This commentary on One Health, written by Merck Manuals physician editor Ernest Yeh, MD, and Merck Veterinary Manual editor Nicholas Roman, DVM, MPH, explores One Health from a veterinarian’s and a physician’s perspective. They tackle topics ranging from the connection between human and animal health, the human-animal bond, and what One Health means for both the medical and veterinary fields moving forward.

1. What is One Health?
   1. NR: One Health is the idea that the health of humans, other animals, and the environment are vitally linked. Therefore, medical and veterinary professionals as well as those in other scientific, health, and environmental disciplines should communicate and collaborate, rather than work in isolation.
   2. EY: I agree with Nick’s summary – One Health can help bridge the gap between human and veterinary health.
2. How is One Health viewed, both in human health and veterinary health?
   1. NR: Many governmental and international agencies (the CDC, FDA, USDA, AVMA, WHO, FAO, and others) support One Health. In the veterinary clinic, however, One Health is not something that is often discussed. The average veterinarian in clinical practice may not have One Health at the top of mind as they go about their day-to-day job.
   2. EY: Within human health, One Health is not often emphasized or discussed. In medical schools, One Health is very rarely specifically called out within the curriculum. They touch on infectious diseases and zoonotic diseases, but do not typically speak about it under the guise of One Health. I think medical schools could do more to bring One Health to the forefront by calling out its importance. Similar to medical students, the average physician probably does not have One Health top of mind. They look at diseases like Lyme disease, which is a One Health issue as it is tick borne, but don’t think of it outside of how it affects humans. We could do more to bridge that gap in thinking.
3. How have human and veterinary medicine helped contribute to the development of one another?
   1. NR: In many ways, human medicine sets the bar that veterinarians strive for—working to make the same level of care available to animals; for instance, advanced imaging modalities like CT and MRI are increasingly commonplace in vet med. Veterinarians also take an oath to use their scientific training for the advancement of medical knowledge. Since the genus Salmonella was named for Daniel Salmon—the first veterinarian to graduate in the US—veterinary research has made many groundbreaking contributions to the science of medicine as applied to human health.
   2. EY: A lot of discoveries in human medicine are now used in veterinary medicine, and vice versa. This is a result of identifying new practices and applying them across species.
4. How is the concept of One Health currently used in practice, both in the veterinary and medical fields?
   1. NR: A sign of growing recognition of the One Health idea is expanding roles for veterinarians, such as at the CDC, FDA, and NIH. Medical school and colleges of veterinary medicine are also seeking to break down traditional silos and foster scholarly collaborations. Despite this, many individual practitioners may believe that One Health is only peripheral to their areas of focus and interest.
   2. EY: I don’t believe One Health is currently used within the medical field in practice – at least not directly. We may be using it (such as when treating animal borne diseases, or advising someone to use a therapy animal), but we’re not necessarily realizing we’re using One Health concepts.
5. How does the human-animal bond impact human health?
   1. NR: Human-animal interaction promotes health in many ways, such as increased physical and social activity (like a trip to the dog park). The physiological benefits of pet ownership, or even just regular brief interaction with animals, have been well-documented. At the loss of a pet, pet owners often experience severe grief, with accompanying physiological effects like sleep disturbances—a testimony to the reality and the strength of the human-animal bond.
   2. EY: As Nick spoke about, human-animal interaction can promote health in many ways. A lot of people view their pets as a family member, which can play a large role in promoting mental health.
6. More specifically, how does the human-animal bond impact mental health?
   1. NR: Animal companionship enriches our lives and improves our mental and emotional well-being. Animal-assisted therapy is helpful in a wide array of mental health issues, including post-traumatic stress, mood disorders, and OCD. On the dark end of the spectrum, mental health issues contribute to animal hoarding, and animal cruelty and abuse are linked to violence against people.
   2. EY: As Nick spoke about, companionship with animals can be beneficial to human health. On the other hand, it can have a negative affect on mental health when we lose a pet. Similar to losing a family member or friend, we go through the stages of grief.
7. How do service & therapy animals benefit human health?
   1. NR: Service animals provide comfort, companionship, and a healthy distraction in settings like nursing homes, hospitals, and prisons. Assistance animals are trained to help individuals with sensory and other impairments. Some service dogs can even alert their owners to oncoming seizures.
   2. EY: Service animals are trained to perform a specific task for people with specific conditions. For example, a person with seizures may have a service animal trained to detect that the person is about to have a seizure and alert the person (or anyone else around) that a seizure is about to start. The person might then sit or lie down to avoid injuries from falling while having a seizure. The service animal may also try and protect the person’s head thereby preventing any additional injury. On the other hand, therapy dogs provide the opportunity for stress relieving interactions (petting, playful interactions) in a variety of settings such as hospitals, nursing homes and assisted living centers. Therapy dogs receive training to ensure they can interact appropriately and safely in their intended environments.
8. How does environmental health impact human and animal health?
   1. NR: Environmental factors drive outbreaks of disease in wildlife, domestic animals, and humans, and environmental change affects disease risk for animals and human beings alike. Farmers raising animals in greater numbers and density than ever before to feed a growing population… conservationists working to save endangered species and ecosystems… a canine rescue working to rehome stray dogs following a hurricane…these scenarios highlight how the environment, humans, and animals are all interconnected. To quote The Lion King, “Everything [...] exists together in a delicate balance. [...] We are all connected in the great circle of life.”
   2. EY: As population density increases, we are encroaching upon wild, uninhabited areas. This can lead to interactions with wildlife that could be determinantal, both to environmental, human, and animal health. Our increasing population also has impacts on our food supply, which is largely animal dependent at the moment. These are just some examples of the interconnected nature of human, animal, and environmental health.
9. How does One Health affect our understanding of diseases, such as COVID-19 and monkeypox (Mpox)?
   1. NR: Most emerging infectious diseases are zoonotic (spread between people and animals). Non-human animals may be the reservoirs of infection in nature; disease in animals can be a warning that disease might emerge in people. Appreciation of the connection between how humans impact the environment (climate change, loss of wildlife habitat) and the way this in turn affects human health is a cornerstone of the One Health concept. Interdisciplinary, science-based collaborations are going to be needed to tackle these kinds of problems.
   2. EY: We realize there is a huge interaction between the animal and human populations. As we continue surveillance and research we are going to discover more about the interaction and connection between the two.
10. How does the One Health approach affect the long-term potential for medical & veterinary progress?
    1. NR: One application of the One Health concept is biomedical research linking human and veterinary health. The comparative medicine approach has been very productive and holds great promise for progress in the medical and veterinary sciences. Increased veterinary/medical cross-interactions have potential for advancing the development of new therapeutics.
    2. EY: I think One Health has the potential to better connect human and veterinary health, and how we understand one another. Some work needs to be done within the human field, specifically within medical schools, to better emphasize One Health.
11. How does One Medicine relate to One Health?
    1. NR: One Medicine is the collaborative effort of veterinary and human health professionals to combat shared diseases. One Health builds on that concept to include the health of the environment. Prevention and elimination of lead contamination in the environment is one example, affecting both people and animals and encompassing more than the practice of medicine but also the health of the environment.
    2. EY: I agree with Nick’s summary of One Medicine, and how it relates to and impacts our understanding of One Health.
12. How can the average physician or veterinarian embrace One Health in their practice?
    1. NR: Many may see One Health as only applicable to large, complex, transboundary issues, like a global pandemic. Administering vaccines, prescribing parasite preventatives or antimicrobials, talking to an owner about preventing dog bites in children, or spaying a feral cat—these are all One Health activities. There are myriad opportunities for every veterinarian to protect public health and the environment.
    2. EY: As One Health advocates continue to expand their reach, it will impact the average physician more. This may lend itself to a broader understanding of One Health and help bridge the silos between human and animal health. This could lead to greater understanding and collaboration.