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). These STH species are normally addressed as a group because they need similar diagnostic procedures and respond to the same medicines.
Strongyloides stercoralis is an intestinal helminth with peculiar characteristics: the parasite requires different diagnostic methods than other soil-transmitted helminthiases, and for this reason is frequently not identified. In addition, the parasite is not sensitive to albendazole or mebendazole and therefore not impacted by large-scale preventive treatment campaigns targeting other soil-transmitted helmonthiases.
Global distribution and prevalence
More than 1.5 billion people, or 24% of the world’s population, are infected with soil-transmitted helminth infections worldwide. 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 267 million preschool-age children and over 568 million school-age children live in areas where these parasites are intensively transmitted, and are in need of treatment and preventive interventions.
Globally over 600 million people are estimated to be infected by S. stercoralis however, since also this parasite is transmitted in areas where sanitation is poor, its geographical distribution overlaps with the one of the other soil-transmitted helmonthiases.
Transmission
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.
In addition, 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 3 weeks to mature in the soil before they become infective.
A. lumbricoides, T. trichiura and hookworms do not multiply in the human host, re-infection occurs only as a result of contact with infective stages in the environment. S. stercoralis can reproduce in the host and in immunocompromised individuals, its uncontrolled multiplication can be fatal.
Nutritional effects
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.
- Hookworms in addition cause chronic intestinal blood loss that can result in anaemia.
- 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.
Morbidity and symptoms
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.
WHO strategy for control
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.
WHO recommends periodic medicinal treatment (deworming) without previous individual diagnosis to all at-risk people living in endemic areas. This intervention reduces morbidity by reducing the worm burden. In addition:
- 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.
Periodical treatment aims to reduce and maintain the intensity of infection, and to protect infected at-risk populations from morbidity.
Deworming can be easily integrated with child health days or supplementation programmes for preschool children, or integrated with school health programmes. In 2018, over 676 million school-aged children were treated with anthelminthic medicines in endemic countries, corresponding to 53% of all children at risk.
Schools provide a particularly good entry point for deworming activities, as they allow the easy provision of the health and hygiene education component, such as promotion of handwashing and improved sanitation.
WHO added the control of morbidity due to S. stercoralis as an objective for 2030. This will be possible because of the expected availability of pre-qualified ivermectin at affordable cost. Distribution of ivermectin is expected through the platforms used to control the other soil-transmitted helminthiases. Pilot interventions are presently in place to refine the strategy and evaluate its impact.
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). 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.
Global target
There are six 2030 global targets for soil-transmitted helminthiases:
- Achieve and maintain elimination of STH morbidity in pre-school and school age children
- Reduce the number of tablets needed in preventive chemotherapy for STH
- Increase domestic financial support to preventive chemotherapy for STH
- Establish an efficient STH control programme in adolescent, pregnant and lactating women
- Establish an efficient strongyloidiasis control programme in school age children
- Ensure universal access to at least basic sanitation and hygiene by 2030 in STH-endemic areas