The World Health Organization (WHO) has updated its fact sheet on soil-transmitted helminth infections.
The term helminth denotes a parasitic worm.
Soil-transmitted helminth infections are among the most common infections worldwide and affect the poorest and most deprived communities.
They are transmitted by eggs present in human faeces which in turn contaminate soil in areas where sanitation is poor.
The main species that infect people are
- the roundworm (Ascaris lumbricoides),
- the whipworm (Trichuris trichiura) and
- hookworms (Necator americanus and Ancylostoma duodenale).
More than 1.5 billion people, or 24% of the world’s population, are infected with soil-transmitted helminth infections worldwide.
The greatest numbers occur in sub-Saharan Africa, the Americas, China and East Asia.
Over 270 million preschool-age children and over 600 million school-age children live in areas where these parasites are intensively transmitted, and are in need of treatment and preventive interventions.
Soil-transmitted helminths are transmitted by eggs that are passed in the faeces of infected people.
Adult worms live in the intestine where they produce thousands of eggs each day. In areas that lack adequate sanitation, these eggs contaminate the soil. This can happen in several ways:
- eggs that are attached to vegetables are ingested when the vegetables are not carefully cooked, washed or peeled;
- 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
There is no direct person-to-person transmission, or infection from fresh faeces, because eggs passed in faeces need about 3 weeks to mature in the soil before they become infective.
Since these worms do not multiply in the human host, re-infection occurs only as a result of contact with infective stages in the environment.
Morbidity and symptoms
People with light infections usually have no symptoms.
Heavier infections can cause a range of symptoms including
- intestinal manifestations (diarrhoea and abdominal pain),
- general malaise and weakness, and
- impaired cognitive and physical development.
Hookworms cause chronic intestinal blood loss that can result in anaemia.
Soil-transmitted helminths impair the nutritional status of the people they infect in multiple ways.
- The worms feed on host tissues, including blood, which leads to a loss of iron and protein.
- The worms increase malabsorption of nutrients. In addition, roundworm may possibly compete for vitamin A in the intestine.
- Some soil-transmitted helminths also cause loss of appetite and, therefore, a reduction of nutritional intake and physical fitness. In particular, T. trichiura can cause diarrhoea and dysentery.
The strategy for control of soil-transmitted helminth infections is to control morbidity through the periodic treatment of at-risk people living in endemic areas.
People at risk are:
- preschool children,
- school-age children,
- women of childbearing age (including pregnant women in the second and third trimesters and breastfeeding women),
- adults in certain high-risk occupations such as tea-pickers or miners.
WHO recommends periodic medicinal treatment (deworming) without previous individual diagnosis to all at-risk people living in endemic areas.
Treatment should be given
- once a year when the baseline prevalence of soil-transmitted helminth infections in the community is over 20%, and
- twice a year when the prevalence of soil-transmitted helminth infections in the community is over 50%.
This intervention reduces morbidity by reducing the worm burden.
- health and hygiene education reduces transmission and reinfection by encouraging healthy behaviours; and
- provision of adequate sanitation is also important but not always possible in resource-poor settings.
WHO recommended medicines
The WHO recommended medicines –
- albendazole (400 mg) and
- mebendazole (500 mg)
– are effective, inexpensive and easy to administer by non-medical personnel (e.g. teachers).
The global target is to eliminate morbidity due to soil-transmitted helminthiases in children by 2020.
Link to the updated fact sheet:
Link to World Health Assembly Resolution on Schistosomiasis and soil-transmitted helminth infections (English) [PDF]:
Link to Global Health Observatory data on soil-transmitted helminths: