By Simran Patel
Thinking about what happens to our waste after flushing the toilet is a revolting thought. However, without proper treatment, sewage can disrupt biodiversity and threaten public health. One of the reasons is pharmaceutical pollution because the active ingredients in medicines still remain active after excretion and are not fully cleared by wastewater treatment systems.1 This means the pharmaceuticals are released into the natural environment, disrupting biodiversity.
Sources of pharmaceutical pollution are not only domestic. If a waterway is near a hospital2 or pharmaceutical manufacturing site, it can be contaminated with medicines.3 Two-thirds of antibiotic production are aimed at livestock not only to treat or prevent infection but to promote livestock growth.2 Livestock excrement, including antibiotics, can then run off into nearby water bodies4. Arable agriculture also causes pharmaceutical pollution, because some farms are irrigated with treated wastewater which is not completely free of medicines.4 This allows pharmaceuticals to enter waterways when the soil erodes during a flood or storm.4 These pollution sources are especially important in lower-income countries, without adequate water treatment infrastructure3, and arid areas3, where wastewater is more likely to be reused.4 Overall, pharmaceutical pollution is worst in areas of high economic activity and high modern medicine usage.3
The most common pollutants are compounds that are not under strict regulation, such as caffeine, nicotine and paracetamol3. However, prescribed medicines such as antibiotics and endocrine disruptors are more likely to have devastating ecological impacts. These two pharmaceutical categories will be discussed in the rest of this article.
Antibiotics were designed to tamper with bacterial metabolism. Thus, unsurprisingly, antibiotic pollution affects the microbiome. Because they are only partially broken down during wastewater treatment1, antibiotics can enter natural ecosystems and inhibit ecologically important bacterial enzymes.2 The worst antibiotic pollutants are ciprofloxacin and ofloxacin because not only are they highly concentrated in freshwater, but they are some of the most toxic per unit concentration.5 Ciprofloxacin has a high rate of excretion, likely explaining its high freshwater concentration.5 But antibiotics do not have to be at high concentrations to affect bacteria – they can slow growth, change gene expression and disrupt quorum sensing at subinhibitory concentrations.2 Tetracycline, for example, changes microbial community composition, abundance and productivity at subinhibitory concentrations.5
Exposure to a low concentration of antibiotics over a long time also allows antibiotic resistance to evolve.1 For example, bacteria resistant to erythromycin and sulfamethoxazole were isolated from the agriculturally intensive and population-dense Red River delta in Vietnam.6 Once it has evolved, microplastics in wastewater help antibiotic resistance spread, by providing a surface on which biofilms can form and horizontal transfer of resistance genes can occur.7 Even the wastewater treatment process can encourage horizontal gene transfer, depending on the chemicals and conditions the waste is treated with.2 Furthermore, some evidence suggests that antibiotic exposure helps bacteria evolve resistance to protozoan predators.5 If humans are infected with antibiotic-resistant bacteria from polluted water bodies, and existing treatments are ineffective against the new pathogen, an epidemic could start. Therefore, antibiotic pollution could have both health and ecological consequences.
The ecological consequences of antibiotics are not limited to bacteria though. Sulfathiazole inhibits the growth of algae species Ulva lactuca and Lemna gibba, while other antibiotics disrupt algal photosynthesis by damaging chlorophyll and/or preventing chloroplast production.8 Enrofloxacin, erythrocin and roxithromycin are neurotoxic to fish by affecting acetylcholinesterase activity.8 Sulfamethoxazole and norfloxacin affect the ethoxyresorufin-O-deethylase pathway in fish, reducing their ability to metabolise dangerous compounds.8 Macrolide and tetracycline antibiotics cause developmental deformations in zebrafish, such as an uninflated swim bladder.8 Even if the fish survive antibiotic exposure, the toxic chemicals can bioaccumulate inside them. This means when humans eat contaminated fish, they ingest a more concentrated dose of the antibiotic than the fish were exposed to.9 Thus, antibiotics threaten the health of organisms across taxa and across the food chain, including humans.
In contrast, endocrine disruptors such as birth control, anti-inflammatory and performance enhancement drugs10mainly decimate fish fertility and development. Polluted waterways have more female and intersex fish than normal, and genotyping the intersex fish using a sex marker reveals that they are genetically male11. This ‘feminisation’ of male fish occurs because endocrine disruptors induce vitellogenin production9 and, in the case of the synthetic hormone 17α-ethinyloestradiol, egg cell development in the testes.11 As well as developing female characteristics, ‘feminised’ fish lose their male characteristics. Roaches in wastewater-contaminated rivers have lower sperm density and fertility, for example.11 The lower fertility is especially problematic if endangered fish live in polluted areas, because it reduces the population growth rate11 and recovery rate from any disturbances. ‘Masculinisation’ is also possible when xeno-androgen compounds contaminate waterways, but this is less common.12 Regardless, the reduced fertility and skewed sex ratio of fish in polluted water bodies makes their populations unstable, complicating conservation efforts.9
Although the above examples show how pharmaceuticals threaten microbial and animal life, the extent of damage depends on the context. A fish species’ response to endocrine disruptors depends on its diet, metabolism and ecology.9 Some microorganisms can degrade pharmaceuticals to reduce their toxicity, but the rate of degradation depends on abiotic factors like water temperature and oxygen level.4 In salty estuaries, pharmaceuticals collect near the river bed so animals higher up the water column are hardly affected.4 If multiple pharmaceuticals are present in the same ecosystem, they can interact synergistically or antagonistically.5 Therefore, all the biotic and abiotic aspects of an ecosystem need to be considered when tackling pharmaceutical pollution.
To conclude, contamination of water bodies from pharmaceuticals has a variety of negative impacts. Antibiotic resistance and bioaccumulation threaten human health, while feminisation and pharmaceutical toxicity can damage bacterial, algal and fish populations. This can be prevented by investing in sanitation projects, using treated wastewater with caution, researching an area’s ecology before making economic development decisions, and promoting the use of naturally derived medicines3. How about that for something to think about when flushing the toilet?
1. Ortúzar M, Esterhuizen M, Olicón-Hernández DR, et al. Pharmaceutical pollution in aquatic environments: A concise review of environmental impacts and bioremediation systems. Frontiers in Microbiology; 13. Epub ahead of print 2022. DOI: 10.3389/fmicb.2022.869332.
2. Le TH, Truong T, Tran L-T, et al. Antibiotic resistance in the aquatic environments: the need for an interdisciplinary approach. International Journal of Environmental Science and Technology. Epub ahead of print 29 June 2022. DOI: 10.1007/s13762-022-04194-9.
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12. Teta C, Holbech B, Norrgren L, et al. Occurrence of oestrogenic pollutants and widespread feminisation of male tilapia in peri-urban dams in Bulawayo, Zimbabwe. African Journal of Aquatic Science 2018; 43: 17–26.