The One Health of Animals, Humans, and Our Planet: It’s All Microbially Connected

July 26, 2019

The phrase "One Health" has grown in the scientific conscious over the past decade. But what does this near-ubiquitous phrase mean? One Health is defined as “the collaborative effort of multiple disciplines – working locally, nationally, and globally – to attain optimal health for people, animals and the environment." In other words, everything is connected: what affects our environment can have effects on animals and humans and vice versa. One Health is a broad concept, including topics from antimicrobial resistance, mental health, biodiversity, climate, and more.
Diagram with 3 interlocking circles depicting a mountain, a person hiking and a bison.
A One Health emphasis for the National Parks ecosystems.
Source: National Park Service

One Health: A History

Though the phrase has been used in scientific literature for 10 years, the notion of One Health is not new. The concept echoes the writings of ancient philosophers. As early as c.460-c.377 B.C., Hippocrates wrote that human health depends on the environment in his book On Airs, Waters, and Places. Since then, others have taken note about the connection between health and the environment in examples such as oil spills and ozone depletion.

Scientists have noted similarities in diseases between animals and humans since the 1800s, thousands of years after Hippocrates. The animal and human medical communities largely remained separate, until 1964, when the veterinary epidemiologist Calvin Schwabe proposed that veterinary and human health professions come together to combat zoonotic disease. In 2007, the American Veterinary Medical Association, the American Medical Association (AMA), and the American Public Health Association came together to bring the animal and human medical communities together. They formed the One Health Initiative Task Force to create 12 recommendations for advancing and attaining One Health practices to address the broad scope of topics falling within One Health. Soon, One Health became an international framework for addressing global problems.

An Antimicrobial Resistance Gene Travels the Globe

One such global issue is antimicrobial resistance (AMR), an especially pressing problem in bacteria. The rise of antimicrobial resistance has doctors racing the clock to find antibiotics that can treat their patients while at the same time preventing antibiotic overuse. Scientists are also turning to soils, ants, and more as new sources for antimicrobials.

Antimicrobial resistant pathogens aren’t limited to human infectious disease, of course. In fact, antimicrobial resistance genes are very promiscuous, circulating through humans, animals, and the environment. Because of its multifaceted impact, microbial resistance is a perfect case for the One Health approach to tackle.

The genetic basis of microbial resistance is also important for understanding the role of One Health. Genes conferring antibacterial resistance typically lie on mobile genetic elements, such as plasmids. These plasmids can be shared between bacterial cells, strains, and even different species, whereby new isolates will be able to withstand the presence of antibacterial drugs. At the same time, the host may shed these AMR bacteria, which can release their AMR genes into soil and water to provide an environmental reservoir of resistance determinants. These genes can then be picked up by other microbes and eventually circulate into new hosts.

This is how a gene from India may have traveled all the way to the Arctic. The carbapenemase gene, blaNDM-1, was first identified in 2007 in a hospitalized patient in India, and was accordingly named New Delhi metallo-beta-lactamase 1. Three years later, the gene had made it to the surface waters in urban India. How did it get there? Likely via fecal contamination of local water and food supplies. This can then lead to consumption of contaminated foodstuffs by humans and wildlife. In 2013, a team of researchers arrived in the Arctic to sample soil for an unrelated study to investigate microbial methane release in thawing permafrost. They had no intention of finding AMR genes in their samples, but sequencing revealed the blaNDM-1 gene among their samples.

The One Health perspective helps us understand how blaNDM-1 and other AMR genes came to be found worldwide. Microbially-produced antimicrobial compounds selected for resistance on the microbial scale long before humans discovered penicillin. According to the producer hypothesis, increasing the scale of antimicrobials used in agriculture and medicine changed the selective pressures for AMR genes, giving advantages to new bacterial populations that carried these genes. Without determining exactly who is transmitting resistance to whom, the AMR connections among humans, animals and the environment are clear.

Vector-borne Diseases: A Look at Disease Transmission and Climate

At the start of the One Health initiative, there were 1,461 recognized human diseases caused by pathogens. Of those, 60% are due to pathogens that infect multiple animal hosts, and approximately 75% of new emerging human infectious diseases in the last 3 decades are spread between animals and humans. In terms of the global burden of all infectious diseases, 17% of cases are vector-borne.

Vector-borne diseases are those that are transmitted between humans and animals and rely on another organism (a vector) for transmission. Some examples of disease vectors include mosquitoes, flies, fleas, and even freshwater snails. Vector-borne diseases are well suited for studies and intervention using a One Health approach: changes in human population and mobility, climate, and animal populations all affect vector-borne disease transmission.

World map with continents colored according to their mean temperature.
Predicted spread of mosquitos Aedes aegypti and Ae. albopictus and the months of transmission time per year. Change in the ecosystem, like those that affect temperature or rainfall, can alter these spreads.
Source: Ryan S. J., et al./PLoS Neglected Tropical Diseases, 2019
Rift Valley fever (RVF), first described in Kenya in 1931, is a prime example of how climate affects mosquito-borne disease transmission. This viral infection appears to follow heavy rainfall in the tropical regions. Higher rainfall has coincided with major regional RVF outbreaks in the last several decades as flood water increases breeding areas for infected Aedes mosquitoes to lay eggs, which are also infected. Increased rainfall also spurs the growth of dense vegetation that can be detected by satellites, helping to predict outbreaks of RVF 5 months before they happen. The connection between climate and disease can thus be used to predict and prepare for disease outbreaks.

Aside from RVF, the effects of climate change on mosquito populations is a broad area where the One Health approach applies. Mosquitoes can carry pathogens such as those that cause Dengue fever, malaria, and Zika in tropical and warmer climates. As the temperature rises across the globe, the mosquito population can spread to new areas and persist year-round in places they may not have previously. The spread of mosquito populations (along with increased human travels) means that mosquito-borne illnesses may also spread more globally.

Viewing biologically important issues through the One Health lens is only becoming more necessary as humans, animals, and the environment increasingly converge through population growth and travel. Nearly any environmental change can affect the tiny microbes living amongst us and the flora and fauna that surrounds us. The One Health approach shows us that in addressing seemingly isolated issues, there’s more than what meets the eye. 
Share your research! ASM's mSphere journal is partnering with AGU journal GeoHealth on a joint call for papers on One Health, Microbes and Climate Change. With the accelerating pace of climate change leading to an increased risk of global disease outbreaks, there is a need to understand the link between geophysical processes, microbes and human, animal and plant health. Submissions close April 1, 2024. 


Author: Jennifer Tsang, Ph.D.

Jennifer Tsang, Ph.D.
Dr. Jennifer Tsang works in science communications and marketing and writes her own microbiology blog called "The Microbial Menagerie." She completed a Ph.D. in microbiology studying bacterial motility.