shifting to snail control to maintain progress

Over the past decade, significant progress has been made in the prevention and control of schistosomiasis (also known as bilharziasis) globally. The disease affects more than 230 million people, mainly in sub-Saharan Africa.

The progress is mainly due to the extensive periodic treatment of the affected population with praziquantel, the only drug recommended by the World Health Organization (WHO) against all forms of schistosomiasis. Regular treatment treats mild symptoms and prevents them
progression to severe, chronic illness.

Extension of treatment is largely supported by donated praziquantel from Merck KGaA of up to 250 million tablets per year. In 2018, nearly 78 million children were treated, representing 63% of all school-eligible children.

Transmission: The role of snails

Schistosomiasis is an acute, chronic parasitic disease caused by infection with the trematode worms of the genus Schistosoma, Prevails in tropical and subtropical areas, especially in poor rural communities with little or no access to it
safe drinking water and basic sanitation. The infection is transmitted when people suffering from schistosomiasis contaminate fresh water sources when evacuating urine and faeces containing parasite eggs.

Once the egg comes in contact with fresh water, it releases myrcidia (hatching) that enter the snail and produce hundreds of cercariae (larvae) a day. Cercariae in contact with the skin enter the human body and develop into adult earthworms.

In addition to treatment with praziquantel, removing the snail host and improving sanitation are important methods for the sustainable prevention of the disease. Dam and irrigation projects are known to be a potential outbreak. Snail management,
often achieved by molluscs, can help reduce transmission by removing these native snail snails.

Vector control

In 2017, the World Health Assembly – the WHO’s decision-making body – adopted a resolution WHA70.16 on global vector control
Responses, urging Member States to develop or adapt national vector control strategies and operational plans. The goal of the strategy is to reduce by at least 40% the incidence of vector-borne diseases, including schistosomiasis, by 2025.

In response, WHO is stepping up snail control as part of its strategic approach to achieving the goal of eliminating schistosomiasis as a public health problem and, ultimately, the goal of eliminating transmission. Snail control will replenish the mass
treatment campaigns and sustain the impact of this fundamental public health intervention.

To facilitate the implementation of snail control, WHO was published in 2019 on specific, standardized procedures and performance criteria
testing and evaluation and in 2017 a manual on the use of molluscs in the field. WHO also plans to strengthen capacity
medical staff in medical malacology to enhance snail control activities in countries.

In the past, molluscicides (especially nicosamide¹ widespread use products) have been widely used in schistosomiasis suppression programs. In China, for example, programs rely heavily on snail control for more than 30 years to overcome the disease in public
health problems. In other parts of the world, however, snail suppression programs have gradually ceased and competencies have declined.

An integrated control strategy that includes mass praziquantel treatment, snail control and environmental modification, wider access to sanitary and safe water, economic and environmental development, health education and poverty eradication can greatly
contribute to the sustainable control and elimination of schistosomiasis.

A new WHO path map on neglected tropical diseases for 2021-2020, due to be presented later this year, calls for increased financial and political support to achieve the goal of interrupting transmission. Additional research to address deficiencies,
develop tools and optimize the impact of existing snail control programs will complement the basic strategic approach.

Schistosomiasis: forms of the disease

There are two main forms of schistosomiasis – intestinal and urogenital.

Intestinal schistosomiasis usually results in diarrhea and blood in the stool; enlargement of the liver and spleen and portal hypertension are common in advanced cases.

Urogenital schistosomiasis is characterized by the presence of blood in the urine. Chronic infection results in bladder and ureteral fibrosis, which can evolve to hydronephrosis and create conditions for bladder cancer.

In women, urogenital schistosomiasis can cause vaginal bleeding, pain during intercourse and nodules in the vulva, a condition now described as female genital schistosomiasis. In men, urogenital schistosomiasis can cause disease in the semen
vesicles and prostate gland.

¹Restrictions on the use of niclosamide formulations include restrictions on the use of water immediately after administration and temporary toxicity to non-target aquatic organisms, including
fish, amphibians and invertebrates.