What are Zoonotic Diseases?
People generally come into contact with animals in one way or another, often on a daily basis, such as through raising livestock or owning pets. While there are often many benefits to this, like food production and income generation for farmers, these interactions can be risky under certain circumstances. This is where zoonotic diseases come in as they are transmitted from animals to humans. In fact, most infectious diseases that affect humans are zoonotic in origin: according to the Centers for Disease Control and Prevention (CDC), more than 60% of known infectious diseases can be spread from animals, and 75% of new infectious diseases came from animals.
What are the impacts of zoonotic diseases, and why are they concerning?
Zoonoses have significant socioeconomic costs on global scales. From 2000-2010, the direct cost of zoonotic diseases was estimated to be over $20bn with over $200bn indirect losses. Zoonoses are responsible for an estimated 2.5bn cases of human illness and 2.7mn human deaths annually: for example, rabies, which is difficult to manage due to its long latency period in humans, resulted in roughly 59,000 annual human deaths and $8.6bn global economic losses. The 2014-2016 Ebola epidemic in West Africa also resulted in significant economic and human costs through the deaths of 11,316 people and an economic loss of $2.2bn.
Recently, zoonotic diseases have made a resurgence in global health and environmental crises because of increased contact between humans and wildlife. Due to joint issues of urbanisation and forest habitat destruction, humans spread into and/or destroy wildlife habitats, facilitating human-wildlife interactions leading to zoonosis spread. This is especially apparent in case studies such as COVID-19, which is believed to have originated from wildlife markets in China and spread quickly through international travel and trade.
Additionally, environmental catastrophes can result in the emergence of zoonoses through increasing the abundance of wildlife reservoir hosts. El Niño (1997-98) led to increased rainfall and food availability for rodent reservoirs, which increased rodent population densities, human-rodent contact, and hantavirus cases in southwestern USA. As such, climate change, environmental degradation, and zoonotic diseases are inextricably linked.
Understandably, zoonoses are major public health concerns. But who is most vulnerable?
Low-income communities are particularly vulnerable to zoonoses, particularly in the Global South, for a few key reasons. Firstly, low-income workers are more at risk of contracting zoonoses in the first place. There is a correlation between poverty and living closely with animals, with some zoonotic diseases viewed primarily as “occupational” diseases that affect a specific occupation. For example, farmers are often most exposed to anthrax, bovine tuberculosis, and brucellosis spread by livestock like cattle, and outdoor workers are often most exposed to West Nile Virus, Hepatitis E Virus, and Crimean-Congo Hemorrhagic Fever.
Secondly, once infected, those in poverty are least likely to properly seek treatment. This is due to the difficulty of obtaining treatment due to various reasons such as the lack of diagnostic facilities or affordable testing, or the fact that zoonoses disproportionately affect remote rural populations that would incur more expensive treatment when factoring in travel time or repeated trips to health centres. For example, in Uganda, those suffering from trypanosomiasis (sleeping sickness) caused by microscopic parasites transmitted by the tsetse fly had made three prior trips to a health centre before being diagnosed, even among the most affluent patients who were more likely to advocate for a proper diagnosis. Thirdly, seeking treatment for zoonoses is directly dependent on the capacity of households or affected persons to take time off from potentially income-generating activities. In low-income households, spare labour and spare funds are significantly less common, and the burden of caring for an ill family member may push the household into poverty or destitution.
For example, in Uganda, those suffering from trypanosomiasis (sleeping sickness) caused by microscopic parasites transmitted by the tsetse fly had made three prior trips to a health centre before being diagnosed, even among the most affluent patients who were more likely to advocate for a proper diagnosis.
Thus, zoonoses impact everyone, but their impacts disproportionately impact low-income communities and careful attention must be paid to address the increasing resurgence of global and local outbreaks of zoonotic diseases.
So what’s being done about them?
Combatting zoonotic diseases can look different depending on the scale of action. Globally, approaches that recognise and optimise the health of humans, animals, and ecosystems have become more popular. The One Health approach, for example, is predicated on the idea that human health is connected to animal and environmental health and requires a holistic approach to disease prevention, surveillance, response, and management. It addresses issues of emerging zoonoses and biological invasions by integrating them into management strategies at different stages, from research priorities for proactive mitigation to management planning for ongoing control.
The OH recognises that human-wildlife interactions are mediated by direct and indirect conditions, such as policy and climate change impacts. However, the feasibility of OH approaches have been critiqued, as it requires transdisciplinary collaboration, so major bottlenecks in implementation include poor stakeholder coordination, poor data-sharing mechanisms across sectors, or the absence of legal implementation frameworks.
While these criticisms can be true, they mainly apply on a broader, global scale. However, the One Health approach has also been applied to smaller scales. In 2006, the Health for Animals and Livelihood Improvement project was implemented in Ruaha, Tanzania. Ruaha supports Africa’s third largest population of endangered Lycaon Pictus (The African Wild Dog). Agriculture accounted for 80% of the community’s livelihood needs, yet illegal bushmeat hunting was common for livestock-dependent households. Consequently, zoonoses such as rabies and bovine tuberculosis are persistent threats. The OH framework advocated for a multilevel approach including the testing of animals and water sources for zoonotic pathogens/diseases, assessing wildlife population health and demography, evaluating livestock and human disease impacts on pastoralist households, and introducing new diagnostic techniques for zoonotic disease detection.
It successfully identified zoonoses such as bovine tuberculosis and brucellosis in livestock and wildlife, and identified geographic areas where transmission risk may be the highest. This fed into local action, as these findings informed behaviour change and policy, leading to a reduction in cases of zoonotic diseases.
Conclusively, the rise of zoonotic diseases provides a prevalent reminder of the interplay between human, environmental, and animal health, and the need for a comprehensive and collaborative approach to mitigate their impacts. As we navigate a world marked by increasing environmental changes and human activities, it is imperative that we recognize the interconnectedness of all living beings. By embracing a holistic approach and collectively working towards a healthier coexistence with nature, we can hopefully curb the rise of zoonoses.
