Nematodes (Roundworms)

Nematodes are parasitic worms with long, cylindrical bodies. Roundworm infections are widespread throughout the world.

There are separate articles on Nematode skin infestations, Ascaris lumbricoides, Angiostrongyliasis, Strongyloides Stercoralis, Threadworm, Toxocariasis, Onchocerciasis and Filariasis.

Ascariasis and trichuriasis are more common in warm, moist climates where people use human or animal faeces for fertilizer. Anisakiasis is most common in countries where raw or pickled fish or squid is a popular food item. Humans acquire most types of roundworm infection from contaminated food or by touching the mouth with contaminated and unwashed hands.

• Loa loa filariasis is a skin and eye disease caused by the nematode worm, loa loa filaria. Humans contract this disease through the bite of a horsefly. The deer fly and the Mango Fly are also a vector of Loa loa. Humans are the only known natural reservoir. It is estimated that 2-13 million humans are infected with the Loa loa larvae.
• Human loiasis geographical distribution is restricted to the rain forest and swamp forest areas of West Africa, especially Cameroon and on the Ogowe River.
• Life cycle:
  • During a blood meal, an infected fly (genus Chrysops, day-biting flies) introduces larvae onto the skin of the human host. The larvae develop into adults in subcutaneous tissue.
  • The female worms measure 40 to 70 mm in length and 0.5 mm in diameter, while the males are slightly smaller.
  • Adults produce microfilariae, which have been recovered from spinal fluid, urine and sputum, but are mainly found in peripheral blood and in the lungs. The fly ingests microfilariae during a blood meal.
• Presentation:
  • Lymphatic filariasis such as loiasis most often consists of asymptomatic microfilaremia. Some patients develop lymphatic dysfunction causing lymphoedema.
  • Red itchy swellings below the skin (Calabar swellings) in the arms and legs, caused by immune reactions are common. The swellings may last for 1-3 days, and may be associated with surrounding urticaria and pruritus.
  • Subconjunctival migration of an adult worm to the eyes occurs frequently.
  • Dead worms may cause chronic abscesses, which may lead to the formation of granulomatous reactions and fibrosis.
• Investigations:
  • Eosinophilia is often prominent.
  • Identification of microfilariae by microscopic examination is the most practical method of diagnosis.
  • Antigen detection using immunoassay is also useful because microfilaremia can be low and variable.
  • Antibody detection is of limited value because there is significant cross reactivity between filaria and other helminths, and a positive serologic test does not distinguish between past and current infection.
  • Identification of adult worms is possible from tissue samples collected during subcutaneous biopsies or worm removal from the eye.
• Treatment:
  • Loa Loa is treated with diethylcarbamazine.
  • In heavy infections there may be a febrile reaction and there is a small risk of encephalopathy. Treatment must be stopped at the first indication of cerebral involvement.

• Life cycle:
  • The female Dracunculus worm emerges through the skin of its human host one to two years after infection. Emergent worms may enter sources of drinking water release larvae into the water. The larvae are ingested by water fleas.
  • Inside the water fleas, the larvae develop into the infective stage in 10-14 days. Humans may then become infected by drinking contaminated water.
  • Human stomach acid digests the water flea but not the guinea worm larvae.
  • The larvae enter the small intestine and then into the body.
  • During the next 10-14 months, the female mates with a male guinea worm. The male dies and is absorbed into the larger female.
  • The female develops into its full length of 60-100 centimetres.
  • The adult female migrates to the area of the body from which it will emerge, which is usually one of the lower limbs.
  • A blister develops on the skin at the site where the worm will emerge. This blister causes a very painful burning sensation and ruptures within 1-3 days, exposing one end of the emergent worm.
  • When the blister soon becomes an ulcer or open sore, and when placed in water, the adult female releases hundreds of thousands of guinea worm larvae into the water.
• Treatment:
  • The most common treatment still involves wrapping the worm around a stick at the skin surface, wrapping or winding the worm a few centimetres each day. This process can take many days, but must be slow to avoid breakage and leaving behind a portion of the worm. Leaving a portion of the dead worm within the host's body increases the risk of infection, and can trigger immune responses resulting in pain and swelling.
  • Metronidazole or thiabendazole (in adults) are also used and facilitates the extraction process but may be associated with aberrant migration of worms, causing infection elsewhere in the body.
  • The worm also can be excised surgically.
• Eradication:
  • The Dracunculiasis Eradication Program has been very effective.
  • Asia has been declared free of dracunculiasis since 2005.
  • Worldwide, there were about 3.5 million cases reported in 1986, over 30,000 cases in 2003, and only about 16,000 cases in 2004.
  • Dracunculiasis now occurs only in sub-Saharan Africa.
• Prevention:
  • Drink only water from underground sources free from contamination, e.g. borehole or hand-dug wells.
  • Prevent persons with an open Guinea Worm ulcer from entering ponds and wells used for drinking water.
  • Always filter drinking water, e.g. cloth or nylon mesh filter.
  • Additionally, unsafe sources of drinking water can be treated with an approved larvicide.

• The hookworm is a parasitic nematode worm that lives in the small intestine of its host, e.g. dog, cat or human.
• Two species of hookworms commonly infect humans, Ancylostoma duodenale and Necator americanus.
• Necator americanus predominates in the Americas, Sub-Saharan Africa, Southeast Asia, China and Indonesia.
• A. duodenale predominates in the Middle East, North Africa and India.
• Hookworms are thought to infect 800 million people worldwide.
• Life cycle:
  • Eggs are excreted from the host in faeces and thrive in warm earth with heavy rainfall. Only if these conditions exist can the eggs hatch.
  • Larva hatch and penetrate skin.
  • Once in the host gut, Necator tends to cause a prolonged infection and adult worms may live for 15 years or more.
  • Ancylostoma adults survive on average for only about 6 months but infection can be prolonged because dormant larvae can develop from tissue stores over many years.
  • It takes 5-7 weeks for adult worms to mature, mate and produce eggs, and so in the early stages of very heavy infection, acute symptoms may occur without any eggs detected in the patient's faeces, making diagnosis very difficult
• Presentation:
  • Most individuals with hookworm infection are asymptomatic.
  • Symptoms are due to inflammation in the bowel, e.g. nausea, abdominal pain and intermittent diarrhoea, and to progressive anaemia in prolonged disease.
  • Ankylostomiasis is the disease caused by hookworms. It is caused when hookworms, present in large numbers, produce an iron deficiency anemia by sucking blood from the host's intestinal walls.
  • The blood loss in the stools is occult and not visibly apparent.
• Investigations:
  • Full blood count: eosinophilia, iron deficiency anaemia
  • Diagnosis depends on finding characteristic worm eggs on microscopic examination of the stools, although this is not possible in early infection.
• Treatment:
  • Hookworm can be treated with local cryotherapy when it is still in the skin.
  • Mebendazole is effective both in the intestinal stage and during the stage the parasite is still migrating under the skin.
  • In case of anaemia, iron supplementation (folic acid or vitamin B₁₂ may also be required as red blood cells are replenished).
• Complications:
  • Hookworm is a leading cause of maternal and child morbidity in developing countries.
  • Hookworms cause intellectual, cognitive and growth retardation in children, intrauterine growth retardation, prematurity, and low birth weight in newborns born to infected mothers.
  • Hookworm infection is rarely fatal, but anemia can be significant if heavily infected.
• Prevention:
  • Prevent skin/soil contact: do not walk barefoot
  • Do not defecate outside latrines, toilets etc.
  • Do not use human excrement or raw sewage as manure/fertilizer in agriculture

• Anisakiasis is caused by the accidental ingestion of larvae of the Anisakis simplex and Pseudoterranova decipiens.
• There is a higher incidence in areas where raw fish is eaten (e.g., Japan, Pacific coast of South America, the Netherlands).
• Life cycle:
  • Adult stages of Anisakis simplex or Pseudoterranova decipiens live in the stomach of marine mammals.
  • Eggs produced by adult females are passed in the faeces of marine mammals.
  • First-stage larvae are formed in the eggs when in the water. The larvae molt, becoming second-stage larvae and, after the larvae hatch from the eggs, they become free-swimming.
  • Larvae released from the eggs are ingested by crustaceans. The ingested larvae develop into third-stage larvae that are infective to fish and squid. The larvae migrate from the intestine to the tissues in the peritoneal cavity and grow up to 3 cm in length.
  • When the host dies, larvae migrate to the muscle tissues, and are transferred from fish to predator.
  • When fish or squid containing third-stage larvae are ingested by marine mammals, the larvae molt twice and develop into adult worms. The adult females produce eggs that are shed by marine mammals.
  • Humans become infected by eating raw or undercooked infected marine fish. After ingestion, the anisakid larvae penetrate the gastric and intestinal mucosa, causing the symptoms of anisakiasis.
• Presentation:
  • Within hours after ingestion of infected larvae, violent abdominal pain, nausea, and vomiting may occur.
  • If the larvae pass into the bowel, a severe eosinophilic granulomatous response may also occur 1 to 2 weeks after infection, causing symptoms resembling Crohn's disease.
• Investigation: diagnosis can be made by:
  • Gastroscopy: 2 cm larvae are seen and can be removed.
  • Histology of tissue removed at biopsy or during surgery.
• Treatment:
  • The treatment of choice is surgical or endoscopic removal.

• Trichinosis is a parasitic disease caused by eating raw or undercooked pork and wild game products infected with the larvae of a species of Trichinella spiralis.
• Trichinosis is endemic in central and eastern Europe, the whole of the Americas, parts of Africa and Asia.
• Life cycle:
  • The worm can infect any species of mammal that consumes its encysted larval stages. When an animal eats meat that contains infective Trichinella cysts, the acid in the stomach dissolves the hard covering of the cyst and releases the worms.
  • The worms pass into the small intestine and, in 1-2 days, become mature.
  • After mating, adult females produce larvae, which break through the intestinal wall and travel through the lymphatic system to the circulatory system to find a suitable cell. Larvae can penetrate any cell, but can only survive in skeletal muscle.
  • Within a muscle cell, the worms curl up and direct the cell functioning much as a virus does.
• Presentation:
  • Usually asymptomatic but in heavy infection causes gastrointestinal symptoms, fever, tachycardia and hypersensitivity reactions during the migration phase.
  • Within 1-2 days of infection, nausea, heartburn, dyspepsia, and diarrhoea may occur, the severity depending on the number of worms ingested.
  • Later other manifestations may occur, e.g. headache, fever, chills, cough, eye swelling, joint pain, muscle pain, petechiae, and itching.
  • Worms may enter the central nervous system, causing serious neurological deficits, e.g. ataxia or respiratory paralysis, and even death. Infestation of the heart may also lead to death.
  • However most symptoms subside within a few months.
• Investigation:
  • Full blood count shows eosinophilia in virtually all patients.
  • Creatine kinase is elevated in most patients.
  • Parasite-specific indirect immunoglobulin G (IgG) enzyme-linked immunosorbent assay (ELISA) titres and anti-newborn larvae antibodies: recommended but may not be positive initially and there is some cross-reactivity with other parasitic disorders and so specificity is less when results are weakly positive.
  • Stool studies can identify adult worms, with females being about 3 mm long and males about half that size.
  • A muscle biopsy is the definitive diagnostic test.
  • Other tests may be indicated depending on presentation, e.g. CT scan for suspected central nervous system involvement.
• Treatment:
  • Symptoms can be treated with aspirin and corticosteroids.
  • Mebendazole can kill adult worms in the intestine; however, there is no treatment that kills the larvae.
• Prevention:
  • Cooking meat products until the juices run clear or to an internal temperature of 62 °C.
  • Freezing pork kills larval worms. Freezing wild game meats, even for long periods of time, may not effectively kill all worms.
  • Cooking wild game meat thoroughly.
  • Cooking all meat fed to pigs or other wild animals.
  • Cleaning meat grinders thoroughly when preparing ground meats.

• Trichuriasis is a parasitic disease caused by infection of the large intestine by a parasite whipworm (Trichuris trichiura).
• Trichuriasis is common worldwide, especially in countries with warm, humid climates. Trichuriasis mainly affects children, who may become infected if they ingest soil contaminated with whipworm eggs.
• The main risk factor for infection is ingestion of eggs from soil contaminated with faeces.
• Larvae hatch from swallowed eggs in the small intestine and move on to the upper part of the large intestine. The adult worms produce eggs that are passed in the faeces and mature in the soil.
• The ingested eggs hatch, and the whipworm embeds in the wall of the large intestine.
• Presentation:
  • Light infestations may lead to few or no symptoms.
  • Heavy infestations may result in bloody diarrhoea, abdominal pain, iron deficiency anemia and, in severe infestations, rectal prolapse.
• Investigations:
  • Stool examined for ova and parasites reveals the presence of typical whipworm eggs.
• Treatment:
  • Oral treatment with mebendazole for 3 days is often used in symptomatic infections.
• Prognosis:
  • Full recovery is expected with treatment.
• Prevention:
  • Improved facilities for faeces disposal have decreased the incidence of whipworm.
  • Hand washing before food handling, and avoiding ingestion of soil by thorough washing of food that may have been contaminated with egg-containing soil.