In the NCEAS SNAPP Ecological Levers for Health working group, we’re collecting examples of local or regional interventions that can have direct, measurable benefits for human health (via reduced infectious disease) AND the environment – win–win solutions. The case studies that we’ve collected thus far are so cool that we just can’t wait to share them! In September, I shared a story about vulture conservation and rabies. This week, I’ll tell you about plants and diarrheal disease.
Two years ago, I adopted a tiny blue Aussie puppy: a wild, brilliant beast with a thirst for adventure… and water. I mean he really likes water – jumping in it, biting it, blowing bubbles in it, fishing sticks out of it, etc. And the dirtier that water is, the better. So I set my puppy loose at the duck pond at our local gem of a dog park, where I had seen my previous dog and dozens of other dogs safely drink the water. Days later, my precious puppy developed severe diarrhea, and just hours after the onset of his symptoms, he became terrifyingly lethargic. The enteric pathogens that he had guzzled in the pond water might have killed him if we had not immediately sought out veterinary care. But fortunately, antibiotics and rehydration allowed Carrot to make a full recovery, and he grew up to be the healthy mud monster pictured below. From that experience, I re-learned an important lesson from disease ecology: pathogens often have minimal effects on adult animals, which have developed resistance/immunity during prior exposure, but the same pathogens can be deadly for juveniles during their first exposure.
This principle doesn’t just apply to puppies: diarrheal disease is the second leading cause of global childhood mortality for children under five years old. That’s hundreds of thousands of children dying every year because they did normal childhood things – like eating, drinking, and playing outside – and became infected by waterborne or foodborne pathogens like rotaviruses, Vibrio cholerae, and Salmonella. Many more children become infected by these pathogens and survive their illnesses, only to experience lasting physiological impacts. For instance, diarrhea leads to malnourishment, and malnourishment increases a child’s risk of future infection and diarrhea, creating a vicious cycle of ill health that can retard physical and mental development.
How can we remedy this huge global burden of childhood morbidity and mortality? The good news is that we already have substantially reduced the global impacts of childhood diarrheal disease by (1) improving hygiene and sanitation to reduce peoples’ exposure to the pathogens and (2) using oral rehydration therapy to treat children who are suffering from diarrhea, so that they do not die from dehydration. However, millions of people still lack access to clean water resources and quality healthcare, and an unthinkable number of children are still dying each year, and thus there is still much to do. Today, I want to broaden the scope of potential solutions: are there ecological solutions that can help reduce human exposure to enteric pathogens as a complement to current public health efforts?
But before we discuss specific ecological solutions, it’s worth discussing how these pathogens enter and persist in water sources in the first place. In some cases, the pathogens are pumped into public water sources directly from sewage pipes or human bodies (e.g., people swimming and defecating at water access points). In other cases, the pathogens reach public water sources via runoff from the environment after they’ve been excreted by humans and/or animals. When these pathogens reach a waterbody, they do not necessarily find and infect a human. For instance, if the pathogen is buried under the sediment in a stream, degraded by sunlight, consumed by microorganisms, or destroyed by plant biocides, it will never reach a human host. So ideally, ecological solutions will reduce the number of pathogens reaching waterbodies and/or increase pathogen death rates in those waterbodies. With this in mind, let’s talk about one class of ecological solutions for waterborne enteric pathogens: can plants be a win–win solution for conservation and human health?
In my freshmen year as an undergrad, my favorite professor made us “draw and describe, in excruciating detail, the difference between an urban and rural hydrograph.” I received full marks, so let’s assume I’m an expert: in urbanized areas with lots of buildings and paved, impervious surfaces, stormwater reaches streams and rivers quickly, whereas in rural areas with lots of trees and permeable soil, stormwater reaches streams and rivers relatively slowly (see below). And of course, it isn’t just water that reaches those streams and rivers. In urban environments, pollutants and pathogens within the stormwater also make it to downstream waterbodies faster, meaning that fewer pathogens die before reaching water sources where they can encounter and infect people. Therefore, human-caused hydrological changes should affect human disease burdens.
And we’re seeing that. For instance, in a massive study of 300,000 children in 35 nations, deforestation upstream from a child’s house was found to be strong predictor of whether the child had high risk of diarrheal disease, presumably because many pathogens were entering the waterbodies upstream (Herrera et al. 2017). (But this was only true for the poor children – the wealthier children living in cities probably had better access to sanitation infrastructure.) Similarly, in Brazil, children living near protected forests were less likely to experience diarrheal disease (Bauch et al. 2015). These large-scale correlational studies suggest that protecting forests might be a win–win solution for conserving biodiversity and reducing childhood diarrhea!
Of course, many forests have already been cut down, so it’s too late to preserve them for human health. In those cases, reforestation/restoration might be a win–win solution. For instance, Herrera et al. (2017) predicted that increasing upstream forest cover by 30% would reduce childhood diarrheal risk as much as improved sanitation and hygiene!
But re-forestation is a big undertaking, and as far as I know, no one has experimentally evaluated the effects of re-forestation on human disease yet. An easier/faster intervention to slow the rate that pathogens and other pollutants reach streams and rivers might be replanting vegetation just within riparian buffers. It’s still unclear whether replanting riparian vegetation can reduce human infection, but in some studies, the number of enteric pathogens and/or fecal indicator bacteria within streams has decreased after riparian buffers were restored, which suggests that human infectious risk would be reduced by stream-side vegetation. This remains an important avenue for future research.
So, preserving or restoring forests and/or riparian buffers can reduce the number of pathogens reaching waterbodies and potentially reduce human infection, but can plants also reduce the number of pathogens that reach human hosts after reaching waterbodies? Potentially! For instance, at Indonesian islands without wastewater treatment systems, there are fewer human bacterial pathogens in seagrass meadows than in nearshore waters that lack seagrass meadows (Lamb et al. 2017). Furthermore, disease burdens in corals are lower near seagrass meadows, too, suggesting that preserving or restoring seagrass meadows could be a win–win for human health and conservation. This is a great correlational study, but is there any experimental evidence that aquatic/marine plants reduce environmental pathogen loads or human disease burdens?
Yep! You may have seen something similar to the photograph below in a town near you. It’s a constructed wetland. Specifically, it’s the Dominguez Gap Wetland, which was created to treat stormwater before it reached the LA River and then the Pacific Ocean. Constructed wetlands like this one are typically designed to filter heavy metals, excess nitrogen and phosphorous, and other chemical pollutants from stormwater. But they can also remove viruses, bacteria, protozoans, and other pathogens from runoff waters. For instance, by forcing viruses to hang out in the slow-flowing water for a while, the wetlands ensure that many viruses die from UV exposure long before they reach downstream waterbodies. Several studies have shown that constructed wetlands successfully reduce environmental pathogen loads, and now we need studies that link constructed wetlands and human disease risk.
However, there are many varieties of constructed wetlands – they vary in retention time, turbidity, whether they contain plants or not, whether there is subsurface or surface water flow, etc. And some designs are better at removing pathogens from stormwater than others. Furthermore, even really efficient constructed wetlands might fail to reduce pathogen loads to levels that are safe for human use, depending on how many pathogens are entering the environment. Therefore, if we want to use constructed wetlands to reduce human exposure to enteric pathogens, we need to design them carefully.
So there you have it! “Plants” – or environmental characteristics associated with plants – can reduce the number of human pathogens that reach waterbodies and pathogen survival time within waterbodies. And lower pathogen loads in waterbodies presumably reduce human disease, especially childhood diarrheal risk. As far as I can tell, no one is currently using forest protection/restoration or constructed wetlands on a large scale to try to prevent childhood diarrhea, but “plants” could be “ecological levers for health” that advance both conservation and human health goals.
If you know of any existing, planned, or in-progress forest protection, reforestation, or constructed wetland interventions aimed at reducing human diarrheal diseases, please let me know! And if you can think of any other win–win solutions for conservation and human health, we’d love to hear about them.
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