Thames typical of superbug-breeding rivers, says new study

By Dr. Liji Thomas, MDSep 5 2019

The River Thames is rapidly becoming a breeding pool for antibiotic-resistant bacteria, according to a new study, which underlines the urgent need to cut down the amount of antibiotics currently being discharge into the river by at least 80% to arrest this trend.

Just a week prior to the publication of the study, Dame Sally Davies, head of England’s medical service, warned that superbugs were the chief threat to human survival, costing 10 million lives a year if not more. These superbugs are spreading through the human environment, because of antibiotics released directly or indirectly to the earth, air and water.

River Thames and London from above. Image Credit: Zoltan Major / Shutterstock

Antibiotics and the environment

The direct application of antibiotics to the water or plants in fish and fruit production processes, and their indirect release via effluents from animal farms, sewage and pharmaceuticals, account for the major part of environmental water exposure to antibiotics.

Antibiotics used by humans mostly end up in the public sewers as a constituent of urine and feces. At least 50% finally ends in the rivers where the effluent is discharged. Since the rivers also contain bacteria from body secretions and wastes, a mixture of drugs and bacteria is formed. This allows multiple genes that confer antibiotic resistance to be transferred between bacterial strains, either as a result of mutations or by recombination of bacterial chromosomes. Either way, strains that contain powerful combinations of antibiotic-resistant genes will be formed. This leads to high antibiotic levels in rivers that receive such sewage, enough to select for genes that make the bacterial population resistant to these antibiotics. The minimum antibiotic concentration needed for antibiotic resistance to be selected for is called the Predicted No Effect Concentration (PNEC).

However, since antibiotics in such settings come as a package with the fecal bacteria from the human gut, that already show antibiotic resistance, scientists sought a way to tease out the true contribution of these antibiotics to new antibiotic resistance arising in the river itself.

How was the study done?

The study, carried out at the Centre for Ecology & Hydrology (CEH), used a modeling tool on the River Thames to identify how antibiotic prescriptions would affect the bacteria in a river system. The study looked at prescriptions for two antibiotic classes, namely, the macrolides and the fluoroquinolones. The Thames was used as its basin is the most thickly populated catchment area in the UK, and has the lowest dilution factor, which makes it a “realistic worst-case scenario.”

The macrolides are generally prescribed for respiratory and sexually transmitted infection. Typified by erythromycin and azithromycin, they are taken for infections like pneumonia and chlamydial infection of the reproductive tract. They are remarkable for their stability to degradation, which means they remain active even after being excreted and as part of the effluent that reaches the river. Fluoroquinolones are represented by drugs like ciprofloxacin and levofloxacin. Penicillins, in contrast, are quickly broken down and not found in river water.

About 30% and 60% of macrolide and fluoroquinolone antibiotics, respectively, are thought to be excreted in urine and feces after ingestion. PNECs for fluoroquinolones ranged from 0.064 to 0.5 μg/L and for macrolides 0.25 to 1 μg/L. The scientists used the model to arrive at a predicted environmental concentration (PEC) for macrolides and fluoroquinolones. When these exceeded the PNEC for the most powerful drug in each class, namely, azithromycin/clarithromycin and ciprofloxacin for macrolides and fluoroquinolones respectively, they labeled that part of the river as being ‘at risk’ for resistance selection. On the other hand, a ‘critical risk’ of resistance selection was found if the PEC was above the PNEC for the least potent antibiotic within each class (erythromycin and norfloxacin/ofloxacin respectively).

What does the study show?

The results show that in about 75% of the Thames’ catchment area, the effluent discharge contains antibiotics at concentrations that are high enough to encourage the development of resistant bacteria. About 65% and 75% of the river’s catchment area is either at risk or critical for resistance to macrolides and fluoroquinolones, respectively. In fact, almost 120 km of the Thames, which is about 8% of the total length, has macrolides at concentrations 5 times the ‘at risk’ PNEC. This figure is doubled in the case of fluoroquinolones. When it comes to ‘critical’ PNEC, 20% of the catchment length in the case of macrolides, and 5% with fluoroquinolones, meets the definition.

Earlier studies have shown that antibiotic-resistant bacteria are living in the River Thames. Possible climate change, older populations, intense storms and new sources of run off all need to be factored into future studies to provide a more accurate risk prediction tool.

What can be done?

Scientists suggest several possible solutions:

• Stop prescribing antibiotics unless they are really required; that is, they will actually treat the infection, and are not continued even when they could be safely stopped. The need is to reduce overall macrolide and fluoroquinolone prescriptions by about 75% and 85% respectively to achieve the goal of preventing the emergence of antibiotic resistant bacteria in the river. However, taking steps to achieve the following measures as well could alleviate this demand to some extent.
• Take preventive steps against infection: earlier diagnosis and better immunization coverage, as well as preventing infection spread in hospitals and other healthcare facilities
• Develop better ways to treat wastewater which will ensure that the sewage effluent doesn’t contain bacteria or antibiotics, as well as other substances like metals and biocides which co-select for antibiotic resistance.

Following a concerted effort to prevent inappropriate antibiotic prescribing, the total amount of antibiotics used in both primary and secondary levels of care has been reduced by about 6% from 2014 levels. However, people in the UK still have a much higher per capita consumption of prescribed antibiotics compared to European countries. In fact, antibiotic usage in the Netherlands is only half of that in England, because of much stricter regulatory control and hospital hygiene control which has resulted in lower rates of antibiotic resistance.

The national water body Thames Water has taken notice of the increasing danger posed by antibiotic resistance in water bodies, and says it will work on finding better ways to manage it. But it’s not just about rivers; rather, as researcher Andrew Singer says, “Rivers are a 'reservoir' for antibiotic-resistant bacteria which can quickly spread to people via water, soil, air, food and animals. Our beaches offer a similar risk. It has been shown that surfers are four times more likely to carry drug-resistant bacteria than non-surfers. Environmental pollution from drugs and bugs is a serious problem that we need to find solutions to.”

The study was published in the journal PLOS ONE on September 4, 2019.