- Wormlike parasites are called helminths.
- Worms usually live in the gastrointestinal tract.
- Common worms include roundworms, tapeworms and hookworms.
- Infection is usually more common in areas with poor sanitation, poor personal hygiene, or where human faeces are used as fertiliser.
- Children are commonly infected.
- Infections are treated with drugs called antihelminthics.
What are worms?
A parasite is an organism that lives in a host organism (e.g. a human) and which gets its food at the expense of its host. Helminths (worms) are large organisms that are usually visible to the naked eye in the adult stage. Worms usually (but not always) live in the gastrointestinal tract (the gut). Some helminths can also infect the bloodstream or tissues. They can cause symptoms either by their presence (they cause a blockage in the gut, the bile duct in the liver or the lymphatic vessels), or they deprive the body of nutrients. In the latter case, it is only when there is heavy infestation, or if you are malnourished to start with, that you would have obvious symptoms. Some worms also cause bleeding in the gut, and with continued loss of small amounts of blood, you can become anaemic.
Helminth life cycles include a stage in the human host, and sometimes a stage outside the human host – either in another animal, or free-living in soil. Parasites often have different forms at different stages in their life cycles.
Two groups of helminths commonly cause gastrointestinal infestations. These are nematodes (roundworms), and platyhelminthes (flatworms). Common examples of roundworm include Ascaris lumbricoides; flatworms include tapeworms such asTaenia solium and Taenia saginata; and bilharzia (schistosomiasis, caused by Schistosoma haematobium and S. mansoni). Worm infections are the most prevalent infections on the world.
Over one billion people are thought to be infected with roundworm. The condition is also called ascariasis. You get the worm by ingesting food or water which contains worm eggs. The eggs hatch in your gut, and the larvae are released. These larvae penetrate the gut wall, and move into the bloodstream. They reach the lungs, are coughed up and swallowed, and thus return to the gut. The larvae then mature into adult worms, which live in the gut and reproduce. Mature females are 25-35 cm long, with the males a bit smaller. They are about 2 – 6 mm in diameter.
The female lays eggs which are passed in the faeces and stay in the soil until another host ingests them. One female worm can produce nearly 250 000 eggs per day!
As with many helminth infections, symptoms are generally mild and vague, and may include abdominal pain, nausea, weight loss and possibly diarrhoea. Some people develop a cough and mild fever from the larvae moving through the lungs. Large numbers of adults in the gut may block the gut itself. Sometimes the worms migrate around the gut, and move into structures such as the bile duct and appendix, causing cholangitis (inflammation/infection of the bile duct) and appendicitis (inflammation/infection of the appendix).
The diagnosis is made in the laboratory by observing ascaris eggs from a stool sample, or by identifying worms that have been passed.
Drugs are available to treat ascariasis: albendazole and mebendazole are probably the commonest. This treatment is highly effective and is associated with few side effects. Hygienic disposal of faeces is very important in controlling this and other worm infections. In some countries, mass treatment campaigns of children are carried out to try to reduce the worm burden in the population.
Taenia solium (pork tapeworm), T. saginata (beef tapeworm), and T. asiatica (Asian tapeworm)
As the name suggests, these worms are flat, ribbon-like and segmented. They can reach up to 10 metres in length with over 1000 segments, each segment capable of reproducing. Tapeworms are hermaphroditic – each worm contains both male and female sex organs, and one worm is able to reproduce on its own. Tapeworm infections are common in Eastern Europe, Russia, Africa, Latin America and Asia.
The adult worms reproduce in the gut, and mature segments (called proglottids) break off the worm and migrate out through the anus into the soil. These segments are filled with eggs, and are ingested by a herbivorous animal host (usually cows for T. saginata and pigs for T. solium). In the host animal’s gut, the eggs hatch to release larvae, which move through the gut wall into the tissues. Once in the tissues (e.g. muscle) of the herbivore, the larvae fill with fluid and form a structure called a cysticercus. If a human eats undercooked or raw beef or pork, these cysticerci can hatch, and a mature tapeworm can develop in the gut after two to three months.
An important variation can occur in Taenia solium infections. If the eggs are eaten by another human (or by the original host), the eggs hatch in the human host’s gut and larvae can reach the tissues and form cysticerci there as well. This gives rise to the condition called cysticercosis. The cysticerci for some reason tend to migrate mostly to the brain, but may lodge in other tissues too.
Infections with adult worms normally produce no or mild symptoms, such as some abdominal pain, weight loss, and nausea. However, cysticercosis (cysts in human tissues) can cause a variety of problems, such as epilepsy and hydrocephalus (fluid on the brain). However, many people with cysticercosis have no symptoms at all. Eventually the cysticerci die, and become calcified.
The diagnosis can be confirmed by observing proglottids or tapeworm eggs in a stool sample. Praziquantel is a drug that is used to treat tapeworm infections.
(Schistosoma haematobium, S. mansoni)
An estimated 200 million people worldwide are infected with Schistosoma, the parasite that causes bilharzia. The worms are approximately 15 to 18 mm long, with the females slightly longer than the males. The female worms lie within a fold on the surface of the male.
The life cycle is fairly complex. Eggs are excreted in the human host’s urine or faeces. If the eggs reach water, they hatch and release a miracidium – the larval form. The miracidium is able to infect fresh water snails, in which it undergoes various developmental changes to become a cercaria (a small, free-living, motile form of the parasite), which swims from the snail. Cercariae are able to penetrate human skin, so if you’re swimming in water which has infected snails, you may become infected. The snails live in still or slowly moving water. (There is little risk of bilharzia in rapidly running water, where the snails are seldom found.)
Once the cercariae penetrate the skin, they develop into yet another larval form in the bloodstream. They move through the lungs, and eventually reach the blood vessels around the liver. As they migrate through the bloodstream, the worms mature. Once they reach the vessels in the liver, they mate, and then move to the blood vessels around either the gut or the bladder. The females lay eggs, which pass into the gut or bladder, and are excreted in the faeces or the urine. S. mansoni usually finds its way to blood vessels around the gut, whileS. haematobium reaches vessels around the bladder.
Symptoms can be due to the cercariae entering the skin, or to the effects of the adult worms and eggs. Some people develop an itchy rash at the site the cercariae penetrated the skin. This usually lasts about a week.
Katayama fever is a syndrome that starts a few weeks after first exposure to the parasite, and can last a few weeks. It is not very common with either S. mansoni or S. haematobium infections, but is seen more commonly with a form of schistosomiasis that occurs in the Far East (S. japonicum). Symptoms include cough, loss of appetite, abdominal pain, fever, chills, diarrhoea, and enlargement of the spleen, liver and lymph nodes.
In chronic schistosomiasis due to S. mansoni, symptoms are initially uncommon, but may include fatigue, nausea, diarrhoea , abdominal pain, and blood in stool. However, with time, the presence of the parasites and eggs in the vessels around the gut and liver can cause liver damage and enlargement of the spleen.
In S. haematobium infection, symptoms are initially of a burning sensation on passing urine, and blood in the urine. With time, there can be damage to the urinary tract and the kidneys.
The infection can be diagnosed by looking for the eggs in stool or urine samples. Alternatively, blood tests are available to see if the patient has antibodies to the parasite. This test is not as reliable as finding the eggs, however.
Bilharzia can be treated with the drug praziquantel. Like many infections, prevention can be aided through sanitary waste disposal. Where bilharzia is common, the water can be treated with chemicals to help kill the snail population.
(Toxocara canis, T. cati)
Toxocariasis is caused by infection with the roundworms that normally infect dogs or cats – Toxocara canis and Toxocara cati respectively. Infection of humans with T. canis is far more common than infection with T. cati. These roundworms, like Ascaris, are cylindrical in shape and can be short or long. Infected dogs or cats shed the ova of the worms into the soil. These ova are very hardy, and can remain in the soil for weeks. Children who eat sand can therefore easily ingest roundworm eggs, although the eggs can be ingested by anyone if foods (such as vegetables) are eaten raw and unwashed. In general, children are more commonly infected than adults as they are more likely to eat dirt and play in an outdoor environment where dog/cat faeces can be found.
Infection of humans with these worms is common, especially in developing countries, where sanitation is often poor and access to clean drinking water limited. However, even in areas of the USA, some studies have found that up to 20% of children are infected.
Infection with either of these worms can cause a syndrome called visceral larva migrans. After ingestion of the ova by a human, the eggs hatch to release larvae into the gut. These larvae then burrow through the intestinal wall, and migrate to various organs in the body – hence the name “visceral larva migrans”. Most infections are in fact asymptomatic – the infected person has no idea that he or she is infected.
If people do become symptomatic, symptoms include:
- Enlarged liver or spleen
- Skin nodules and itchiness
- Seizures (very uncommon)
- Involvement of the eye if a larva becomes trapped in the eye. This syndrome is called ocular larva migrans. Inflammation and scarring of the retina caused by the worm can result in blindness.
The above-mentioned symptoms may alert a doctor to the presence of Toxocara. A high level of certain white blood cells called eosinophils is another warning sign. Blood tests may reveal antibodies to the larvae, and this is the most commonly used method to make the diagnosis. If doubt exists, a biopsy of liver tissue may reveal the presence of roundworm larvae in some cases.
To prevent infection, dogs and cats should be dewormed regularly. People who have been infected with Toxocara usually rid themselves of the parasite without treatment in six to 18 months. Albendazole and mebendazole (Vermox) is sometimes prescribed for people infected with this parasite.
(Necator americanus, Ancylostoma duodenale)
Hookworm is an intestinal parasite which infects approximately 576-740 million people worldwide.
The life cycle of hookworms begins and ends in the small intestine. Worms in the intestine produce large numbers of eggs, which are passed in the faeces. The eggs need warm, moist, shaded soil to hatch into larvae. The larvae can infect a person if they come into contact with human skin, such as someone walking barefoot. They penetrate the skin and are carried to the lungs. The larvae then go through the respiratory tract to the mouth, are swallowed, and eventually reach the small intestine. Here they develop into adult worms that are approximately 1cm long. The worms attach themselves to the intestinal wall and suck blood. Adult worms produce thousands of eggs that are passed in the faeces. If the eggs contaminate soil, they can hatch and develop into infective larvae again after five to ten days.
People are usually infected when they walk barefoot on soil that contains human faeces. Hookworm cannot be spread from person to person.
Itching and a rash are usually the first signs of infection. Although many people have no symptoms, others with heavy infection may become anaemic and suffer abdominal pain, diarrhoea, loss of appetite and weight loss. Blood loss can also lead to protein deficiency. The degree of anaemia or malnutrition depends on the number of worms infecting the person as well as the amount of iron and protein in the diet. Heavy, chronic infections can cause stunted growth and mental development. It can be fatal, especially among infants.
Diagnosis is normally made by looking for the eggs under a microscope in a stool specimen. Treatment consists of medication (albendazole/ mebendazole/thiabendazole) which needs to be taken for up to three days. Iron supplementation may also be needed if the patient is anaemic.
Pinworms occur worldwide and are often seen in children. They are small, thin, white intestinal worms which live in the large intestine or rectum of humans. When an affected person sleeps, the worms leave the intestines through the anus and deposit eggs on the surrounding skin. The eggs can survive up to two weeks on clothing, bedding, toys, and other objects. People can become infected if they touch contaminated surfaces and then swallow the eggs. These eggs then hatch in the small intestine, and develop into the adult worms after 40 – 50 days. Once matured, the female migrates to the colon and lays eggs around the anus at night.
Main symptoms include itching around the anus, irritability, restlessness and insomnia caused by the disturbed sleep, weight loss, poor appetite, abdominal pain, nail biting and grinding of teeth.
Diagnosis can be made by seeing the worms as they migrate out of the anus to lay their eggs – however, they are often mistaken for bits of thread. The normal method is to press a bit of sticky tape to the anal area early in the morning, and examine it for eggs. By doing this a few times, up to 99% of infections can be diagnosed.
Treatment consists of a two-dose course of medication, usually pyrantel pamoate, albendazole, or mebendazole. The second dose must be given two weeks after the first. General hygiene should be improved- fingernails should be cut short, daily washing of the infected individual’s bedding and sleep garments, and regular baths are recommended. Close family members should also be treated.
Threadworms occur mainly in moist tropical regions, especially in rural areas, institutional settings and lower socio-economic groups.
Rhabditiform larvae in the intestine are excreted in faeces and contaminate soil. From there, some larvae develop into adult worms and continue to breed. Others develop into infective filariform larvae which may penetrate the human skin, usually the feet. They travel to the lungs and from there to the small intestine where they become adult female worms. Female worms lay eggs which become rhabditiform larvae, which once again get passed in the faeces. Sometimes, however, the larvae mature into filariform larvae while still in the gut, and can reinfect the same person by migrating through the mucosa into the bloodstream, into the lungs, etc. This “autoinfection” route can lead to an overwhelming larval invasion. This hyperinfection syndrome is often found in patients with immune defects – such as leukaemia, lymphoma, steroid treatment and HIV infection.
Threadworm infection is frequently asymptomatic. Symptoms which may be present include itching, urticarial rashes in the buttocks and waist areas, mild diarrhoea alternating with constipation, abdominal pains, nausea and vomiting, and cough with bloodstained sputum.
People who have massive larval invasion due to hyperinfection often have more severe symptoms. These include severe abdominal pain, diffuse involvement of the lung, and sometimes associated bacterial infections with shock and severe sepsis, and this form of the infection can be fatal.
The diagnosis is made by finding larvae in stool or duodenal fluid. The infection is usually treated with the medication ivermectin, thiabendazole or albendazole. In people with the hyperinfection syndrome, treatment may be life-saving.
This roundworm occurs mainly in the subtropics and tropics, especially in areas where sanitation is poor. It causes about 500 million infections. Children are usually infected.
Whipworm eggs are deposited in the soil where they mature and become infective. People become infected when they handle contaminated soil and touch their mouths without washing their hands. They can also be infected when they eat food that contains the eggs.
The larvae hatch in the small intestine. On reaching adulthood, the worms move to the large intestine where they burrow into the intestinal lining. Female worms lay eggs which are excreted in the faeces.
If the infection is light, there may be few or no symptoms. Moderate infection causes abdominal pain, loss of appetite, nausea and vomiting, and diarrhoea. Heavy infection causes bleeding from the intestine, severe abdominal cramps, anaemia and can also result in appendicitis. Rectal prolapse is another complication of whipworm infection.
As with most worm infections, diagnosis is best made by examining a stool sample for the ova, which have a very characteristic shape.
Although in the past, no treatment was considered necessary for light infections, the availability of safe effective drugs has changed that. Nowadays, light, moderate and heavy infections can be treated with the antihelminthics such as albendazole or mebendazole. Whipworm infection can be prevented by good personal hygiene, e.g. washing hands, proper cleaning of fruit and vegetables and good sanitation.
Written by Dr Andrew Whitelaw, University of Cape Town and Groote Schuur Hospital
Reviewed by Dr Mischka Moodley, Microbiologist, UCT, March 2011
Also view our Worms in humans Gallery
- www.cdc.gov/parasites/about.html. Retrieved 29 March 2011.
- wwwnc.cdc.gov/travel/yellowbook/2010/chapter-5/intestinal.helminths.aspx. Retrieved 29 March 2011.
- Mandell, GL, Bennet, JE, Dolin, R. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases. 7th Edition. 2009: Churchill Livingstone.Vol 2, Chapter 286, 287, 289-291.