Soil-transmitted helminthiases

Soil-transmitted helminth (STH) 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 the hookworms (Necator americanus and Ancylostoma duodenale).

Intestinal worms produce a wide range of symptoms including intestinal manifestations (diarrhoea, abdominal pain), general malaise and weakness. Hookworms cause chronic intestinal blood loss that result in anaemia.

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. 

Morbidity is related to the number of worms harboured. People with infections of light intensity (few worms) usually do not suffer from the infection. Heavier infections can cause a range of symptoms including intestinal manifestations (diarrhoea and abdominal pain), malnutrition, general malaise and weakness, and impaired growth and physical development.

Infections of very high intensity can cause intestinal obstruction that should be treated surgically.

S. stercoralis may cause dermatological and gastro-intestinal morbidity and is also known to be associated with chronic malnutrition in children. In case of reduced host immunity, the parasite can cause the hyperinfection/dissemination syndrome that is invariably fatal if not promptly and properly cured and is often fatal despite the treatment.

In 2001, delegates at the World Health Assembly unanimously endorsed a resolution (WHA54.19) urging endemic countries to start seriously tackling worms, specifically schistosomiasis and soil-transmitted helminths.

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 reproductive 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.

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). They have been through extensive safety testing and have been used in millions of people with few and minor side-effects.

Both albendazole and mebendazole are donated to national ministries of health through WHO in all endemic countries for the treatment of all children of school age.

Ivermectin for the control of S. stercoralis is expected to be available at affordable price from 2021.