Characteristics of Helminths

Helminths are parasitic worms that live and feed on living hosts to receive nourishment and protection which results in illness of the host.

Learning Objectives

Describe the attributes of helminths

Key Takeaways

Key Points

  • Helminths can cause disruption of the hosts nutrient absorption by utilizing all nutrients that pass through the intestinal tract.
  • There are four major groups of parasitic worms: monogeneans, cestodes (tapeworms), nematodes (roundworms), and trematodes (flukes).
  • Helminths are characterized by the presence of attachment organs which include suckers, hooks, lips, teeth, and dentary plates.

Key Terms

  • parasitic worm: Parasitic worms are referred to helminths as they live and feed on living hosts. Helminths receive both nourishment and protection by disrupting the hosts ability to absorb nutrients resulting in weakness and disease of the host.
  • bothridia: A sucker or attachment organ on a parasitic worm.
  • helminth: A parasitic roundworm or flatworm.

Parasitic worms that inhabit the intestinal tract (blood, tissue and organs) of humans are referred to as helminths. They receive nourishment and protection by living within the host where they cause disease. The parasitic intestinal helminths can be divided into three groups which include Nematodes (roundworms), Cestodes (tapeworms), and Trematodes (flukes). Helminths share numerous characteristics that contribute to their parasitic quality including the presence of attachment organs. These attachment organs include bothria (sucking grooves: Cestodes or tapeworms, which may also have a rostellum (crown of thorns with hooks); Old World Hookworms: cutting teeth; New World Hookworms: cutting plate. These attachment organs allow these particular helminths to reside within their human host. It should be noted, however, that blood and tissue roundworms (Nematodes) exist that will not be discussed in this section.


Suckers on Taenia solium: A micrograph showing the morphology of a Taenia solium tapeworm scolex with its four suckers, and two rows of hooks.

The three commonly studied and well-known groups include the intestinal Nematodes (round worms), tapeworms (Cestodes), and blood, tissue and organ flukes (Trematodes).

Intestinal helminths are commonly transmitted through fecally contaminated food and water and these parasites include Ascaris lumbricoides, Trichuris trichiura (whipworm), and Enterobius vermicularis (pin worm).

Hookworms include Ancylostoma duodenale and Necator americanus, whose larvae penetrate the skin after their ova have been passed in human feces. These soil-transmitted helminths are associated with climates that are warm and moist and where sanitation and hygiene are poor. The eggs are passed in the feces of the infected individual, the larvae hatch, contaminate the soil, and can penetrate the skin when contact is made.


Hookworms: An image of a hookworm attached to intestinal mucosa.

Ascaris lumbricoides, the cause of Ascariasis, is a type of soil transmitted helminth. Ascaris, the largest roundworm, lives in the intestine and the eggs are passed in the feces of the infected person. Ascariasis is caused by ingestion of eggs from food and water contaminated with feces from humans infected with Ascaris, and ingestion allows for continuation of the life cycle. Ascariasis may not be symptomatic, but may become symptomatic or fatal if intestinal blockage occurs without surgical intervention.

Hookworms, another type of soil-transmitted helminth, reside in the small intestine and eggs are passed in the feces of the infected individual. The eggs will mature and hatch in the soil and the immature worms (larvae) will penetrate the skin of humans if contact is made. The hookworm is transmitted by exposing bare skin to contaminated soil.

Whipworms (Trichuris trichiura), a type of soil-transmitted helminth, resides in the large intestine and eggs are passed in the feces of the infected individuals. The eggs will mature into an infective form in the soil and transmission occurs by ingestion of eggs in fecally contaminated food and water. Individuals with whipworm may have light or heavy infections. Light infections are usually not significantly symptomatic. However, heavy symptoms include frequent, painful passage of stool that contains mucus, water, and blood.

This summary does not contain an exhaustive compilation of all human parasitic nematodes, but merely a representation of several species.

Classification and Identification of Helminths

Helminths, or parasitic worms, are eukaryotic parasites characterized by their ability to feed and live on living hosts.

Learning Objectives

Recall the attributes of helminths

Key Takeaways

Key Points

  • The major groups of parasitic helminths include: platyhelminths (flatworms), acanthocephalins (thorny-headed worms) cestodes (tapeworms), trematodes (flukes) and nematodes (roundworms).
  • The classification and identification of helminths are dependent on numerous factors including body shape, body cavity, body covering, digestive tubing, sex and type of attachment organs.
  • Parasitic worms disrupt the ability of a host to receive and obtain nourishment.

Key Terms

  • acetabulum: A large posterior sucker, for example that of leeches.
  • tegument: A natural covering of the body or of a bodily organ.
  • bothridia: A sucker or attachment organ on a parasitic worm.

Helminthsare large, multicellular organisms that are visible to the eye once in the adult stage of their life cycle. Helminths and multicellular eukaryotes, can either be free-living or parasitic.

In their adult form, helminths are unable to multiply in humans and utilize numerous mechanisms of transmission to ensure reproductive success. These parasites live in and feed on hosts which allow them to obtain nourishment while disrupting the hosts’ nutrient absorption. Parasitic worms are commonly found within the intestine and thus, are called intestinal parasites. They are able to live in both humans and animals.

The major groups of parasitic helminths include:

  • platyhelminths (flatworms)
  • acanthocephalins (thorny-headed worms)
  • cestodes (tapeworms)
  • trematodes (flukes)
  • nematodes (roundworms).

Helminths: Image of a tapeworm, specifically an adult Ascaris lumbriocoides worm.

The classification and identification of helminths are dependent on numerous factors including body shape, body cavity, body covering, digestive tubing, sex and type of attachment organs.

Platyhelminths (flatworms) include both trematodes (flukes) and cestodes (tapeworms). Specifically, tapeworms are characterized using the above criteria and are organized in a segmented plane. They lack a body cavity and have a tegument body covering. Tapeworms lack a digestive tube and are hermaphroditic. They utilize suckers or bothridia, and rostellum with hooks for an attachment organ.

Trematodes are characterized by an unsegmented plane for body shape. They also lack a body cavity and have a tegument for body covering. However, the digestive tube for trematodes ends in the cecum. Trematodes are hermaphroditic and utilize oral suckers, ventral suckers or acetabulum for attachment organs.

Nematodes are characterized by a cylindrical body shape and do indeed have a body cavity. Its body covering is a cuticle and the digestive tube ends in the anus. The sex of nematodes is dioecious (distinct male and female organisms). Lastly, their attachment organs range from lips, teeth, filariform extremities and dentary plates.

Distribution and Importance of Parasitic Worms

Parasitic worms, distributed worldwide, are hypothesized to have importance in immune system regulation.

Learning Objectives

Explain how parasitic worms may be beneficial

Key Takeaways

Key Points

  • Parasitic worms appear to have medicinal properties as well which has opened a new field of research by studying the use of parasitic worms in disease treatment.
  • Scientists hypothesize that parasitic worms can be used to combat autoimmune diseases by damping down the immune system of the host by using parasitic worms to active eosinophils and downstream targets.
  • Parasitic worms have been linked to a protective role in autoimmune disease development and prevention of metabolic syndrome.

Key Terms

  • eosinophils: a type of white blood cell used to fight parasitic infection

Image of Necator Americanus: Infectious Necator americanus larva are applied to the skin in therapy, either in a single dose or in multiple smaller doses over the course of two or three months.

Parasitic worms, often the result of horrible illness and disease, appear to have medicinal properties as well. The importance of parasitic worms has come to light in regards to treating various diseases which may benefit from their presence. It is argued that humans have evolved with parasitic worms and there is a mutualistic relationship which mandates the need for parasitic worms to contribute to a healthy immune system. The most common use of parasitic worms for medicinal purposes is in the use against diseases characterized by an overactive immune response. An overactive immune response is often seen in individuals with allergies and hay-fever, specifically in developed countries where parasites have been under strict prevention and control. It is speculated that parasitic worms have the ability to damp down the immune system, which promotes an environment where they can thrive without being attached. In return, the damping down of the immune system is believed to be beneficial, as this may prevent the development of allergies.

An additional study links an increase in metabolic syndrome in the Western world and the success in preventing and eliminating parasites. The study demonstrates that immune system cells, eosinophils, that are present in fat tissue play a role in the prevention of insulin resistance via secretion of interleukin 4. The interleukin 4 is then able to activate macrophages that function in the maintenance of glucose homeostasis. The study showed that parasitic worm infection results in an increase in eosinophils, thus, promoting control of glucose maintenance.

The hypothesis that parasites are necessary for a healthy immune system is currently under investigation and still requires multiple lines of evidence.

Arthropods as Vectors

Arthropods are capable of functioning as vectors by transmitting diseases.

Learning Objectives

Demonstrate how arthropods act as disease vectors

Key Takeaways

Key Points

  • Arthropods will transmit diseases via their ability to function as hematophagous vectors which is characterized as their ability to feed on blood at some or all stages of their life cycles.
  • Arthropod vectors include mosquitoes, fleas, sand flies, lice, fleas, ticks and mites.
  • Arthropods transmit parasites either by injection into the blood stream of the host directly via their salivary glands, or by forcing parasites into a pool of blood which develops when chewing the skin.

Key Terms

  • vector: A carrier of a disease-causing agent.
  • hematophagous: feeding on blood

Arthropods are capable of serving as vectors, indicating that they play a major role in disease transmission. Arthropods that serve as vectors include mosquitoes, fleas, sand flies, lice, ticks, and mites. These arthropods are responsible for the transmission of numerous diseases. These types of vectors are considered to be hematophagous. These arthropod vectors are characterized as feeding on blood at some or all stages of their life cycles. The arthropods feed on the blood which typically allows parasites to enter the bloodstream of the host.

The Anopheles mosquito serves as a vector for malaria, filariasis, and arboviruses as well (arthropod-borne-viruses). The Anopheles mosquito inserts its mouthpart under the skin and feeds on the hosts. As the mosquito is feeding on the host blood, the parasites which are carried by the mosquito, are typically located within its salivary glands. The mosquito is able to directly transfer the parasites into the blood stream of the host. Pool feeders, which include both the sand and black fly, responsible for Leishmaniasis and Onchocerciasis diseases, will chew the hosts skin. The chewing action produces a well which promotes the formation of a small pool of blood from which they feed. In the case of sand flies, responsible for Leishmaniasis, the parasites infect the host through the saliva. In the case of black flies, responsible for Onchocerciasis, the parasites are forced out of the insects head into the pool of blood. Tsetse flies are vectors of the human African trypanosomiasis, called “African sleeping sickness”. Additional examples of mosquitoes include the Aedes mosquito which is a vector for avian malaria, dengue fever, and yellow fever.

Fleas are another type of arthropod vector that transmit numerous diseases. The human flea, Pulex irritans, and the Oriental rat flea, Xenopsylla cheopis, are responsible for the transmission of the bubonic plague, murine typhus, and tapeworms.


Male Xenopsylla cheopis (oriental rat flea) engorged with blood.: Image of Xenopsylla chepsis (oriental rat flea) engorged with blood. The oriental rat flea is an example of an arthropod vector as it is the primary vector of plague. This vector has been the cause of large plague epidemics in Asia, Africa, and South America. Both male and female fleas can transmit the infection.