What are helminths?


What are helminths?

What are helminths?

Helminths 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 a variety of different helminths from the very large to the microscopic.

What are helminths?

  • ‘Helminth’ is a general term meaning worm.
  • All helminths are invertebrates with long, flat or round bodies.
  • There are many different kinds of helminth ranging in length from less than one millimetre to over one metre.
  • Helminths infect a range of hosts, including humans.
  • Their effects inside their host also vary, causing a wide spectrum of diseases, from mild to potentially deadly.
  • Helminths are one of the leading causes of morbidity in the developing world with over two billion people affected. That’s almost a third of the world’s population!
  • Infection can cause physical, nutritional and cognitive impairment in young, developing children.
  • The global burden of helminth disease exceeds that of conditions such as malaria and tuberculosis.

Why study the genetics of worms?

  • Caenorhabditis elegans, a roundworm (or nematode) with around 1,000 cells, was the first animal to have its genome sequenced.
  • Although C. elegans is a very simple organism, sequencing its genome paved the way to a comprehensive view of its development and behaviour.
  • It is hoped that knowing more about the genomes of other helminths could provide useful insights into their biology.
  • Knowing how they function and cause disease is expected to lead to new, more effective ways of treating these infections.

Below is an introduction to some of the best characterised helminth worms. They are grouped according to their general shape and the part of the host they tend to affect and live in during infection.


What are they?

  • Roundworms, or nematodes, are slender worms that can live, feed and reproduce in the human intestine.
  • The worms can get into a host’s body when:
    • the host eats food or drinks water containing the worms
    • the worm directly penetrates the host’s skin
    • an insect, such as a mosquito, carrying the worm bites a host.
  • Roundworms range in length from less than a millimetre to a metre.
  • Infections of roundworm happen more often in warm, tropical climates in poor, rural communities.
  • Roundworm infections usually do not cause noticeable symptoms unless the worms are present in very large numbers.
  • Disease can be caused by the worm living in the gut, blood, lymph or tissues.
  • Gut worms are the most prevalent and can be diagnosed by looking under a microscope for worms or eggs in samples of faeces.
  • Roundworm infections can be successfully treated with medication, either a course of tablets or a powder that is dissolved in water for young infants. The medication often works by interfering with the worm’s movement or metabolism.
  • There are thousands of different species of roundworm, several of which infect humans.

Giant roundworms

  • Giant roundworms are very long (often over 30 centimetres in length) and vary in diameter from two to six millimetres.
  • Ascaris lumbricoides is the giant roundworm that infects humans and can grow up to 35 centimetres long.
  • Their eggs are deposited into soil in faeces and are transmitted to humans via contaminated food or drink. The eggs hatch in the intestines and migrate to the lungs. They are then coughed up out of the lungs into the mouth and consequently swallowed. They travel to the stomach before entering the intestines where they live.
  • Ascaris lumbricoides is most prevalent in tropical and sub-tropical regions of the world, particularly those areas with poor sanitation.
  • Around 85% of people who are infected with Ascaris lumbricoides do not show any symptoms, particularly if the number of worms in the body is low.
  • If a large number of worms are present in the body, the host may experience vomiting, shortness of breath, swelling in the abdomen or blockages in the intestines.

A dish of Ascaris lumbricoides worms. Many grow up to 30 centimetres long.
Image credit: Professor Philip Cooper, Liverpool School of Tropical Medicine, UK


  • Whipworms are so called because of their distinctive whip-like shape with a long, thin front end and a thicker tail.
  • Trichuris trichiura is the human whipworm.
  • Whipworms reach a maximum length of five to eight centimetres.
  • Like giant roundworms, Trichuris trichiura eggs are deposited into the soil in faeces and transmitted to humans in contaminated food or drink. They hatch and develop inside the small intestine and move to the large intestine as adults.
  • It is estimated that 600-800 million people in the world are infected with whipworm.
  • Whipworms are found in warm, humid climates, particularly in Asia.
  • Understanding the biology of whipworms shows promise for the development of therapeutic treatments for inflammatory diseases such as ulcerative colitis and Crohn’s disease.
    • Diseases like ulcerative colitis and Crohn’s disease are caused by the body’s own immune system attacking areas of the gut.
    • These diseases are rarely seen in developing countries where whipworm infection is very common. Scientists believe this could be because the worms help to control the immune response and stop the immune system from attacking the gut lining.

The human whipworm, Trichuris trichiura.
Image credit: AJ Cann via Flickr CC BY-SA


  • Hookworms are a greyish white or pink colour. Their name comes from the shape of their heads which is slightly bent in relation to the rest of its body, like a hook.
  • Ancyclostoma duodenale have well-developed mouths with two pairs of teeth whereas Necator americanus have a pair of cutting plates, rather than teeth, that help them latch on to the intestinal wall.
  • The most common way people become infected with hookworm is through direct skin contact with contaminated soil, often when walking barefoot.
  • After being infected people can experience an itchy rash around the area where the worm penetrated the skin.
  • Hookworms are much smaller than giant roundworms so cause less tissue damage and obstruction.
  • In severe infections, hookworm can cause abdominal pain, diarrhoea, fatigue and weight loss. Hookworm are bloodsucking worms and in some cases can cause enough blood loss to result in anaemia (iron deficiency).
  • The damage caused by hookworms to the intestine can reduce its ability to properly absorb nutrients from food leading to protein deficiency and malnutrition. This is particularly significant in children and pregnant women.
  • Hookworm infection affects over half a billion people globally and is the leading cause of maternal and childhood disease in developing countries in the tropics and subtropics.

The head of the hookworm, Ancyclostoma duodenale.
Image credit: Daniel and Anita Saunders via Flickr CC BY-NC-SA 2.0

Filarial worms

  • Examples of thread-like filarial roundworms are Wuchereria bancrofti and Onchocerca volvulus.
  • Wuchereria bancrofti:
    • is transmitted via a mosquito bite and affects over 120 million people, mainly in Africa, South America and Asia.
    • lives in the lymph nodes. It is a major cause of lymphatic filariasis, a painful and profoundly disfiguring disease that results in swelling and fluid retention in the limbs and genitals.
  • Onchocerca volvulus:
    • is spread by the bite of the black Simulium fly and affects over 18 million people worldwide with the majority of cases in Africa.
    • lives in the fat layer of the skin causing onchocerciasis. It is more commonly known as river blindness because the Simulium fly breeds rapidly in fast-flowing water, such as streams and rivers. If the worm reaches the eyes it can cause blindness.
  • Scientists are currently working on sequencing the full genomes of filarial worms to learn more about their genetics and how they function in their hosts.


What are they?

  • Tapeworms are flat, ribbon-like worms that live in the intestines of their host. Their scientific name is cestodes.
  • Humans can become infected with tapeworm by eating raw contaminated pork, beef or fish, or by consuming food contaminated by faeces from infected animals.
  • Tapeworms are most commonly found in developing countries and are rare in the UK.
  • The tapeworm life cycle involves at least two hosts, the adults live in one host (for example, a human), while the larvae live in another host (for example, a pig).
  • Infection with adult tapeworm rarely causes serious symptoms and therefore people often won’t know they’re infected until they see segments from the worm in their faeces. These segments look a bit like white grains of rice and contain tapeworm eggs.
  • Infections with tapeworm larvae can be deadly if the larvae create cysts within the tissue of their host.
  • Different kinds of tapeworm cause different symptoms in humans.
    • For example, fish tapeworm absorb vitamin B12 leading to vitamin B12 deficiency in the host. Vitamin B12 deficiency can lead to mouth ulcers, depression and the skin developing a pale yellow tinge.

An adult Taenia saginata tapeworm.
Image credit: Centers for Disease Control and Prevention


What are they?

  • Trematodes, more commonly known as flukes, are flattened oval or leaf-shaped worms ranging in length from a few millimetres to seven or eight centimetres.
  • Infections are most common in Asia, Africa, South America and the Middle East.
  • Flukes can be classified into two groups – tissue flukes and blood flukes.

Tissue flukes

  • These flukes infect the bile ducts, liver or other tissues.
  • One example is the Chinese liver fluke, Clonorchis sinensis:
    • This fluke lives in the bile ducts of humans and is thought to feed on the epithelial cells there.
    • Symptoms include abdominal pain, weight loss and jaundice.
    • These symptoms are caused by swelling or blockage of the bile ducts, the tubes that carry bile from the liver to the intestine.

Chinese liver fluke.
Image credit: Wellcome Images

Blood flukes

  • All blood flukes belong to the genus Schistosoma.
  • As their name suggests, at some stages of their life cycle blood flukes inhabit the blood of their host .
  • Schistosoma parasites cause schistosomiasis (also known as bilharzia), a tropical disease that affects about 240 million people worldwide.
  • People in sub-tropical and tropical areas of the world can become in infected with Schistosoma when playing or bathing in fresh water contaminated with schistosome larvae. The larvae burrow directly into the skin.
  • Symptoms of infection are the result of the body’s immune system reacting to the eggs of the parasite:
    • Generally flu-like symptoms are common, including a high temperature and muscle aches. Symptoms may also include a skin rash, cough and abdominal pain.
    • In more chronic infections, inflammation occurs in areas of the body where eggs have lodged, such as the bladder, intestine, lungs, liver and even the brain. If left untreated, this can lead to long-term damage to these organs.

Male and female schistosomes. The female can be seen lying within a groove on the surface of the male (stained pink).
Image credit: Shutterstock

This page was last updated on 2021-07-21

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