Soil-transmitted helminths (STH) are parasitic worms that are, as their name suggests, transmitted through soil. But not just any soil — soil that has been contaminated with infected human feces.

There are four main species of STH: Ascaris lumbricoides, which is also known as the roundworm or giant roundworm because it is the largest of helminths; Trichuris trichiura, sometimes called the human whipworm; and Necator americanus and Ancylostoma duodenale, which are jointly known as the hookworms — both are indistinguishable, even under a microscope, according to Dr. Antonio Montresor, the medical officer in charge of STH at the World Health Organization (WHO) Department of Neglected Tropical Diseases in Geneva, Switzerland.

STH are grouped together because these parasites all infect humans in a similar way, and many of those who are infected can be infected by more than one type of worm.

Take Action: Ask Swiss Leaders to Tackle Neglected Tropical Diseases

They are all transmitted the same way — and they can be prevented by the same sanitation measures, and are sensitive to the same medicines, according to Montresor.

The symptoms of a person infected with STH are what Montresor describes as “aspecific.” They include weakness and growth stunting, and many people are often unaware they are even infected, and therefore do not seek treatment. STH significantly affect children and their abilities in school, which in turn can have long-term effects on their success in life.

"The morbidity caused by STH is mainly a disturbance of the normal nutritional processes," Montresor told Global Citizen by email.

The severity of the illness is directly related to the number of worms in the person. In addition to loss of nutrients, infected people can also experience intestinal bleeding, loss of appetite, diarrhea, or dysentery. It depends on which worm species has entered the body.

Hookworms and whipworms can cause blood loss that can result in anaemia, whereas roundworms can cause malabsorption of nutrients, loss of appetite and reduced nutrient intake, according to Montresor. 

Pregnant women are espeically at risk of nutritional deficiencies caused by helminth infections, and those who are anemic are more likely to die in childbirth. Several large-scale studies have demonstrated that deworming and iron supplementation reduced anemia among pregnant women and have led to positive birth outcomes.

Some larger parasites on the soil-transmitted helminths table - human and pig-hosted roundworms pictured #NTDProgresspic.twitter.com/ynCpZqbF8Q

— NaturalHistoryMuseum (@NHM_London) June 25, 2015

Those who walk barefoot are vulnerable to infection with hookworms because immature worms living in contaminated soil are able to penetrate human skin. Roundworm and whipworm are transmitted by ingestion of eggs through food, water or dirty hands that have been in contact with contaminated soil.

The WHO considers preschool-age children, school-age children, and women of reproductive age as highest risk.

Montresor points out that these population groups are also the most vulnerable because they are most in need of micronutrients and vitamins.

Because STH are transmitted in environments that are contaminated with human feces, it will take significant change to improve and completely prevent future infection.

"Only an improvement in sanitation and appropriate changes in human behavior to a level that impedes environmental contamination with human excreta will permanently interrupt transmission of these parasites," Montresor said.

Unfortunately, in most areas where STH infections are endemic, substantial improvements in living standards and sanitation conditions may not be feasible in the short term.

For this reason, the WHO recommends regular administration of deworming medicines in countries that remain endemic.

The treatment recommended by WHO is periodic treatment (every six months or every year, depending on prevalence in the area) with albendazole or mebendazole, according to Montresor.

Treatment is safe and inexpensive. In fact, tablets are donated by companies like GlaxoSmithKline (GSK) and Johnson & Johnson. Global commitments to the elimination of neglected tropical diseases like STH could go a long way in ensuring treatment reaches the people who need it most.

International coverage of this intervention has progressively increased in children from less than 30% in 2010 to more than 60% in 2017. The WHO has set a goal for treatment coverage to reach 75% by 2020.

"What You Don’t Know About X" is a new series focusing on neglected tropical diseases (NTDs). NTDs are a cluster of parasitic and bacterial diseases. While you may have heard of a few of them, it’s likely you know very little about their actual effects or why they are so often overlooked. This series looks to shed light on these devastating — and preventable — diseases.