Insights From Nyari Lake: Some local snail captures
I was clearing the lake outlet/hatchery yesterday evening (23/11/2025) when I saw these snails (image below) in large numbers in the fish hatchery that I maintain at the outlet point. I keep this hatchery clear of leaves and 3 to 4 times a year have to dredge the leaves and sediment out of it or it dries up and kills off the young fry that hatch and seek refuge here from the big, invasive black bass that we have in the main body of water. The water coming in here is filtered through cattail reeds which also keep the big brutes out of the hatching pond. Another maintenance task is to keep boats and people out of this little pond for obvious reasons and to install and maintain a sluice gate (built by a kind and very capable neighbour and yet to be installed as water levels have been high and unabating) that will allow me to control the water level in the lake during the biannual flooding at the rainy seasons and the subsequent dry period when levels fall catastrophically. Unfortunately - there are some visitors to the lake (and even some residents) that don't understand the value of this pond/hatchery and the small inlet pond at the other end (a siltation trap) and who sometimes wander into these catchment areas destroying moths of effort and care.
As you read this blog post please note: Nyari Residents Welfare Society has regularly tested the water in the main lake and each time the tests come back as positive for Schistosoma parasite eggs. This may mean that anyone handling the water, handling fish/lures/wet fishing line, letting it splash on them, swimming, watering their gardens, cleaning boats etc., could be at risk of aquiring this deadly disease. This series of blog posts here is meant to educate our members and the general public on the species of plants and animals found in our enviroments and to inculcate a feeling of responsibility with the backing knowledge needed for preserving the biomes we all live and thrive in.
I encountered two species of freshwater snails. They are elongated, conical-shelled creatures that I identified as Melanoides tuberculata (left) and Tarebia granifera (right) (see images below). These particular snails are not bilharzia vectors.
Why These Snails Cannot Carry Bilharzia
Schistosomiasis (bilharzia) is a disease transmitted by specific freshwater snails that act as intermediate hosts for Schistosoma parasites. The primary carriers in Africa belong to the family Planorbidae, with Bulinus and Biomphalaria genera being the critical players. These carrier snails possess a distinctive flat, spiral-shaped shell and thrive in certain slow-moving water habitats.
On the other hand, Melanoides tuberculata and Tarebia granifera are turreted snails from the Thiaridae family. Their shell structure is markedly different—high-spired, elongated, and pointed. Crucially, these Thiaridae snails cannot host or transmit Schistosoma parasites, which require the physiology of planorbid snails for part of their lifecycle. Even though Melanoides and Tarebia are abundant and widespread in tropical waters, they do not participate in the transmission pathway of bilharzia.
The mechanism that prevents bilharzia transmission via Melanoides tuberculata and Tarebia granifera is fundamentally biological: Schistosoma parasites require specific snail hosts for their development, and these Thiaridae snails are incompatible for several reasons.
Key Mechanisms:
1. Host-Parasite Compatibility
Schistosoma species (the parasites causing bilharzia) have evolved to infect only certain snail families—primarily Planorbidae (e.g., Bulinus, Biomphalaria). The parasite’s miracidia (larval stage) seek out and penetrate only compatible snail hosts based on chemical signals and surface proteins.
2. Immune Response
Melanoides and Tarebia possess innate immune defenses that recognize the Schistosoma miracidia as foreign and rapidly destroy them. The parasite is unable to evade or suppress these snails’ immune systems—unlike with planorbid snails, where co-evolution has enabled the parasite to survive and develop.
3. Tissue and Molecular Barriers
Successful Schistosoma infection requires that the miracidia reach the snail’s hepatopancreas and transform into sporocysts. Thiaridae snails have different tissue composition, receptor molecules, and internal environments, which do not support the parasite’s development or transformation.
4. Absence of Suitable Internal Conditions
Development of Schistosoma within the snail host is highly specialized. Thiaridae snails lack critical biochemical and physiological factors found in vector snails. As a result, even if a miracidia enters the snail, it cannot mature or reproduce, halting the transmission cycle.
Schistosoma parasites cannot complete their lifecycle in Melanoides or Tarebia snails due to host incompatibility at both immune and cellular levels. Only certain snails (Planorbidae family) provide the environment and molecular features needed for the parasite’s development and transmission to humans.
Ecological Benefits: Competitors Of Carrier Snails
Interestingly, the presence of Melanoides and Tarebia provides an important ecological benefit: competition. Both species are prolific breeders and highly adaptable (there are thousands in our lake and that's a good thing!), often outcompeting native planorbid snails for resources such as food and habitat space. This competition can suppress populations of Biomphalaria and Bulinus in shared environments, potentially reducing the overall risk of schistosomiasis transmission. In some regions, the spread of Melanoides tuberculata has correlated with a drop in the number of carrier snails, and thus a decreased incidence of bilharzia.
How Could These Snails Come To Be In The Lake
The introduction of Melanoides and Tarebia into lakes in Africa reflects a blend of natural dispersal and human activity. These snails are native to tropical Asia and Africa, but have expanded their range:
- Through aquatic plant trade (hitchhiking on imported plants)
- With the movement of waterfowl and other animals
- As aquarium escapees or deliberate introductions
Their hardiness allows them to colonize new habitats quickly, especially where environmental disturbances (like pollution or habitat modification) provide opportunities for adaptable species to thrive. Once established, they become dominant members of the local snail fauna.
Finding Melanoides tuberculata or Tarebia granifera in our lake is not a cause for concern regarding schistosomiasis. In fact, their dominance may lower parasite risk by keeping bilharzia-vector snails at bay. Their success story in our lake combines fascinating aspects of biological competition and the ongoing reshaping of aquatic ecosystems, reminding us that not all snails are alike—and some might just be unwitting allies in the fight against parasitic disease.
References:
- Rollinson, D., & Stothard, J.R. (2009). “Interactions between intermediate snail hosts and schistosomes: molecular and immunological insights.” Parasite Immunology.
- Brown, D. S. (1994). “Freshwater Snails of Africa and Their Medical Importance.”
- Other inline references to Wikipedia article on Bilharzia, image search links to snail species.