The use of antibiotics in the care of farm animals includes treating sick animals (therapeutic use), treating a group of animals when at least one has a confirmed infection (metaphylaxis), and giving antibiotics to prevent illness before it occurs (prophylaxis). Antibiotics are important for treating diseases in both animals and humans, protecting animal health, and ensuring safe food. However, if used carelessly, antibiotics can lead to antibiotic resistance, which harms human, animal, and environmental health.

Antibiotic use varies widely between countries. For example, some Northern European countries use very few antibiotics for animals compared to humans. Globally, about 73% of antibiotics are used in farm animals. A 2015 study also predicts that worldwide use of antibiotics in agriculture will increase by 67% between 2010 and 2030, mainly due to rising use in developing BRIC countries.

Higher antibiotic use is a growing concern because antibiotic resistance is seen as a major threat to human and animal health. More antibiotics or 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.

Laws and rules to limit 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 Food and Drug Administration made it illegal in 2017 to use sub-therapeutic doses of important antibiotics in animal feed or water for growth promotion, requiring drug companies to update labels 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 had a big impact on human medicine and on food production. On farms, in fishing and whaling operations, and in food processing and aquaculture, antibiotics were used to treat and prevent disease, improve how animals convert food into body weight, and keep food safe. Their use spread quickly across nearly all areas of food production and processing and was seen as a sign of progress in both the Eastern and Western blocs during the Cold War."

Before antibiotics, people knew about natural antibacterial substances, but antibiotics became widely used during World War II to treat injured soldiers. Records show that antibiotics were first used in farming near the end of the war, in the form of penicillin applied to treat bovine mastitis in cows. At that time, milk was considered a food product that was easily contaminated by bacteria, and farmers welcomed antibiotics to help make milk safer for consumers. Later, concerns shifted from the amount of bacteria in food to the leftover antibiotic traces that might remain after treatment.

The use of antibiotics to treat and prevent disease in animals followed a path similar to their use in human medicine. Antibiotics were used to treat illness, prevent disease in groups of animals, and improve the health of animal populations. Farmers also used targeted treatments for animals at higher risk of disease. In the late 1940s, studies found that adding B12 made from a type of antibiotic (Streptomyces aureofaciens) to chick feed helped the birds gain weight faster and use less food to reach market size. Similar results were later found in other farm animals. As antibiotics became cheaper, they were added in small amounts to animal feed to help increase the production of affordable animal protein to meet the needs of a growing population after the war. This happened at the same time that farms became larger and animals were kept in more confined spaces, making routine antibiotic use the most cost-effective way to prevent disease. Veterinary medicine began to widely accept the use of antibiotics for treating disease, preventing illness in groups, and using them strategically for prevention. Routine antibiotic use for improving growth and preventing disease also increased.

In the UK, the use of antibiotics in food production has been banned since 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 caused people to protest and refuse to buy 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 short time to meet new needs from consumers. In the 1940s, scientists found that giving animals small amounts of antibiotics that do not treat illness helped animals use food more efficiently and grow faster. After this discovery, a company called American Cyanamid shared research that supported the use of antibiotics to help animals grow. By 2001, this practice became so common that a report by the Union of Concerned Scientists showed that about 90% of all antibiotics used in the United States were for purposes other than treating illness in farming. Certain antibiotics, when given in small, non-treatment doses, help animals produce more meat or milk from the same amount of food and may increase growth, likely by changing the bacteria in their stomachs. Some of these drugs can improve how well animals convert food into weight gain, but they are no longer legally allowed for this use in the United States. Some of these drugs are called ionophores, which are used to control a type of disease in animals and are not considered antibiotics in many countries. These drugs have not been shown to increase the risk of antibiotic resistance in humans.

The use of antibiotics to help animals grow has caused problems for these reasons:

• It is the largest use of antibiotics worldwide.
• Using antibiotics in small amounts leads to bacteria becoming resistant to them.
• All major types of antibiotics are used this way, making them less effective.
• The bacteria that change can harm humans.

Antibiotic resistance

Antibiotic resistance is a type of antimicrobial resistance (AMR), which happens when bacteria develop ways to survive attacks from antibiotics meant 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 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 the harmful effects of antibiotics. Bacteria can still grow even when faced with many environmental threats, including antibiotics. This ability is called genetic plasticity. Bacteria 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 cause of antibiotic-resistant bacteria in animals and humans. Bacteria can gain these 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 the direct transfer of DNA between bacteria through cell-to-cell contact, often using plasmids. Conjugative plasmids carry useful genes that allow bacteria to share them, helping them survive antibiotics. This sharing can lead to bacteria that are resistant to multiple drugs.

Antibiotic resistance also happens naturally as a response to threats. 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 survive and pass their resistance genes to future generations. This can lead to infections that antibiotics cannot treat.

The World Health Organization (WHO) released an updated list in 2019 called "Critically Important Antimicrobials for Human Medicine, 6th revision." This list aims to help prioritize strategies to reduce antimicrobial resistance from human and non-human use. It lists the highest priority antibiotics as: 3rd, 4th, and 5th 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 are categorized in veterinary medicine based on their risk to humans and their use in 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 resort. These include quinolones (like fluoroquinolones), 3rd and 4th 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 diseases in animals, such as infections in poultry, pigs, cattle, and sheep.

While human use of antibiotics is the main source 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 include: 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. Evidence shows resistance can move from animals to humans in all three cases, but the scale is often small or difficult to prove. As Chang et al. (2014) noted, "The topic of agricultural antibiotic use is complex. While many believe agriculture is a major threat, the problem may be exaggerated. There is no proof that agriculture is 'largely to blame' for rising resistance, and we should focus 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 way to monitor antibiotic use in livestock and track how much is used. This model could help reduce antibiotic resistance in animals over time.

Health issues

The use of antibiotics in livestock is a health issue that affects people, animals, and the environment. Antibiotic use in animals used for food production contributes to the development and spread of antimicrobial resistance (AMR), which can impact many biological systems and processes.

Antibiotic-resistant bacteria from livestock can spread to humans in several ways, such as eating contaminated food, touching animals, or being exposed to the environment. Resistant bacteria like Salmonella, Campylobacter, and certain types of Escherichia coli have been linked to livestock farming and can cause infections in humans that are harder to treat. Resistance genes can move between bacteria, making it harder to control the spread of resistance beyond specific species or environments.

Antibiotics are used in livestock to treat, prevent, and control diseases, and in some areas to improve growth. While these practices can help animals stay healthy and increase productivity, their overuse or misuse can speed up the development of resistant microorganisms in animal populations. This creates challenges for veterinary care, as infections in animals may become harder to manage, potentially harming animal welfare and food supplies.

The environment plays a key role in spreading antimicrobial resistance. Antibiotics and resistant bacteria can enter soil and water through animal waste, farm runoff, and wastewater. These substances may remain in the environment, encouraging the development and sharing of resistance genes among bacteria in the environment. These environmental sources can then reintroduce resistant bacteria into human and animal populations.

Reducing antibiotic use in livestock requires teamwork across different fields, following the One Health approach, which connects human, animal, and environmental health. International groups like the World Health Organization (WHO), the Food and Agriculture Organization (FAO), and the World Organisation for Animal Health (WOAH) recommend reducing non-therapeutic antibiotic use in animals, improving monitoring of antibiotic use and resistance, and creating programs to manage antibiotic use responsibly. These efforts aim to balance animal health and agricultural needs with protecting public and environmental health.

Direct contact with livestock can spread antibiotic-resistant bacteria. Studies show that people who work with or manage livestock, such as farm workers, are at higher risk. For example, a study found that farm workers and neighbors near chicken farms had higher levels of resistant bacteria after the chickens were given antibiotics in their feed. Manure from animals may also contain resistant bacteria like Staphylococcus aureus, which can infect humans. In 2017, the WHO listed methicillin-resistant S. aureus (MRSA) as one of 12 priority antibiotic-resistant bacteria, highlighting the need for new treatments. Some resistant bacteria, such as livestock-associated MRSA (LA-MRSA), have been linked to livestock and farm animals and can spread to humans through air and soil. A study found that livestock workers and veterinarians were much more likely to have LA-MRSA infections compared to people not exposed to livestock. Although the number of people infected is low, LA-MRSA is becoming more common, harder to treat, and a growing public health concern. Monitoring for antibiotic resistance should expand, as limits on antibiotic use for livestock are sometimes applied to pets without clear reasons.

Bacteria on or in food can expose humans to antibiotic-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 can carry both resistant and non-resistant bacteria. Studies of retail meats like turkey, chicken, pork, and beef have found Enterobacteriaceae. When resistant bacteria are eaten and enter the body, they can cause infections that are harder to treat if they are resistant to common antibiotics. Some research has linked antibiotic-resistant infections to food-producing animals, while other studies have struggled to prove a direct cause. Simple steps like pasteurizing food, cooking meat thoroughly, using proper food storage methods, and washing hands can help reduce the spread of harmful bacteria.

Environmental antibiotic resistance

Humans can also be exposed to antibiotic resistance through the environment. Most antibiotics given to livestock are not absorbed by the animals and are passed in their urine and manure. This can harm animal, human, and environmental health. A study found that using manure as fertilizer increased the amount of antibiotic resistance genes (ARGs) in soil by four times. Poor storage or application of manure can spread resistant bacteria to crops and water. Urine and fecal waste also transfer resistance through wastewater, especially on farms. Wind can carry bacteria from farms into surrounding areas, especially in winter. Research shows that the amount of ARGs in manure decreases over time with fermentation. Composting can reduce antibiotics by 20–99%, but some studies found that chlortetracycline, an antibiotic used in livestock feed in China, degrades at different rates depending on the animal it was given to. Composting may not always fully break down antibiotics in manure.

Mechanisms of resistance

Antimicrobial resistance occurs when bacteria gain and share new genetic material. This sharing, called horizontal gene transfer, allows bacteria to pass genes between species. One way this happens is through conjugation, where bacteria transfer genetic information directly through plasmids. This process does not require reproduction. Studies suggest that antibiotics like Macrolides and Fluoroquinolones, used to treat infections in pigs, may increase the chance of horizontal gene transfer between species. Continued use of antibiotics may also increase genetic changes in bacteria.

Global positions on antibiotic use in farm animals

In 2017, the World Health Organization (WHO) suggested reducing the use of antibiotics in animals raised for food. Because of the growing risk of bacteria that do not respond to antibiotics, the WHO strongly advised limiting antibiotics used to help animals grow or treat healthy animals. Animals that need antibiotics should receive those that are least harmful 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 a financial reason to keep using large amounts of antibiotics, especially in overcrowded or unclean environments.

The World Organisation for Animal Health agreed that protecting antibiotics is important but argued against completely banning their use in animal farming. A total ban could greatly reduce the amount of protein available in some parts of the world. When antibiotics are reduced or stopped in livestock through laws or voluntary actions, both animal health and economic impacts may suffer. For example, farms that reduced or stopped using antibiotics to meet consumer demand for "antibiotic-free" or "raised without antibiotics" products often saw harmful effects on animal health and welfare. When antibiotics are used to improve animal performance, growth, or feed efficiency, the cost of meat, eggs, and other animal products is lower. One major concern about limiting antibiotics is the possible financial difficulties for livestock and poultry producers, which could also 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 would cost consumers about $1.2 billion to $2.5 billion each year. To understand the overall economic impact of limiting antibiotic use, the financial costs must be compared to the health benefits for people. Since it is hard to estimate the value of these health benefits, the study concluded that the complete economic impact of restricting antibiotics has not yet been determined.

Although measuring health benefits is challenging, the economic effects of limiting antibiotics in animals can also be evaluated by looking at the economic costs of antibiotic resistance in humans, which is a major result of antibiotic use in animals. The WHO identifies antibiotic resistance as a cause of longer hospital stays and higher medical costs. When infections cannot be treated with common antibiotics, more expensive treatments are needed. When illness lasts longer due to antibiotic resistance, the increased healthcare costs create a greater economic burden for families and societies. The Center for Infectious Disease Research and Policy estimates that antibiotic resistance causes about $2.2 billion in healthcare costs each year. While limiting antibiotics in animals causes significant economic challenges, the economic burden of antibiotic resistance in humans, which is worsened by antibiotic use in animals, is similar in cost.

Use and regulation by country

The use of medicines to treat disease in animals that produce food is controlled in most countries. In some countries, only trained veterinary doctors can prescribe and give certain medicines, such as antibiotics. These rules were created to stop harmful chemicals from entering food like meat, milk, eggs, and honey. When sick animals are treated with medicines, those medicines can stay in their bodies and be passed into food products unless a waiting time is followed. This waiting time ensures the medicines leave the animal’s body safely. Scientists test each medicine to learn how long it stays in an animal’s body and how the body breaks it down. By using "drug withdrawal periods" before animals are slaughtered or their milk and eggs are used, doctors and farmers make sure food is safe and free from harmful chemicals. Some countries have also banned or limited the use of antibiotics for growth or to prevent disease caused by poor farming conditions. These rules are not about leftover chemicals but about stopping the spread of antibiotic resistance.

Brazil is the world’s largest exporter of beef. The government controls how antibiotics are used in the cattle industry. Most beef cattle in Brazil are grass-fed and belong to the Nellore breed. Official numbers about antibiotic use in Brazil are not published. Studies on farms are the only way to estimate how much antibiotics are used. Brazil has a national plan to reduce antibiotic resistance and limit antibiotic use in farming. Not all antibiotics are banned; they can still be used to treat sick animals or prevent disease.

In Canada, the government ensures that food from animals does not contain harmful levels of antibiotics. Two government agencies, Health Canada and the Canadian Food Inspection Agency (CFIA), manage rules about medicines in food. Testing for leftover medicines in food includes checking samples through monitoring, surveillance, and compliance. There are also tests called "Swab Test On Premises (STOP)" to find antibiotic traces in animal kidneys.

China uses and produces the most antibiotics of any country. Scientists have measured antibiotic use by testing water near farms and animal waste. In 2012, about 38.5 million kilograms of antibiotics were used in China’s pig and chicken farming. This overuse caused pollution in soil and water in northern China. In 2012, a news report said China’s rules for controlling antibiotics in farming were weak.

In the United Kingdom, a 5-year plan (2013–2018) focused on reducing antibiotic resistance in both humans and animals. The UK also partnered with other countries to address global antibiotic resistance. China created a national plan in 2017 to control antibiotic resistance in animals, linking it to the "one health" approach that includes people, animals, and the environment.

In 1999, the European Union started monitoring antibiotic resistance and planned to stop using antibiotics for growth by 2006. In 2006, the EU banned antibiotics as growth agents. Germany used more antibiotics in animals (1,734 tons) than in people (800 tons) in 2011. Sweden banned antibiotics for growth in 1986 and helped the EU ban them in 2006. Sweden also uses antibiotics carefully, treating individual animals instead of groups. Denmark and the Netherlands reduced antibiotic use after the EU ban. In 2011, the European Parliament voted to stop using antibiotics for prevention in farming. A new EU rule, effective in 2022, limits antibiotics used to prevent or treat disease in animals.

In 2011, India created a plan to reduce antibiotic resistance. Rules require animals to stop receiving antibiotics before their food is sold. A 2014 study found antibiotic traces in chicken in India and warned that overuse in farming could make people resistant to antibiotics. The study said India needs better rules, including banning antibiotics for growth.

In 1999, New Zealand said it would not ban antibiotics in farming. In 2017, New Zealand created a plan to fight antibiotic resistance, focusing on both humans and farming. New Zealand has low antibiotic resistance because it uses few antibiotics in animals.

In 1998, researchers found that antibiotic use in farming caused high levels of antibiotic-resistant bacteria in Korea. In 2007, Korea used a lot of antibiotics in farming. In 2011, Korea banned antibiotics for growth. Like other European countries, Korea stopped using antibiotics for growth in 2006. In the UK, less than one-third of antibiotics sold are now used in farming, based on 2017 data.

Research into alternatives

Concern 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 types, that are being tested to help improve animal health and growth.

Prebiotics are types of carbohydrates that the body cannot digest. These carbohydrates are often 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 feed. MOS helps prevent harmful bacteria from attaching to the intestines by acting as a place for bacteria to bind instead, allowing them to be removed from the body.

Bacteriophages are viruses that can infect bacteria. They are found in many places where bacteria live and are also being 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. While more research is needed, these methods have shown promise in controlling infections in animals.

Other ways to reduce the need for antibiotics include improving animal living conditions, giving animals better nutrition to strengthen their natural immunity, increasing biosecurity (measures to prevent disease), using better management and hygiene practices, and making sure vaccines are used properly.