It’s a familiar ecology story: Human dam-building activities in the 1980s wiped out a species of prawn in the Senegal River by blocking its migration routes. But this tale takes an unexpected turn into human health. A pilot study suggests that reintroducing the prawns to the river wouldn’t be good just for biodiversity—it could also help to control a parasite that causes disease in humans.
The research, published in PNAS, found that when river prawns were reintroduced to a village’s water supply, the number of parasite-carrying water snails dropped substantially compared to a village with no prawns. This drop had a significant impact on the disease levels of the villagers.
The construction of the Diama Dam in 1986 caused the outbreak of a nasty disease called schistosomiasis, occurring in the villages along the river. Schistosomiasis, also known as bilharzia, is a waterborne disease with a lifecycle after the larvae penetrate human skin, they go on to mature in the intestine or bladder. The sharp, spiny eggs laid by the female are able to penetrate tissues, allowing them to travel through the body.
If their travel is interrupted, they can get stuck in an organ and stay there, causing inflammation. Complications can be extreme, causing death from liver failure or bladder cancer. Children can suffer cognitive impairment and stunted growth.
The parasites that cause schistosomiasis have a complex life cycle, spending part of their lives using vertebrates (such as humans) as their hosts and an intermediate stage living in molluscs like snails. When the Dam was built, the number of river prawn was much reduced which seems to have created a paradise for the snails that act as intermediate hosts. So no more prawns upstream in the river where villagers use the water. In the absence of predators, the snails flourished.
Lab experiments have shown that prawns can control snail populations and to test whether it would be worth investing in an effort to reintroduce prawns to the Senegal River, the researchers chose two villages along the river bank. One village would receive schistosomiasis medicine; the other would get schistosomiasis medicine, plus prawns. To keep the prawns in an enclosed area, nets were installed near the river bank, creating a patch of prawn-abundant water alongside the village. After 18 months, snail populations had halved in the area that had prawns than the control village. Also there seem to be lower infection rates among people using water from the prawn enclosed area when compared to the start of the trial.
The results aren’t proof that the prawns were the change that brought about the reduction in disease rates, the authors write. It’s possible for other changes in the villages that left unnoticed or maybe effects from the net that caused the reduction in snail population regardless of prawns. But the researchers say that the results did match up with what we know about the parasite’s life cycle as well as lab research and the fieldwork in Kenya.
It will be necessary to replicate these results before forging ahead with prawn restoration, but in the meantime, it’s possible to think about how this restoration could be achieved.
Natural ecosystems have river prawns at higher number than necessary to control snail populations, so building a prawn “ladder” to restore the natural population seems like an ideal solution. Restoring the population would have other benefits too. Because larger prawns prey on smaller prawns, catching and eating the larger prawns could have economic benefits and provide a new food source, while still leaving enough smaller prawns to eat the snails, the researchers suggest. Working out whether this assumption is correct and what the economic effects would be are necessary steps before taking the project further.
If this small scale preliminary study holds up for further research under critical observation, then it could be a cost-efficient and highly beneficial way to combat schistosomiasis.
More on this can be read here.