Farm animals and antibiotic resistance
Recent headline figures have highlighted that we are about to reach the point where more antibiotics will be consumed worldwide by livestock than humans...
This rate of consumption is fuelling concerns over the link between antibiotic use in livestock and antibiotic resistance in humans. But why are antibiotics being used in prodigious quatities on farms, and what are the implications for antibiotic resistance?
How are antibiotics used in farm animals?
There are three main ways in which antibiotics can be used in livestock farming: treatment, disease prevention and growth promotion. Treatment involves giving antibiotics to animals that are sick with bacterial infections, in much the same way that antibiotics are used to treat people with bacterial infections. This is an important part of maintaining good animal welfare, and would generally be classified as responsible use.
Prophylaxis means using antibiotics to prevent animals from picking up bacterial infections in the first place. Farmers might use antibiotics this way if they know that their animals are particularly at risk from infection. Many people would argue, however, that prophylactic antibiotic treatment is used to compensate for other problems, such as poor hygiene. But owing to the growing human population and increased consumer demand for cheap food, farmers are being urged to drive down costs while maintaining productivity. Regrettably, this often means that farming systems become more intensive, with large numbers of animals being kept close together. And it's this high density living that increases the transmission of infectious disease, and necessitates the use of antibiotics.
Growth promotion involves adding low doses of antibiotics to the feed of all animals in a group to make them more productive. Growth promotion is the most controversial use of antibiotics in livestock and has been banned in about half the countries that report their antibiotic usage to the World Organisation for Animal Health (OIE). One of the reasons that the use of antibiotics as growth promoters is problematic is because antibiotic resistance is more likely to develop when bacteria are exposed only to low doses of an antibiotic for an extended period of time. In fact, the dangers of using antibiotics as growth promoters, at least in terms of antibiotic resistance, were highlighted as early as 1969. The EU banned the practice in 2006 and the USA followed suit in 2017 but the ban there is only partial and related to the use of antibiotics considered important for human health; other growth promoters are still permitted.
What quantity of antibiotics are we actually using in livestock?
The total estimated global usage of antibiotics in food animals in 2013 was 131,109 tons. This sounds like a lot, but, as a figure on its own, it is not particularly informative. Antibiotic usage figures are most often reported as milligrams per kilogram population correction unit or mg/PCU. This is a way of adjusting the amount of antibiotic used to account for the number of animals in the system and their weight at the time of treatment. For example, 50mg/PCU means that, on average, 50mg of antibiotic was used for every kg of bodyweight at time of treatment. This measure allows us to more easily compare different countries, time points and farming sectors.
There are differences in antibiotic use between different farming sectors. In both the UK and Denmark, the pig industry is the highest user of antibiotics. This is partly due to the nature of pig farming: because it is more intensive, there is a greater risk of infectious disease, and because animals are often housed together in groups, medications are often added to feed or water in order to treat the whole group rather than individuals. In contrast, in sheep, which are traditionally farmed more extensively in the UK, medications tend to be given to individual animals, and infection pressures are lower.
Antibiotic usage also varies by country. Of the countries that report use to the OIE, China is the highest user of antibiotics in livestock, at 318mg/PCU, while Norway, at 8mg/PCU, is the lowest. There is clearly a lot of variability here, and the reasons for it are complex and multifactorial but they are likely to include the types of farming system used, the hygiene and infection control standards of those systems, and the amount of regulation of antibiotic usage. A consistent trend, however, is that antibiotic usage is particularly problematic in developing countries. Again, this is complex but involves lack of regulation (antibiotics are often sold by pharmacists or middle men, rather than through veterinary prescriptions), variable quality of products, and a lack of education about responsible antibiotic use among stakeholders.
How does antibiotic use in animals affect humans?
We know that use of antibiotics in animals leads to an increased number of antibiotic resistant bacteria in those animals. There are then many ways that these bacteria can theoretically be transferred to the environment and to humans. Studies have shown that antibiotic resistant bacteria are present on meat, in soil and dust around farms, in animal manure spread on fields, and in water courses. It doesn’t necessarily need to be bacteria that cause disease in humans for this to be a problem. Antibiotic resistance is encoded and carried on small mobile pieces of DNA called plasmids, which bacteria routinely swap among themselves, transferring antibiotic resistance instructions as they do so. This means that a harmless bacterium could transfer resistance to one that causes disease at any point in the chain linking animals and humans.
Is there evidence that the transfer of resistant bacteria from animals to humans actually happens?
By studying the occurrence of specific strains of antibiotic resistant bacteria in humans and animals, we can see evidence for a link. People who work closely with animals tend to have higher levels of strains of antibiotic resistant bacteria found in animals compared to the general human population. A study in 2013 showed that people who live near pig farms, or in areas where manure is spread, have higher levels of MRSA infections than the general population. However, this evidence demonstrates an association rather than causality, and the strains of bacteria found in these people did not include the most common strain of MRSA usually found in pigs. On a larger scale, there is a general trend that countries with higher antibiotic use in livestock have higher levels of antibiotic resistant bacteria in people, for example countries in southern and eastern Europe tend to have higher levels of both antibiotic use and resistance than those in northern Europe. However, this is complicated because there are many antibiotics, many species of bacteria and many types of livestock, and other factors such as poor infection control practices can contribute to levels of resistance.
So it's not a simple question; we know that antibiotic resistant bacteria can be transferred between animals and humans, but quantifying this is extremely difficult, partly because it is so variable and partly because it is not the only driver for the presence of antibiotic resistant bacteria in people. The general consensus is that it is better to address antibiotic use in animals now, rather than wait until we have managed to quantify its contribution only to realise that it is too late.
What is the best way to address the problem?
The obvious answer is to reduce the amount of antibiotic we use in livestock, and steps are already being taken to address this. The first stage is to get more detailed evidence about how much antibiotic we use, allowing us to then set targets for reducing that use. Because of the variability in antibiotic use described above, this needs to be country and sector specific. In 2015 the 180 member countries of the OIE made a commitment to tackle antibiotic resistance and to develop national action plans. In the UK, the government has set targets for antibiotic use, and a task force has been set up to monitor this and make recommendations for compliance in each sector. Substantial reductions in antibiotic use have already been seen across all UK livestock sectors.
As discussed earlier, there are multiple ways in which antibiotics can be used in livestock. Therefore, one part of reducing usage is to focus on encouraging responsible use of antibiotics. This requires education of those dispensing and using antibiotics, particularly in countries where there aren’t regulations in place to limit antibiotic sales. A concern for farmers is that if they cannot use antibiotics, their livestock will be at risk of disease, and therefore efforts need to be focused on improving disease control measures and the development and deployment of vaccines to prevent disease. If we can minimise the risk of infectious disease, our antibiotic use will decrease and the welfare and productivity of our livestock will improve.
But is reducing antibiotic use in livestock enough? A recent study using mathematical modelling showed that focusing on reducing the transmission of resistant bacteria between animals and humans is more important than simply reducing antibiotic use in animals. Close collaboration between human and animal sectors will be crucial to achieve this. Since 2010 the Tripartite Alliance (comprising the World Health Organisation, the OIE and the Food and Agriculture Organisation of the United Nations) has made antimicrobial resistance one of its three priority issues. It isn’t too late to tackle antibiotic resistance, but the clock is ticking, and it will require commitment from all parties to achieve this vital goal...