Veterinary · Public health, regulation and professional practice · also One Health
Animal, human and environmental health as one system: One Health governance, AMR surveillance and stewardship, emerging diseases, climate and the Sustainable Development Goals.
In the exams: RPSC One Health unit; EAEVE Day One competences (One Health, SDGs); proposed MSVE revision (AMR and One Health awareness)
One Health is the recognition that the health of people, animals, plants and ecosystems is one connected system, and that problems at their interfaces, zoonoses, food safety, antimicrobial resistance and climate-driven disease, cannot be solved by any one sector working alone. This subject is about how that idea is governed and put to work: by the four international agencies (FAO, UNEP, WHO and WOAH), by national governments through action plans and coordination mechanisms, and by veterinarians in ordinary practice.
It is a systems subject. It does not repeat the drug-by-drug pharmacology of antimicrobials, which belongs to chemotherapy and stewardship, or the measures of disease frequency and the zoonoses one by one, which belong to epidemiology and zoonoses. Instead it covers the concept and its history, global governance, zoonoses prioritisation and joint outbreak response, AMR as a shared problem, the global and national surveillance systems (GLASS, ANIMUSE, InFARM, integrated programmes), environmental AMR, stewardship as policy, emerging diseases and the food-system and wildlife interfaces, climate and health, the Sustainable Development Goals, risk analysis and the economics of One Health.
Worldwide, the subject is examined in different ways. Indian state service examinations such as the Rajasthan Veterinary Officer paper now carry an explicit One Health unit, and ICAR postgraduate entrance syllabi include AMR. European schools assess it through the EAEVE Day One Competences, which since 2023 include One Health and the SDGs; the RCVS Day One Competences list One Health under population care. North American licensing examinations test it under public health, and the PAVE qualifying examination from 2027 has a One Health and professional standards domain. Specialist boards in veterinary public health and preventive medicine test it in depth, especially surveillance design, risk analysis and programme economics.
The idea that human and animal medicine are one science is old. Rudolf Virchow, the nineteenth-century German pathologist who is credited with the word zoonosis, argued that there is no dividing line between animal and human medicine. William Osler, trained in that tradition, taught comparative pathology at a veterinary college in Montreal. In 1964 the American veterinary epidemiologist Calvin Schwabe used the phrase 'one medicine' in his book Veterinary Medicine and Human Health, arguing that veterinarians and physicians share the same biology, the same methods and many of the same diseases. The modern phrase came later. In 2004 the Wildlife Conservation Society convened a meeting in New York that produced the Manhattan Principles under the banner 'One World, One Health', which added wildlife and ecosystems to the human-animal pair. The H5N1 avian influenza crisis from 2003 onwards turned the idea into policy, because no single ministry could control a virus that moved between wild birds, poultry markets and people. In 2010 FAO, the then OIE and WHO signed a tripartite concept note on sharing responsibilities at the human-animal-ecosystem interface, and in 2022 UNEP joined to form the Quadripartite.
The definition now used by all four agencies was drafted in 2021 by the One Health High-Level Expert Panel (OHHLEP). It reads, in substance: One Health is an integrated, unifying approach that aims to sustainably balance and optimise the health of people, animals and ecosystems. It recognises that the health of humans, domestic and wild animals, plants and the wider environment, including ecosystems, are closely linked and interdependent. The approach mobilises multiple sectors, disciplines and communities at varying levels of society to work together to foster well-being and tackle threats to health and ecosystems, while addressing the collective need for clean water, energy and air, safe and nutritious food, taking action on climate change and contributing to sustainable development. Three things in it matter for examinations and for practice. First, plants and the environment are named, not just humans and animals, so plant health, soil and water are inside the frame. Second, it is an approach, not a discipline: it describes how sectors work together, not a new body of knowledge. Third, it links health to sustainable development and climate, which is why One Health is now taught alongside the UN Sustainable Development Goals. Related terms are worth separating. EcoHealth emphasises ecosystem approaches and community participation; Planetary Health, popularised by a 2015 Rockefeller Foundation-Lancet Commission, focuses on human health within the limits of Earth's natural systems. The three overlap heavily and are best seen as different emphases.
One Health is practical at interfaces where infection, residues or resistance cross from one compartment to another. The livestock-human interface covers occupational zoonoses such as brucellosis and bovine tuberculosis, and close contact in backyard systems. The food-system interface runs from farm to fork: pathogens such as non-typhoidal Salmonella and Campylobacter, residues of veterinary medicines, and resistant bacteria and resistance genes travel with meat, milk, eggs and fish. The wildlife interface is where most novel pathogens begin; land-use change, hunting and the wildlife trade increase contact between wild animals, livestock and people. The environmental interface includes water, soil and air contaminated by manure, sewage and industrial effluent. The companion-animal interface matters because dogs and cats share homes, beds and sometimes resistant bacteria with their owners, and the dog remains the source of almost all human rabies deaths. A veterinarian working at any of these interfaces is doing One Health work whether or not it carries the label.
Global One Health is governed by four organisations with different legal bases. The Food and Agriculture Organization of the United Nations (FAO), founded in 1945 and based in Rome, leads on agriculture, food security, livestock production and animal disease control in food systems, and with WHO hosts the Codex Alimentarius Commission, which sets international food standards. The United Nations Environment Programme (UNEP), founded in 1972 after the Stockholm Conference and based in Nairobi, leads on the environment, pollution, biodiversity and ecosystems. The World Health Organization (WHO), founded in 1948 and based in Geneva, leads on human health and administers the International Health Regulations (IHR 2005), a legally binding instrument that requires countries to detect, assess and report public health events that may constitute a public health emergency of international concern (PHEIC). The World Organisation for Animal Health (WOAH), founded in 1924 as the Office International des Epizooties (OIE) and based in Paris, is an intergovernmental organisation outside the UN system. It sets standards for animal health and zoonoses in its Terrestrial and Aquatic Codes and Manuals, runs the World Animal Health Information System (WAHIS) to which members must report listed diseases, officially recognises disease status for some diseases, and evaluates national veterinary services through the Performance of Veterinary Services (PVS) Pathway. It adopted the name World Organisation for Animal Health in 2003 and the acronym WOAH in 2022.
FAO, OIE and WHO began formal collaboration in 2010 with a concept note on sharing responsibilities and coordinating global activities at the human-animal-ecosystem interface. They chose three early priorities: rabies, zoonotic influenza and antimicrobial resistance. A joint early warning system, GLEWS, linked their outbreak intelligence. In 2018 the three signed a memorandum of understanding, and in March 2022 UNEP joined through a new Quadripartite memorandum, recognising that the environment had been the weakest link. In 2021 the agencies created the One Health High-Level Expert Panel (OHHLEP) to give scientific advice, and in 2022 they launched the One Health Joint Plan of Action (OH JPA), covering 2022 to 2026. The plan has six action tracks: (1) enhancing One Health capacities to strengthen health systems; (2) reducing the risks from emerging and re-emerging zoonotic epidemics and pandemics; (3) controlling and eliminating endemic zoonotic, neglected tropical and vector-borne diseases; (4) strengthening the assessment, management and communication of food safety risks; (5) curbing the silent pandemic of antimicrobial resistance; and (6) integrating the environment into One Health. Students should know the six tracks by theme rather than by number.
Several other instruments sit around the Quadripartite. The World Trade Organization Agreement on the Application of Sanitary and Phytosanitary Measures (the SPS Agreement, in force from 1995) names Codex, WOAH and the International Plant Protection Convention as the reference standard-setters, so a country that follows their standards is presumed to be trade-compliant, and one that imposes stricter measures must justify them by risk assessment. The IHR were revised in 2005 after SARS, and amendments adopted in 2024 added a 'pandemic emergency' alert level. The WHO Pandemic Agreement was adopted by the World Health Assembly in 2025, with an annex on pathogen access and benefit sharing still to be negotiated at that time. Country capacity is assessed by WHO's Joint External Evaluation (JEE) for the IHR and by WOAH's PVS Pathway for veterinary services; IHR-PVS National Bridging Workshops bring the two sides together to plan joint action. For antimicrobial resistance, the Quadripartite has a joint secretariat, a Global Leaders Group on AMR (set up in 2020) and an AMR Multi-Partner Trust Fund. The UN General Assembly held high-level meetings on AMR in 2016 and 2024; the 2024 political declaration set a target of reducing deaths associated with bacterial AMR by 10 per cent by 2030 against a 2019 baseline of 4.95 million.
No country can run active surveillance and control for every zoonosis, so it must choose. Prioritisation is a structured, transparent process in which human health, animal health and environment sectors agree which zoonoses deserve joint resources. The most widely used method is the One Health Zoonotic Disease Prioritization (OHZDP) tool developed by the US Centers for Disease Control and Prevention. In an OHZDP workshop, voting members from each sector draw up a long list of zoonoses relevant to the country, agree on five or so criteria (for example severity of human disease, epidemic or pandemic potential, economic impact, presence in the country, and the availability of prevention and control measures), weight those criteria by group ranking, score each disease against them, and produce a ranked list. The top five or so become the national priority zoonoses, with an agreed plan for joint surveillance, laboratory capacity, outbreak response and communication. Rabies, zoonotic influenza, brucellosis, anthrax and bovine tuberculosis appear on many national lists, but the result is country-specific by design; a list from one country should never be copied to another. The value of the exercise lies as much in the process, which builds trust between ministries, as in the list itself.
In 2019 FAO, OIE and WHO published Taking a Multisectoral, One Health Approach: A Tripartite Guide to Addressing Zoonotic Diseases in Countries, known as the Tripartite Zoonoses Guide (TZG). It describes what a country needs: a multisectoral coordination mechanism (MCM), strategic planning and emergency preparedness, coordinated surveillance and information sharing, joint risk assessment, coordinated investigation and response, joint risk reduction, risk communication and community engagement, and workforce development. Operational tools support specific parts. The Joint Risk Assessment Operational Tool (JRA OT) guides human, animal and environment experts through a shared assessment of a zoonotic hazard, so that each sector's data and judgement feed one agreed risk statement and one set of recommendations. The Surveillance and Information Sharing Operational Tool helps countries design how data flow between sectors. A further tool helps set up and run the coordination mechanism itself. The principle throughout is that each sector keeps its legal mandate while sharing information early and acting on one agreed picture.
A joint response follows a familiar outbreak-investigation logic, but each step is shared. Alerts may come from either side: a cluster of abortions in goats reported by a field veterinarian, or a cluster of fever cases in farm workers reported by a district hospital. Under a working One Health arrangement, whichever sector receives the signal informs the other promptly, and a joint team is formed. The team agrees a case definition for people and for animals, collects samples from both, and uses shared laboratory capacity. It then carries out a joint risk assessment, agrees control measures (animal movement restrictions, vaccination, culling with compensation, post-exposure prophylaxis for people, food recalls, vector control) and speaks with one voice to the public. Anthrax, Rift Valley fever, avian influenza, Nipah virus and rabies outbreaks are the classic cases. The commonest failures are delayed notification between sectors, separate press statements that contradict each other, and the absence of compensation, which drives farmers to hide sick animals. Countries notify animal events to WOAH through WAHIS, with immediate notification within 24 hours for events such as the first occurrence of a listed disease, and human events to WHO under the IHR.
Antimicrobial resistance (AMR) is the ability of a microorganism to survive or multiply in the presence of an antimicrobial concentration that would normally kill or inhibit it. The best-known global estimate, the Global Research on Antimicrobial Resistance (GRAM) study published in 2022, estimated that in 2019 bacterial AMR was directly attributable to about 1.27 million deaths and associated with about 4.95 million. Attributable deaths are those that would not have happened if the infection had been susceptible; associated deaths are deaths in people with a resistant infection, whatever the cause. A 2016 UK review chaired by Jim O'Neill projected up to 10 million deaths a year by 2050; that projection is widely quoted but rests on scenario assumptions and is best cited as a projection rather than a forecast. The World Bank estimated in 2017 that, in a high-AMR scenario, global economic output could fall by several per cent by 2050, with poor countries losing most. AMR is a One Health problem because resistant bacteria and resistance genes move between people, animals, food and the environment, and because antimicrobial use in any one sector selects for resistance that can reach the others. A widely cited modelling study estimated that roughly three-quarters of antimicrobials sold worldwide by weight are used in animals, though estimates vary by method and country.
Resistance travels in three main ways. Resistant bacteria move directly: livestock-associated methicillin-resistant Staphylococcus aureus (MRSA) clonal complex 398 colonises pig farmers and veterinarians; resistant Salmonella and Campylobacter reach people through food. Resistance genes move horizontally on mobile genetic elements, plasmids, transposons and integrons, so a gene selected in an animal commensal can end up in a human pathogen. The 2015 discovery of the plasmid-borne colistin resistance gene mcr-1 in Escherichia coli from pigs and pork in China, and its subsequent detection in many countries in people, animals and the environment, is the textbook illustration and directly prompted several countries to restrict colistin in animals. Thirdly, residues of antimicrobials, biocides and heavy metals in manure, sewage and industrial effluent can co-select resistance in environmental bacteria. Because of these pathways, controlling AMR needs coordinated action on use, infection prevention, sanitation and waste in every sector; reducing use in one sector alone delays but does not solve the problem.
The World Health Assembly adopted the Global Action Plan on AMR (GAP) in May 2015, developed with FAO and OIE and later endorsed by their governing bodies. It has five strategic objectives: to improve awareness and understanding of AMR through communication, education and training; to strengthen knowledge and the evidence base through surveillance and research; to reduce the incidence of infection through sanitation, hygiene and infection prevention; to optimise the use of antimicrobial medicines in human and animal health; and to develop the economic case for sustainable investment and increase investment in new medicines, diagnostic tools, vaccines and other interventions. Countries committed to producing multisectoral national action plans aligned to it. FAO followed with its own Action Plan on AMR, and WOAH with a Strategy on AMR and the Prudent Use of Antimicrobials (2016). The Quadripartite has since published a One Health Priority Research Agenda for AMR (2023) and a monitoring and evaluation framework for the plan. The GAP structure is the most frequently examined item: be able to list the five objectives and match an intervention to each.
AMR surveillance has two arms. Antimicrobial use (AMU) surveillance measures how much is used, ideally by species, route and class; antimicrobial consumption is often measured from sales data, which tell you what was sold but not which animal received it. Resistance surveillance measures how often target bacteria are resistant. In animals, programmes usually test three groups: zoonotic bacteria (Salmonella, Campylobacter), indicator commensal bacteria (Escherichia coli and enterococci), which reflect selection pressure in the healthy population, and animal pathogens from clinical submissions, which guide treatment but are biased toward treatment failures. Results are interpreted against two kinds of threshold. Clinical breakpoints predict whether treatment is likely to succeed. Epidemiological cut-off values (ECOFFs) separate the wild-type population, with no acquired resistance, from the non-wild-type population; surveillance programmes prefer ECOFFs because they detect the earliest acquired resistance, even when the drug would still work clinically. Methods must be harmonised, because a laboratory using different breakpoints, sampling points or methods produces numbers that cannot be compared.
Each Quadripartite agency runs a global data system for its sector. WHO launched the Global Antimicrobial Resistance and Use Surveillance System (GLASS) in 2015. It collects standardised national data on resistance in priority human bacterial pathogens, mainly from blood, urine, stool and genital specimens, and on antimicrobial use, and has added modules for emerging resistance and for a One Health ESBL-producing E. coli survey. WOAH collects data on antimicrobial agents intended for use in animals, a collection that began in 2015 and now runs through the ANIMUSE (Animal antiMicrobial USE) global database. Quantities are reported by antimicrobial class and adjusted for animal biomass, so that countries with very different livestock populations can be compared in milligrams of active ingredient per kilogram of animal biomass. FAO runs the International FAO Antimicrobial Resistance Monitoring (InFARM) system for resistance data from food-producing animals and food. The Tracking AMR Country Self-assessment Survey (TrACSS), run jointly and repeated annually since 2016, asks countries to score their own progress on national action plan implementation across sectors. Together these systems are meant to let the world see AMR across humans, animals, food and, increasingly, the environment.
Global data have known weaknesses. Participation is voluntary and uneven; many low-income countries report little or no data because laboratory capacity is limited. Human data from hospital laboratories over-represent severe and treatment-failure cases. Animal use data are mostly sales, not use on the farm, and often cannot be split by species. Resistance data from animals are fewer still outside high-income countries. Different programmes use different denominators, sampling frames and interpretive criteria. Exam answers about surveillance should therefore name a bias or limitation along with the system, and propose what would fix it: representative sampling, harmonised methods, laboratory quality assurance, and linked data. A second common examination trap is to treat a rise in reported resistance as a real rise, when it may reflect more laboratories reporting, a change in the patients sampled or a change in breakpoints. Trends should be read only within a series that kept the same methods. Finally, surveillance is worth funding only if someone acts on it: the results should feed prescribing guidelines, regulatory decisions on which classes may be used in animals, and the targets in national action plans, and the action should in turn be evaluated by the next round of surveillance.
Integrated surveillance means collecting and analysing AMU and AMR data from humans, animals, food and the environment in a coordinated way, with harmonised methods, so that trends can be compared across sectors and the sources of resistance can be traced. The WHO Advisory Group on Integrated Surveillance of Antimicrobial Resistance (AGISAR) published guidance in 2017 on integrated surveillance of resistance in foodborne bacteria. Its core recommendations are to sample at several points along the food chain (healthy animals at slaughter, retail meat, and people), to use the same bacterial species and the same testing methods in each, to report antimicrobial use alongside resistance, and to analyse the data together. The WHO 'Tricycle' protocol (2021) is a deliberately simple version for countries starting out: it uses a single indicator, ESBL-producing E. coli, measured in human bloodstream infections, pregnant women or healthy people, poultry caecal samples, and environmental water, with simple culture methods. Its value is that even a low-resource laboratory can produce comparable One Health data on one organism. Whole-genome sequencing now adds a further layer: it identifies genes, plasmids and clones, and so can show whether resistance in people and animals is shared or only looks similar.
National programmes differ in design but follow the same logic. The Danish Integrated Antimicrobial Resistance Monitoring and Research Programme (DANMAP), started in 1995, was the first fully integrated programme, reporting animal and human use and resistance in one annual report; its prescription data from veterinarians allowed Denmark to monitor the effect of its growth-promoter withdrawal and later its 'yellow card' system for pig farms with high use. The United States National Antimicrobial Resistance Monitoring System (NARMS), started in 1996, is a partnership of the food, public health and agriculture agencies that tests enteric bacteria from people, retail meat and food animals. Canada's CIPARS and Japan's JVARM follow similar models, and the European Union runs harmonised monitoring of resistance in zoonotic and indicator bacteria from food animals and meat, with joint reports from its food safety and disease control agencies. Many low- and middle-income countries have built national networks with FAO and WHO support, often starting with a few sentinel laboratories and the Tricycle protocol. Students do not need the details of every national programme; they need to know what makes one integrated and what each component contributes.
Because a tonne of antibiotic means something very different in a country of chickens than in a country of cattle, use is expressed per unit of animal. The European Union's former sales-monitoring project used the population correction unit (PCU): the number of animals in a category multiplied by a standard estimated weight at the time of treatment, summed across categories, so 1 PCU equals 1 kg of animal. Use is then expressed as milligrams of active ingredient per PCU (mg/PCU). WOAH reports milligrams per kilogram of animal biomass in a similar way. Both are crude: they ignore differences in potency between drugs, so a shift from an old high-dose drug to a modern low-dose drug can lower mg/PCU without any fall in treatments. Dose-based units, such as defined daily doses for animals (DDDvet) and defined course doses (DCDvet), count treatments rather than weight and are preferred for farm-level benchmarking, but they need standard doses for each product and species. When comparing countries, always ask which denominator and which unit were used.
The environment is both a reservoir of resistance genes and a route between hosts. Soil bacteria have carried resistance genes for millions of years, because many antibiotics are natural products of soil organisms; the total pool of resistance genes in a habitat is called its resistome. Human activity adds three things to this background: resistant bacteria from human and animal faeces, antimicrobial residues that select for resistance, and co-selecting agents such as biocides and heavy metals, including copper and zinc used in animal feeds. The main hotspots are wastewater treatment plants, which receive faeces, residues and bacteria from whole cities and hospitals and are places where horizontal gene transfer is likely; manure and slurry spread on land; aquaculture ponds; and effluent from antibiotic manufacturing, where concentrations of active ingredient in some rivers near production sites have been measured far above the levels that select for resistance. UNEP's 2023 report Bracing for Superbugs set out the environmental dimension and called for action on these three pollution sources: sanitation and municipal waste, agriculture and food systems, and pharmaceutical manufacturing.
Environmental surveillance is the least developed arm. It can use culture of indicator bacteria such as ESBL-producing E. coli from rivers or sewage, quantitative PCR for chosen resistance genes, or shotgun metagenomics to describe the whole resistome. Wastewater-based surveillance has the advantage of sampling a whole population at once and was greatly expanded during the COVID-19 pandemic; the same infrastructure can be used for AMR. Control measures are mostly ordinary sanitation and pollution control: safely managed sanitation, effective wastewater treatment, proper manure storage and composting before land application, controls on discharge from farms and aquaculture, and limits on effluent from manufacturing. WHO published guidance in 2024 on wastewater and solid waste management for antibiotic manufacturing, giving risk-based targets for discharges. For a veterinarian, the practical points are the handling of manure and dead stock, the disposal of unused medicines and medicated feed, and the hygiene of farm water. Composting and anaerobic digestion of manure reduce, but do not eliminate, resistant bacteria and genes, so storage time and treatment method matter.
Antimicrobial stewardship is the coordinated set of actions that keep antimicrobials effective by ensuring they are used only when needed, with the right drug, dose, route and duration, while preventing infection so that fewer treatments are needed. Drug-level detail belongs to pharmacology; this chapter deals with the system. At policy level, stewardship works through five levers. Regulation: prescription-only status, bans on growth promotion, restrictions on prophylactic and group use, and rules on medicated feed. Lists: the WHO List of Medically Important Antimicrobials (successor to its list of critically important antimicrobials) ranks drug classes by their importance to human medicine, and the WOAH List of Antimicrobial Agents of Veterinary Importance ranks them for animal health; a class that is highest priority on both, such as third-generation cephalosporins, fluoroquinolones and polymyxins, should be used in animals only when nothing else works and ideally after susceptibility testing. Surveillance and benchmarking: farms or prescribers compared with peers. Infection prevention: biosecurity, vaccination, hygiene, good husbandry and nutrition. Education and behaviour: of prescribers, farmers, feed millers and pharmacists. Reducing use without improving husbandry simply moves disease and suffering elsewhere, so welfare must be monitored alongside use.
The UK Swann Report of 1969 first recommended that antibiotics used in human medicine should not be used as growth promoters. Sweden banned antibiotic growth promoters in 1986; the European Union banned avoparcin in 1997, after vancomycin-resistant enterococci were linked to it, and all remaining antibiotic growth promoters from January 2006. The United States ended the use of medically important antimicrobials for growth promotion in food animals in 2017 and placed their in-feed use under veterinary oversight. Many other countries followed with bans or phase-outs, though enforcement varies widely. After the mcr-1 report, China banned colistin as a growth promoter in 2017 and India prohibited colistin and its formulations for food-producing animals, poultry, aquaculture and animal feed supplements in 2019. European Union Regulation 2019/6 on veterinary medicinal products, applied from January 2022, went further: it bans routine prophylaxis in groups, restricts metaphylaxis, allows antimicrobials to be reserved for human use only, and requires imported animal products to meet equivalent conditions. These steps show the sequence of policy: evidence of harm, restriction of the most important classes, removal of non-therapeutic use, then restriction of preventive use.
International standards give countries a template. The Codex Code of Practice to Minimize and Contain Foodborne Antimicrobial Resistance (first adopted in 2005 and revised in 2021) sets out responsibilities for regulators, the veterinary profession, the pharmaceutical industry, distributors, feed manufacturers and farmers. Codex Guidelines on integrated monitoring and surveillance (2021) and on risk analysis of foodborne AMR (2011) complete the food side. The WOAH Terrestrial Code contains chapters on harmonisation of national AMR surveillance, monitoring of quantities and use, responsible and prudent use, and risk analysis for AMR arising from use in animals. Professional bodies in many countries publish stewardship principles, often summarised as using antimicrobials 'as little as possible and as much as necessary'. The veterinarian's systemic role is to diagnose before treating, keep accurate treatment records, set and review herd health plans, report suspected lack of efficacy and adverse events, advise on vaccination and biosecurity, and resist commercial pressure to prescribe without examination. Detailed drug choice, dosing and withdrawal periods are covered in the chemotherapy subject and must always be confirmed against the product label and the national formulary.
An emerging infectious disease is one that has newly appeared in a population, or that has existed but is rapidly increasing in incidence or geographic range. WOAH's Terrestrial Code uses the term emerging disease for a new infection or infestation resulting from the evolution or change of an existing pathogen, a known infection spreading to a new geographic area or population, or a previously unrecognised pathogen or disease diagnosed for the first time, which has a significant impact on animal or public health. A re-emerging disease is a known disease that had declined and is returning, often because control has broken down or the agent has changed. Two studies are cited everywhere. Taylor and colleagues in 2001 catalogued about 1,400 human pathogens and found that about 61 per cent were zoonotic and that zoonotic pathogens were about twice as likely as others to be associated with emerging diseases, making up about three-quarters of emerging pathogens. Jones and colleagues in 2008 analysed 335 emerging disease events between 1940 and 2004 and found that about 60 per cent were zoonoses, of which about 72 per cent originated in wildlife.
The US Institute of Medicine's 2003 report on microbial threats grouped the drivers of emergence into factors that remain the standard list: microbial adaptation and change; human susceptibility to infection; climate and weather; changing ecosystems; human demographics and behaviour; economic development and land use; international travel and commerce; technology and industry; breakdown of public health measures; poverty and social inequality; war and famine; lack of political will; and intent to harm. For veterinarians, the most important are land-use change, which brings people and livestock into contact with wildlife reservoirs; intensification of livestock production, which creates large dense populations of genetically similar hosts; long-distance trade in animals and animal products; live animal markets where many species meet; and the wildlife trade. Spillover is the transmission of a pathogen from a reservoir host population to a new host population; bridge hosts, often domestic animals, can amplify a wildlife pathogen to the point where it reaches people, as pigs did for Nipah virus and horses for Hendra virus.
Several events are standard examination material. Hendra virus appeared in 1994 in Australia, spreading from fruit bats (flying foxes) to horses and from horses to people. Nipah virus emerged in Malaysia in 1998 to 1999 among pig farmers; fruit bats infected pigs, pigs infected people, and control required the culling of about a million pigs; later outbreaks in Bangladesh and India arose from date palm sap contaminated by bats and from person-to-person spread. SARS emerged in southern China in 2002 to 2003, with civets in live animal markets implicated as an intermediate host and bats as the likely reservoir. MERS was first identified in 2012 in Saudi Arabia, with dromedary camels as the source of human infection. Highly pathogenic avian influenza H5N1 caused human deaths in Hong Kong in 1997; the clade 2.3.4.4b lineage spread across continents from about 2020, infected wild birds, poultry and many mammal species, and was confirmed in dairy cattle in the United States in 2024. The West African Ebola epidemic of 2014 to 2016 and the COVID-19 pandemic showed the cost of late detection. Re-emerging examples include dengue, yellow fever and mpox. On the positive side, rinderpest was declared eradicated in 2011, the second disease ever eradicated after smallpox, and the global strategy against dog-mediated rabies, 'Zero by 30', aims to end human rabies deaths from dogs by 2030 mainly through mass dog vaccination.
Two interfaces deserve their own attention. The food system carries a large disease burden: the WHO Foodborne Disease Burden Epidemiology Reference Group estimated that in 2010 unsafe food caused about 600 million illnesses and 420,000 deaths, with children under five bearing a large share. Food safety control relies on good practices at farm level, meat inspection and hygiene at slaughter, hazard analysis and critical control points (HACCP) in processing, and the Codex standards that underpin trade. The wildlife interface needs surveillance that most countries lack. WOAH's Wildlife Health Framework (2021) asks members to build wildlife health into national surveillance, and WAHIS accepts reports of selected non-listed wildlife diseases. Regulating the wildlife trade, improving biosecurity in markets where live wild and domestic animals are sold together, and protecting habitats to reduce contact are now recognised as pandemic prevention at source. This 'upstream' prevention is far cheaper than responding once a pathogen is spreading in people.
Climate change affects health through direct, ecological and social pathways. Direct effects include heat stress, floods, droughts and storms. Livestock under heat stress eat less, produce less milk, grow more slowly, have lower fertility and higher mortality; the temperature-humidity index (THI), which combines air temperature and relative humidity, is the usual field measure, and thresholds for action differ between species, breeds and production levels. Ecological effects come through vectors and pathogens. Warmer temperatures can extend the range and season of mosquitoes, ticks and biting midges and shorten the time a pathogen needs to develop in the vector. The emergence of bluetongue serotype 8 in northern Europe in 2006, transmitted by native Culicoides midges far north of the disease's historical range, is the classic veterinary example. Rift Valley fever epidemics in East Africa follow unusually heavy rainfall, which floods the depressions where Aedes eggs survive; outbreaks in the late 1990s and 2006 to 2007 followed El Nino-related rains, and rainfall-based early warning is now used. Social effects include migration, conflict over grazing and water, and food insecurity, all of which weaken disease control.
Livestock are also a cause of climate change. FAO's 2013 assessment estimated that livestock supply chains accounted for about 14.5 per cent of human-induced greenhouse gas emissions; later FAO modelling with updated data and methods gave a somewhat lower share, so the figure is best quoted as a range of estimates. Enteric methane from ruminants is the largest single source; feed production and processing taken together (fertiliser, land-use change and energy) is of similar or larger size, and manure adds a smaller share. Methane is a powerful but short-lived greenhouse gas, so reducing it has a relatively quick effect on warming. Mitigation options include improving animal health and productivity so that fewer animals produce the same output, better feeding and forage quality, manure management, and, in some systems, feed additives that reduce enteric methane. Healthy animals are low-emission animals: disease that kills or stunts animals wastes the emissions already spent raising them, which is one of the strongest arguments linking veterinary services to climate policy. Adaptation measures include shade, ventilation and cooling, heat-tolerant breeds, water security, vector surveillance and climate-informed early warning systems.
Biodiversity loss and disease are linked, though the details are debated. Habitat fragmentation increases the edge where people, livestock and wildlife meet. The 'dilution effect' hypothesis proposes that diverse host communities reduce transmission of some pathogens because many hosts are poor reservoirs; it is supported for some systems, such as Lyme disease in parts of North America, but disputed as a general rule. The safest statement for an examination is that biodiversity loss and land-use change increase opportunities for spillover, and that the direction of the effect on any one disease depends on the system. The intergovernmental science-policy platform on biodiversity, IPBES, concluded in a 2020 workshop report that the same drivers that cause biodiversity loss, notably land-use change and wildlife trade, also drive pandemic risk. For veterinary practice the message is not that every wild area must be closed, but that land-use planning, livestock expansion into forest margins and the siting of farms near bat roosts or wetlands used by migratory waterfowl are health decisions as well as economic ones. Nipah virus and avian influenza are reminders that the location and design of farms can make spillover more or less likely, and that veterinary advice belongs in those planning decisions.
The UN General Assembly adopted the 2030 Agenda for Sustainable Development in September 2015. It contains 17 Sustainable Development Goals (SDGs) with 169 targets, monitored through a global indicator framework. Unlike the earlier Millennium Development Goals, which focused on poor countries, the SDGs apply to all countries. They matter to veterinarians because the EAEVE Day One Competences for European veterinary graduates now expect awareness of the SDGs, and because funders and governments frame animal health programmes in SDG terms. Animal health contributes to many goals. SDG 1 (no poverty) and SDG 2 (zero hunger) depend on livestock, which support the livelihoods of hundreds of millions of poor households and provide high-quality protein and micronutrients. SDG 3 (good health and well-being) includes target 3.3, ending epidemics of neglected tropical diseases and combating other communicable diseases, which covers many zoonoses. SDG 5 (gender equality) is relevant because women keep much of the world's small livestock. SDG 6 (clean water and sanitation) matters for environmental AMR and waterborne zoonoses. SDG 8 covers decent work, including occupational zoonoses. SDG 12 (responsible consumption and production) covers food loss and waste. SDG 13 (climate action), SDG 14 (life below water) and SDG 15 (life on land) cover emissions, aquaculture and fisheries, and wildlife and biodiversity. SDG 17 (partnerships) is the formal home of multisectoral collaboration.
Antimicrobial resistance entered the SDG indicator framework through indicator 3.d.2, the percentage of bloodstream infections due to selected antimicrobial-resistant organisms. The two organisms specified are methicillin-resistant Staphylococcus aureus and Escherichia coli resistant to third-generation cephalosporins, and the data come through GLASS. This is a human-health indicator; there is no SDG indicator for antimicrobial use in animals, which is one reason animal data systems such as ANIMUSE and InFARM matter. Neglected zoonoses, including rabies, echinococcosis, taeniasis and cysticercosis, foodborne trematodiases and some others, fall within WHO's neglected tropical disease road map and so contribute to target 3.3. In writing about the SDGs, show the trade-offs as well as the synergies: expanding livestock production can serve SDG 1 and 2 while working against SDG 13 and 15, and the One Health approach is the way to manage those trade-offs by bringing all affected sectors into the decision. An examiner may also ask which goal a given programme serves: a dog rabies vaccination campaign serves SDG 3 directly and SDG 17 through its partnerships, while a programme to cut antimicrobial use in aquaculture touches SDG 3, 12 and 14 at once.
Every country that endorsed the Global Action Plan committed to a multisectoral national action plan on AMR (NAP-AMR). A good plan follows the five GAP objectives, names a lead and a budget for each activity, covers human health, animal health, food, agriculture, aquaculture and the environment, and has a monitoring and evaluation framework. Most countries have produced plans; far fewer have funded and implemented them, which TrACSS and the Quadripartite reports repeatedly note. India's first NAP-AMR (2017) followed the GAP structure and added a sixth priority on India's leadership in AMR; several Indian states have produced their own plans. Beyond AMR, countries build One Health into policy through a national coordination mechanism (a committee, platform or secretariat with members from health, agriculture, livestock, environment, wildlife and finance), through jointly owned priority zoonoses lists, through combined preparedness plans for pandemics, and through legislation that obliges data sharing. Capacity assessments, JEE for the IHR and PVS for veterinary services, identify gaps, and National Bridging Workshops turn those gaps into joint roadmaps. The usual obstacles are separate budgets, separate laws, and the low status of veterinary services in ministries of health.
Risk analysis is the framework used to make decisions about hazards under uncertainty, and both WOAH and Codex define it. In the WOAH Terrestrial Code, import risk analysis has four components: hazard identification, risk assessment, risk management and risk communication. Risk assessment is further divided into entry (formerly release) assessment, exposure assessment, consequence assessment and risk estimation. In Codex, risk analysis consists of risk assessment, risk management and risk communication, and risk assessment comprises hazard identification, hazard characterisation, exposure assessment and risk characterisation. Assessments can be qualitative (negligible, low, medium, high) or quantitative (probability distributions and simulation). A hazard is the agent that can cause harm; risk is the likelihood and magnitude of the harm. Risk management decides on measures in proportion to the assessed risk, and the SPS Agreement requires measures stricter than international standards to be justified by such an assessment. AMR has its own guidance: WOAH's chapter on risk analysis for AMR arising from the use of antimicrobial agents in animals, and Codex's guidelines on risk analysis of foodborne AMR. Risk communication is continuous, two-way and starts at the beginning, not after the decision.
Economic arguments win budgets. The World Bank report People, Pathogens and Our Planet (2012) argued that investing in prevention and control of zoonoses through integrated systems costs far less than the losses from major outbreaks, and that shared laboratories, cold chains, field staff and surveillance reduce duplication. Economic tools used in One Health include cost-benefit analysis, which compares the monetary value of benefits with costs and reports a benefit-cost ratio (BCR) or net present value (NPV); cost-effectiveness analysis, which reports cost per health outcome; and cost-utility analysis, which uses disability-adjusted life years (DALYs). A DALY is the sum of years of life lost to premature death (YLL) and years lived with disability (YLD). The zoonotic DALY (zDALY) adds animal losses, converted to DALY-equivalents, so that one metric captures burden on both sides. The central One Health economic insight is cross-sector benefit: when livestock vaccination against brucellosis was evaluated in Mongolia, it was not worth funding from the human health budget alone, but was clearly worthwhile for society once agricultural and human health benefits were counted together, and the authors proposed that sectors share the cost in proportion to their benefit. The same logic applies to rabies: mass dog vaccination protects people at lower cost than post-exposure prophylaxis alone. When future costs and benefits are compared, they are discounted to present value using a stated discount rate, and results should be tested in a sensitivity analysis because rates and assumptions vary.
Learn the vocabulary exactly: the OHHLEP definition, the four Quadripartite agencies and what each one does, the five objectives of the Global Action Plan on AMR, the six action tracks of the One Health Joint Plan of Action, and the difference between GLASS, ANIMUSE, InFARM and TrACSS. Be able to explain, with one example each, how resistance moves between animals, food, people and the environment. Practise the small calculations, mg/PCU, DALYs, benefit-cost ratio, until you can do them quickly and say what each one hides. When you see an emerging disease in the news, name its reservoir, its bridge host and the agency that must be notified.
In a mixed or farm practice the subject becomes concrete. You record every antimicrobial you prescribe so that farm use can be benchmarked, keep highest-priority critically important classes for cases where culture and sensitivity show nothing else will work, and put vaccination and biosecurity into herd health plans so that less treatment is needed. You notify suspected listed diseases at once, share findings with the district public health officer when a zoonosis is suspected, and advise on manure handling and safe disposal of unused medicines. You explain to farmers why a growth-promoter or colistin ban exists and what replaces it.
A senior veterinarian in government, industry or a large practice group designs and runs systems. Tasks include chairing or sitting on a national One Health or AMR committee, writing the animal-health chapters of a national action plan with costed activities and indicators, setting up sentinel laboratories and data flows to ANIMUSE and InFARM, leading joint outbreak responses with the health ministry, and negotiating compensation schemes so that farmers report disease. Decisions turn on budgets and law, so this person needs the risk analysis and economic tools to argue for joint investment and to defend trade measures under the SPS Agreement.
A researcher with long field experience asks the questions the systems cannot yet answer. Examples include attributing human resistant infections to animal, food and environmental sources with genomic and modelling methods; evaluating whether a national use reduction changed resistance or only shifted disease; building and validating zDALY or other cross-sector metrics; testing early-warning models that link climate data to Rift Valley fever or other vector-borne disease; and studying the political economy of why plans are written but not funded. Such work informs Quadripartite guidance and needs strict attention to bias, uncertainty and the limits of observational data.
This manual describes global systems in general terms and does not list every country's national action plan, surveillance programme or One Health law; national details change often and should be checked against the relevant ministry. It does not give the newest numerical outputs of GLASS, ANIMUSE or InFARM, which change with each report. It deliberately leaves drug choice, dosing and withdrawal periods to the chemotherapy and stewardship subject, and the zoonoses one by one to epidemiology and zoonoses. It does not yet cover plant health in depth, the WHO Pandemic Agreement annex negotiations after 2025, or quantitative source-attribution modelling, and the economics chapter gives methods rather than country-specific cost figures because published estimates vary widely with assumptions.
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