When we think about debilitating diseases, snails don’t often come to mind. However, in the world of parasitology, freshwater snails play an integral role, especially in the transmission of schistosomiasis, the second most important human parasitic disease after malaria. Control of intermediate snail hosts is one of the primary schistosomiasis control strategies but a lack of comprehensive understanding of biology and ecology of these snails hinders its success. So how do we approach snail control, what are the associated limitations, and how do we move forward?
Schistosomiasis is a major NTD
Schistosomiasis is both an acute and chronic neglected tropical disease (NTD) caused by parasitic flatworms called schistosomes. The disease affects over 250 million people globally, causes over 200,000 deaths annually, and poses a severe economic burden. Yet there are limited prevention and treatment options.
Schistosomes have a complex life cycle that depends on freshwater snails, commonly belonging to the Biomphalaria, Bulinus, and Oncomelania genera, that serve as intermediate hosts for the parasite. After being excreted into freshwater from infected human or mammalian hosts, schistosome eggs hatch out miracidia larvae that seek and infect their snail host. Human-infective cercariae larvae develop within the snails and are shed into freshwater when exposed to light. Infection occurs through contact with water contaminated with cercariae that penetrate the skin and enter the bloodstream. The primary pathology of the disease is caused by an inflammatory response to schistosome eggs that are laid in veins and become accidentally entrapped in nearby organs. Chronic infection is associated with abdominal pain, organ fibrosis and failure, bladder cancer, and, in severe cases, death.
Snail host control for schistosomiasis elimination
Targeting vectors has been an approach to eliminate diseases for over a century. In fact, its history is linked to the completion of the Panama Canal, after Major Walter Reed had proved a theory proposed by Carlos Finlay that mosquitoes transmit yellow fever, a disease that debilitated builders of the canal. This discovery spurred early public health initiatives to eradicate yellow fever through mosquito control efforts. Since then, vector control has remained central to management and eradication efforts of diseases like malaria, dengue, and Zika. Similarly, schistosomiasis snail host control approaches date back to 1917 when snail-picking campaigns initiated the elimination of schistosomiasis in Japan.
The 2022 World Health Organization (WHO) guideline on control and elimination of human schistosomiasis advocates for an integrated approach involving chemotherapy, increased water, sanitation, and hygiene (WASH) programs, improved education, and host control. This approach addresses the need to both prevent infections and break the transmission cycle. It is important to highlight that a single miracidium larva that successfully infects a snail host can go on to generate thousands of human-infective cercariae larvae. So, in addition to control strategies that focus on life stages that parasitize humans, the intermediate snail hosts pose as accessible targets within the complex transmission trajectory and offer an intervention route to break the transmission cycle.
Today, snail host control methods primarily include the use of chemical molluscicides, habitat modification, and biological control. While disrupting snail populations that transmit schistosomiasis has proven to be successful at reducing transmission and shows promise toward eliminating schistosomiasis, there are a number of ecological and logistical concerns associated with these methods. For instance, studies of effects on non-target organisms and the greater ecology of endemic regions are limited. Additionally, financial challenges in low-resource settings question the feasibility and sustainability of implementing these control measures.
Gliding forward at a snail’s pace
In recent years, scientists have explored modern avenues for more efficient snail host control methods, such as increasing molluscicide specificity and efficacy, pursuing “gene-drive” technology, or more holistic approaches based on snail and community behavior. Already, harnessing recent advances in genomic resources of snail hosts has revealed potential mechanisms of infection compatibility and provided a pathway toward biological control options. Additionally, developments in surveillance and diagnostic tools will aid in understanding the relationship between snail behavior and infection.
In order for these discoveries to become integrated solutions, though, further research is needed to comprehensively understand snail biology and behavior, as well as interactions with their environments, parasites, and humans. Increased awareness of the prevalence, impact, and global threat of NTDs like schistosomiasis can help advocate for novel and advance ongoing research aiming to lay the foundations for developing effective, achievable, and sustainable NTD intervention and elimination routes.