The use of antibiotics in the care of farm animals includes treating sick animals (therapeutic use), treating a group of animals when one is found to be sick (metaphylaxis), and giving medicine to prevent illness before it happens (prophylaxis). Antibiotics are important for treating diseases in both animals and humans, protecting animal health, and ensuring safe food. However, if used carelessly, this can lead to antibiotic resistance, which may harm human, animal, and environmental health.
Antibiotic use varies widely between countries. For example, some countries in Northern Europe use very small amounts of antibiotics for animals compared to humans. Globally, about 73% of antimicrobials (mostly antibiotics) are used in farm animals. A 2015 study also predicts that worldwide agricultural antibiotic use will increase by 67% between 2010 and 2030, mainly due to rising use in developing BRIC countries.
More antibiotic use is a concern because antibiotic resistance is seen as a major threat to human and animal health in the future. Higher levels of antibiotics or antibiotic-resistant bacteria in the environment could lead to more infections that are hard to treat. Bacterial diseases are a major cause of death, and a future without effective antibiotics would change how medicine is practiced for both humans and animals.
Rules and limits on antibiotic use in farm animals are being introduced worldwide. In 2017, the World Health Organization recommended reducing antibiotic use in animals raised for food. The European Union banned using antibiotics to promote animal growth in 2006. In the United States, the use of low doses of important antibiotics in animal feed and water for growth and better feed use became illegal on January 1, 2017, through changes made by the Food and Drug Administration (FDA). These changes asked drug companies to relabel their antibiotics voluntarily.
History
The 2018 book Pharming animals: a global history of antibiotics in food production (1935–2017) explains how antibiotics have been important in agriculture: "Since the 1930s, antibiotics have greatly influenced both human medicine and food production. On farms, in fishing operations, and during food processing, antibiotics were used to treat and prevent illness, improve how animals use food, and keep food safe. Their use spread quickly across nearly all areas of food production and was seen as progress on both sides of the Iron Curtain."
Before the 1930s, people knew about natural substances that could kill bacteria, but antibiotics became widely used during World War II to help treat injured soldiers. Records show that antibiotics were first used in farming near the end of the war, when penicillin was applied to treat mastitis in cows. At that time, milk was considered a product that could easily become contaminated with bacteria, and farmers used antibiotics to make their milk safer for people. Later, concerns shifted from bacteria in the product to leftover antibiotic traces from improper use.
The way antibiotics were used to treat and prevent disease in animals followed a path similar to their use in humans. They were used to treat illness, prevent disease in animals at risk, and improve overall health. In the late 1940s, studies showed that adding B12 made from a type of bacteria called Streptomyces aureofaciens (which is also used in human medicine) helped chicks gain weight faster and use less feed. Similar results were found in other animals, leading to the use of small amounts of antibiotics in animal feed to improve growth and food efficiency. As antibiotic costs dropped, this practice became common to produce more affordable animal protein for growing populations. This happened at the same time as farms grew larger and animals were kept in more confined spaces, making routine antibiotic use a cost-effective way to prevent disease. Veterinarians began to support using antibiotics for treating illness, preventing disease in at-risk animals, and improving health.
In the UK, the use of antibiotics in food production was banned in 2006. However, in 2017, 73% of all antibiotics sold worldwide were used in animals for food production.
Growth stimulation
In 1910 in the United States, a lack of meat led to protests and people refusing to buy certain products. After this and other food shortages, the public asked the government to study ways to keep food supplies steady. Since the 1900s, farms in the United States have had to raise more animals in a shorter time to meet new needs from consumers. In the 1940s, it was found that giving animals small amounts of antibiotics helped them use food more efficiently and grow faster. After this discovery, American Cyanamid shared research that showed using antibiotics to help animals grow was a common practice. By 2001, this practice had become so widespread that a report by the Union of Concerned Scientists showed that nearly 90% of all antimicrobial use in the United States was for purposes other than treating illnesses in animals. Certain antibiotics, when given in low doses, help animals produce more muscle or milk from the same amount of food and may increase growth by changing the bacteria in their stomachs. The drugs listed below can help improve how efficiently animals use food and increase weight gain, but they are no longer legally allowed for these purposes in the United States. Some of the drugs listed are ionophores, which are used to prevent certain parasites and are not classified as antibiotics in many countries; they have not been shown to increase the risk of antibiotic-resistant infections in humans.
The use of antibiotics to help animals grow has been considered a problem for these reasons:
- It is the biggest use of antimicrobials worldwide
- Using antibiotics in small amounts makes bacteria harder to treat
- All major types of antibiotics are used this way, which makes them less effective
- The bacteria affected can harm humans
Antibiotic resistance
Antibiotic resistance is a part of antimicrobial resistance (AMR), which happens when bacteria develop ways to survive the effects of antibiotics, medicines used to kill them. AMR includes resistance by all microbes, such as viruses, fungi, parasites, and bacteria, to medicines. This is a growing problem because antibiotic resistance is seen as a serious future threat to human health. AMR is increasing in both developed and developing countries. If this trend continues without proper action, it could lead to infectious diseases that cannot be treated with medicines or vaccines.
Bacteria that live in the same environment as antibiotic-producing organisms have developed old methods to survive harmful antibiotics. These bacteria can still grow even when faced with many environmental dangers, including antibiotics. This ability to change their genes is called genetic plasticity. Bacteria can change their genetic information in two ways to become more resistant: by changing their own genes or by gaining foreign DNA that carries resistance genes. The second method is the main reason for antibiotic-resistant bacteria in animals and humans. Bacteria can gain these foreign genes through three processes: transformation, transduction, and conjugation.
Transformation is when bacteria take in DNA from the environment. In transduction, bacteria receive DNA from viruses through bacteriophages. Conjugation is when bacteria transfer DNA directly from one cell to another, often using plasmids. Conjugative plasmids carry useful genes that help bacteria share them with others, improving their survival against antibiotics. This sharing can lead to bacteria that are resistant to multiple medicines.
Antibiotic resistance also happens naturally as a response to threats. Antibiotic-resistant bacteria have been found in untouched environments, such as the remains of woolly mammoths, polar ice caps, and deep underground caves. Resistance can occur if antibiotics are not strong enough to stop bacterial growth, which may cause bacteria to develop ways to survive. These bacteria can then reproduce and pass their resistance genes to future generations, increasing their numbers and causing infections that cannot be treated with antibiotics.
The World Health Organization (WHO) released an updated list in 2019 called "Critically Important Antimicrobials for Human Medicine, 6th revision." This list helps guide efforts to manage antimicrobial resistance by prioritizing strategies to protect the effectiveness of current medicines. It lists the highest priority antimicrobials as: third, fourth, and fifth generation cephalosporins; glycopeptides; macrolides and ketolides; polymyxins, including colistin; and quinolones, including fluoroquinolones.
The European Medicines Agency (EMA) Antimicrobial Advice Ad Hoc Expert Group (AMEG) also updated how antibiotics used in veterinary medicine are categorized based on their risk to human health and their importance for treating animal diseases. In Europe, Category A antibiotics are not used in food-producing animals. Category B antibiotics, called Highest Priority Critically Important Antibiotics, are only used as a last option. These include quinolones (like fluoroquinolones), third and fourth generation cephalosporins, and polymyxins, including colistin. A new Category C has been added for antibiotics that should be used only when no Category D antibiotics (which are used with caution) are effective. Category C includes macrolides and aminoglycosides, except spectinomycin, which remains in Category D.
There is little evidence that macrolide-resistant microbes move from animals to humans. Most harmful bacteria in humans come from humans themselves, with rare cases of transfer. Macrolides are also important for treating certain infections in animals, such as Mycoplasma in poultry, Lawsonia in pigs, respiratory infections in cattle, and lameness in sheep.
While human use of antibiotics is the main cause of antibiotic resistance in humans, people can also gain resistance genes from animals, including farm animals, pets, and wildlife. Much of this resistance comes from overusing antibiotics in farm animals. Three ways agricultural antibiotic use might lead to human disease are: 1) direct infection from resistant bacteria in animals; 2) transmission of resistant strains from livestock to humans after crossing species barriers; and 3) transfer of resistance genes from agriculture to human pathogens. While evidence exists for all three, the scale is limited or causality is unclear. As Chang et al. (2014) noted, "The topic of agricultural antibiotic use is complex. While many believe agricultural antibiotics are a major threat to human health, the problem may be overstated. There is no proof that agriculture is 'largely to blame' for rising resistance, and attention should remain on ensuring proper antibiotic use in all areas, especially in medicine."
The future of antibiotic use in livestock depends on cooperation between local governments and agricultural workers. The Livestock Biomass Conversion (LBC) method is a new technique to monitor antibiotic use in livestock and track how much is used. This model aims to help reduce antibiotic resistance in animals in a sustainable way.
Health issues
The use of antibiotics in livestock is a major health concern, showing how human, animal, and environmental health are connected. Using antibiotics in animals used for food helps create and spread antimicrobial resistance (AMR), which can harm many systems in the body.
Antibiotic-resistant bacteria from animals can spread to humans in several ways, such as eating contaminated food, touching animals directly, or being exposed to the environment. Bacteria like Salmonella, Campylobacter, and some types of Escherichia coli are linked to livestock and can cause infections in humans that are harder to treat. Resistance genes can move between bacteria, making it harder to control these infections because resistance can spread beyond certain animals or environments.
Antibiotics are used in livestock to treat, prevent, and control disease, and sometimes to help animals grow faster. While these practices can improve animal health and productivity, overuse or misuse of antibiotics can speed up the development of resistant bacteria in animals. This makes it harder for veterinarians to treat animal infections over time, which can harm animal welfare and food supplies.
The environment plays a key role in spreading AMR. Antibiotics and resistant bacteria can enter soil and water through animal waste, runoff from farms, and wastewater. These substances can stay in the environment, helping bacteria share resistance genes. These environmental sources can then spread resistant bacteria back to humans and animals.
To address antibiotic use in livestock, scientists, doctors, and farmers must work together using the One Health approach. Organizations like the World Health Organization (WHO), the Food and Agriculture Organization (FAO), and the World Organisation for Animal Health (WOAH) have called for reducing unnecessary antibiotic use in animals, better tracking of antibiotic use and resistance, and creating programs to use antibiotics responsibly. These efforts aim to protect public and environmental health while still supporting animal health and farming.
Direct contact with livestock can spread antibiotic-resistant bacteria. People who work with animals, such as farm workers, are at higher risk. For example, a study found that farm workers and neighbors near farms where chickens received antibiotics in their feed had higher levels of resistant bacteria. Manure can also carry resistant bacteria, such as Staphylococcus aureus, which can infect humans. In 2017, the WHO added methicillin-resistant S. aureus (MRSA) to its list of 12 priority antibiotic-resistant bacteria, highlighting the need for new treatments. Some strains of MRSA linked to livestock, called livestock-associated MRSA (LA-MRSA), can be found in air and soil samples, infecting workers. A study showed that livestock workers and veterinarians were 9.64 times more likely to have LA-MRSA infections than people not exposed to livestock. Although few people are infected, LA-MRSA is becoming more common and harder to treat, raising public health concerns. Surveillance of antibiotic resistance should expand, as limits on antibiotics for livestock are sometimes applied to pets without clear reasons.
Bacteria on or in food can expose humans to resistant bacteria. Livestock may carry resistant bacteria, or contamination can happen at any stage of food production, such as during slaughter, processing, storage, or handling. Common foodborne bacteria include Campylobacter, Salmonella, E. coli, and Listeria. Foods like dairy products, ground beef, and poultry often carry these bacteria. Studies have found resistant bacteria in retail meats like turkey, chicken, pork, and beef. When resistant bacteria are eaten and enter the gut, they can cause infections that are harder to treat if they resist common antibiotics. Some research has linked resistant infections to food-producing animals, but others have struggled to prove a direct connection, even when looking at resistance linked to plasmids. Simple steps like pasteurizing food, cooking meat thoroughly, using food preservation methods, and washing hands can help reduce the spread of harmful bacteria.
Environmental antibiotic resistance
Humans can be exposed to antibiotic resistance through the environment. Most antibiotics given to animals, about 70% or more, are not absorbed and are passed through urine and manure. This can harm animals, humans, and the environment. A study found that using manure as fertilizer increased antibiotic resistance genes (ARGs) in soil by four times. Poor storage or use of manure as fertilizer can spread resistant bacteria to crops and water. Urine and feces also transfer resistance through wastewater, especially on farms. Wind can carry bacteria from farms to nearby areas, especially in winter. Research shows that the amount of ARGs in manure decreases with time during composting. Composting can reduce antibiotics in manure by 20–99%, but some studies found that chlortetracycline, an antibiotic used in China, breaks down at different rates depending on the animal it was given to, and composting may not fully remove the antibiotic.
Mechanisms of resistance
Antimicrobial resistance happens when bacteria gain or share genetic material that helps them survive antibiotics. This transfer between bacteria is called horizontal gene transfer. One way this happens is through conjugation, where bacteria pass genetic material using plasmids directly from one cell to another. This does not require the bacteria to reproduce. Studies have found that antibiotics like Macrolides and Fluoroquinolones, used to treat pig infections, may increase the spread of resistance between species. Continued use of antibiotics can also increase genetic changes in bacteria.
Global positions on antibiotic use in farm animals
In 2017, the World Health Organization (WHO) advised reducing the use of antibiotics in animals raised for food. This recommendation was made because of the growing risk of bacteria that no longer respond to antibiotics. The WHO strongly encouraged limiting the use of antibiotics for improving animal growth or treating healthy animals. When antibiotics are needed for sick animals, they should be chosen based on how little risk they pose to human health. In October 2018, HSBC released a report stating that using antibiotics in meat production could have very harmful effects on humans. The report noted that many dairy and meat producers in Asia and the Americas had financial reasons to continue using large amounts of antibiotics, especially in environments where animals live in crowded or unclean conditions.
The World Organisation for Animal Health has recognized the importance of protecting antibiotics but believes a complete ban on their use in animal farming is not the right solution. A total ban could reduce the supply of protein in some parts of the world. When antibiotics are reduced or stopped through laws or voluntary actions, both animal health and economic outcomes may suffer. For example, farms that have reduced or stopped using antibiotics to meet consumer demand for "antibiotic-free" or "reared without antibiotics" products have shown negative effects on animal health and well-being. When antibiotics are used in small amounts to improve animal performance, growth, or feed efficiency, the cost of meat, eggs, and other animal products decreases. One major concern about limiting antibiotic use is the possible economic hardship for livestock and poultry producers, which could lead to higher prices for consumers. A study analyzing the economic cost of the FDA restricting all antibiotic use in animals estimated that such a restriction could cost consumers about $1.2 billion to $2.5 billion each year. To understand the full economic impact of limiting antibiotic use, the financial costs must be compared to the health benefits for people. Because it is hard to estimate the value of these health benefits, the study concluded that the total economic impact of restricting antibiotic use has not yet been fully determined.
Although measuring health benefits can be difficult, the economic effects of limiting antibiotic use in animals can also be assessed by looking at the economic costs of antibiotic resistance in humans, which is a major result of antibiotic use in animals. The World Health Organization identifies antibiotic resistance as a cause of longer hospital stays and higher medical costs. When infections can no longer be treated with standard antibiotics, more expensive treatments are needed. When illness lasts longer due to antibiotic resistance, the increased healthcare costs create a greater financial burden for families and societies. The Center for Infectious Disease Research and Policy estimates that antibiotic resistance-related healthcare costs in the United States are about $2.2 billion each year. While limiting antibiotics in animals causes significant economic costs, the effects of antibiotic resistance in humans, which are linked to antibiotic use in animals, also carry similar economic burdens.
Use and regulation by country
The use of medicines to treat diseases in animals that produce food is controlled in most countries. In some countries, only trained veterinary doctors can prescribe and give certain antibiotics. These rules were created to stop harmful chemicals from getting into food like meat, milk, eggs, and honey. When sick animals are treated with medicine, the medicine can stay in their bodies and enter the food they produce, such as meat or milk, unless a waiting period is followed. These waiting periods tell how long the medicine needs to leave the animal’s body to be safe. Scientists study each medicine to find out how long it stays in an animal’s body and how the body breaks it down. By using "drug withdrawal periods" before an animal is slaughtered or its milk or eggs are used, doctors and farmers ensure the food is safe. Some countries have banned or limited the use of antibiotics for growing animals or preventing disease caused by poor management. This is not because of medicine left in the food, but because of the risk of antibiotic resistance.
Brazil is the world’s largest exporter of beef. The government controls how antibiotics are used in cattle farming. Most cattle in Brazil are grass-fed and belong to the Nellore breed. Official numbers about how much antibiotics are used in Brazil are not published. Studies on farms in Brazil are the only way to estimate antibiotic use. Brazil created a National Action Plan to reduce antibiotic resistance and limit antibiotic use in animals. Not all antibiotics are banned in Brazil; they can still be used to treat sick animals, prevent disease in groups, or stop diseases before they start.
In Canada, the government ensures that food from animals does not contain harmful levels of antibiotics. Two agencies, Health Canada and the Canadian Food Inspection Agency (CFIA), enforce rules about medicines in food. They test food samples using three methods: monitoring, surveillance, and checking for compliance. One method is the Swab Test On Premises (STOP), which checks for antibiotics in animal kidneys.
China uses the most antibiotics of any country. Scientists have measured antibiotic use by testing water near farms and animal waste. In 2012, it was found that 38.5 million kilograms of antibiotics were used in China’s pig and chicken farming. Overuse of antibiotics has caused serious pollution in northern China’s soil and water. In 2012, a report said China’s rules for controlling antibiotics in farming were weak.
In the UK, a 5-year plan from 2013 to 2018 focused on reducing antibiotic resistance in both humans and animals. The UK and China started working together to fight antibiotic resistance. China created a national plan in 2017 to control antibiotic resistance in animals. This plan includes research and efforts to improve public health and food safety.
In 1999, the European Union started monitoring antibiotic resistance and planned to stop using antibiotics for growing animals by 2006. In 2006, the EU banned using antibiotics as growth agents. Germany used 1,734 tons of antibiotics for animals in 2011, compared to 800 tons for humans. Sweden banned all use of antibiotics for growing animals in 1986 and helped push for a similar ban in the EU. Sweden also uses antibiotics carefully, treating individual animals instead of groups. Denmark reduced antibiotic use by 60% since 1994. In the Netherlands, antibiotic use for treating diseases increased after a ban on using them for growing animals in 2006.
In 2011, the European Parliament passed a resolution calling for an end to using antibiotics to prevent disease in animals. A new rule was proposed in 2014 to limit antibiotics used for prevention and group treatment. This rule was agreed to in 2018 and will take effect in 2022.
In 2011, India introduced a plan to control antibiotic resistance. Some rules require animals to stop using antibiotics before their food is sold. A study in 2014 found antibiotic residues in chicken in India. This study said overuse of antibiotics in farming is causing people to become resistant to medicines, making some illnesses harder to treat. India has not set limits for antibiotic residues in chicken and needs to create better rules, including banning antibiotics for growing animals.
In 1999, New Zealand said it would not ban antibiotics in farming. In 2007, reports showed antibiotics were used in chicken farming. In 2017, New Zealand created a plan to fight antibiotic resistance. This plan has five goals that address resistance in both humans and animals. New Zealand has low antibiotic resistance in animals and plants because it uses few antibiotics in farming.
In 1998, researchers found that antibiotic use in farming was linked to high levels of antibiotic-resistant bacteria in Korea. In 2007, Korea used a lot of antibiotics in farming. In 2011, Korea banned using antibiotics for growing animals.
Like other European countries, antibiotic use for growing animals was banned in the UK in 2006. Less than one-third of all antibiotics sold in the UK are now used for treating or preventing disease in farmed animals.
Research into alternatives
Growing worry about antibiotic-resistant bacteria has caused scientists to search for other ways to care for animals instead of using antibiotics.
Probiotics are living bacteria, either one type or a mix of different types, that are being tested in animals to help improve their health and growth.
Prebiotics are types of carbohydrates that the body cannot digest. These carbohydrates are mostly made of short chains of simple sugars called oligosaccharides. The two most studied prebiotics are fructooligosaccharides (FOS) and mannanoligosaccharides (MOS). FOS has been tested in chicken food. MOS works by acting as a place for bacteria to attach, so the bacteria do not stick to the intestine and are removed from the body.
Bacteriophages are viruses that can infect bacteria. They are found in many places where bacteria live and have also been studied as possible solutions.
In one study, scientists found that using probiotics, competitive exclusion (a method that stops harmful bacteria from growing), enzymes, immunomodulators (substances that help control the immune system), and organic acids can stop the spread of bacteria and may replace antibiotics. Another group of researchers tested bacteriocins (natural substances that kill bacteria), antimicrobial peptides (small proteins that fight bacteria), and bacteriophages to control infections. Although more research is needed, these alternatives have shown promise in helping animals fight bacterial infections.
Other options include ways to keep animals healthier and reduce the need for antibiotics. These include improving living conditions for animals, giving them better food to strengthen their natural immunity, increasing biosecurity (measures to prevent disease), using better management and hygiene practices, and making sure vaccinations are used properly.