Tracing the relationship between human disturbances and the emergence of zoonotic diseases
This web-article is a part of an upcoming publication on biosecurity and zoonotic diseases. | Image source: Shutterstock
The present article serves as an introduction to a three-part publication revolving around the compelling theme of zoonotic diseases (or zoonoses) and the adverse impact of human activities on the emergence of pandemics.1 This project is a result of the collaboration between research units (namely the RU Illicit Wild Flora and Fauna Trade and the RU Environment and Climate) within the Invictus Corporation’s Department on Security Challenges at Seaports (SCS Department).
The aim of the project is twofold. First, to consolidate information available on the causal relationship between selected human activities and the risks associated with the emergence of zoonotic diseases. In particular, through the analysis of human interferences at the human-animal-environment interface and the occurrence of spillover events, that is the transmission of a pathogen from one species (usually a natural host) to another In particular, the authors have focused their attention on the analysis of human interferences at the human-animal-environment interface and the repercussions on the occurrence of spillover events, that is the transmission of a pathogen from one species (usually a natural host) to another. These interferences consist of trade in wildlife and products of animal origin, and anthropisation. Notably, the term anthropisation encompasses a series of activities such as unsustainable urbanisation, land-use change and agricultural/farming intensification. Second, the project aims to provide an overview of the current regulatory systems for identifying, surveillance, prevention and containment of major outbreaks and pandemics.
Figure1: Scheme representing the genesis of a pandemic. In the initial phase, animal pathogens bred in natural hosts “jump” from (non-human) animal species to humans (spillover) and cause the disease. When a human-to-human transmission is possible, contamination can turn into the global spread of the disease (Source: Johnson C.K. et al., 2015;5:14830).
The emergence of COVID-19
In early 2020, the emergence of SARS-CoV-2 (COVID-19) corroborated the thesis according to which human activities play a tangible role in enabling the spread of emerging infectious zoonoses (EIZs) – a subgroup of so-called emerging infectious diseases (EIDs) – by setting the conditions for the manifestation of spillover events.2 Over the last decade, the incidence of EIZs has increased in parallel with an irrefutable escalation of human disturbances.3 It is particularly evident in those contexts where the sustained consumption and trade in wildlife, alongside deforestation and biomes degradation, have exposed human populations to a higher risk of coming into contact with zoonotic pathogens.4
As of the 6th of January 2021, countries around the globe and especially in Europe are experiencing a “second wave” of increased cases of COVID-19. With an average of around 630,000 daily cases and more than 11,000 deaths every day over the last week, both the cases and deaths tolls have reached peaks not seen since the 1918-20 tremendous Spanish flu.5 While it is too early to determine whether there will be a third widespread surge in the number of infections, public health systems remain under constant pressure to cope with the current situation. Looking at the hypothesis of the origin of the COVID-19 pandemic, which allegedly traces it back to the exposure of people to bats in a wet market in the Wuhan region (China) it is possible to identify some staples characterising the emergence of pandemics in our times. According to Huynen et al., globalisation concurs to enhance the transmissibility or pathogenicity of a disease.6 Goods and people can move from one place to another easily and smoothly, and so do pathogens. Furthermore, disruptive human activities (anthropisation) continue to alter landscapes despite efforts to limit their impact, therefore putting a strain on biodiversity and the delicate balance that regulates the cohabitation of species. Consequently, the chances of transmission become unpredictable as animals and humans get closer to each other and experts worldwide are beginning to look at the likelihood of a next pandemic no more as a matter of “if”, but “when” and “how”.7
Transmissibility and risks associated with zoonoses
Humans and animals naturally share many micro-organisms, the majority of which are harmless.8 However, some zoonotic pathogens – namely viruses, parasites, fungi, or bacteria – residing in animal reservoirs carry a potential for adapting to human hosts and causing diseases. In other words, these can “jump” species from (non-human) animals to people. The passage can take place in different forms: through direct contact with an infected specimen or its excreta; indirectly through contaminated fomites such as clothes and utensils; via intermediate species known as vectors. Other indirect ways of transmission consist in the consumption of infected food and the ingestion of contaminated water.9 The World Health Organisation (WHO) estimates that about 60 % of human infections have an animal origin, as do 75 % of all EIDs.10 In addition, it divides zoonoses into three classes: a) endemic zoonoses which are regularly found in a specific area or among particular people; b) epidemic zoonoses which are more sporadic; and c) emerging and re-emerging zoonoses.11
The WHO has warned about the negative repercussions linked to the increased rate of zoonoses owing to their high case fatality ratio and, in most cases, the absence of specific treatment and vaccines.12 However, while not all zoonotic diseases carry an epidemic potential, they might not be endowed with high human-to-human pathogenicity, few can constitute a major threat to public health.13 With regards to the infamous coronaviruses (CoVs), so-called for the presence of crown-like thorns on their surface, scientists have demonstrated that once the pathogen has made the jump from their natural reservoir to other epicentres new variants with higher transmissibility rates often emerge.14 This process of genetic reassortment, implying the acquisition of new genes or modification of existing ones, makes the emergence of significant outbreaks more likely, for the microbial density at the new host can be much higher than that at the reservoir.15
A database of zoonoses and the adherence to a One Health approach
Experts agree on the existence of a positive correlation between human health, animals and the environment.16 In particular, there is consensus surrounding the claim that the emergence of novel zoonoses intrinsically links to human activities. In this regard, growing trade in wildlife and products of animal origin alongside environment degradation and intensive livestock production constitute primary factors.17 In order to substantiate this assumption, the authors of this report not only relied on an intensive literature review, but they also sought to provide for a selected set of relevant case studies. The first step for the identification of case studies consists of setting series of specific parameters. Thanks to which it was possible (table 1) to build an extensive database of zoonoses featuring characteristics suitable for the inception of major outbreaks or, in the worst of cases, pandemics (with the only exception of SARS-CoV-2 owing to the ongoing debate surrounding its account).
- Animal-to-human transmission.
1.1 Transmission facilitated by anthropogenic disturbances.
1.1.1 Transmission route
1.2 Infection as a result of trade in wildlife or contaminated food.
1.2.1 Transmission route
- Human-to-human transmission
Table 1 – Parameters utilised for the identification of known zoonoses with higher chances to cause major outbreaks.
Particularly, only animal-to-human and human-to-human transmittable pathogens were taken into account. It means that zoonoses that do not feature human-to-human pathogenicity are unlikely to cause large outbreaks, epidemics or pandemics, thus it is possible to contain them more effectively. In addition, relevant zoonoses were further categorised in accordance with their specific route of transmission. This entails leading to a distinction between zoonoses occurring directly, through animal-to-human contact (airborne), and indirectly, through vectors (vector-borne). It also includes oral ingestion of infected food (foodborne), water (waterborne) or via contact with contaminated fomites (faecal-oral). At a second stage, the analysis of major outbreaks for each of the identified zoonoses was contextualised in accordance with the ultimate goal of this publication.
According to the WHO, the key for managing health risks at the human-animal-environment interface consists of cross-sectoral collaboration between different stakeholders. This integrated strategy is also known as the One Health approach. It entails the design and implementation of programmes, policies, legislation and research in which multiple sectors communicate and work together to strengthen public health response.18 Through a One Health approach, different stakeholders across relevant sectors and disciplines concur to successfully design prevention, detection and response protocols to EIDs in a comprehensive, efficient and more sustainable manner as shown in the table below.19
A more timely and effective responseInformation sharingDecisions are reliableRegulations, policies, and guidelines are realistic and acceptableEffective collaboration
Technical, human and financial resources are well managed Identification and filling of infrastructural gaps More effective advocacy for funds, policies and programs
Table 2 – Benefits of using a One Health approach identified at the Tripartite High-Level Technical Meeting to Address Health Risks at the Human-Animal-Ecosystems Interfaces held in Mexico in 2011.20
The Food and Agriculture Organisation (FAO) and the World Organisation for Animal Health (OIE) also collaborate with the WHO to encourage proactive cooperation amongst institutions and governments to foster international intervention and implement timely and effective responses to pandemics. Following the same approach, in 2018, the World Bank established the “Operational Framework for Strengthening Human, Animal and Environmental Public Health Systems at their interface” to achieve more resilient and prepared public health administrations.21
On a local basis, aligning a national multisectoral One Health approach with existing international and regional frameworks will secure a more timely and effective response to the emergence of zoonoses. In addition, more efficient use of funds and resources will also help boost sustainability with particular emphasis on the most vulnerable contexts.22
Overall, the project seeks to dissect the causal relationship between human activities at the human-animal-environment interface and the emergence (and transmission) of zoonotic diseases. It ultimately aims at doing so by providing for both an accessible compilation of relevant sources and, through the adoption of a data-driven approach; a thorough analysis leading to the identification of the presence of such a correlation in selected zoonoses. For this reason, the concept of One Health is positioned as the fulcrum around which the entire study revolves. Furthermore, it provides a holistic and multidisciplinary approach in line with the challenges set forth by the interdependency between human health, animals and the environment.
The following part of the report will examine the risks associated with legal and illegal wildlife trade. In particular, it will analyse some of the forces driving wildlife demand and the corresponding zoonotic threats. Then, the last part will assess the negative impact of anthropogenic disturbances on natural landscapes and biodiversity in order to demonstrate that, in areas where such activities are more relevant, the risk of the emergence of zoonotic diseases is proportionally higher.
1 Zoonotic disease is an infectious disease that spreads between species from animals to humans.
2 According to the WHO, an emerging infectious disease is “one that either has appeared and affected a population for the first time, or has existed previously but is rapidly spreading, either in terms of the number of people getting infected, or to new geographical areas”. Available here.
3 Jones et al., “Global trends in emerging infectious diseases”, Nature, 2008; 451(7181): 990–993
4 United Nations Environment Programme (UNEP), ‘Preventing the next pandemic: Zoonotic diseases and how to break the chain of transmission’, United Nations Environment Programme, 2020
5 Statistics on COVID-19 pandemic from https://www.worldometers.info/ and elaborated by the author.
6 Huynen M.M. et al., “The health impacts of globalisation: a conceptual framework”, Globalization and health, 2005;1(1), p.14
7 Emro.who.int. 2019. WHO EMRO | The Next Flu Pandemic: A Matter Of ‘When’, Not ‘If’ | News | Epidemic And Pandemic Diseases Available at: (http://www.emro.who.int/pandemic-epidemic-diseases/news/the-next-flu-pandemic-a-matter-of-when-not-if.html) [Accessed 7 January 2021].
8 United Nations Environment Programme (UNEP), ‘Preventing the next pandemic: Zoonotic diseases and how to break the chain of transmission’, United Nations Environment Programme, 2020, p.11
9 World Health Organisation (WHO) et al. ‘Taking a Multisectoral, One Health Approach: A Tripartite Guide to Addressing Zoonotic Diseases in Countries’ World Health Organisation (WHO), Food and Agriculture Organisation of the United Nations (FAO) and World Organisation for Animal Health (OIE), 2019
10 United Nations Environment Programme (UNEP), ‘Preventing the next pandemic: Zoonotic diseases and how to break the chain of transmission’, United Nations Environment Programme, 2020, p.11
11 World Health Organization (Eastern Mediterranean Regional Office), “Zoonotic disease: emerging public health threats in the Region”, Health Organisation (WHO), 2014. Available at (http://www.emro.who.int/about-who/rc61/zoonotic-diseases.html)
13 Fraser C. et al., “Pandemic potential of a strain of influenza A (H1N1): early findings”, Science, 2009;324(5934), pp.1557-1561.
14 Van der Giessen J.W.B. et al. “Zoonoses in Europe: a risk to public health”, The National Institute for Public Health and the Environment, 2004, p.22
16 Karesh W.B. et al., “Ecology of zoonoses: natural and unnatural histories”, The Lancet, 2012; 380(9857), pp.1936-1945.
17 Vorou R.M. et al., “Emerging zoonoses and vector-borne infections affecting humans in Europe”, Epidemiology & infection, 2007;135(8), pp.1231-1247
18 ‘One Health’ (Who.int, 2017) (https//www.who.int/news-room/q-a-detail/one-health) accessed 26 August 2020
21 Berthe F.C. J. et al., ‘One health: operational framework for strengthening human, animal, and environmental public health systems at their interface’, World Bank Group, 2018
22 The World Health Organisation (WHO) et al. ‘Taking a Multisectoral, One Health Approach: A Tripartite Guide to Addressing Zoonotic Diseases in Countries’ World Health Organisation (WHO), Food and Agriculture Organisation of the United Nations (FAO) and World Organisation for Animal Health (OIE), 2019