Pesticides promote the spread of parasitic disease

Pesticides and other chemicals used in agriculture are speeding up the spread of schistosomiasis, a debilitating disease caused by parasitic worms. Researchers at UC Berkeley report that agrochemicals disrupt the balance of aquatic ecosystems in ways that expose more animals and people to infection.

Schistosomiasis develops when parasitic worms multiply inside of freshwater snails. The infection, also known as snail fever, is transmitted through contaminated water. 

This disease affects hundreds of millions of people each year, potentially causing lifelong liver and kidney damage. Among parasitic diseases, the global impact of schistosomiasis on human health is second only to malaria.

The study authors report that agrochemicals can facilitate the transmission of the disease in different ways. 

First, the chemicals promote the survival of the schistosome worms by killing aquatic predators that feed on infected snails. In addition, pesticides and fertilizers alter the composition of algae in the water, providing an abundance of food for the snails.

“We know that dam construction and irrigation expansion increase schistosomiasis transmission in low-income settings by disrupting freshwater ecosystems,” said study lead author Christopher Hoover.

“We were shocked by the strength of evidence we found also linking agrochemical pollution to the amplification of schistosomiasis transmission.”

The findings come at a time when the COVID-19 pandemic is shining a spotlight on the need to better understand the connections between environment and infectious diseases.

“Environmental pollutants can increase our exposure and susceptibility to infectious diseases,” said study senior author Justin Remais. 

“From dioxins decreasing resistance to influenza virus, to air pollutants increasing COVID-19 mortality, to arsenic impacting lower respiratory tract and enteric infections – research has shown that reducing pollution is an important way to protect populations from infectious diseases.”

The research team identified 144 studies that have scientifically linked agrochemical concentrations to schistosomiasis. The experts analyzed this data using a mathematical model that accounted for the transmission dynamics of the parasite. 

The model also simulated the concentrations of common agrochemicals in surrounding agricultural fields and estimated the resulting impacts on the nearby human population.

The study revealed that even low concentrations of common pesticides, including atrazine, glyphosate, and chlorpyrifos, increase rates of transmission and interfere with efforts to control schistosomiasis. 

In communities along the Senegal River Basin in West Africa, the extent of schistosomiasis infection tied to agrochemical pollution was found to be comparable to the disease burden of lead exposure, high sodium diets, and low physical activity.

“We need to develop policies that protect public health by limiting the amplification of schistosomiasis transmission by agrochemical pollution,” said Hoover. 

“More than 90% of schistosomiasis cases occur in areas of sub-Saharan Africa, where agrochemical use is expanding. If we can devise ways to maintain the agricultural benefits of these chemicals, while limiting their overuse in schistosomiasis-endemic areas, we could prevent additional harm to public health within communities that already experience a high and unacceptable burden of disease.”

The study is published in the journal Lancet Planetary Health.

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By Chrissy Sexton, Earth.com Staff Writer