A new modeling study highlights the potential threat that human antibiotic use may pose to global waterways, researchers reported yesterday in the journal PNAS Nexus.
The presence of antibiotic residues in the environment, and the impact that those residues may have on aquatic microbial diversity and antimicrobial resistance (AMR), has become a growing concern among scientists and public health experts in recent years.
Much of the focus has been on the antibiotic manufacturing process, which is known to release antibiotic-laced wastewater directly into nearby waterways. That has resulted in efforts to monitor and limit pollution from antibiotic manufacturing sites.
But since antibiotics consumed by people aren't fully metabolized, wastewaters from households and hospitals are also potential sources of antibiotic contamination. And while wastewater treatment plants can remove some antibiotic residues before the treated water is released into surface waters, they can't remove all of them.
Furthermore, many parts of the world have limited or no wastewater treatment. With human antibiotic use projected to rise 200% by 2030, people could potentially be a significant source of antibiotic pollution.
Modeling can provide initial global outlook
Using a model developed to predict the distribution and fate of chemical pollutants from wastewater treatment plants and untreated wastewater into the aquatic environment, a team of researchers from McGill University in Montreal and the One Health Trust calculated how much of the estimated annual human consumption of the 40 most used antibiotics ends up in rivers and oceans.
Their hope was to get a clearer picture of the problem and pinpoint regions where environmental management strategies and regulations might be needed.
"Given the potential risks to the environment and human health due to antibiotics exposure, it is crucial to assess the prevalence of these compounds in wastewaters and surface waters globally," the researchers wrote. "In the absence of adequate large-scale monitoring schemes, modeling approaches can provide an initial global outlook on the possible risks caused by a wide range of chemical substances, including antibiotics."
Nearly a third of consumed antibiotics end up in rivers
Based on global antibiotic sales data for 2012 through 2015, the researchers estimated that people consume roughly 29,200 tonnes (32,187 tons) of the 40 most used antibiotics each year and excrete 20,500 tonnes (22,597 tons) after metabolism.
Of this amount, 8,500 tonnes (9,370 tons; 29%) are discharged into surface waters after treatment or natural attenuation in soil, and 3,300 tonnes (3,638 tons; 11%) reach the world's oceans or inland sinks (such as lakes or reservoirs) via rivers after lake removal and instream decay.
In most rivers, the total amounts of antibiotics residue translate into very low concentrations that fall below assessed levels of potential environmental impact. But during low-flow conditions, when there is less dilution, the researchers estimated that 6 million kilometers (km; 3.7 million miles) of rivers have concentrations of antibiotics that exceed the threshold of high risk. India, Pakistan, and countries in Southeast Asia were found to be the most heavily impacted.
Given the potential risks to the environment and human health due to antibiotics exposure, it is crucial to assess the prevalence of these compounds in wastewaters and surface waters globally.
In the 3.8 million km (2.7 million miles) of river with at least one antibiotic presenting high environmental risk at low-flow conditions, amoxicillin, ceftriaxone, and cefixime were the main contributors to high exposure levels.
The researchers also estimated that 750 million people—roughly 10% of the global population—are exposed to the top 1% of surface waters with the highest cumulative concentrations of antibiotics.
"These populations are thus potentially subjected to chronic antibiotic intake at harmful levels if surface waters are used for direct human consumption," the study authors wrote.
Problem likely much worse than model indicates
The authors say the findings suggest an "urgent need" for further research on the environmental fate and impact of the antibiotics that their model predicts are most prevalent in surface waters, particularly those that pose a high potential risk to aquatic ecosystems and human health.
And they note that the problem is likely much worse, since their model doesn't account for residues from antibiotics used in food-producing animals—many of which are also used in human medicine—or from pharmaceutical manufacturing.
"Of course, while it would be desirable to account for these other sources of antibiotic contamination, the modeling of such sources is not currently possible due to the lack of data both globally and regionally with respect to local manufacturing and animal husbandry practices, waste management practices, and characteristics that govern the transport, degradation, and emissions of antibiotic residues into surface waters," they wrote.
"Nevertheless, by establishing a baseline level of contamination arising from human consumption, we aim to pave the way for more comprehensive studies in the future, including the modeling of other antibiotic sources," they added.
In the meantime, they said, the findings highlight the need to develop and implement appropriate wastewater management plans, particularly in areas of the world where they found the highest exposure levels. They also call for more appropriate antibiotic use, noting that the most high-risk areas identified by their model are countries where antibiotics can be easily obtained without a prescription.
"As such, improved healthcare practices that ensure the appropriate use of antibiotics are recommended," they wrote.
