Zoonoses and One Health
What zoonoses are, how they spread, rabies, brucellosis and bovine TB, why new ones emerge, the One Health approach, and a dog vaccination sum.
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A farmer drinking raw milk from an infected goat can catch brucellosis. A child bitten by an unvaccinated dog can die of rabies. Slaughter workers can catch anthrax from sick animals, and new influenza viruses can jump from birds or pigs to people. Diseases that pass naturally between animals and humans are called zoonoses, and they make animal health a public health issue. Because people, animals and the environment are so closely linked, the One Health approach brings human doctors, veterinarians and environmental scientists together to tackle them.
Zoonoses and One Health are core topics in veterinary public health, epidemiology and livestock courses.
What zoonoses are
WHO defines a zoonosis as any disease or infection naturally transmissible from vertebrate animals to humans. Studies have estimated that about 60 per cent of known human infectious diseases are zoonotic, and around 75 per cent of newly emerging ones have animal origins (Taylor and colleagues, 2001; Jones and colleagues, 2008).
They can be classified:
- By agent: bacterial (brucellosis, bovine tuberculosis, anthrax, leptospirosis, salmonellosis, campylobacteriosis), viral (rabies, avian influenza, Nipah, Rift Valley fever, Japanese encephalitis), parasitic (toxoplasmosis, taeniasis and cysticercosis, hydatid disease, trichinellosis) and prion diseases (bovine spongiform encephalopathy, linked to variant Creutzfeldt–Jakob disease).
- By route: direct contact, bites, inhalation, food and water, and vectors such as mosquitoes and ticks.
- Some diseases also pass from humans to animals (reverse zoonoses), such as human tuberculosis in cattle or elephants.
Important zoonoses in agriculture
- Rabies: almost always fatal once symptoms appear, and responsible for tens of thousands of human deaths each year, mostly in Asia and Africa and mostly from dog bites. Preventable by vaccinating dogs and by prompt post-exposure treatment of bite victims.
- Brucellosis: from cattle, buffalo, sheep, goats and pigs, through raw milk and contact with birth fluids; causes undulant fever in people and abortion in animals.
- Bovine tuberculosis (Mycobacterium bovis): through raw milk and close contact; controlled by testing and slaughter and by pasteurising milk.
- Anthrax: spores persist in soil for decades; outbreaks follow floods or droughts; people are infected by handling or eating carcasses of animals that died suddenly.
- Leptospirosis: from rodent and livestock urine in water and mud, common among rice farmers and in floods.
- Food-borne zoonoses: Salmonella, Campylobacter, pathogenic E. coli and parasites in meat, milk and eggs.
- Avian and swine influenza: occasional infections in people in close contact with infected animals, with the risk of new pandemic strains.
Why zoonoses emerge
- Land-use change and deforestation bringing people and livestock into contact with wildlife.
- Intensification of livestock, with large, genetically uniform populations.
- Wildlife trade, live animal markets and bushmeat hunting.
- Global travel and trade of animals and products.
- Climate change shifting the range of vectors.
- Antimicrobial misuse, selecting resistant bacteria that move between animals and people.
Preventing zoonoses on the farm
- Vaccinate animals where vaccines exist, such as brucellosis vaccines for young female cattle, anthrax vaccines in endemic areas and rabies vaccines for dogs.
- Test and remove infected animals in control programmes.
- Boil or pasteurise milk and cook meat thoroughly.
- Wear gloves when assisting births or handling aborted foetuses and carcasses, and never open the carcass of an animal that died suddenly.
- Control rodents and keep animals and their dung out of household drinking water sources.
- Use antimicrobials prudently and only under veterinary advice.
The One Health approach
One Health recognises that human, animal and environmental health are interdependent. In 2022, FAO, UNEP, WHO and WOAH (the Quadripartite) launched a One Health Joint Plan of Action. In practice One Health means:
- Joint surveillance of disease in animals and people, sharing data quickly.
- Coordinated response to outbreaks.
- Control at the animal source, often cheaper and more effective than treating people.
- Food safety systems from farm to fork.
- Tackling antimicrobial resistance across sectors.
- Community education on safe practices: boiling milk, safe slaughter, hand washing, protective clothing.
How many dogs must be vaccinated against rabies?
Field studies suggest that vaccinating about 70 per cent of dogs can interrupt rabies transmission.
A district has 10,000 dogs.
- Dogs to vaccinate = 0.70 × 10,000 = 7,000.
Dog populations turn over quickly. If about 40 per cent of dogs are replaced each year by births and new dogs, and no further vaccination is done:
- Coverage after one year ≈ 70 × (1 − 0.40) = 42 per cent, well below the threshold.
That is why mass dog vaccination is repeated every year. At a cost of 3 dollars per dog, annual vaccination of 7,000 dogs costs 21,000 dollars, often less than the cost of post-exposure vaccines for all the bite victims in an uncontrolled outbreak.
Key points
- Zoonoses pass naturally between vertebrate animals and humans; about 60 per cent of human infectious diseases and most emerging ones are zoonotic.
- Classified by agent (bacterial, viral, parasitic, prion) and by route (contact, bites, food, water, vectors).
- Key examples: rabies, brucellosis, bovine TB, anthrax, leptospirosis, food-borne pathogens, avian influenza.
- Emergence is driven by land-use change, intensification, wildlife trade, travel, climate and antimicrobial misuse.
- One Health links human, animal and environmental health through joint surveillance, response and prevention.
What the exams ask
- School-leaving and entrance papers: what zoonoses are, examples and prevention.
- Degree and research-fellowship entrances: classification, epidemiology and control of major zoonoses, emerging diseases, and the One Health concept.
- Veterinary and public health examinations: surveillance systems, outbreak investigation, food safety and rabies elimination programmes.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Is pasteurised milk safe from brucellosis and TB?
Yes. Pasteurisation kills Brucella and Mycobacterium bovis, which is why it has been so important in reducing these diseases in people.
What should I do after a dog bite?
Wash the wound immediately with soap and running water for about 15 minutes, and seek medical care quickly for post-exposure vaccination and, if needed, rabies immunoglobulin.
Why involve vets in human health?
Many human infections start in animals. Controlling them in animals, such as vaccinating dogs or testing cattle, prevents human cases more cheaply than treating people afterwards.
Sources
- Taylor, L. H., Latham, S. M. and Woolhouse, M. E. J. "Risk factors for human disease emergence." Philosophical Transactions of the Royal Society B 356, 983–989 (2001).
- Jones, K. E. et al. "Global trends in emerging infectious diseases." Nature 451, 990–993 (2008).
- FAO, UNEP, WHO and WOAH. One Health Joint Plan of Action (2022–2026). Rome, 2022.
- Coleman, P. G. and Dye, C. "Immunization coverage required to prevent outbreaks of dog rabies." Vaccine 14, 185–186 (1996).
- WHO. WHO Expert Consultation on Rabies: Third Report. WHO Technical Report Series 1012, 2018.
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