One Health: Human, Animal, and Environmental Health

Laurie Anne Walden, DVM

Zoonotic diseases are diseases transmitted between people and animals. An estimated 75% of emerging infectious diseases in humans have an animal origin.[1] Because people and animals share habitats, environmental health is linked to both human and animal health. Changes in environmental conditions can increase the risk of zoonotic disease spread.
 
One Health is the concept that human, animal, and environmental health are connected and that professionals in all of these areas need to work together to solve global health problems. The COVID-19 pandemic, caused by the zoonotic virus SARS-CoV-2, has highlighted the importance of using a One Health approach to manage emerging diseases.[2]
 
Connections Between Human and Animal Health
COVID-19 is not the first pandemic caused by a zoonotic agent. The Black Death was a bacterial infection spread in part by rat fleas, and the 1918 flu pandemic was caused by an avian influenza virus. The more recent coronavirus epidemics SARS (severe acute respiratory syndrome) and MERS (Middle East respiratory syndrome) both had animal origins.[2] Many other diseases originating in animals have serious consequences in people. These are just a few examples:

• Rabies
• Anthrax
• Ebola
• HIV/AIDS
• Brucellosis
• Salmonella infection
• H1N1 swine influenza

 
People spread pathogens to animals too. For example, humans can transmit SARS-CoV-2 to companion animals like cats and dogs. Some infectious agents (bacteria, for instance) move in both directions between humans and nonhuman animals. Contaminated water endangers people and animals alike.
 
Cross-species disease transmission is not necessarily direct. In some cases, viruses circulating in an animal population undergo genetic changes and are then able to infect a new species. Some pathogens spread from one species to another through multiple intermediate host species. Others require vectors—carriers like mosquitoes—to infect new hosts.
 
The One Health approach recognizes multiple factors that can influence disease spread:

• Expansion of humans into new geographic areas, encroaching on wildlife habitats
• Close contact between humans and livestock
• Shared living spaces for humans and companion animals
• Increased travel and trade, allowing pathogens to move worldwide
• Antimicrobial resistance, which endangers animal health, human health, and the food supply
• Contamination of the environment
• Deforestation and habitat disruption through inappropriate farming practices
• Effects of climate change on animal populations and ecosystems[3]

 
Emerging Diseases and the Environment
The reasons new infectious diseases emerge in humans can be lumped into 3 categories:

• Changes in the pathogen, such as a virus mutating
• Host factors: individual factors (like immune function) and population factors (like crowding behavior and population growth)
  
• Environmental changes, mostly caused by human behavior[4]

 
Climate change and extreme weather affect the movement of wildlife that carry zoonotic diseases. The geographic spread of some vector-borne diseases has increased as climate change (warming) expands the range of the insects that transmit them.[3] Any human behavior that increases human contact with wildlife (like legal or illegal wildlife trade), decreases natural habitats, alters ecosystems, or reduces species diversity can increase the risk of zoonotic disease spread.
 
International Collaboration
A number of agencies worldwide are involved in One Health collaborations. In the United States, the One Health Office is located in the National Center for Emerging and Zoonotic Infectious Diseases, part of the Centers for Disease Control and Prevention. The World Health Organization, Food and Agriculture Organization of the United Nations, and World Organisation for Animal Health work together to develop global health strategies. Many other organizations are collaborating across disciplines to study and manage zoonotic diseases.
 
References
1. Zoonotic diseases. Centers for Disease Control and Prevention. Accessed November 3, 2020. https://www.cdc.gov/onehealth/basics/zoonotic-diseases.html
2. Deem SL, Brenn-White M. One Health—the key to preventing COVID-19 from becoming the new normal. Mol Front J. Published online September 30, 2020. doi:10.1142/S2529732520400039
3. One Health basics. Centers for Disease Control and Prevention. Accessed November 3, 2020. https://www.cdc.gov/onehealth/basics/index.html
4. Morens DM, Fauci AS. Emerging pandemic diseases: how we got to COVID-19. Cell. 2020;182(5):1077-1092. doi:10.1016/j.cell.2020.08.021
 
Image source: Centers for Disease Control and Prevention, National Center for Emerging and Zoonotic Infectious Diseases

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