24
Jul
Environmental Pollutants, including Pesticides, Can Increase Susceptibility to Infectious Diseases
(Beyond Pesticides, July 24, 2020) As the novel coronavirus pandemic has heightened awareness of infectious diseases, there is increased attention to connections between environmental concerns and such diseases, including factors that may exacerbate their transmission. New research shows one such relationship: the transmission of schistosomiasis, a tropical disease caused by contact with the larvae of parasitic worms (schistosomes), is likely accelerated by the use of pesticides and other agrochemicals (such as synthetic fertilizers). The study, published in The Lancet Planetary Health, also shows that contamination of freshwater bodies with these chemicals disturbs ecological balances that can actually limit schistosome infections. This new research underscores the urgency of the needed transition, in affected tropical and subtropical areas, to agricultural approaches that do not involve synthetic agrochemicals that pollute local waterways and put people’s health at increased risk.
Beyond Pesticides recently covered another study, published in Nature’s Scientific Reports in February 2020, that indicates that agricultural pesticide runoff indirectly increases rates of transmission of schistosomiasis. The transmission landscape for this disease is complex, in part because one of the parasite’s vectors are freshwater snails, which: (1) play an important role in schistosomes’ life cycle, (2) are relatively resistant to the effects of pesticides, and more tolerant of them than their predators, and (3) seem to handle one of the damaging effects of agricultural runoff — eutrophication — well.
These schistosome worms — Schistosoma haematobium spp (the “spp” indicating that this species, unlike others in the genus, has separate female and male organisms) — are internal parasites of molluscs and vertebrates, and require two hosts to reproduce successfully. They develop and proliferate inside freshwater snails (the intermediate host), and release their larvae into their resident freshwater body. The larvae can then penetrate the skin of humans who come into contact with that water, causing schistosomiasis in the final, human, host. When infected people, in turn, contaminate freshwater sources with their excreta, transmission continues. Schistosomiasis, also known as bilharzia and “snail fever,” affects hundreds of millions of people annually, primarily in sub-Saharan Africa, and can cause lifelong liver and kidney damage. Schistosomiasis is second only to malaria in its global health impacts.
The study — “Effects of agrochemical pollution on schistosomiasis transmission: a systematic review and modelling analysis” — was led by researchers from the University of California, Berkeley, and included members affiliated with Stanford University, Notre Dame, and the School of Sciences, Royal Melbourne Institute of Technology University. It comprises a systematic literature review of more than 1,000 studies on impacts of agrochemicals on the lifecycle of Schistosoma haematobium spp, one of the trematodes (blood flukes, or flatworms) responsible for the disease. (There are a number of species within the genus Schistosoma that can play a role in the disease; the main species infecting humans are S. haematobium, S. japonicum, and S. mansoni.)
Researchers were able to identify 144 experiments that provides data connecting stages of the S. haematobium lifecycle with concentrations of agrochemicals in freshwater bodies. They then integrate both the dose-response functions and environmentally relevant agrochemical concentrations (post-application to agricultural fields) into a mathematical model to estimate agrochemical effects on schistosomiasis transmission in nearby human populations.
Christopher Hoover of UC Berkeley, second lead author, comments, “We were shocked by the strength of evidence we found . . . linking agrochemical pollution to the amplification of schistosomiasis transmission.” The study findings indicate that agrochemicals, such as pesticides and synthetic fertilizers, can catalyze the transmission of the schistosome worm in several ways: “by directly affecting the survival of the waterborne parasite itself, by decimating aquatic predators that feed on the snails that carry the parasite, and by altering the composition of algae in the water, which provides a major food source for snails.” These synthetic chemicals can destabilize freshwater ecosystems. For example, predators in the ecosystem that would typically consume snails infected with the parasite may find pesticide-contaminated water uninhabitable — setting the stage for overpopulation of snails that can fulfill their “duty” as first hosts of the parasites.
The study reveals that even small concentrations of pesticides in common use, such as glyphosate, chlorpyrifos, and atrazine, can increase transmission rates. The study’s modeling also shows that short-lived spikes in pesticide concentrations can cause faster rebounds in infection after application of chemical controls (for S. haematobium) to the water. Further, the team estimated that agrochemical effects on schistosomiasis transmission causes, in some situations, upwards of 142 additional DALYs (disability-adjusted life-years) lost annually per 100,000 people. Researchers found that the magnitude of the increased rates could be significant: in communities in West Africa’s Senegal River Basin, the additional disease incidence attributable to agrochemical pollution was roughly equivalent to that caused by diets high in sodium, exposure to lead, and low levels of physical activity.
Many pesticides are implicated across a variety of mechanisms of impact. Some specific findings include:
- herbicides, and especially atrazine and glyphosate, can increase transmission at expected environmental concentrations (EECs)
- a number of insecticides, including chlorpyrifos, permethrin, carbaryl, profenofos, and others, boost mortality among snail predators
- insecticides can decrease transmission via direct impacts on survival of miracidia (the free-swimming, ciliated larval stage of a schistosome, in which a fluke passes from the egg to a host snail) and cercariae (the free-swimming larval stage of a schistosome, in which the fluke passes from a snail to a final, vertebrate host), as well as impacts on snail reproduction and survival.
Investigators note that, although taken alone, some agrochemicals might actually have an ameliorating effect on transmission, both the span of trematode life stages and the variability of persistence of some chemical pesticides may contravene that influence. For example, glyphosate appears to reduce transmission because of its reproductive toxicity to snails (at environmentally relevant concentrations). Yet, the study says, “Indirect effects overwhelm and reverse the transmission-reducing, direct effects on snails and schistosome cercariae, miracidia, and eggs and might also be more likely to dominate in real-world settings, because they occur at lower concentrations that are more commonly observed. [Additionally,] the net effect of chlorpyrifos, profenofos, and atrazine is to amplify transmission through the dominance of indirect effects on snail predators and algal dynamics, which are longer lasting than direct effects on snails and schistosome larvae.”
In addition to the direct relationship to disease transmission, the toxic agrochemical pollution of freshwater sources, as noted, disturbs important ecosystem balances, and can cause profoundly damaging trophic cascades. Such cascades are catalyzed by the interplay between the toxicity of chemical pollution and their indirect impacts, and can significantly reduce, change the behavior of, or destroy populations of plants and animals.
The findings of the study are concerning, and all the more so because the sub-Saharan region — whose agricultural enterprises have until recently been mostly small farms that have not necessarily used pesticides as readily or as intensively as agricultural enterprises in other parts of the world — is changing. The agrochemical industry no doubt sees an “under-exploited” market there, and farmers are increasingly moving to the use of pesticides and synthetic fertilizers. The published research states, “Increases in rural population density and the availability of modern agricultural inputs suggest that sub-Saharan Africa is on the verge of a rapid expansion of agrochemical use, suggesting that agrochemical pollution is likely to become more common in schistosomiasis-endemic areas.”
Several researchers advocate for more-stringent controls on any synthetic agricultural chemical use in schistosomiasis-prone regions. Mathias Liess, PhD, co-author of the 2020 Nature’s Scientific Reports study, confirmed its findings in ScienceDaily, saying: “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.”
Senior author of the UC Berkeley study Justin Remais, PhD comments, “Environmental pollutants can increase our exposure and susceptibility to infectious diseases. 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.” Finally, subject study co-author Mr. Hoover adds, “We need to develop policies that protect public health by limiting the amplification of schistosomiasis transmission by agrochemical pollution. . . . [By] 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.”
Globally, the public is understandably more attentive to protection from infectious diseases in the COVID-19 context. Though many people do not live in climes where schistosomiasis is a problem, this research reinforces the reality: the broader issue of the havoc that agrochemical pollution wreaks on ecosystems and human health needs far more precautionary attention from government officials and regulators. This is true in the U.S. and in most countries, and is particularly acute in a time when it is clear that exposure to these chemicals, directly and indirectly, can put humans at greater risk from the novel coronavirus and other infectious diseases.
To learn more about pesticide use as it relates to the “new coronavirus normal,” see Beyond Pesticides coverage of disinfectant compounds. For general information on protection from pesticides in the community, see the Center for Community Pesticide and Alternatives Information.
Sources: https://www.thelancet.com/journals/lanplh/article/PIIS2542-5196(20)30105-4/fulltext and https://news.berkeley.edu/2020/07/17/pesticides-speed-the-spread-of-deadly-waterborne-pathogens/
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
