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Parasitic worm
A commonly used term to describe certain parasitic worms with some similarities, many of which are intestinal worms

Parasitic worms, or helminths, are a polyphyletic group of large macroparasites often visible to the naked eye. Many helminths are intestinal worms that are soil-transmitted and infect the gastrointestinal tract, while others like schistosomes inhabit blood vessels. They live in hosts, gaining nourishment but causing disease by disrupting nutrient absorption. Helminths manipulate the host’s immune response to survive for years and reproduce by laying durable eggs that remain viable externally for long periods. Infections, called helminthiasis, are named by adding "-asis" to the worm’s name, such as Ascaris causing ascariasis.

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Taxonomy

Helminths are a group of organisms which share a similar form but are not necessarily evolutionarily related. The term "helminth" is an artificial term.56 There is no real consensus on the taxonomy (or groupings) of the helminths, particularly within the nematodes.7 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).8 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.910

There may be as many as 300,000 species of parasites affecting vertebrates,11 and as many as 300 affecting humans alone.12

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.13

Ringworm (dermatophytosis) is actually caused by various fungi, and not by a parasitic worm.1415

Reproduction and life cycle

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.16

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.17 All helminths produce eggs (also called ova) for reproduction.18

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.19 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 feces, fecal sludge (historically called night soil), and sewage sludge – a period that is much longer compared to other microorganisms.2021

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:22

  1. the 1 to 2 outer layers are formed of mucopolysaccharides and proteins,
  2. the middle layers consist of chitinous material and serve to give structure and mechanical resistance to the eggs, and
  3. the inner layer is composed of lipids 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.23242526

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.27

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:

Tapeworms(Cestodes)Flukes(Trematodes)Roundworms(Nematodes)
ExamplesTaenia solium, Taenia saginata, Hymenolepis spp., Echinococcus granulosus, Echinococcus multilocularis, Multiceps multicepsSchistosoma mansoni, Schistosoma japonicum,

Fasciola hepatica

Ascaris spp., Enterobius, Filarioidea, Onchocerca spp., Rhabditis spp., Trichuris spp., Necator americanus, Ancylostoma spp.
Pathological conditions caused in humansTapeworm infection, echinococcosis, alveolar echinococcosisSchistosomiasis, swimmer's itchAscariasis, enterobiasis (pinworm infection, oxyuriasis), filariasis, dracunculiasis (guinea worm), elephantiasis, enterobiasis (pinworm), filariasis, hookworm infection (includes Necatoriasis and Ancylostoma duodenale infection), onchocerciasis, trichinosis, trichuriasis (whipworm)
ShapeSegmented planeUnsegmented planeCylindrical
Body cavityNoneNonePresent
Body coveringTegumentTegumentCuticle
Digestive tubeNoneEnds in cecumEnds in anus
SexHermaphroditicHermaphroditic, except schistosomes which are dioeciousDioecious
Attachment organsSucker or bothridia, and rostellum with hooksOral sucker and ventral sucker or acetabulumLips, teeth, filariform extremities, and dentary plates

Number of species

600028Estimated > 15,00029 Registered > 9,00030Estimated > 800,000 to 1,000,000

Registered > 25,00031

Number of species known to infect humans40321633> 12,00034
Species

Hymenolepis nana

Taenia solium /Taenia saginata

Fasciola hepatica

Ascaris lumbricoides

Hookworm

Trichuris trichiura

Toxocara spp.
Timeline of lifecycle stagesLarval formation

Some days (eggs can survive for months)35

9–15 days36

18 days to several weeks37

1–2 days38

15–30 days39

Larval growth

After hatching, the larvae develop into cysticercoid, which can survive for years in an animal40

5–7 weeks as cercariae in snails and longer periods in wet environments as encysted metacercariae41

10–14 days42

5–10 days (after maturing can survive for weeks outside the host)43

60–70 days (from hatching to mature state)44

5–6 days45

Maturation to adult

2 months (from cysticercoid to adult)46

3–4 months47

2–3 months48

2–8 weeks49 (can become dormant for months)

Lifespan of adult worm

4–6 weeks

Several years50

8–10 years51

1–2 years52

Several years53

1 year54

Eggs laid per day250,00055 to 700,000563,000 to 25,000573,00058 to 250,00059
Egg depositionFrequency

up to 6 times a day60

daily61

daily62

daily63

Number of eggs per event

50,000-100,00064

200,0006566 to 250,000 or more67

5,000-10,00068

3,000-20,00069

Larvae per egg11300 cercariae (Schistosoma), 250,000 metacercariae (Fasciola)701111

Draft genomes for all categories of helminth have been sequenced in recent years and are available through the ParaSite sub-portal of WormBase.71

Use in medicine

Main article: Helminthic therapy

Parasitic worms have been used as a medical treatment for various diseases, particularly those involving an overactive immune response.72 As humans have evolved with parasitic worms, proponents argue they are needed for a healthy immune system.73 Scientists are looking for a connection between the prevention and control of parasitic worms and the increase in allergies such as hay-fever in developed countries.74 Removal of parasitic worms from areas is correlated with an increase in autoimmune disorders in those areas.75 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.76 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 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 macrophages 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.77 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....78

Human stool samples

For medical purposes, the exact number of helminth eggs is less important and therefore most 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 smears. 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.

Relevance for sanitation

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.7980 Helminth eggs are regarded as the main biological health risk when applying sewage sludge, fecal sludge or fecal matter on agricultural soils.81 The eggs are the infective stage of the helminths' life cycle for causing the disease helminthiasis.

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.8283 Helminth eggs of concern in wastewater used for irrigation have a size between 20 and 90 μm and a relative density of 1.06–1.23.84 It is very difficult to inactivate helminth eggs, unless temperature is increased above 40 °C or moisture is reduced to less than 5%.85 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).86 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.878889

Removal versus inactivation

Processes that remove particles, such as sedimentation, filtration or coagulation-flocculation physically remove helminth eggs from wastewater (but do not inactivate them).9091 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%.92 Best results can be obtained when both of these conditions are met together for an extended period of time.93 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.94 Helminth eggs are considered highly resistant biological structures.95

Measurements

Indicator organism

This section is an excerpt from Indicator organism § Indicator helminth eggs.[edit]

The eggs from helminths (parasitic worms) are a commonly used indicator organism to assess the safety of sanitation and wastewater reuse systems (such schemes are also called reuse of human excreta).96: 55  This is because they are the most resistant pathogens of all types of pathogens (pathogens can be viruses, bacteria, protozoa and helminths).97 It means they are relatively hard to destroy through conventional treatment methods. They can survive for 10–12 months in tropical climates.98 These eggs are also called ova in the literature.99

Helminth eggs that are found in wastewater and sludge stem from soil-transmitted helminths (STHs) which include Ascaris lumbricoides (Ascaris), Anclostoma duodenale, Necator americanus (hookworm), and Trichuris trichiura (whipworm).100 Ascaris and whipworm that are identified in reusable wastewater systems can cause certain diseases and complications if ingested by humans and pigs.101 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.102

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 or fecal sludge is often used to assess the efficiency of diverse wastewater and sludge treatment processes in terms of pathogen removal.103: 55  In particular, the number of viable Ascaris 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.104

The technique used for testing depends on the type of sample.105 When the helminth ova are in sludge, processes such as alkaline-post stabilization, acid treatment, and anaerobic 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.106

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 toilets.

See also

  • Biology portal
  • Medicine portal

Further reading

Wikimedia Commons has media related to Helminths.

References

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