Antimicrobial resistance (AMR) in aquaculture has been a longstanding issue that has only recently garnered significant attention, emerging as a central concern within the scientific community. In fact, AMR is now widely acknowledged as a critical global public health threat (Mesalhy Aly and Albutti 2014). Aquaculture, with its variety of farm sizes and cultivation methods, presents challenges in collecting comprehensive global data on antimicrobial use, which makes it difficult to draw clear conclusions regarding the causes and consequences of AMR. However, despite these challenges, research in this area continues to grow, and this article aims to introduce the concept of AMR to non-specialists.What is antimicrobial resistance (AMR)?
To begin with, let’s define AMR. The term “antimicrobial” refers to the chemical agents used to combat diseases caused by microbial activities. When we talk about AMR, we are addressing bacteria that have developed resistance to the chemicals originally designed to eliminate them. In this article, we focus on these tiny organisms that have essentially decided to fight back. Instead of surrendering to antimicrobial agents, they have adapted by altering their genetic material, effectively arming themselves with new capabilities to counter the chemical defenses.
When a fish tank gets infected, and all individuals are at risk of being killed, it is common practice to use antibiotics to cure the disease. However, using antibiotics in large quantities or at high frequency without first implementing proper prevention measures can lead to significant issues. Moreover, treating an entire population when only a few individuals are infected exacerbates the problem.
The bacteria, constantly under attack, begin to adapt. Over time, they learn from their aggressor, observing and understanding how to fight back. Eventually, these bacteria pass on their knowledge, teaching their counterparts how to resist the antibiotics. This adaptation process is the driving force behind antimicrobial resistance.
A selective pressure occurs, with resistant bacteria surviving and multiplying, passing to the next generation the weapons (gene mutation) that allow them to resist the antibiotics. This phenomenon is called vertical gene transfer. Another possibility for bacteria to become resistant is to receive weapons from their neighbors. A resistant bacteria can ‘teach’ a non-resistant bacteria how to fight. The following figure taken from Lundborg and Tamhankar (2017) illustrates this phenomenon that we call horizontal gene transfer.
Figure 1: How a plasmid, which might contain antibiotic genes, gets copied from one bacterium to another
Once pathogens develop resistance to antimicrobials, it becomes increasingly difficult to treat the associated diseases. This is the most direct consequence of the development of resistant bacteria: it takes longer to cure diseases, and in some cases, it might not even be possible to find an effective treatment. This means production loss, which leads to bad economic consequences.
When human activities drive the development of resistant bacteria, we not only risk running out of solutions for certain strains, but we also create a broader threat. These resistant bacteria don’t stay confined to their original environment. Instead, they seek to spread. At the first opportunity, they move into natural watercourses, further amplifying the danger.
Resistant bacteria are much more likely to emerge near aquaculture facilities because the selective pressure is artificially increased by human activities. There are now growing concerns around the globalization of AMR, and it becomes clear that some human pathogens have probably originated in aquatic bacteria (Cabello et al. 2013). The development of resistant bacteria is nothing new in our knowledge of evolution. It is however worrying how fast new resistant strains are developing (Santos and Ramos 2018).
The primary role of antibiotics is to control the growth of harmful bacteria in fish, allowing the individual’s immune system to respond effectively. However, before resorting to antibiotics, it is crucial to first identify and eliminate any sources of stress. Additionally, selecting the appropriate antibiotic is a key step in the process. Ideally, a fish health specialist should conduct sensitivity tests by applying antibiotics to a sample of infected fish (Mesalhy Aly and Albutti 2014). While probiotics are being explored as an alternative to antibiotics, caution is necessary, as these bacteria can also develop resistance over time (Watts et al. 2017).
There is a clear link between animal welfare and antimicrobial resistance. Specifically, stress plays a crucial role in weakening immunity. Consequently, factors that negatively impact animal welfare, such as poor water parameter control, inadequate feed quality, and unsanitary enclosures, directly lead to higher infection risks. This, in turn, results in the overuse of antibiotics. Therefore, it is essential to prioritize proper aquaculture practices, such as implementing effective biosecurity measures and ensuring high-quality water and feed, which can prevent disease outbreaks (Preena et al. 2020).
Figure 2: Prophylaxis may help a shrimp farm to reduce issues linked to AMR
Around the world, the use of antibiotics differs greatly from one country to another. A major variable influencing antibiotic misuse is how strictly governmental organizations regulate the practices. Europe, North America, and Japan have the strictest regulations on antibiotic use (Watts et al. 2017). However, many countries still face significant challenges in achieving this level of regulation. Similar to fisheries management, addressing this pressing issue requires a clear and sustained commitment from governmental entities.
In parallel, the work of certification bodies can help to translate a global consumers’ demand into an actionable list of requirements. Strong partnerships between certification bodies, governmental organizations, and technology companies are the way forward to breach the gap of extensive data collection, effective data analysis, best practices advocacy, better regulation, and stronger verification.
The scientific community is increasingly recognizing the concept of One Health. It emphasizes that the overuse of antibiotics is a global issue, one that cannot be viewed solely from the lens of agriculture or aquaculture (Cabello et al. 2016). To address this challenge, we must shift our focus to the broader global picture. Our world is interconnected, meaning that anything we introduce into the water can eventually end up on our plates. To combat antimicrobial resistance (AMR), we need to promote cooperation and coordinated planning on a global scale.
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