Antimicrobial resistance (AMR) has emerged as one of the principal public health problems of the 21st century. This has resulted in a public health crisis of international concern, which threatens the practice of modern medicine, animal health and food security. The substantial problem of AMR is especially relevant to antibiotic resistance, although antifungal resistance is increasing at an alarming rate. Although the phenomenon of AMR can be attributed to many factors, there is a well-established relationship between antibiotic prescribing practices and the emergence of resistant bacteria. Antibiotic overuse is occurring in multiple sectors (human, animal, agriculture). Bacteria faced with antibiotic selection pressure enhance their fitness by acquiring and expressing resistance genes, then sharing them with other bacteria and by other mechanisms, for example gene overexpression and silencing, phase variation. Most bacteria and their genes can move relatively easily within and between humans, animals and the environment and several interconnected human, animal and environmental habitats can contribute to the emergence, evolution and spread of antibiotic resistance, and the health of these contiguous habitats may represent a risk to human health. The drivers of antimicrobial resistance include antibiotic use and abuse in human, animal, and environmental sectors and the spread of resistant bacteria and resistance determinants within and between these sectors and around the globe. Most of the classes of antimicrobials used to treat bacterial infections in humans are also used in animals. Only few antibiotic classes are reserved exclusively for humans (e.g., carbapenems). There are also few classes limited to veterinary use (e.g., flavophospholipols, ionophores) because of their toxicity to humans. There are marked differences in the ways antibiotics are used in human compared to non-human sectors. In people, antibiotics are mostly used for treatment of clinical infections in individual patients, with some limited prophylactic use in individuals (e.g., surgical prophylaxis) or occasionally in groups (e.g., prevention of meningococcal disease). Antibiotics use in animals (e.g., dogs, cats, pet birds, horses) are broadly similar to those in humans. Antibiotics are mostly administered on an individual basis to treat infection, and occasionally for prophylaxis, such as surgical prophylaxis. In the food-producing animal sector, antibiotics are also used therapeutically to treat individual clinically sick animals. However, in intensive farming and aquaculture, for reasons of practicality and efficiency, antibiotics are often administered through feed or water to entire groups (e.g., pens of pigs, flocks of broilers), either for prophylaxis (to healthy animals at risk of infection) or metaphylaxis (to healthy animals in the same group as diseased animals). Growth promotion, prophylaxis and metaphylaxis account for by far the largest volumes of antimicrobials used in the food-producing animal sector. Pollution from inadequate treatment of industrial, residential, and farm waste is expanding the resistome in the environment. Large quantities of antibiotics are produced annually and find their way into the environment. Waste from treatment plants and pharmaceutical industry, particularly if inadequately treated, can release high concentrations of antimicrobials into surface water. Residues of antibiotics are constituents of human sewage, livestock manure, and aquaculture, along with fecal bacteria and resistance genes.
Given the important and interdependent human, animal, and environmental dimensions of antimicrobial resistance, it is logical to take a One Health approach when addressing this problem. This includes taking steps to preserve the continued effectiveness of existing antibiotics by eliminating their inappropriate use and by limiting the spread of infection. Major concerns in the animal health and agriculture sectors are mass medication of animals with antibiotics that are critically important for humans, such as third-generation cephalosporins and fluoroquinolones, and the long-term, in-feed use of medically important antimicrobials, such as colistin, tetracyclines, and macrolides, for growth promotion. Many international agencies have included a One Health approach within their action plans to address antimicrobial resistance. Necessary actions include improvements in antibiotic use regulation and policy, surveillance, stewardship, infection control, sanitation, animal husbandry, and alternatives to antibiotics. The approach has been formally endorsed by the World Health Organization (WHO), European Commission, the US Department of State, US Department of Agriculture, US Centers for Disease Control and Prevention (CDC), World Bank, Food and Agriculture Organization of the United Nations (FAO), World Organization for Animal Health (OIE), United Nations System Influenza Coordination (UNSIC), various Universities, NGOs and many others.