Schistosomiasis elimination: refocusing on snail control to sustain progress
Substantial progress has been made during the past decade to prevent and control schistosomiasis (also known as bilharziasis) globally. The disease affects more than 230 million people, mostly in sub-Saharan Africa.
Progress is mainly due to large-scale periodic treatment of affected populations with praziquantel, the only medicine recommended by the World Health Organization (WHO) against all forms of schistosomiasis. Regular treatment cures mild symptoms and prevents
progression to severe, chronic disease.
“Through progressive scale-up of periodic large-scale treatment
to control morbidity in many endemic countries, we are closer to
achieving the minimum target of 75% global coverage of all school‐aged
children,” said Dr Amadou Garba Djirmay, who leads WHO’s global schistosomiasis control programme. “But
if we want to achieve elimination, we must go a step further to
introduce snail control in hot spots and in areas targeting elimination.
This is bound to increase impact and accelerate progress towards
interruption of transmission.”
Expansion of treatment is largely supported by donated praziquantel from Merck KGaA of up to 250 million tablets annually. In 2018, almost 78 million children were treated, representing 63% of all school‐aged children eligible for treatment.
Transmission: role of snails
Schistosomiasis is an acute, chronic parasitic disease caused by infection with trematode worms of the genus Schistosoma. It is prevalent in tropical and subtropical areas, especially in poor, rural communities that have little or no access to
safe drinking‐water and basic sanitation. Infection is transmitted when people suffering from schistosomiasis contaminate freshwater sources when evacuating urine and faeces containing the parasite’s eggs.
Once an egg comes into contact with fresh water, it releases miracidia (hatchlings) which enter a snail and produce hundreds of cercariae (larvae) daily. The cercariae enter the human body on contact with the skin and develop into adult worms.
Besides treatment with praziquantel, eliminating snail hosts and improving sanitation are important methods for sustainable prevention of the disease. Dam and irrigation projects are known to be potential sites for outbreaks of the disease. Snail management,
often achieved through mollusciciding, can help to reduce transmission by eliminating these intermediate snail hosts.
In 2017, the World Health Assembly – WHO’s decision making body – adopted resolution WHA70.16 on the Global Vector Control
Response, calling on Member States to develop or adapt national vector control strategies and operational plans. The strategy aims to reduce by at least 40% the incidence of vector borne diseases, including schistosomiasis, by 2025.
In response, WHO is reinforcing snail control as part of its strategic approach to achieve the target of eliminating schistosomiasis as a public health problem and, ultimately, the goal of elimination of transmission. Snail control will complement mass
treatment campaigns and sustain the public health impact of this core intervention.
To facilitate implementation of snail control, in 2019 WHO published on specific, standardized procedures and criteria for efficacy
testing and evaluation and in 2017 a manual on the use of molluscicides in the field. WHO is also planning to strengthen capacities
of health staff in medical malacology in order to reinforce snail control activities in countries.
In the past, molluscicides (particularly niclosamide1 based products) were widely used in schistosomiasis control programmes. For example, in China programmes largely relied on snail control for over 30 years to overcome the disease as a public
health problem. In other parts of the world, however, snail control programmes were gradually discontinued and competencies faded away.
“Snail control is a crucial component of the global schistosomiasis elimination strategy,” said Dr Rajpal Yadav, Scientist, WHO Department of Control of Neglected Tropical Diseases. “It
is time we look for new snail control products that are cost–effective
and of low risk to humans and the environment. Some efforts are being
made to explore molluscicide compounds from plants, marine algae and
An integrated control strategy involving mass treatment with praziquantel, snail control and environmental modification, wider access to sanitation and safe water, economic and environmental development, health education and poverty-alleviation can greatly
contribute to sustainable control and elimination of schistosomiasis.
WHO’s new road map on neglected tropical diseases for 2021–2030, due to be launched later this year, advocates enhanced financial and political support to achieve the target of interruption of transmission. Additional research to address gaps,
develop tools and optimize the impact of existing programmes with snail control will complement the core strategic approach.
Schistosomiasis: forms of the disease
There are two major forms of schistosomiasis – intestinal and urogenital.
Intestinal schistosomiasis usually results in diarrhoea and blood in the stool; enlargement of the liver and of the 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 fibrosis of the bladder and ureter that can evolve to hydronephrosis and create conditions for bladder cancer.
In women, urogenital schistosomiasis can cause vaginal bleeding, pain during sexual intercourse and nodules in the vulva, a condition now described as female genital schistosomiasis. In men, urogenital schistosomiasis can induce disease in the seminal
vesicles and prostate.
1Limitations with the use of formulations based on niclosamide include restrictions on the usage of water immediately after application and temporary toxicity to non-target aquatic organisms including
fish, amphibians and invertebrates.