One Health, One Future: Navigating the Interconnected Challenges of Modern Diseases

Intro

The world is dealing with a severe health crisis worsened by quick changes in our climate and the increasing effects of human activity. This situation has led to the spread of both known and new diseases, which pose risks to human health and the ecosystems we depend on. It is crucial to understand that these emerging infectious diseases, caused by various pathogens, have crossed local borders and become a global issue, with potential widespread social and economic effects.

The connection between habitat loss, food security, and influenza may initially seem complex. Yet, humans have altered landscapes and destroyed wetlands that have served as rest stops for migrating birds for thousands of years. These areas, often turned into poultry production sites, facilitate the mixing of influenza strains, increasing the risk of disease transmission to humans.?

We are only starting to comprehend the full range of challenges that threaten human well-being, as well as the health of domestic and wild animals, and the resilience of ecosystems. These challenges are global and intersect in numerous ways. For instance, a single suitcase of bushmeat from non-human primates flown from Cameroon to New York could spread zoonotic diseases worldwide, such as monkeypox or simian foamy virus.

This problem is not limited to certain places; it is a worldwide issue affecting all countries. These pathogens enter our lives through soil, water, food, and insects, playing a crucial role in their spread.? The ongoing risk of these diseases requires a unified global response and cooperation across borders and cultures, as the consequences reach beyond human health. They affect wildlife and agriculture, closely tied to our societies and economies.?

If we're wondering why these diseases are frequently in the news, it's because our planet's climate is rapidly changing, accelerating past what was predicted for the year 2100. However, we often treat these diseases as isolated cases affecting only humans and react only after they strike. We focus much on known diseases, neglecting emerging ones. Most global diseases are yet to be discovered, and they are increasingly finding us, as shown by ongoing outbreaks. This issue is not new, but our preparations for these outbreaks have been inadequate.

However, global solutions are also possible. Preventative actions, like implementing laws to reduce environmental pollutants, can have broad and beneficial health impacts. This chapter explores the health of all living organisms, from isolated natural areas to densely populated cities, emphasizing their connectivity. The One Health initiative focuses on the interconnected health of humans, animals, and environments. Understanding these connections is crucial for addressing health challenges effectively.

Nature, humans and the butterfly effect

Consider the role of bats. While commonly associated with rabies or most recently with Covid, bats actually provide critical ecosystem services that benefit planetary health. In the last decade in North America, around 6 million insect-eating bats that live in caves have perished due to a fungus brought into the United States in 2006, likely by humans. This fungus, Pseudogymnoascus destructans, native to Europe, was probably carried over on a caver's footwear and has since spread across the continent.?

It causes white-nose syndrome in bats, which leads to a build-up of fungal spores on their noses and wings during hibernation. This irritation awakens them in winter when food is scarce, causing many to starve as their primary food source, insects like mosquitoes, are unavailable.

The absence of these bats has significant repercussions. Bats typically consume about 350 mosquitoes each per night. Without bats, mosquito populations may increase, possibly leading to greater use of pesticides or higher incidences of mosquito-borne diseases like West Nile virus and Zika virus—both of which are also recent introductions by humans to the New World. Furthermore, the rise of "green" energy solutions such as wind farms has led to increased mortality among tree-roosting bats.?

This situation exemplifies the interconnected challenges of pathogen spread, energy solutions, and wildlife conservation. It illustrates the urgent need for solutions that consider the health of both humans and the environment, highlighting the broad implications of losing bat populations, not only for disease control but also for broader ecological and human health concerns.

And the unintended effects we have on the environment aren’t new, but rather something that has been happening throughout history. A great example of this is how in 1539, Hernando de Soto became the first European to interact with the many tribes surrounding the Mississippi River. During his quest for gold, his interactions with the native peoples often led to conflict due to cultural and language differences. De Soto's expedition, which included, imposed severe demands on the local communities they encountered, often consuming a year’s worth of local food supplies within days and enslaving many, leading to significant violence and devastation.

The expedition’s actions caused widespread environmental damage, deforestation, loss of biodiversity, and pollution. His expedition’s movement across several states left a destructive trail, especially the 900 pigs that grew from an original 100, causing significant ecological damage by rooting and trampling vegetation and spreading pathogens through their feces.

The gravest impact of de Soto’s journey, however, was the introduction of diseases like smallpox and measles, which devastated native populations, killing an estimated 95% of the people in affected communities.

What does "One Health" refer to?

The most popular definition of "One Health" describes it as a combined effort involving multiple disciplines that work together at local, national, and global levels to achieve optimal health outcomes for humans, animals, and the environment.?

Amid frequent global health concerns, such as climate change, pollution, and infectious diseases affecting various species, the One Health concept emerges as a strategy to seek solutions rather than merely worrying about these issues.?

Although the interconnection among human, animal, and environmental health might seem apparent to some, its importance might not be as clear to those new to their fields. Historically, scientific disciplines have been quite segregated, but learning from cross-disciplinary collaboration is crucial to address complex challenges. After years of isolated work within specific domains like human or veterinary medicine, it is now essential for experts from diverse fields to collaborate in our complex, interconnected modern world.

Central to the One Health approach is the involvement of various health-related disciplines that are crucial for this comprehensive strategy. This discipline emphasizes the interconnectedness between humans, other animals, and the environment. Another perspective within One Health includes themes such as translational and ecological medicine.?

Translational medicine in the One Health context involves a cross-species approach to health challenges, utilizing shared medical knowledge across humans and animals. On the other hand, the ecological aspect focuses on understanding the interactions between living organisms and their environments, highlighting how environmental health influences overall planetary health, including both human and animal health.

Why is one-health becoming more and more important?

Emerging infectious diseases (EIDs), which are infections that have become more common, spread to new areas, or affect new species, now represent a significant concern for species conservation, including human health.

Humans and domestic animals now constitute 98% of vertebrate biomass. This includes 7.6 billion humans and billions of domestic animals such as chickens, pigs, sheep, cattle, goats, and camels, many of which are raised for food.

The decline in biodiversity and natural resources incurs significant health costs. One example is the "dilution effect," which suggests that infectious diseases are more likely to emerge in areas with reduced biodiversity. For instance, Lyme disease, first identified in 1976, has become more prevalent in places lacking diverse non-human hosts that could otherwise help control the spread of disease-carrying ticks.

EIDs in humans, which are of global public health importance, often stem from interactions between humans and animals. A critical aspect of this issue, from a One Health perspective, is that 75% of human EIDs are zoonotic, meaning they can be transmitted between animals and humans. About 70% of these originate from wild animals. Around 1600 EIDs, such as avian influenza, West Nile Virus (WNV), SARS, Ebola, monkeypox, and recently Covid are now widely recognized and impact human and animal health worldwide.

EIDs do not only affect humans but also non-human animals and plants. For instance, White Nose Syndrome in bats and chytridiomycosis in amphibians are examples of EIDs affecting wildlife, sometimes leading to local or even species extinctions. Similarly, EIDs affecting food crops—often linked to monoculture farming practices—threaten agricultural output and have driven the use of genetically modified organisms and potent chemical treatments, which carry their own health risks for all forms of life and the environment.?

Moreover, the loss of biodiversity impacts human nutrition, particularly through the decline of bees and other pollinators, which are crucial for about one-third of the food we consume. The reduction in pollination services not only affects human health but also the pollination of native plants.

The economic burden of pathogens

Pathogens, cause extensive damage not only to health but also to economies worldwide. The economic impact they create is immense. For example, plant fungal diseases and nematodes resulted in losses of $77 billion from 1988 to 1990. In the United States, invasive species, including pathogens, cost the economy about $120 billion annually. Animal pathogens lead to $14 billion in damages, while viral diseases like influenza cost $6.5 billion—and this doesn't even include newer threats like West Nile virus, Zika, Yellow Fever virus, and dengue, all of which significantly strain our economies.

Emerging infectious diseases, often originating from animals (zoonoses), pose a particularly serious concern. In the last 15 years, more than 15 deadly outbreaks, such as Ebola and avian influenza, have occurred globally. Since 1980, over 87 new zoonotic and vector-borne diseases have appeared, costing economies heavily. The World Bank notes that outbreaks between 1997 and 2009 cost about $80 billion, and a potential pandemic influenza could cause losses up to $3 trillion. These figures emphasize the need for proactive investments in disease prevention.

Water-borne pathogens in the United States cause 8.5% to 12% of acute gastrointestinal illnesses annually, affecting 12 to 19 million people. The financial losses from healthcare and lost productivity are substantial. In mariculture and fisheries, pathogens cause economic losses ranging from hundreds of millions to over a billion dollars, highlighting the need for strategies to address these threats in aquatic environments.

Viral pathogens like influenza, dengue, and Zika not only create health emergencies but also economic ones. In the U.S., influenza alone costs between $71.3 billion and $166.5 billion annually. The economic impact of dengue, affecting 50–100 million people yearly, is significant though less precisely quantified. Avian influenza in Asia in 2005-2006 and bluetongue virus in Europe have led to billions in damages. Zika virus, recently, imposed short-term costs of $3.5 billion in the Caribbean and Latin America. These diseases affect more than just healthcare; they impact productivity, trade, and economic stability.

Protozoan and bacterial pathogens also contribute significantly to economic burdens. Malaria leads to a productivity loss of $12 billion annually in Africa. Chagas disease costs include $627.46 million in healthcare and $7.18 billion in lost productivity. Lyme disease incurs more than $3.5 billion in costs in the U.S. annually. Bacterial pathogens like Escherichia coli and cholera cause hundreds of millions in losses, varying by regional life expectancies.

Helminth pathogens, affecting both humans and animals, represent a significant economic threat. Schistosome blood flukes affect 125 million people, costing over $641 million annually. Echinococcus granulosus leads to $764 million in human health costs and $2.2 billion in livestock losses. In the U.S., helminths cost the cattle industry $2 billion, with similar losses in Australia and Brazil where they affect domestic ruminants. These microscopic agents, be they viral, bacterial, protozoan, or helminth, have profound economic impacts that go well beyond immediate healthcare costs.?

Why is it so hard to solve the problem?

Solving complex problems like climate change and emerging diseases is inherently challenging due to the complexity of climate, biology, and human society. These systems consist of many interacting components, and the outcomes of their interactions depend on specific contexts and quantities, similar to altering a recipe's ingredients and their proportions, which leads to different results.

Climate change and emerging diseases are influenced by multiple factors, making simple solutions inadequate. For instance, improvements in food production may increase demand and lead to higher poverty levels. Better healthcare results in longer lifespans but strains medical systems. Reduced birth rates can lessen population pressures but may impact public health funding negatively. These complexities and the competing demands on land use and climate often lead to conflicts.

Critical factors in addressing these issues include understanding tipping points and threat multipliers. Tipping points represent irreversible changes, often unrecognized until they are upon us, such as the diminishing capacity to respond to disease outbreaks. Threat multipliers like climate change, population growth, and weakened healthcare systems exacerbate existing problems and add complexity to issues like poverty and migration, viewing emerging diseases as a national security concern.

Bureaucratic resistance to innovation often hinders progress. Bureaucrats may resist new ideas that outperform established methods, as changes can make current leaders appear ineffective. Moreover, modern management practices, which often involve avoiding decision-making and sticking to past "best practices," are ill-suited for our rapidly changing world.?

The costs of dealing with emerging diseases are significant and growing, impacting health, ecosystems, and essential resources like food and water. While difficult to quantify precisely, these costs indicate that the current methods of managing these diseases may not be economically sustainable. The annual financial burden is substantial, and our preparedness for addressing these challenges in a changing climate is lacking.

The potential for devastating pandemics, similar to the 1918 influenza, exists today with a denser global population. We should also be concerned about diseases that may seem minor but have a high probability of occurrence and can lead to ongoing economic burdens, termed "pathogen pollution." For example, diseases like West Nile virus and norovirus, though not causing severe pandemics, require continual resources and lead to significant productivity losses.

And our lack of understanding of pathogens, with only about 10% documented, is troubling because it hampers our ability to prepare for unknown threats.?

Climate change and healthcare in Africa

The scientific community agrees that human activities are significantly increasing greenhouse gases, leading to global temperature rises and altering the Earth's water cycle. These changes severely impact natural resources like water, land, and vegetation, especially in regions already facing poverty, inequality, and weak infrastructure. Africa is expected to suffer most from these climate impacts, largely because it contributes least to the problem yet has minimal capacity to adapt.

The Intergovernmental Panel on Climate Change (IPCC) highlights that Africa will face the most severe effects of the global climate crisis. This vulnerability was recognized as early as 1992 by the United Nations Framework Convention on Climate Change (UNFCCC), which noted the disproportionate impact on the most vulnerable nations.

Climate change poses a major developmental challenge, particularly affecting the poorest populations in low- and middle-income countries. Notably, of the world's 50 poorest countries, 36 are in sub-Saharan Africa, according to the International Monetary Fund (IMF). The unpredictable nature of climate change effects makes planning difficult, exacerbated by unreliable data and multiple environmental and societal stressors.

In recent years, the continent saw economic growth rates above 5.2% annually from 2004 to 2012, though these were affected by the global financial crisis. Some countries have seen declines in new HIV infections and AIDS-related deaths, reflecting progress in public health. But climate change might jeopardise many of these advances.?

Climate change has a significant influence on diseases like malaria and dengue fever. Rising temperatures affect the spread of disease-carrying mosquitoes, increasing the risk of infection. Malaria, for example, is projected to affect millions more people by 2080 due to mosquito migration caused by higher temperatures.?

In February 2024, the World Health Organization highlighted a severe cholera outbreak in southern Africa as unprecedented, with nations like Malawi, Zimbabwe, and Mozambique experiencing over 1,600 deaths and a case surge four times higher than the previous year. The outbreak was fueled by warmer climates and increased rainfall. Cholera, which spreads through contaminated food and water, posed global public health challenges, especially in areas lacking clean water and sanitation.

The WHO reported 40,900 cases and 775 deaths from cholera in January alone, affecting multiple regions worldwide. The prior year saw significant outbreaks in 30 countries, with nine recording over 10,000 cases each. A critical issue was the shortage of Oral Cholera Vaccines (OCV), with a demand for 76 million doses but only 38 million available due to limited production capacity, notably from EuBiologics in South Korea.

The situation worsened with more than 667,000 cases and 4,000 deaths reported globally in 2023, heavily impacting eastern and southern Africa. The rapid spread was exacerbated by poor sanitation and water infrastructure, particularly affecting vulnerable children.

Dengue fever is also sensitive to climate change, with increased temperatures and precipitation elevating the infection risk. Climate change also threatens food security and increases the likelihood of diseases like diarrhea, which are major health concerns in sub-Saharan Africa. There's also evidence linking climate change to increased conflicts in Africa due to resource scarcity and migration, with predictions of a significant rise in armed conflicts by 2030.?

Why we will continue facing new EIDs

Evolutionary radiations address significant environmental changes. Rapid climate shifts may cause many species to perish if they cannot relocate or adapt quickly. Yet, adaptable species that move or evolve could thrive. This adaptation might involve finding more suitable habitats or developing new survival skills, initiating a biodiversity increase as disruptions worsen current conditions. Such challenges also connect previously isolated habitats, offering each species opportunities to thrive and evolve diverse capabilities for future challenges.

Historically, severe events have led to mass extinctions, resetting evolutionary timelines and fostering diversity among surviving species and pathogens. This demonstrates evolution's potential to generate varied life forms after catastrophes.

In conservation biology, opinions about pathogens differ. Some see them as harmful and advocate for their elimination, while others believe pathogens play essential roles in ecosystems, such as regulating host populations. Some parasitologists argue that pathogens themselves are vulnerable to climate change and require protection. Descriptions like "arms races" and "plagues" are common in these discussions.

Pathogens should be viewed more objectively. They exist not with the intent to cause disease or manage populations but as natural entities striving for survival. Often unnoticed unless impacting economically significant or health-related species, pathogens are always part of the environment.

Evolution does not aim to eliminate pathogens or host species. It resolves conflicts by removing unsustainable interactions, allowing pathogens to find new hosts during ecological changes. Initially, these hosts may lack resistance, leading to outbreaks, but over time, resistance develops, and severe diseases decline, integrating pathogens into the environment. This dynamic forms functional, albeit imperfect, relationships between pathogens and hosts, explaining why Emerging Infectious Diseases (EIDs) are expected as the biosphere adapts to climate change.

Climate change and the evolution of pathogens

Research on how life evolved during past climate changes offers hope that we can learn from history to address today's climate-related challenges. The issue of disease is just one aspect of how nature adjusts to significant changes.

Significant environmental changes can lead to the creation of many new species, increasing biodiversity. However, major changes also lead to new diseases. As we address climate change, we might also see more diseases emerging.

The Stockholm Paradigm suggests that when the environment changes, living beings adapt using existing abilities (ecological fitting), leading to a more generalized survival strategy. When the climate stabilizes, isolated groups of species become more specialized, regardless of their interactions, which could be either competitive or cooperative and do not follow a uniform pattern globally.?

For example, in some areas, intense coevolutionary battles between predators and prey could lead to new adaptations, while in other areas, the same species might interact more passively due to different environmental conditions. This variation in interactions across different landscapes results in a complex mosaic pattern of coevolution, where each area has a unique evolutionary story.?

Gene flow between regions further influences these dynamics, making this theory a robust framework for understanding the diversity of ecological and evolutionary relationships globally (geographic mosaic theory of coevolution).

This specialization helps species survive subsequent environmental changes. For instance, if there are many rabbits (prey), the wolf population (predator) may grow due to abundant food. However, as wolves increase, they consume more rabbits, reducing the rabbit population.?

With fewer rabbits available, the wolf population also decreases, allowing the rabbit population to recover, and thus starting the cycle anew. The oscillation hypothesis suggests that these population fluctuations occur in a predictable, cyclical manner, although not all ecological systems exhibit this pattern. This hypothesis helps scientists understand the dynamic relationships between species in nature.

Taxon pulses and the evolutions of pathogens

The concept of "taxon pulses" is significant in understanding the evolution of pathogens due to its implications on how species diversity and interaction patterns can lead to the emergence of new diseases. Taxon pulses refer to periods of rapid biodiversity changes, where species evolve separately and later mix, often resulting in significant evolutionary shifts and new species formations. These events are crucial for several reasons:

1. Rapid Diversification and Pathogen Emergence: During taxon pulses, the rapid increase in the number of species provides more opportunities for pathogens to evolve and adapt to new hosts. This diversification can lead to the emergence of novel pathogens as they find new ecological niches or hosts.
2. Isolation and Mixing: Initial periods of isolation allow species to evolve distinct traits independently, which might include resistance to certain pathogens or the development of unique pathogens within isolated populations. When these populations later mix, the pathogens can be introduced to new hosts that may not have the same resistance, potentially leading to outbreaks of new diseases.
3. Geographic and Ecological Scale: Taxon pulses are not confined to small areas; they can span entire regions or continents, affecting a vast array of species and ecosystems. This widespread impact can facilitate the rapid spread and evolution of pathogens across broad geographic areas, increasing the likelihood of pathogens finding suitable environments and hosts to thrive and evolve.
4. Variability in Triggers: The varied factors that can trigger taxon pulses—such as climate change, geographic shifts, and changes in resource availability—also influence pathogen dynamics. For instance, climate change can alter the range of both hosts and vectors, bringing previously separated species into contact and creating new pathways for disease transmission.
5. Coexistence and Ecological Fitting: The ability of closely related species to coexist with minimal competition during taxon pulses suggests that pathogens might also adapt to multiple hosts within these newly formed communities. Ecological fitting, where species adjust to living together by adapting to shared environmental pressures, can also apply to pathogens adapting to new hosts within these communities.

The one-health approach

It can be disheartening to consider the vast challenges affecting life on Earth, but there is reason for hope. In our complex and interconnected world, the One Health initiative represents a crucial approach to addressing these challenges. This initiative recognizes the need for a holistic, multifaceted, and interdisciplinary strategy, similar to how medical professionals in the 20th century came to understand the complex nature of diseases.

For example, medical doctors treating children for lead poisoning need to address environmental causes, such as lead paint, knowing that medication alone will not solve the problem. Similarly, veterinarians dealing with cattle herds suffering from leptospirosis must consider broader environmental factors like animal reservoirs and water sources to effectively tackle the outbreak. Simply administering doxycycline won't be enough without incorporating preventative environmental measures into the plan.

We now have multiple examples showing the benefits of a holistic One Health approach in solving modern health issues. For instance, the economic impact of losing bats to white-nose syndrome and wind energy development, estimated to cost between $4 and $53 billion, has spurred efforts to protect bat habitats and develop bat- and bird-friendly wind energy solutions. This includes restricting human access to caves where bats hibernate to prevent the spread of pathogens.

In translational medicine, teams of oncologists from both human and veterinary health professions are joining forces to advance cancer diagnostics and treatments. Since many cancers affect both humans and animals and are similar across species, understanding their environmental triggers and mechanisms can lead to breakthroughs that benefit everyone.

Another critical application of the One Health approach is in managing zoonotic diseases, which are infections that move between species and can cause significant suffering in both humans and animals. By considering the reservoirs, hosts, and environments together, we can develop effective prevention strategies.?

While there is no one off solution, what remains true is that we have to start considering more than just the diseases as we have done until now, and start considering the environment, the species that inhabit it and their interactions. Only then will we be able to reduce the damage we cause, while improving our health.

Reference:

World Health Organization. (2024, February 12). Multi-country outbreak of cholera, External situation report #11 - 12 February 2024. WHO. https://www.who.int/emergencies/disease-outbreak-news/item/2024-DON385

Wikipedia. (n.d.). Overview of different evolution theories, and pathogen related data. Retrieved [date you accessed the article], from https://en.wikipedia.org/ [Exact URL needed]

Respiratory Disease Series. (2019). Diagnostic tools and disease management.

The Theory of Evolution. (2020). Principles, concepts, and assumptions.

Climate Change and Microbial Ecology. (2016). Current research and future trends.

Universal Health Coverage for Inclusive and Sustainable Development. (2014). A synthesis of 11 country case studies.

A Short History of Medicine. (2008).

The Stockholm Paradigm. (2019). Climate change and emerging disease.

Climate Change and Health. (2016). Improving resilience and reducing risks.

The DAMA Protocol. (2022). An introduction: Finding pathogens before they find us.

Rayhel, E. A., Deem, S. L., & Lane-deGraaf, K. E. (2018). Introduction to One Health: An interdisciplinary approach to planetary health. Wiley-Blackwell.