- Soil-transmitted helminth infections are caused by different species of parasitic worms.
- They are transmitted by eggs present in human faeces, which contaminate the soil in areas where sanitation is poor.
- Approximately two billion people are infected with soil-transmitted helminths worldwide.
- Infected children are physically, nutritionally and cognitively impaired.
- Control is based on:
- periodical deworming to eliminate infecting worms
- health education to prevent reinfection
- improved sanitation to reduce soil contamination with infective eggs.
- Safe and effective medicines are available to control infection.
Soil-transmitted helminth infections are among the most common infections worldwide and affect the poorest and most deprived communities. They are caused by parasitic worms (helminths) that are transmitted to people through contaminated soil. The main species of soil-transmitted helminths that infect people are the roundworm (Ascaris lumbricoides), the whipworm (Trichuris trichiura) and the hookworms (Necator americanus and Ancylostoma duodenale).
Global distribution and prevalence
Approximately two billion people, or almost 29% of the world’s population are infected with soil-transmitted helminth infections worldwide. Soil-transmitted helminth infections are widely distributed in tropical and subtropical areas, with the greatest numbers occurring 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. People become infected with A. lumbricoides and T. trichiura by ingesting infective parasite eggs. 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 soil and then put their hands in their mouths without washing them.
Hookworm eggs hatch in the soil, releasing larvae that mature into a form that can actively penetrate the skin. People become infected with hookworm primarily by walking barefoot on the contaminated soil.
There is no direct person-to-person transmission, or infection from fresh faeces, because eggs passed in faeces need about three weeks to mature in the soil before they become infective. Since these worms do not multiply in the human host, reinfection occurs only as a result of contact with infective stages in the environment.
Signs and symptoms
Morbidity is related to the number of worms harboured. People with light infections usually have no symptoms. Heavier infections can cause a range of symptoms including intestinal manifestations (diarrhoea, 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 nutritional impairment caused by soil-transmitted helminths is recognized to have a significant impact on growth and physical development.
WHO strategy for control
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); and
- adults in certain high-risk occupations, such as tea-pickers or miners.
WHO recommends periodic drug treatment (deworming) without previous individual diagnosis to all at-risk people living in endemic areas. Treatment should be given once a year when the 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. In addition:
- health and hygiene education reduces transmission and reinfection by encouraging healthy behaviours;
- provision of adequate sanitation is also important but not always possible in resource-poor settings.
The aim of control activities is morbidity control: periodic treatment of at-risk populations will reduce the intensity of infection and protect infected individuals from morbidity.
Periodic deworming can be easily integrated with child health days or supplementation programmes for preschool children, or integrated with school health programmes. In 2009, over 300 million preschool and school-age children were dewormed in endemic countries, corresponding to 35% of the children at risk.
Schools provide a particularly good entry point for deworming activities, as they allow easy provision of the health and hygiene education component such as the promotion of hand washing and improved sanitation.