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Daily News Blog

18
Mar

Infectious Human Disease, Snail Fever, Worsened by Pesticide Run-Off into Fresh Waterways

(Beyond Pesticides, March 18, 2020) Freshwater habitats are threatened now—more than ever—by the adverse effects of pesticide pollution, according to a report published in Scientific Reports by a collaborative research team from the Helmholtz Centre for Environmental Research (UFZ) and the Kenya-based International Centre of Insect Physiology and Ecology (ICIPE). Pesticide pollution, attributed to runoff from agricultural farms, indirectly increased the rate of the tropical disease schistosomiasis, which infects over 280 million people (2018). This research underlines the range of uncertainties that exist as a result of pesticide contamination, making it critically important that subtropical areas where this disease threat exists move toward organic and pesticide-free approaches. 

Increased prevalence of this disease is devastating to socioeconomic development in affected regions, as life expectancy, employment rate, and gross domestic product (GDP) decreases.

Schistosomiasis (snail fever), or bilharzia, is a tropical disease caused by parasitic flatworms (trematodes) in the genus Schistosoma and transmitted via freshwater snail (genus Biomphalaria) to its definitive human host. Freshwater snails act as a vector for schistosomiasis as they play a vital role in the lifecycle of the parasitic flatworm.

Professor Matthias Liess (Ph.D.), Head of the Department of System Ecotoxicology at the UFZ, and his research team investigated pesticide pollution’s impact on the macroinvertebrate community composition in regions where schistosomiasis persisted. The research sampled forty-eight freshwater study sites in the Kenyan Lake Victoria Basin with a range of habitats suitable for schistosomiasis transmission. To confirm freshwater snail’s high pesticide tolerance, UFZ investigated acute neonicotinoid, organophosphate, and pyrethroid sensitivity in all macroinvertebrate taxa, in 6 different regions within the study area. Freshwater snails dominated over the less-tolerant invertebrate opponents exclusively in habitats characterized by pesticide pollution and eutrophication.

Laboratory testing discovered freshwater snails have a higher tolerance toward commonly used agriculture pesticides, like neonicotinoids (neonics) and organophosphates. This tolerance enabled the host snail to persist in an environment where non-tolerant (macro)invertebrates could not. In turn, the population of parasitic flatworms increased with its snail host. Human exposure to infested water, through skin contact, adversely impacts human health. Infection associated with this disease can initiate abdominal pain, diarrhea, bloody stool, liver failure, and long-term disabilities. UFZ and ICIPE researchers concluded, “…[for] the first time that in the field, pesticide concentrations considered ‘safe’ in environmental risk assessment have indirect effects on human health.”

Freshwater snails acted as an intermediate host for parasitic flatworms that presented the deadly tropical pathogen. Increased input of pesticides from agriculture paired with an already unhealthy marine environment bolsters host snail population for the parasitic worm to develop. Host snails are more tolerant of pesticide inputs than their predators. Predators act as a biological control agent for the host snail population, and a sustainable approach to mitigate schistosomiasis disease events.

This study is just one example of pesticide use causing a trophic cascade in unhealthy marine environments. A research study published in PLOS ONE  directly links a top-down trophic cascade to pesticide use in aquatic ecosystems dominated by predatory invertebrates. High pesticide concentrations results in vast predatory invertebrate mortality; freshwater snails exhibit no observable mortality at pesticide’s maximum concentration. A 2018 mesocosm study also displayed that pesticides indirectly favored the freshwater snail that hosted the human-pathogenic schistosomes. Neonicotinoid use eradicates the host snail’s predators and supports destructive planktonic algae (periphyton), a food source for snails. An unhealthy marine environment caused by improper waste/sewage disposal exacerbates conditions for the parasitic worm to thrive. The resilience of the intermediate host snail, coupled with the loss of predator biodiversity, allowed the parasite to flourish and infect its human host at an elevated rate.

Control strategies must focus on a sustainable approach to controlling the intermediate host population to mitigate schistosomiasis transmission. Currently, control strategies for the disease treat the human host by administering the medication praziquantel to kill the adult flatworms, but this does not curb the reinfection rate. The disease can rebound in humans in an area where schistosomiasis is endemic.

Biological control for freshwater snails can reduce schistosomiasis events as the host snail is susceptible to predation by various organisms like shrimp, water bugs, and ostracods. These natural predators are sensitive to anthropogenic inputs like pesticides. Tropical regions that commonly practice extensive farming and are prone to heavy rainfall experience schistosomiasis more regularly.

Dr. Liess confirmed the study’s findings in ScienceDaily, “With our study, we were able to demonstrate that even low pesticide concentrations constitute a serious environmental risk and, in this respect, not only contribute to the decline in insect populations, but also indirectly promote dangerous diseases in humans… The results underline the urgent need for reassessing the environmental risk of low pesticide concentrations and for integrated disease management that includes a focus on the regulation and management of pesticides in areas where schistosomiasis is endemic or might be introduced due to potentially favorable ecological conditions.”

Regulation and elimination of pesticides can aid in reducing the propagation of harmful diseases exacerbated by pesticide use. There are a wide range of resources, which can be used to help gain knowledge and apply practices avoid pesticides use and their adverse effects. These include news stories, local organizations, school pesticide policies, regulatory contacts, and least-toxic pest control operators. Organic practices can successfully eliminate toxic pesticide use, especially in agriculture. Organic farming protects water quality as it reduces pesticide and nutrient runoff. Clean water is essential for human health, wildlife, and a balanced environment. Increased global participation in organic agriculture can eliminate the use of toxic chemicals in agriculture, promote biodiversity, and improve water quality to protect human and animal health. Buying organic products (food and non-food items), and advocating for organic regulations in the marketplace can help eradicate pesticide use and promote a healthy, sustainable future.

All unattributed positions and opinions in this piece are those of Beyond Pesticides.

Source: Scientific Reports 

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