Soil Transmitted Helminth Infections

The soil-transmitted helminths are a group of parasitic nematode worms causing human infection through contact with parasite eggs or larvae that thrive in the warm and moist soil of the world's tropical and subtropical countries. As adult worms, the soil-transmitted helminths live for years in the human gastrointestinal tract. Soil-transmitted helminths do not reproduce within the host.

Of particular worldwide importance are the roundworms (Ascaris lumbricoides), whipworms (Trichuris trichiura), and hookworms (Necator americanus or Ancylostoma duodenale).¹ Strongyloides stercoralis is discussed in a separate article.

• Adult Necator and Ancylostoma live in the upper part of the human small intestine.
• Ascaris roundworms are parasites of the entire small intestine.
• Adult trichuris whipworms live in the large intestine, especially the caecum.
• The parasites can live for several years in the gastrointestinal tract.
• Human beings are the only major definitive host for these parasites, although ascaris infections can also be acquired from pigs.
• After mating, each adult female produces thousands of eggs per day, which leave the body in the faeces.
• People become infected with T. trichiura and A. lumbricoides by ingesting the fully developed eggs. The released larvae moult and travel to the colon where they burrow into the epithelia and develop into adult whipworms within 12 weeks.
• Ascaris larvae penetrate the intestinal mucosa and enter the liver and then the lungs, before passing over the epiglottis to re-enter the gastrointestinal tract and develop into egg-laying adult worms about 9-11 weeks after egg ingestion.
• N. americanus and A. duodenale hookworm eggs hatch in soil.
• The larvae moult twice to become infective third-stage larvae, which are non-feeding but motile organisms.
• After skin penetration, they enter the blood stream and ultimately the larvae become trapped in pulmonary capillaries, enter the lungs, pass over the epiglottis, and migrate into the gastrointestinal tract.
• About 5-9 weeks are needed from skin penetration until development of egg-laying adults.
• A. duodenale larvae are also orally infective.

• More than one billion people are infected with at least one species.
• Soil-transmitted helminth infections are widely distributed throughout the tropics and subtropics. Adequate moisture and warm temperature are essential for larval development in the soil.
• Recent estimates suggest that A. lumbricoides infects 1.221 billion people, T. trichiura 795 million, and hookworms 740 million people. The greatest numbers of soil-transmitted helminth infections occur in the Americas, China and East Asia, and Sub-Saharan Africa. 85% of the number of infected people in the world are estimated to be in sub-Saharan Africa.²
• Equally important risk factors are poverty, and inadequate water supplies and sanitation.
• Soil helminth infections should be considered in immigrants and also travellers or military personnel returning from affected areas.³

The clinical features of soil-transmitted helminth infections can be classified into the acute manifestations associated with larval migration through the skin and viscera, and the acute and chronic manifestations resulting from parasitism of the gastrointestinal tract by adult worms.

Early larval migration

• Migrating soil-transmitted helminth larvae provoke reactions in many of the tissues as they pass, e.g.
  • Ascaris larvae that die during migration through the liver may cause eosinophilic granulomas.
  • In the lungs, ascaris larval antigens cause eosinophilic infiltrates, leading to pneumonia with wheeze, dyspnoea, non-productive cough and fever. Blood-tinged sputum is produced during heavy infections. Children are more susceptible to pneumonitis, and the disease is more severe with reinfection.
  • After skin invasion, hookworm third-stage larvae travel through blood vessels to the lungs, but the resulting pneumonitis is not as severe as in ascaris infection.
• Several cutaneous syndromes result from skin-penetrating larvae.
  • Repeated exposure to N. americanus and A. duodenale hookworm third-stage larvae results in ground itch, a localised pruritic erythematous and papular rash on the hands and feet.
  • When zoonotic hookworm third-stage larvae (typically A. braziliense) enter the skin, they produce cutaneous larva migrans, which is characterised by the appearance of serpiginous tracks on the feet, buttocks, and abdomen.
• Oral ingestion of A. duodenale larvae can cause Wakana syndrome, which presents with nausea, vomiting, pharyngeal irritation, cough, dyspnoea, and hoarseness.

• Usually only moderate or high intensity of soil-transmitted helminth infections in the gastrointestinal tract produce clinical manifestations. The highest intensity infections occur most commonly in children.
• Often presents with the appearance of worms in stool or vomit.
• Each of the major soil-transmitted helminths produces characteristic disease syndromes.

Ascariasis

• Large numbers of adult ascaris worms in the small intestine can cause abdominal distension and pain. They can also cause lactose intolerance, and malabsorption of vitamin A and other nutrients, which may contribute to growth failure.
• In young children, adult worms can aggregate in the ileum and cause partial obstruction. Intussusception, volvulus, and complete obstruction may occur, leading to bowel infarction and intestinal perforation.
• The resulting peritonitis may be fatal, but if not, the adult worms may die and cause a chronic granulomatous peritonitis.
• Adult worms can enter the lumen of the appendix, leading to a clinical picture indistinguishable from appendicitis.
• Adult ascaris worms also tend to move in children with high fever, resulting in the worms emerging from the nasopharynx or anus.
• Hepatobiliary and pancreatic ascariasis results when adult worms in the duodenum enter and block the ampullary orifice of the common bile duct, leading to biliary colic, cholecystitis, cholangitis, pancreatitis and hepatic abscess. Hepatobiliary and pancreatic ascariasis occur more commonly in adults, especially women.

Trichuriasis

• Adult whipworms mainly live in the caecum, but in heavy infections may also be seen throughout the colon and rectum.
• Inflammation at the site of attachment from large numbers of whipworms results in colitis.
• Long-standing colitis resembles inflammatory bowel disease, including chronic abdominal pain and diarrhoea, as well as impaired growth, anaemia of chronic disease, and finger clubbing.
• Trichuris dysentery syndrome results in chronic dysentery and rectal prolapse.

Hookworm infection

• The appearance of eosinophilia coincides with the development of adult hookworms in the intestine.
• Intestinal blood loss results in iron-deficiency anaemia.
• The clinical manifestations of hookworm disease therefore resemble those of iron-deficiency anaemia from other causes.
• Chronic protein loss can result in hypoproteinaemia and anasarca (widespread swelling of the skin due to effusion of fluid into the extracellular space).
• Although parasite infections do not in general protect against asthma, infection with hookworm may reduce the risk of asthma.⁴

• Persistent eosinophilia is common, especially with hookworm infection.
• Stool examination: several egg concentration techniques, e.g. formalinethyl acetate sedimentation, can detect even light infections.
• The Kato-Katz faecal-thick smear and the McMaster method are used to measure the intensity of infection by estimating the number of egg counts per gram of faeces.
• Serology: Enzyme-linked immunosorbent assay (ELISA) can be used to identify some helminth species.
• Ultrasonography and endoscopy are useful for diagnostic imaging of the complications of ascariasis,
  including intestinal obstruction and hepatobiliary and pancreatic involvement.
• Chest x-ray: pulmonary infiltrates.

• The drugs most commonly used for the removal of soil-transmitted helminth infections from the gastrointestinal tract are mebendazole and albendazole.
• Both agents are effective against ascaris in a single dose.
• However, in hookworm, a single dose of albendazole is not effective in many cases of trichuriasis.
• Levamisole is very effective against Ascaris lumbricoides.
• For both trichuriasis and hookworm infection, several doses of mebendazole or albendazole are often needed.
• Albendazole is used for the treatment of disorders caused by tissue-migrating larvae such as visceral larva migrans caused by Toxocara canis.
• Both pyrantel pamoate and levamisole are alternative drugs for the treatment of hookworm and ascaris infections, although pyrantel pamoate is not effective for the treatment of trichuriasis.
• Management of associated malnutrition, growth impairment, anaemia, and impaired mental development may also be required.⁵

Morbidity control through deworming

• Anthelmintic drugs are also used for large-scale morbidity reduction in endemic communities. Regular treatment with benzimidazole anthelmintic drugs in school-age children reduces and maintains the worm burden below the threshold associated with disease and improve iron stores, growth and general health, cognitive performance and school attendance.¹
• If women in endemic areas are treated once or twice during pregnancy, there are substantial improvements in maternal anaemia, birth weight and infant mortality at 6 months. In areas where hookworm infections are endemic, anthelmintic treatment is recommended during pregnancy except in the first trimester.
• Reinfection is common but regular antihelminth drug treatment to reduce the worm burden consistently could prevent some of the sequelae associated with chronic infection.

• Evidence from Africa and Asia shows that 30-54% of moderate to severe anaemia in pregnant women is attributable to hookworm. Hookworm also contributes to moderate and severe anaemia in children.¹
• Chronic infection with soil-transmitted helminths results in impaired childhood growth and poor general health and nutritional status.¹
• Reinfection
  • After community-wide treatment, rates of hookworm infection reach 80% of pre-treatment rates within 30-36 months.¹
  • A. lumbricoides infection reached 55% of pre-treatment rates within 11 months.¹
  • T. trichiura infection reached 44% of pretreatment rates within 17 months.¹

• Improved water supply and sanitation
• Deworming
• Mebendazole and albendazole are used for large-scale prevention of morbidity in children living in endemic areas.¹