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Intestinal worms (soil-transmitted helminthiases)

What are soil-transmitted helminthiases?

Soil-transmitted helminthiases (STH) are infections caused by intestinal parasites. These infections are among the most common worldwide, and affect the poorest and most marginalised communities.

How are soil-transmitted helminthiases transmitted?

STH (soil-transmitted helminths) are parasitic intestinal worms that are transmitted between humans through contaminated soil.

The main species of STH that infect people include roundworm (Ascaris lumbricoides), whipworm (Trichuris trichiura), hookworms (Necator and Ancylostoma spp.), and threadworm (Strongyloides stercoralis).

Unlike schistosomiasis, the life cycle for STH does not involve an intermediate host. The roundworm and the whipworm live in the intestines of humans, where they produce thousands of eggs a day, which are then passed out in the faeces of the infected persons, contaminating the soil in areas where sanitation is poor (i.e. where faecal waste is not safely separated from human contact and the environment). The eggs mature in the soil and can infect individuals through several pathways:

  • eggs that are attached to vegetables grown in contaminated soil or fertilised with contaminated manure are ingested when the vegetables are not carefully washed, peeled or cooked;
  • eggs are ingested from contaminated water sources;
  • eggs are ingested by children who play in the contaminated soil and then put their hands in their mouths without washing them; or by any person accidentally ingesting contaminated soil.

The life cycle of soil-transmitted helminths

The absence of safely managed sanitation systems and the widespread use of untreated faecal matter in agriculture, leads to continued contamination of soils with parasite eggs.

The life cycle of hookworm differs because the eggs that are passed in the faeces hatch in the soil. Infection occurs when the free-living larvae attach themselves to the bare feet and ankles of passers-by. They then penetrate the skin, migrate around the body, and end up in the intestine where they attach to the lining and gorge on blood.

Similarly to hookworm, a primary infection with threadworm also occurs through skin penetration by larvae during contact with contaminated soil. But the eggs hatch in the intestine rather than the environment, meaning that although most larvae will be excreted in the faeces, auto-infection may occur when some larvae mature and immediately re-infect the same individual.


Symptoms and long-term complications of soil-transmitted helminthiases (intestinal worm infections)

Intestinal worm infections produce a wide range of syma ptoms depending on disease severity. Severity of disease is directly related to the number of worms within an individual’s body:

  • people with infections of light intensity (few worms) usually do not suffer any syma ptoms as a result of infection;
  • heavier infections can cause intestinal manifestations (diarrhoea and abdominal pain), malnutrition, and impaired growth and cognitive development;
  • infections of very high intensity can cause intestinal obstruction that should be treated surgically.

Soil-transmitted helminthiases treatment

Intestinal worms can be expelled from the body with a single 400 mg tablet of albendazole (or a 500 mg tablet of mebendazole), as recommended by the World Health Organization (WHO). These medicines are effective, relatively inexpensive and easy to administer by non-medical personnel. Both albendazole and mebendazole are donated by manufacturers to ministries of health (MoH) of countries with high burdens of STH through WHO for the treatment of school-age children.

Soil-transmitted helminthiases prevention

Primary prevention of transmission of STH involves improvements to infrastructure, such as sanitation systems and improved housing. It also requires improvements to food chain safety, as well as behavioural aspects such as safe food preparation and consumption including washing of hands and raw fruit and vegetables. Shoe wearing can protect against infection with hookworm and threadworm.

When soil, temperature and humidity conditions are favourable, helminth eggs can survive in the environment for a long time, in some cases for several years. Environmental and behavioural measures should therefore be sustained over the long term in order to result in sustained infection control.


Who is affected by parasitic worm infections and why?

The greatest burden of disease for STH occurs among populations in areas that lack access to clean water and sanitation. As eggs can remain viable in soil for long periods, where soil and temperature are conducive, the distribution of helminth infections is slow to respond to improvements in water and sanitation services. It is for this reason that the nationwide elimination of STH infections goes hand-in-hand with socio-economic development.

What Unlimit Health is doing

We provide technical and financial support to MoH, in line with their own strategies and plans to control or eliminate intestinal worm infections. We work to scale up their government-implemented deworming programmes by conducting research and analysis on outcomes to continually improve and optimise the programmes we support.

In 2021-22, Unlimit Health supported 28,067,699 treatments for intestinal worms, most of which were provided to school-aged children (5-14 years) in Burundi, Côte d’Ivoire, Democratic Republic of Congo, Ethiopia, Madagascar, Malawi, Mauritania, Mozambique, Tanzania, Zambia and Zanzibar.

The work we still need to do

With the launch of WHO’s “Ending the neglect to attain the Sustainable Development Goals: A road map for neglected tropical diseases 2021–2030”, we will continue to align with the plans and strategies of MoH partners, to support stronger health systems. We will also focus on prevention of infection (e.g. WASH), which is crucial to meeting the ambitious elimination targets set out by the road map.