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Helminths are parasitic worms that can infect humans and other animals. They are worm-like parasites that survive by feeding on a living host to gain nourishment and protection, sometimes resulting in illness of the host. There are three types of helminths: flukes (trematodes), tapeworms (cestodes), and roundworms (nematodes). They are classified according to their general shape and the part of the host they tend to affect and live in during infection. Roundworms, for example, can live, feed, and reproduce in the human intestine. Flukes, on the other hand, generally infect the bile ducts, liver, or blood. Tapeworms are commonly found in or around the intestines. In this paragraph, we will focus on helminths that live in muscle tissue.
| Characteristics | Values |
|---|---|
| Definition | Parasitic worms, also known as helminths, are a polyphyletic group of large macroparasites; adults can generally be seen with the naked eye |
| Types | Flukes (trematodes), tapeworms (cestodes), and roundworms (nematodes) |
| Shape | Elongated, flat or round bodies |
| Size | From less than one millimetre to over one metre |
| Location | Intestinal and extra-intestinal sites, blood vessels, lymph nodes, skin |
| Hosts | Humans, animals |
| Transmission | Food or water contaminated with eggs, walking barefoot on contaminated soil, mosquito bites, fly bites |
| Symptoms | Abdominal pain, diarrhoea, abdominal discomfort, abdominal swelling, vomiting, shortness of breath, intestinal blockages, intestinal worms, lung flukes, inflammation, muscle aches, skin rash, cough, anaemia, weakness, disease, physical impairment, nutritional impairment, cognitive impairment, river blindness, elephantiasis |
| Treatment | Antihelminthic medications such as Albenza (albendazole) and Emverm (mebendazole) |
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What You'll Learn
- Flukes (trematodes) are leaf-shaped flatworms with prominent suckers
- Tapeworms (cestodes) are long, segmented flatworms
- Roundworms (nematodes) are slender worms that can live in the intestine
- Thorny-headed worms (acanthocephalans) infect animals, rarely humans
- Hookworms enter the body through the feet and travel to the intestine
Flukes (trematodes) are leaf-shaped flatworms with prominent suckers
Flukes, also known as trematodes, are flatworms that are leaf-shaped and range in length from a few millimetres to 7 to 8 centimetres. They are parasitic in nature and are internal parasites. Flukes are hermaphrodites, except for blood flukes, which are bisexual. They have prominent oral and ventral suckers that help them maintain their position in the host's body. These suckers are also used to adhere to host tissues by penetrating the host's tissue with spines lining the acetabulum organ. The acetabulum is a saucer-shaped organ attached to the oral sucker.
Flukes have a complex life cycle that requires at least two hosts. The intermediate host, where asexual reproduction occurs, is usually a snail, while the definitive host, where sexual reproduction occurs, is a vertebrate. Flukes can infect all vertebrate classes, but they most commonly parasitize fish, frogs, and turtles. They can also parasitize humans, domestic animals, and invertebrates such as molluscs and crustaceans.
Flukes have a dorsoventrally flattened body, bilateral symmetry, and a definite anterior end. Their body surface is covered with a tough syncytial tegument, which helps protect them from digestive enzymes in the gut of larger animals. They do not have a circulatory system, and waste material must be egested through the mouth as they usually lack an anus.
Blood flukes, such as Schistosoma, are a type of trematode that can undergo both asexual and sexual reproduction. They inhabit the blood during some stages of their life cycle and can cause diseases such as schistosomiasis in humans. The urinary blood fluke (S. haematobium), the intestinal blood fluke (S. mansoni), and the Oriental blood fluke (S. japonicum) are three species that infect humans.
Trematodes are commonly referred to as flukes, which comes from the Old English name for flounder, referring to the flattened shape of the organisms. The term trematode comes from the Greek word "trēmatṓdēs", meaning "pierced with holes", referring to the worm's sucker.
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Tapeworms (cestodes) are long, segmented flatworms
Tapeworms, or cestodes, are parasitic worms that are members of the invertebrate class Cestoda (phylum Platyhelminthes). They are long, segmented flatworms, also described as ribbon-like or ribbon-shaped. Tapeworms are internal parasites, affecting the liver or digestive tracts of vertebrates, including humans, and other animals such as domestic animals and fish. Tapeworms can also infect invertebrates.
Tapeworms are divided into two subclasses: Eucestoda and Cestodaria. Eucestoda are the most well-known type of tapeworm and are ribbon-like worms commonly known as tapeworms. Cestodaria are mainly parasites that infect fish. All cestodes are parasitic, and many have complex life cycles involving a definitive host and one or two intermediate hosts. The larval forms of tapeworms lodge in the skin, liver, muscles, the central nervous system, or other organs. The adult worms live in the digestive tracts of vertebrates.
Tapeworms are unique in that they lack an alimentary canal, meaning they absorb nutrients through their surface as their food flows past them. They absorb nutrients directly from the host's gut through their specialised neodermal cuticle, or tegument. Tapeworms are composed of a series of segments called proglottids, which are produced from the neck by mitotic growth. Each proglottid contains an independent reproductive tract, and the sum of the proglottids is called a strobila, which resembles a strip of tape and gives tapeworms their name.
Tapeworms are one of the most common causes of foodborne parasitic diseases in developing countries. Beef tapeworm, for example, is the most common cause of tapeworm disease in Ethiopia. Tapeworms can cause infections in humans, known as cestodiasis, which can last up to 30 years. Symptoms of tapeworm infection include nausea or vomiting, appetite loss, epigastric or umbilical pain, and weight loss.
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Roundworms (nematodes) are slender worms that can live in the intestine
Helminths are worm-like parasites that can be either free-living or parasitic in nature. They are large, multicellular organisms that live at the expense of the host and are generally visible to the naked eye in their adult stages. Many helminths are intestinal parasites, and an infection by a helminth is known as helminthiasis, helminth infection, or intestinal worm infection.
The most common intestinal roundworms are those transmitted through contact with soil, such as Ascaris lumbricoides, Trichuris trichiura, hookworms, and Strongyloides stercoralis. Infections with these worms, along with Enterobius vermicularis, account for three-quarters of all helminthic infections. Most enteric nematodes have established a well-balanced host-parasite relationship with the human host, and humans tolerate these parasites well.
Ascaris lumbricoides is the largest intestinal nematode of humans, with females reaching up to 30 cm in length and males being smaller. Adults in the small intestine produce eggs that pass in feces, embryonate in the soil, are ingested, and hatch. The larvae migrate from the intestine to the lung and back to the intestine, where they mature. Migrating larvae can cause pneumonitis, allergic reactions, and eosinophilia. Heavy infections can lead to intestinal obstruction and malnutrition.
While roundworm infections may not cause any symptoms, some possible symptoms include disturbed sleep, restlessness, severe abdominal pain, malnutrition, growth delay, anemia, and colic. Diagnosis of intestinal ascariasis is typically done by detecting eggs in the stool, and control measures include proper disposal of human feces and improved personal hygiene.
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Thorny-headed worms (acanthocephalans) infect animals, rarely humans
Acanthocephala, also known as thorny-headed worms, are parasitic worms that infect a wide range of vertebrate and invertebrate hosts. They are commonly found in wildlife and some domestic animal species, but rarely infect humans. The human cases of acanthocephaliasis, or acanthocephalan infections, are rare but not unheard of. The infections are usually a result of eating undercooked fish, which are paratenic hosts for Bolbosoma spp. In children, the infections can occur due to the habit of eating insects.
Acanthocephalans are highly modified parasites, and their relationship with other animals is not well understood. They are characterized by an elongated tube-like body with a retractable, hooked proboscis that attaches itself to the host's intestines. The worms can also indulge in "brain-jacking", where they modify the behaviour of their intermediate hosts to increase the likelihood of being eaten by a definitive host. The definitive hosts for Acanthocephala include animals like pigs, raccoons, rats, and humans.
Acanthocephalans can be identified by histologic examination, which reveals their thick tegument and the presence of both circular and longitudinal muscles. They lack a digestive tract, and their body cavity is mostly occupied by the reproductive tract. The female worms have one or two ovaries, while males have two testes. The eggs of Acanthocephalans are shed in the feces of the definitive hosts and contain a fully developed larva, which is then ingested by an intermediate host, always an insect.
The clinical symptoms of acanthocephaliasis can be severe, including abdominal pain and digestive issues. However, low-intensity or early infections may be asymptomatic. The treatment for acanthocephaliasis involves common anti-parasitic or anthelmintics, and prevention includes raising awareness about the risks of eating insects, beetles, and cockroaches.
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Hookworms enter the body through the feet and travel to the intestine
Hookworms are parasites that enter the body through the skin of bare feet and travel to the intestine, causing hookworm disease. This disease is an infection caused by a parasite, and it is widespread, with as many as 740 million people affected worldwide. It is most common in tropical and subtropical areas with poor sanitation, where people walk barefoot on contaminated soil.
Hookworm larvae are found in dirt or soil contaminated by the faeces of an infected person or animal. Larvae hatch from eggs in faeces and grow in the soil, becoming infective after 5 to 10 days. On contact with human skin, they penetrate the skin and enter through tiny cuts or breaks in the skin. From there, they travel through the bloodstream to the heart and then to the lungs. They penetrate the pulmonary alveoli and ascend the bronchial tree to the pharynx, where they are swallowed and reach the small intestine.
In the small intestine, the larvae mature into adult worms, which attach to the intestinal wall with hook-like heads. They feed on blood from the intestinal wall, causing blood loss and, in severe cases, anaemia and protein deficiency. Hookworms can live in the small intestine for a year or more, after which they pass out of the body through faeces.
Hookworm infections are often asymptomatic, especially in otherwise healthy individuals with a low parasite burden and a diet rich in iron. However, in some cases, the attachment of hookworms to the intestinal wall can cause abdominal pain, nausea, and anorexia. Children with frequent hookworm infections may experience slow growth and mental development due to the loss of iron and protein. Treatment for hookworm infections involves medication to destroy the parasites and address nutritional deficiencies caused by the infection.
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Frequently asked questions
Helminths are worm-like parasites that can infect humans and other animals. They are large, multicellular organisms that live off their host.
Helminths that live in muscle include filarial worms, which can live in the lymph nodes, and blood flukes, which can inhabit the blood of their host. Larvae of the beef tapeworm can also develop in the muscles of animals.
Helminths can infect humans through food or drink contaminated with their eggs, or by entering the body through the skin. They can also be transmitted by insect bites, such as mosquito or fly bites.
