NTDs: Schistosomiasis – worms and snails and humans, oh my.

This post is the 14th in a series highlighting the WHO’s list of 17 Neglected Tropical Diseases (now technically 18 as mycetoma was added to the list at the 69th World Health Assembly in May 2016). To read the previous posts in this series click here.

Right and left show freshwater snails which serve as schistosomiasis hosts, and in the center is the schistosomiasis parasite.

Right and left panels show freshwater snails which serve as schistosomiasis hosts, and in the center panel is the schistosomiasis parasite. Courtesy CDC.

Schistosomiasis, also called bilharzia, is a disease caused by parasitic worms. While you may not be very familiar with schistosomiasis, at the very least you should know that over 200 million people are infected worldwide and this disease is second only to malaria as the most common parasitic disease.

The Schistosome parasites (of which there are three main species infecting humans) live in freshwater, using snails as part of their life cycle. The parasites infect snails for a while, then are released into the water where they come into contact with humans who are bathing, washing clothes, etc. and burrow into the skin, once in the body the parasites migrate around, eventually turning into males and females, then the female begins to release eggs, the eggs leave the body in urine and feces, where, if that occurs in freshwater, the eggs find snails to inhabit so they can start the cycle all over again. However, some eggs migrate to other parts of the body where they can inflict major, chronic damage.

Depiction of the Schistosoma parasite life cycle.

Depiction of the Schistosome parasite life cycle. Courtesy CDC.

As you can probably imagine, schistosomiasis is most commonly found in tropical areas, mainly in Africa. It is estimated that 90% of those requiring treatment for schistosomiasis live in Africa. Schistosomiasis mainly affects poor and rural populations, especially those that rely on fishing and agriculture. Women are at high-risk as they frequently come into contact with infected water while doing domestic chores like washing clothes. Children are also at high-risk as they may have inadequate hygiene and they play or bathe in the infected bodies of water. People can get one of two main forms of schistosomiasis, intestinal and urogenital. As I’m sure you can reasonably guess, the intestinal form causes abdominal pain, diarrhea, the possible enlargement of the spleen, among other complications. The urogenital form causes bladder and kidney issues and can result in long-term complications like infertility.

Schistosomiasis distribution worldwide. Courtesy WHO.

Like many of the other NTDs, the devastating impact of schistosomiasis is not on mortality but on morbidity (at least compared to other major killers like malaria, HIV/AIDS and TB). The chronic issues caused by the parasites are serious and cause more disabilities than deaths. Infected children develop anemia, stunting and a reduced ability to learn. The WHO estimates that 20 000 people die from schistosomiasis – related deaths each year.

The current treatment and prevention to control schistosomiasis involves large-scale treatment of at-risk populations, access to safe water, improved sanitation, hygiene education and snail control. The WHO strategy of drug treatment focuses on periodic, targeted, large-scale treatment of at-risk populations. Generally those “at-risk” groups are defined as school-aged children, adults at-risk and those living in endemic areas who have frequent contact with freshwater, and entire communities living in highly endemic areas. How often treatment should be administered generally depends on the prevalence of infection in school-aged children. Lower infection rates result in less – frequent, maybe every other year, treatment while high infection rates require annual treatment for a number of years. However, recently the WHO expanded their recommendations to encourage countries to offer large-scale chemoprophylaxis to all potentially infected people, not just school-children. This expansion was based on evidence from China and Egypt that showed preventive chemotherapy (without testing individual people beforehand) with high coverage can significantly impact infection and reduce transmission. But a major limitation of any of the treatment campaigns has been a limited supply of the drug, as the demand is much higher than the supply. Data from 2014 showed that only about 21% of people needing treatment were reached. So obviously the number of people needing treatment pales in comparison to the amount of people actually treated.

So that leaves us with high numbers of schistosomiasis, the perfect storm of worms, snails, humans and poverty.

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