mature state)[29] | 5–6 days[23] | ||
| Maturation to adult | 2 months (from cysticercoid to adult)[26] | 3–4 months[15] | 2–3 months[27] | 2–8 weeks[23] (can become dormant for months) | ||||
| Lifespan of adult worm | 4–6 weeks | Several years[26] | 8–10 years[23] | 1–2 years[27] | Several years[28] | 1 year[29] | ||
| Eggs laid per day | 250,000[3] to 700,000[23] | 3,000 to 25,000[24] | 3,000[3] to 250,000[23] | |||||
| Egg deposition | Frequency | up to 6 times a day[26] | daily[27] | daily[28] | daily[29] | |||
| Number of eggs per event | 50,000-100,000[26] | 5,000-10,000[23] | 3,000-20,000[29] | |||||
| Larvae per egg | 1 | 1 | 300cercariae (Schistosoma), 250,000metacercariae (Fasciola)[24] | 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 ofWormBase.[30]
Parasitic worms have been used as a medical treatment for various diseases, particularly those involving an overactiveimmune response.[31] Ashumans have evolved with parasitic worms, proponents argue they are needed for a healthy immune system.[31] Scientists are looking for a connection between the prevention and control of parasitic worms and the increase inallergies such as hay-fever in developed countries.[31] Removal of parasitic worms from areas is correlated with an increase in autoimmune disorders in those areas.[32] 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.[31] This may be one mechanism for their proposed medicinal effect.[citation needed]
One study suggests a link between the rising rates ofmetabolic 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 switchesmacrophages 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.[33] 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….[33]
For medical purposes, the exact number of helminth eggs is less important and therefore mostdiagnoses are made simply by identifying the appearance of theworm or eggs infeces. Due to the large quantity of eggs laid,physicians candiagnose using as few as one or twofecal smears.[citation needed] TheKato 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]
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Helminth eggs can reach the soil when pollutedwastewater,sewage sludge orhuman waste are used asfertilizer. 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.[18][34] Helminth eggs are regarded as the main biological health risk when applying sewage sludge, fecal sludge or fecal matter on agricultural soils.[16] The eggs are the infective stage of the helminths’ life cycle for causing the diseasehelminthiasis.[citation needed]
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.[16][17] Helminth eggs of concern inwastewater used forirrigation have a size between 20 and 90 μm and a relative density of 1.06–1.23.[18] It is very difficult to inactivate helminth eggs, unless temperature is increased above 40 °C or moisture is reduced to less than 5%.[18] Eggs that are no longer viable do not produce any larvae. In the case ofAscaris 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).[22] 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.[8][35][36]
Processes that remove particles, such as sedimentation, filtration orcoagulation-flocculation physically remove helminth eggs from wastewater (but do not inactivate them).[37][38] Therefore,waste stabilization ponds (lagoons), storage basins,constructed wetlands, rapid filtration orupflow 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).[citation needed]
Helminth ova can be inactivated insewage sludge treatment if the temperature is increased over 40 °C or moisture is reduced to less than 5%.[18] Best results can be obtained when both of these conditions are met together for an extended period of time.[39] 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.[8] Helminth eggs are considered highly resistant biological structures.[18]
The eggs fromhelminths (parasitic worms) are a commonly used indicator organism to assess the safety ofsanitation andwastewater reuse systems (such schemes are also calledreuse of human excreta).[40]: 55 This is because they are the most resistantpathogens of all types of pathogens (pathogens can beviruses,bacteria,protozoa and helminths).[41] It means they are relatively hard to destroy through conventional treatment methods. They can survive for 10–12 months in tropical climates.[41] These eggs are also calledova in the literature.[42]
Helminth eggs that are found in wastewater and sludge stem from soil-transmitted helminths (STHs) which includeAscaris lumbricoides (Ascaris),Anclostoma duodenale,Necator americanus (hookworm), andTrichuris trichiura (whipworm).[43] Ascaris and whipworm that are identified in reusable wastewater systems can cause certain diseases and complications if ingested by humans and pigs.[44] Hookworms will plant and hatch their larvae into the soil where they grow until maturity. Once the hookworm eggs are fully developed, they infect organisms by crawling through the organism’s skin.[45]
The presence or absence of viable helminth eggs ("viable" meaning that a larva would be able to hatch from the egg) in a sample of dried fecal matter,compost orfecal sludge is often used to assess the efficiency of diverse wastewater and sludge treatment processes in terms of pathogen removal.[40]: 55 In particular, the number of viableAscaris eggs is often taken as an indicator for all helminth eggs in treatment processes as they are very common in many parts of the world and relatively easy to identify under the microscope. However, the exact inactivation characteristics may vary for different types of helminth eggs.[46]
The technique used for testing depends on the type of sample.[42] When the helminth ova are in sludge, processes such as alkaline-post stabilization, acid treatment, andanaerobic digestion are used to reduce the amount of helminth ova in areas where there is a large amount. These methods make it possible for helminth ova to be within the healthy requirements of ≤1 helminth ova per liter. Dehydration is used to inactivate helminth ova in fecal sludge. This type of inactivation occurs when feces is stored between 1-2 years, a high total solids content (>50-60%) is present, items such as leaves, lime, earth, etc. are added, and at a temperature of 30°C or higher.[45]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 infectionhelminthiasis) 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 fromurine-diverting dry toilets.[citation needed]
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