Residency · Residency · Preventive Medicine
One Health: Zoonotic Disease and Human-Animal-Environment Interface
Introduction
One Health is a collaborative, multisectoral, and transdisciplinary approach recognizing the interconnection between human, animal, and environmental health. Approximately 75% of emerging infectious diseases are zoonotic, originating in animals before spilling over to humans. The One Health approach is endorsed by WHO, FAO, WOAH (World Organisation for Animal Health), and UNEP through the One Health Quadripartite. Climate change, deforestation, agricultural intensification, and urbanization are accelerating the frequency of zoonotic spillover events, making One Health increasingly essential.
Zoonotic Disease Epidemiology
Classification of Zoonoses
Direct zoonoses: Transmitted directly from animals to humans through contact, bites, or aerosols (rabies, brucellosis, anthrax) Cyclozoonoses: Require more than one vertebrate host to complete the lifecycle (echinococcosis, taeniasis) Metazoonoses: Transmitted by invertebrate vectors (plague via fleas, West Nile virus via mosquitoes, Lyme disease via ticks) Saprozoonoses: Require a non-animal developmental site such as soil or water (histoplasmosis, leptospirosis)
Major Zoonotic Diseases
Influenza: Avian influenza (H5N1, H7N9) and swine influenza (H1N1) strains pose pandemic risk through reassortment and adaptation; H5N1 highly pathogenic avian influenza has caused significant concern since 2022 with widespread avian and mammalian infections. Rabies: Causes approximately 59,000 human deaths annually, primarily in Africa and Asia; transmitted by dog bites in 99% of cases; universally fatal once symptomatic. Brucellosis: Caused by Brucella species; acquired through contact with infected livestock or unpasteurized dairy products; important occupational zoonosis. Leptospirosis: Caused by Leptospira species; acquired through contact with water contaminated by infected animal urine; major burden in tropical regions. Plague: Caused by Yersinia pestis; transmitted by flea bites from infected rodents; endemic in parts of Africa, Asia, and the western United States.
| Zoonotic Disease | Pathogen | Reservoir | Transmission | Case Fatality Rate |
|---|---|---|---|---|
| Rabies | Lyssavirus | Dogs (99%), bats | Animal bites | ~100% (once symptomatic) |
| H5N1 Avian Influenza | Influenza A H5N1 | Wild/domestic birds | Direct contact, aerosol | ~60% (in humans) |
| Ebola | Ebolavirus | Bats (suspected) | Bodily fluids | 25-90% |
| MERS | MERS-CoV | Dromedary camels | Respiratory droplets | ~35% |
| Nipah | Nipah virus | Fruit bats | Contaminated fruit, pigs | 40-75% |
| Leptospirosis | Leptospira spp. | Rodents, livestock | Contaminated water | 5-15% (severe) |
| Plague | Yersinia pestis | Rodents | Flea bites | 30-60% (untreated) |
Emerging Zoonotic Threats
SARS-CoV-2: Likely originated in bats with possible intermediate host; pandemic caused over 7 million confirmed deaths globally. MERS-CoV: Transmitted from dromedary camels; case fatality rate approximately 35%; ongoing sporadic cases in the Middle East. Ebola virus disease: Bats are the suspected reservoir; outbreaks in Africa with high case fatality (25-90%); human-to-human transmission through bodily fluids. Mpox (monkeypox): Rodent reservoir in Africa; 2022 global outbreak with sustained human-to-human transmission. Nipah virus: Bat reservoir; transmitted through fruit contaminated with bat saliva/urine or via pigs; high case fatality (40-75%); no approved vaccine or treatment.
<image>Diagram showing the One Health triad with three overlapping circles representing human health, animal health, and environmental health, with zoonotic diseases at the intersection of all three, and specific diseases and risk factors labeled in each overlapping region: human-animal interface (occupational zoonoses, companion animal diseases), animal-environment interface (wildlife diseases, antimicrobial resistance in agriculture), human-environment interface (water-borne diseases, vector ecology), and center (emerging pandemic threats)</image>
Drivers of Zoonotic Emergence
Land Use Change and Deforestation
Deforestation and habitat fragmentation increase contact between wildlife, livestock, and humans at the wildlife-livestock-human interface. Bushmeat hunting and wildlife trade expose humans to novel pathogens from diverse animal species. Agricultural expansion into previously forested areas creates ecotonal zones where spillover risk is highest. Palm oil plantations, logging, and mining drive deforestation in biodiversity hotspots with high zoonotic risk (Southeast Asia, Central Africa, Amazon)
Agricultural Intensification
Concentrated animal feeding operations (CAFOs) create conditions for pathogen amplification, mutation, and cross-species transmission. High animal density, genetic homogeneity, and immunological stress in industrial livestock production accelerate viral evolution (influenza, coronaviruses) Antimicrobial use in livestock: Approximately 70% of medically important antibiotics sold in the U.S. are used in food animals, driving antimicrobial resistance (AMR) The livestock revolution in developing countries increases production while often lacking biosecurity infrastructure.
Climate Change
Vector range expansion: Warming temperatures enable mosquitoes, ticks, and other vectors to colonize previously inhospitable areas, introducing diseases to new populations. Altered migration patterns: Changes in bird migration affect avian influenza distribution. Extreme weather events: Flooding increases leptospirosis risk; droughts concentrate wildlife at water sources, facilitating interspecies pathogen transmission.
Globalization and Trade
International wildlife trade (legal and illegal) moves potentially infected animals across continents; the 2003 mpox outbreak in the U.S. was linked to imported African rodents. Global food supply chains enable rapid international spread of foodborne zoonoses. Air travel facilitates rapid dissemination of novel pathogens from sites of emergence to global population centers.
<image>Infographic showing the cascade of factors driving zoonotic disease emergence, starting with root causes (population growth, economic development, climate change) flowing through intermediate drivers (deforestation, agricultural intensification, urbanization, wildlife trade) to proximate mechanisms (increased human-animal contact, pathogen evolution, vector expansion) leading to spillover events and pandemic risk, with intervention opportunities marked at each level</image>
One Health in Practice
Surveillance and Early Warning
Integrated surveillance systems monitor diseases across human, animal, and environmental sectors simultaneously. The Global Early Warning System (GLEWS+) jointly operated by WHO, FAO, and WOAH provides early detection of zoonotic threats. Wildlife disease monitoring: Programs like USGS National Wildlife Health Center and PREDICT (now concluded) conducted proactive surveillance of wildlife for novel viruses. Syndromic surveillance in livestock populations can provide early warning of zoonotic threats before human cases occur.
Antimicrobial Resistance as a One Health Issue
AMR is driven by antibiotic use in humans, animals, and crops; resistant organisms move freely between these domains. The Global Action Plan on AMR (WHO, 2015) calls for coordinated One Health action including antimicrobial stewardship across all sectors. Surveillance: The Global Antimicrobial Resistance and Use Surveillance System (GLASS) tracks AMR in human health; complementary systems monitor animal and environmental sectors. Colistin resistance (mcr-1 gene): First identified in livestock in China, this plasmid-mediated resistance to a last-resort antibiotic exemplifies cross-sector AMR transmission.
Practical One Health Programs
Rabies elimination: One Health approach combining mass dog vaccination with post-exposure prophylaxis has eliminated dog-mediated rabies in many countries. Brucellosis control: Livestock vaccination programs reduce human disease burden. Food safety: From farm to fork surveillance integrates animal health, environmental monitoring, and human disease surveillance. Vector-borne disease control: Integrated vector management considering ecological, agricultural, and human health dimensions.
Key Clinical Pearls
The majority of novel human pathogens originate in animals; preventive medicine physicians must understand zoonotic risk factors and advocate for surveillance at the human-animal-environment interface. Antimicrobial resistance is fundamentally a One Health problem that cannot be solved by human healthcare stewardship alone; agricultural antibiotic use is a major driver requiring cross-sector policy action. Climate change is a zoonotic disease amplifier, expanding vector ranges and increasing spillover risk; public health preparedness planning must incorporate climate projections. The COVID-19 pandemic demonstrated that preventing the next pandemic requires investment in wildlife surveillance, reducing deforestation, regulating wildlife trade, and reforming intensive animal agriculture.
References
- Jones KE, Patel NG, Levy MA, et al. Global trends in emerging infectious diseases. Nature. 2008;451(7181):990-993.
- Karesh WB, Dobson A, Lloyd-Smith JO, et al. Ecology of zoonoses: natural and unnatural histories. Lancet. 2012;380(9857):1936-1945.
- World Health Organization, Food and Agriculture Organization, World Organisation for Animal Health. Taking a Multisectoral, One Health Approach: A Tripartite Guide to Addressing Zoonotic Diseases in Countries. Geneva: WHO; 2019.
- Van Boeckel TP, Brower C, Gilbert M, et al. Global trends in antimicrobial use in food animals. Proc Natl Acad Sci U S A. 2015;112(18):5649-5654.
