Water signals

Antimicrobial resistance (AMR) is the ability of microorganisms that cause infection, such as bacteria and viruses, to become resistant to antimicrobial medicines. While antimicrobial-resistant pathogens and genes emerge naturally, the excessive or inappropriate use of antimicrobials in both animals and humans has led to increasing rates of AMR across the globe. In Europe, both humans and farmed animals have been consuming fewer antibiotics since 2014 (OECD et al., 2022); however, levels of antibiotic resistance in bacteria remain high (WHO Europe and ECDC, 2021). It has been estimated that more than 670,000 infections and approximately 33,000 deaths occur in EU and European Economic Area countries every year due to bacteria resistant to antibiotics (Cassini et al., 2019). The cost to these countries’ health systems is around EUR1.1 billion each year (OECD et al., 2022).

Antimicrobial resistance and freshwater environments: the role of wastewater treatment plants

Water bodies are critical for the emergence and spread of AMR (Carvalho and Santos, 2016). Antimicrobial residues and antimicrobial resistance genes from different sources enter aquatic systems via wastewater or through run-off to surface water and leaching to groundwater (Figure 1). Here, they can drive the development and spread of AMR among the microbial communities with which they interact. Biocides and metals released in water bodies can also drive AMR (Sanseverino et al., 2018). In addition, recent evidence suggests that microplastic pollution may facilitate the selection and transfer of resistant genes (Niegowska et al., 2021).

Figure 1. Overview of human activities producing AMR and main AMR pathways in aquatic environmental compartments

Notes: The pathways are shown as red arrows. For each of the human activity sectors represented in the figure, a list is provided of the main AMR-driving substances (i.e. antimicrobial residues, antimicrobial resistance genes, heavy metals, biocides, inorganic fertilisers) emitted by that sector.

Sources: Based on Singer et al. (2016); Sanseverino et al. (2018); and EFSA (2021).

Click here to view the figure enlarged

Although antibiotic-resistant bacteria and gene levels can be significantly reduced by wastewater treatment processes, some antimicrobial resistance genes might remain or even increase in treated water (Manaia et al., 2018). Treated sewage sludge represents a further reservoir of AMR, which can increase the abundance of antimicrobial resistance genes in soil when applied on farmland (Chen et al., 2016). At present, there is no legal obligation in the EU to monitor and remove antimicrobial residues or antimicrobial resistance genes. Although the specific concentrations of antimicrobials, chemical mixtures and environmental conditions that are more likely to accelerate the emergence of AMR remain poorly understood (Niegowska et al., 2021), a series of ‘safe’ target concentrations has been proposed recently in scientific literature (Bengtsson-Palme and Larsson, 2016; Tell et al., 2019). In a monitoring study, these concentrations for many selected antimicrobials were exceeded in at least some European rivers (Wilkinson et al., 2021).

Antimicrobial resistance in the environment and the EU’s zero pollution ambition

To address the role of wastewater treatment plants and water bodies as potential hot spots for AMR, the EU and its Member States should follow the lifecycle management approach endorsed by the strategic approach to pharmaceuticals in the environment and the zero pollution action plan. First, this entails acting at source to reduce the inappropriate use of antimicrobials, as well as ensuring that they are disposed of and collected properly (OECD, 2019). Second, it means investing in advanced wastewater treatment technologies, although this can be costly and should always be seen as a complement to measures taken at source (Swedish Environmental Protection Agency, 2018). Lastly, it necessitates filling outstanding knowledge gaps. Approaches to do so include introducing regular monitoring in wastewater treatment plants and water bodies, understanding the concentrations of antimicrobials and mixtures that are more likely to induce AMR, and clarifying how AMR spreads in humans and farmed animals (Singer et al., 2016; Niegowska et al., 2021). For example, according to the European Commission, increased monitoring costs in the EU’s larger wastewater treatment facilities would lead to important benefits by reducing the healthcare and occupational costs arising from AMR (EC, 2022).

Cyanobacteria — commonly known as ‘blue-green algae’ — are ubiquitous bacteria found in marine waters and freshwater. Mostly known for their prominent, colourful blooms in lakes and rivers during warm summer months, they can act as a key indicator of excess nutrient load in a water body. An abundance of cyanobacterial biomass can also contribute to a problematic degree of cloudiness in the water.

Cyanobacteria are found in the upper layer of a water body, where sunlight can enter (Figure 1). This can happen in deep lakes or in the entire water column in shallow lakes, depending on the species and the environmental conditions. With their ability to reproduce massively under favourable environmental conditions, cyanobacteria can form harmful algal blooms with possible impacts not only on the environment but also on human health.

Figure 1. Cyanobacterial bloom

© Ingrid Chorus

Nutrients and cyanobacterial proliferation

High nutrient loads cause eutrophication. The extent of cyanobacterial abundance correlates with the level of human-made releases to water bodies — especially high loads of phosphorus and nitrogen delivered through wastewaters and agriculture. Accompanied by favourable conditions — high air temperature, sufficient light and low circulation in the water body (stagnation) — massive cyanobacteria growth is more than likely.

Cyanobacteria and health

Contact with cyanobacteria can cause mild symptoms such as gastroenteritis or skin irritation. Some cyanobacteria are even able to produce hepato- and neurotoxins (cyanotoxins), which can be lethal when ingested in high doses (Chorus and Welker, 2021). Toxic cyanobacteria in surface water, used for recreational purposes or as drinking water, is already regulated in many countries.

In bathing waters, people can be exposed to cyanobacterial bloom and possibly cyanotoxins; this is because it is not possible to distinguish toxic from non-toxic cyanobacteria without further analyses. With this in mind, people should not bathe in very cloudy water. The EU Bathing Water Directive (2006/7/EC) (EU, 2006) states that if a cyanobacterial proliferation carrying a presumed health risk occurs at a bathing water site, the area should be closed off.

In terms of drinking water, the EU Drinking Water Directive requires that a common and widespread cyanotoxin (microcystin-LR) is measured when a cyanobacterial bloom is detected in the drinking water reservoir (EU, 2020).

Cyanobacterial blooms also affect other mammals, and they can be fatal to livestock and domestic animals such as dogs.

Managing and control measures

Although the Urban Waste Water Treatment Directive (UWWTD) and the Water Framework Directive have reduced phosphorus and nitrogen discharge into water bodies, loads are often still too high to combat eutrophication. To compensate for the effects of climate change — for example, longer periods of high air temperature during the summer — efforts to reduce external nutrient loads in water bodies must be increased.

Ultimately, the best way to protect citizens from cyanotoxins is prevention — action before cyanobacterial proliferation occurs. Reducing anthropogenic nutrient inputs should be the primary focus when considering how to mitigate cyanobacteria in surface waters.

In Denmark, data on drinking water quality are compiled in a publicly available database called Jupiter. 100% of the country’s public water supply is sourced from groundwater. A review of available pesticide data from 2002 to 2019 (Voutchkova et al., 2021) identified the following key findings:

• 0.5% of the samples contained a quantified pesticide.
• 29% of drinking water plants identified pesticides at least once between 2002 and 2019. However, for the more recent period 2015-2019, this reduced to 21%.
• It is estimated that 56% of Danish households were potentially exposed to pesticides in drinking water between 2002 and 2019, compared with 41% in 2015-2019.

In Ireland, the supply source of drinking water is quite different from that in Denmark: 83% of drinking water comes from surface water and only 17% is from groundwater (Irish Water, 2021). The most recent monitoring report (EPA, 2021) identified that 33 drinking water supplies (i.e. 4.5% out of approximately 740 supplies) exceeded the standard for pesticides.

A study in the Netherlands and Belgium on 24 recently authorised pesticides excluded from routine monitoring programmes examined their presence in drinking water sources (Sjerps et al., 2019). Fifteen of these newer pesticides were detected, seven of which were at levels above the drinking water quality standard.

The latest report on pesticides in drinking water in France (Ministère des Solidarités et de la Santé, 2021) indicates that over 94% of the population (62.1 million people) is supplied with drinking water that fully complies with the standards for pesticides. However, 3.9 million people live in areas where at least one incidence of drinking water that fails to comply with the pesticides standards was identified in 2020.

Based on the risk assessment approach in France, only 0.02% of the population (12,000 people) was subject to restrictions on water use because of elevated pesticide concentrations.