How Pesticides Increase the Transmission Rate of Schistosomiasis

Both pesticides and fertilizers have had their dark history of harming the environment, yet it is still commonplace today. The continuous usage of agrochemicals carries far more unintended consequences than we expected. Recent discoveries from the University of California, Berkeley research team has revealed that the rising water developmental projects such as dams have allowed a rise in the freshwater snail population, while dispersing its predators which are necessary for keeping its numbers in check. In addition, agrochemicals we utilize today are polluting the environment, therefore increasing our exposure and vulnerability to infectious diseases, particularly schistosomiasis.

What is Schistosomiasis?

Schistosomiasis, known as bilharzia or snail fever, derives from parasitic worms (In this case, Schistosoma haematobium) in tropical and subtropical freshwater environments. This disease earned the moniker “snail fever” due to the schistosome parasites’ use of snails as their hosts. As a result, the freshwater becomes contaminated, when humans make contact with these waters parasites burrow into their bodies. The worms travel through the bloodstream to vital organs such as the liver, kidney, and intestines. Meanwhile, females lay their eggs which are passed through human urine and feces. If these excretions reach freshwater sources, they will repeat the process to inhabit snails and grow before infecting another human. Without this trend, the parasitic eggs remain in the body and are attacked by the immune system. There are different symptoms as they pertain to the infected area. For example, one might experience seizures, headaches, and loss of balance if their nervous system is infected. If not treated properly, short-term or acute schistosomiasis can lead to long-term or chronic schistosomiasis. In this state, females will continue to reproduce and infected organs can be critically damaged.

Findings

Researchers have found that the utilization of agrochemicals has accelerated the transmission process of schistosomiasis. Some effects of agrochemical pollution include eliminating snail predators, increasing algae which are a main food source for the snails, as well as impacting the schistosome parasites’ survival directly. The insecticides, chlorpyrifos, and profenofos are toxic to the predators that hunt these snails which allows the freshwater snail population to increase dramatically, activating a top-down trophic cascade. Atrazine, an agricultural herbicide, was discovered to indirectly aid the growth of the algae which these snails consume, causing a bottom-up trophic cascade. The snail population expanded, allowing more snails to serve as intermediate hosts for the parasites. Sub-Saharan Africa, where over 90% of schistosomiasis cases originate, has been exponentially increasing its application of agrochemicals in hopes for efficient and less arduous methods of farming. With an ever-increasing freshwater snail population, the waterborne parasite population grows as well, resulting in a rise in the human infection rate.

This diagram from The Lancet Planetary Health shows the use of different agrochemicals and their effects in relation to the study.

In addition, researchers input their data into a complex mathematical model in order to have a general form of structure for the situation. Then, they could easily approximate the R0 (basic reproduction number) of the schistosomes. The R0 of S haematobium was about 1.65 while in an agrochemical-free environment. However, the R0 has increased triple the amount when affected by agrochemicals. The model was also capable of estimating the number of DALYs (disability-adjusted life-years) lost per 100,000 people from the altered schistosomiasis. This represents about how many years are lost due to the disease they have. It has been approximated that there have been 142 additional DALYs lost per 100,000 people. By discovering the effects of individual chemicals within the pesticides, the research team could estimate both the R0 and DALYs that each caused.

Conclusion

This isn’t the first time we’ve witnessed the ramifications of using agrochemicals. One notable instance was the widespread usage of the insecticide, DDT, which leaked into waterways, poisoning fish and other aquatic life. When bald eagles consumed these toxic fish, they lost the ability to produce sturdy eggshells for their offspring. As a result, the eggs often did not survive due to its lackluster protection which led to a massive decline in the bald eagle population.

There are countless chemical compounds used in pesticides, all harboring dangerous side effects that can greatly impact the ecosystem. Justin Remais, a leading figure in UC Berkeley’s School of Public Health, explains that reducing agrochemical pollution will not only reduce risk of schistosomiasis, but other infectious diseases as well. Now that we know agrochemicals cause both unwanted direct and indirect effects, it is especially crucial that we find alternative methods to lower the risk of transmission by eliminating agrochemical pollution in regions where schistosomiasis is endemic.

References

Effects of agrochemical pollution on schistosomiasis transmission: a systematic review and modelling analysis,The Lancet Planetary Health, July 2020

https://www.thelancet.com/journals/lanplh/article/PIIS2542-5196(20)30105-4/fulltext