Pesticides are a double-edged sword: They make farming more productive, but they can harm wildlife and people if not used properly. Now, ecologists have identified a new threat from pesticides in the developing world. By killing off insect predators of worm-infested snails, they can raise the risk of schistosomiasis, the second most common parasitic disease after malaria.
“It’s a ground-breaking article,” says Russell Stothard, a parasitologist at the Liverpool School of Tropical Medicine in the United Kingdom, who was not involved in the research.
Schistosomiasis is a debilitating disease caused by a parasitic flatworm. Some 258 million people are infected, mostly in Africa. The worm spends part of its life in freshwater snails, which release larvae that can penetrate the skin of someone swimming, bathing, or washing clothes. The centimeter-long worms spread through blood vessels, causing fever, diarrhea, anemia, and stunted growth. Immune responses can damage the kidneys and other organs. When infected people relieve themselves, the worms’ eggs can spread into streams or ponds via their urine and feces. There, they hatch and seek out new snails, beginning their life cycle again. Schistosomiasis can easily be treated with drugs, but where the parasites are endemic, people quickly become reinfected.
The leader of the new research, ecologist Jason Rohr of the University of South Florida in Tampa, had previously studied a similar parasitic flatworm in amphibians. His research showed that common agricultural chemicals, like fertilizer, can worsen the situation for frogs. When these chemicals enter streams and ponds, they increase the amount of algae, which is then eaten by snails that serve as a host for the flatworms. That boosts their population and leads to more parasite infections in frogs.
The similar life cycles of the amphibian flatworm and the one that causes schistosomiasis made Rohr and his colleagues wonder whether agricultural pollution might also affect disease transmission. They created a simple ecological model inside 60 open tanks. After filling each with 800 liters of pond water, they added two species of snails that spread the schistosomiasis parasite, algae for the snails to eat, and two kinds of predators—crayfish and water bugs. Finally, they spiked the tanks with three kinds of farm chemicals—fertilizer, herbicide, and insecticide—in various combinations. The concentrations were typical of streams and ponds near corn fields in the United States.
As expected, fertilizer increased the amount of algae in the tanks, which in turn swelled the number of snails. The herbicide also led to more food for the snails, because it predominately killed microscopic algae that clouded the water. When these died, the water cleared, allowing more light to reach larger algae growing on the bottom of the pond—the snails’ food. An epidemiological model of schistosomiasis suggested that the increase in snail population from this typical amount of fertilizer would jack up the risk of transmission to humans by 28%.
The insecticide, chlorpyrifos, had an even bigger effect by killing the two predators of the snails. Water bugs stick their heads inside the shell, bite the mollusc, inject digestive enzymes, then slurp up the remains. The 20-centimeter-long crayfish rely on brute force, crushing the 2-centimeter-long snails. “They’re absolutely voracious,” Rohr says. With these predators gone, the snail population exploded. In such a scenario, disease risk to humans would rise 10-fold, the team reports in a preprint posted this week to bioRxiv. Although only one concentration of insecticide was added to the tanks, the model indicated that lower concentrations in ponds would still have substantial impacts on parasite transmission.
The findings identify what looks like a “strong risk factor” for schistosomiasis, says Joanne Webster, a parasitologist at Imperial College London who was not involved.
Dams have also caused an increase in schistosomiasis in many countries, because snails live in the reservoirs and irrigation channels. In some places, dams have also caused a decline in the natural predators of snails, such as fish, crayfish, and prawns. The combination of new habitat from irrigation and runoff of pesticides may be a “perfect storm” for schistosomiasis where agriculture is intensifying in the developing world, Rohr says.
Rohr is now investigating the impact of insecticides on snail predators and disease transmission in northwest Senegal, as part of an experiment run by a research partnership called the Upstream Alliance, based in Pacific Grove, California. This project has reintroduced prawns near several villages to evaluate their efficacy in controlling freshwater snails. Rohr will study whether helping farmers switch to insecticides less toxic to prawns could lessen the burden of schistosomiasis, while maintaining food production. “In schistosomiasis-endemic regions, we need to think more carefully about the impact of agrochemicals,” he says.
The study highlights the complex links between agriculture and disease, says Charles Godfray, a biologist at the University of Oxford in the United Kingdom. By boosting agricultural productivity, pesticides and other chemicals can help raise people out of poverty and lessen malnutrition, which worsens diseases. “The really clear thing is the importance of precision agriculture, in which agrochemicals are used as efficiently as possible, with as little runoff as possible.”