Parasitic worm

This article is about the organism. For the infection caused by this organism, see Helminthiasis.

Hookworms attached to the intestinal mucosa

Two pinworms

Image showing life cycle inside and outside of the human body of one fairly typical and well described helminth: Ascaris lumbricoides

Helminths (/ˈhɛlmɪnθs/), also commonly known as parasitic worms, are large multicellular organisms, which when mature can generally be seen with the naked eye. They are often referred to as intestinal worms even though not all helminths reside in the intestines; for example Schistosomes (causing the disease schistosomiasis) are not intestinal worms, but rather a type of helminth which resides in blood vessels. There is no clear consensus on the taxonomy of helminths, it is more of a commonly used term to describe certain worms with superficial similarities. These are plathelminths (cestodes and trematodes) and nemathelminths (nematodes) - both of these are parasitic worm types – and the annelida, which is not parasitic or at the most ectoparasites like the leeches.

Many, but not all, of the worms referred to as helminths belong to the group of intestinal parasites. An infection by a helminth is known as helminthiasis, soil-transmitted helminthiasis, helminth infection or intestinal worm infection. The same naming convention applies to all helminths whereby the ending "-asis" (or in veterinary science the ending "-osis") at the end of the name of the worm is added to signify the infection with that particular worm, e.g. Ascaris is the name of a particular helminth, and Ascariasis is the name of the infectious disease caused by this helminth.

Helminths are worm-like organisms living in and feeding on living hosts, receiving nourishment and protection while disrupting their hosts' nutrient absorption, causing weakness and disease. Those that live inside the digestive tract are called intestinal parasites. They can live inside humans and other animals. In their adult form, helminths cannot multiply in humans. Helminths are able to survive in their mammalian hosts for many years due to their ability to manipulate the immune response by secreting immunomodulatory products. Helminth ova (or eggs) have a strong shell that protects the eggs very well against a range of environmental conditions.

Helminthology is the study of parasitic worms and their effects on their hosts. The word helminth comes from Greek hélmins, a kind of worm.

Helminth taxonomy

There is no real consensus on the taxonomy (or groupings) of the helminths, particularly with the nematodes. The term "helminth" contains a number of phyla, many of which are completely unrelated. However, for practical considerations the term is still used nowadays to describe four groups with superficial similarities, the phyla Annelida, Platyhelminths, Nematoda and Acanthocephala.

There is in fact no helminth classification, it is an “artificial” term.

The most important helminths in the sanitation field are the human parasites, which is why most people relate the term helminth to them, where they are classified as nemathelminthes (nematodes) and platyhelminthes, depending on whether they possess a round or flat-shaped body respectively. The latter are further divided into cestodes and trematodes depending on whether or not they have a segmented body. 

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

Common characteristics

Helminths are a group of evolutionary unrelated organisms which share a similar form. Helminths include members of the following taxa: monogeneans, cestodes (tapeworms), nematodes (roundworms), and trematodes (flukes).

Characteristics that are common for all helminths include:

• Helminths can be either hermaphrodites (can have both sexes), like tapeworms and the flukes (except the blood fluke which is not a hermaphrodite), or have their sexes differentiated, like the roundworms.
• All helminths produce eggs (also called ova) for reproduction. Generally hundreds of thousands of eggs are produced each time the female worm deposits its eggs - a process called ovoposition. The frequency of egg deposition is generally every few days from an adult helminth.
• Larvae hatch from these eggs (if the eggs are viable), inside or outside the host, depending on the type of helminth. Life cycles of the helminths differ in this and other specific aspects.
• The life time of helminths is several years due to their ability to manipulate the immune response by secreting immunomodulatory products.  
• The number of species of the different helminth types is vast, in the order of 1 million. The nematodes are the most diverse of all the helminths with the highest number of species.

The following table shows the principal morphological distinctions for three of these helminth groups:

	Tapeworms (Cestodes)	Flukes (Trematodes)	Roundworms (Nematodes)
Example diseases in humans	Tapeworm infection	Schistosomiasis, swimmer's itch	Ascariasis, dracunculiasis (guinea worm), elephantiasis, enterobiasis (pinworm), filariasis, hookworm infection (includes Necatoriasis and Ancylostoma duodenale infection), onchocerciasis, trichinosis, trichuriasis (whipworm)
Shape	Segmented plane	Unsegmented plane	Cylindrical
Body cavity	no	no	Present
Body covering	Tegument	Tegument	Cuticle
Digestive tube	no	Ends in cecum	Ends in anus
Sex	Hermaphroditic	Hermaphroditic, except schistosomes which are dioecious	Dioecious
Attachment organs	Sucker or bothridia, and rostellum with hooks	Oral sucker and ventral sucker or acetabulum	Lips, teeth, filariform extremities, and dentary plates
Number of species	6000	Estimated > 15,000 and 9,000 registered	800,000 to 1,000,000 estimated 25,000 registered

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. As humans 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 allergies such as hay-fever in developed countries. 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 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. 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….

Helminth ova (eggs) as pathogen indicator in sanitation

Analysing for helminth eggs in samples of feces from a dry toilet in Kenya

Processed helminth eggs samples from a dry toilet in Kenya

Helminth eggs (or ova) are a good indicator organism to assess the safety of sanitation and reuse systems because they are the most environmentally resistant pathogens of all pathogens (viruses, bacteria, protozoa and helminths) and can in extreme cases survive for several years in soil. Therefore, the presence or absence of viable helminth eggs ("viable helminth egg" means that a larva would be able to hatch from the egg) in a sample of dried faecal matter, compost or faecal sludge is often used as an indicator to assess the efficiency of diverse wastewater and sludge treatment processes in terms of pathogen removal. In particular, the number of viable Ascaris eggs is often taken as an indicator organism 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.

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

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 and many laboratories in developing countries lack the right equipment or the skilled staff required to do so.

Most of the complications in standard protocols for looking for Ascaris eggs in samples from various sanitation systems are due to finding one egg in a large amount of water or soil, but Ascaris eggs in the feces of infected persons when collected in dry toilets are already concentrated and they do not get diluted by water. Each adult female Ascaris produces 200,000 eggs per day. Therefore, the techniques to concentrate the eggs may be less important if the local laboratory is not equipped to do them and simple searches in samples from sanitation systems with dry toilets, e.g. from the faeces vault of a urine-diverting dry toilet (UDDT) could be done quite simply, and if none are found these biosolids can be considered safe. This only applies if Acaris worms commonly infect the users of such toilets.

Helminth ova removal in wastewater treatment

Helminth ova (or eggs) have a strong shell that protects the eggs very well against a range of environmental conditions. This shell consists of three layers: a lipoidal inner layer, a chitinous middle layer and outer proteinic layer. Helminth eggs of concern in wastewater used for irrigation have a size between 20 and 80 mm and a relative density of 1.06–1.15. It is very difficult to inactivate them, unless temperature is increased above 40°C or moisture is reduced to less than 5%. Thus, in conventional wastewater treatment processes the helminth ova are not inactivated but only removed from the wastewater. This is done by processes that remove particles through sedimentation or filtration such as in waste stabilization ponds (lagoons), storage bassins, constructed wetlands, coagulation-flocculation, rapid filtration and upflow anaerobic sludge blanket (UASB) reactors.

Helminth ova inactivation in sludge treatment processes

Inactivation of helminth ova can be achieved in sludge treatment where the temperature is increased over 40°C or moisture is reduced to less than 5%.

External links

• Parasitic Roundworm Diseases
• World Health Organisation (WHO) topic page on helminthiasis

Further reading

• C. Chandler & Clark P. Read (1976) Introduction to parasitology, John Wiley and Sons Inc, 20^th edition