– Re-introducing natural predators to control parasite infections in western Africa
Despite the miracles of modern medicine, nature sometimes still has the best answers to disease control. Schistosomiasis, a parasite-borne disease that affects people from sub-Saharan Africa to Southeast Asia and South America, is one of a slew of infections known by the World Health Organization as “Neglected Tropical Diseases”, so categorized because they are treatable and preventable, but still exist due to poverty and unsanitary living conditions. Susanne Sokolow, a postdoctoral scholar working with professor Giulio De Leo at Stanford’s Hopkins Marine Station, is taking a new, ecology-based approach to the problem of schistosomiasis prevalence in western Africa, by targeting and reducing the hosts that the parasites depend on.
The group of Schistosoma species, which cause schistosomiasis, use several types of freshwater snails to host parasite reproduction. In their larval form, the parasites latch onto snail gonads, diverting snail energy reserves to parasite growth. Once the parasites have developed sufficiently, they release their hold on the now-castrated snails and cast off into the river, where they can penetrate the skin of humans wading in the water. Schistosoma parasites complete their lifecycle in the human body, and release their eggs into fecal matter and urine, which often ends up back in the river, placing parasite eggs within reach of their snail hosts and beginning the cycle anew. The parasites are not lethal to humans, but cause a range of debilitating symptoms, including chronic anemia, stunting, organ damage and cognitive impairment, all of which create a preventable reduction in health and quality of living.
The rate of schistosomiasis infections originally spiked after the Diama Dam was built on the Senegal River in the 1980s to prevent saltwater backwash into agricultural lands along the upper river. The dam had the unintended consequence, however, of preventing species of native African prawns (Machrobrachium vollenhovenii) from traveling between the upper freshwater regions of the river and the lower saline estuaries necessary for reproduction. African prawns feed on the snails that host schistosomiasis, so diminishing prawn numbers led to an increase in freshwater snails, and a corresponding drastic rise in schistosomiasis in people living along the Senegal River Basin.
Because the Diama Dam almost completely eliminated prawn populations along the Senegal River, Sokolow and collaborators are investigating ways to reintroduce these natural predators to the river, in order to reduce host snail numbers. The current project involves flying prawns in from Cameroon, and implementing them into the Senegal River by hand. The prawns are contained in large net enclosures near frequently-used riverbank sites so that they concentrate feeding on nearshore snail populations, and will be more easily accessible for people to harvest.
Sokolow originally began working on the project as a postdoctoral scholar at UC Santa Barbara with Professor Armand Kuris and Kevin Lafferty. Along with Kuris and Lafferty, Sokolow and De Leo’s current collaborators include Projet Crevette and the 20/20 Initiative, both of which focus on reintroducing prawns to African rivers to reduce schistosomiasis parasites, as well as a prawn aquaculture team from Kentucky State University, and Stanford Urology professor Michael Hsieh. The project was recently named one of seven Environmental Venture Projects by Stanford’s Woods Institute for the Environment.
Part of Sokolow’s work involves investigating just how hungry African prawns are for freshwater snails, and how effective the prawns will be as a snail removal tool if reintroduced into rivers. Interestingly, small-to-medium sized prawns consume the highest proportional snail biomass relative to their own biomass, making them the “most effective” prawn size class of snail removers. This finding is valuable because it means that by the time prawns reach their larger market size, they are no longer as effective at removing snails. Since prawns are an important protein source for people living along the river, and are considered a delicacy abroad, being able to harvest the prawns for food and money after they have consumed snails provides a win-win opportunity for local communities (fortunately, the prawns do not serve as a host for schistosomiasis parasites, so humans cannot contract the disease by eating prawns).
The costs of transport from Cameroon, as well as the travel stress, which shortens prawn lifespan, have encouraged Sokolow and project partners to explore methods of prawn aquaculture farms along the Senegal River. Prawn farming within Senegal would reduce travel distance and costs for importing prawns, and create additional local jobs in aquaculture.
Even when drugs to cure a disease such as schistosomiasis can be cheaply administered, they do not prevent re-infection, which must be tackled by addressing the underlying sanitation problems that arise from the ways in which people use their surroundings. Sokolow’s work capitalizes on the multiple benefits that the naturally predatory African freshwater prawns present, to provide long-term biological control and to help re-implement an important harvestable and marketable resource.