Risks associated with zoonotic diseases

Our response to the current acute pandemic has mainly focused on containment and treatment. Prevention is more efficient than treatment, thus we need to better understand the conditions that favour or contribute to the emergence and spread of zoonotic diseases in order to prevent them.

The linkages between hosts, vectors, parasites and pathogens are complex and can be influenced by environmental conditions. When a pathogen infects a host or an invasive species becomes a host for a native pathogen, several combinations of interactions between native and invasive organisms may be involve. Such dynamics are often supported by drivers of global change, such as climate change or ecosystem degradation / recovery.

The most common animal-human interfaces for the emerging zoonotic diseases are: live animal markets, wildlife hunting, intensive wildlife farming (farming of e.g. deer, rodents, civets, mongooses, fur mammals, ostriches), and domestic animals.

Main interactions between biodiversity and zoonotic diseases

Climate change can act as a driver for the introduction of new vectors, such as certain mosquito species in temperate European regions. Pathogens such as West Nile virus may be affected by changes in biodiversity, as this virus can be transmitted not only to humans or primates, but also to many bird species. This also applies to Hanta virus, which can infect not only humans but also many mammals, or leptospirosis, which can be transmitted through excrements from rats. This can be greatly contributed to by the genetic impoverishment of animal populations in degraded, fragmented habitats, which can lead to low immunity, and thus increase the competence for pathogens. Fertile domestic animals with relatively low genetic diversity are less resistant to environmental changes and pathogens. A good example is the Nipah virus, which was first transmitted from wild fruit bats to domestic pigs.

The size of habitats, their connectivity to neighbouring areas, all have an impact on the diversity of host species and the occurrence of typical host species and pathogens. These also increase the potential risk of spillover.

Interactions in areas with limited human influence

Higher levels of biodiversity may provide a high potential source of (novel) pathogens and can facilitate the transmission of pathogens, due to the high diversity and large numbers of vectors (e.g. ticks, mosquitoes). This is the so-called "diversity begets diversity" hypothesis.

If an ecosystem loses species that are incompetent hosts for a particular pathogen, the remaining susceptible hosts will become more abundant. This is the so-called "dilution effect". In a similar way, if many susceptible hosts disappear, many incompetent hosts remain, the transmission may end because pathogens are more likely to end up in incompetent hosts.

Deforestation, human encroachment on nature, results in large-scale destruction of wildlife populations and habitat fragmentation. From fragmented habitats, animals are drawn into urbanised areas, which facilitates increased contact between pathogens, humans and domestic animals.

The different areas of urban systems can be seen as conglomerates, where humans, vectors, domestic animals and wildlife interact to varying degrees. These interfaces may include forest edges, wildlife sanctuary boundaries, landfills, sewage treatment plants, parks and green recreation areas.

A vizes élőhelyek szintén kulcsfontosságúak az olyan vektorok, mint a szúnyogok esetében, amelyeknél nagy a kórokozók átvitelének kockázata. Más ökoszisztémák különböző fajok közösségeit mutatják a sűrűn lakott területek közelében, mint például a városok részét képező (vagy a közelében található) zöld területek.  Európában és más mérsékelt égövi területeken a patkányok, egerek és macskák mellett a prérifarkasok, a rókák és a vaddisznók egyre inkább meghódítják a városi területeket, amelyek a zoonózis-betegségek rezervátumaként szolgálhatnak.

Interactions in areas under anthropogenic influence

Animal husbandry and crop production is a major driver of land use change worldwide, as deforestation is used to provide space for growing crops and grazing animals. As a general rule, competent host species prefer to seek shelter near people rather than in undisturbed natural areas, as they show greater resilience to the human modifications of their ecosystem.

Interactions due to wildlife hunting and wildlife trade

Regulated wildlife trade can also act as a regulatory element, given that there are not enough natural predators to reduce certain wildlife populations.

Local and global trade of wild animals leads to hunting and capture of live animals, which are then offered for sale on markets. In livestock markets, many species of animals are kept together in large numbers, often in poor sanitary conditions, subjecting the animals to high levels of stress, which can lead to a decline in the animals' immune systems. The close proximity of wild and domesticated animal species in livestock market conditions may also allow the encounter of distant coronaviruses and novel viruses with characteristics different from those found in either parent.

Wildlife trade may have been one of the most important factors in the outbreak of the COVID-19 pandemic.

A ban on hunting and trapping wild animals would have a negative impact on the livelihoods of millions of indigenous people in Asia, Africa and Latin America. It would be more sustainable to increase monitoring and enforcement of the ban on unsustainable wildlife trade and international wildlife trade.

Another factor to be considered is the practice of legal intensive farming of wild animals. Farmed mammals include deer, rodents, civets and fur-bearing mammals, which are often kept in inappropriate conditions, thus contributing to the emergence of zoonoses.

Conclusions and policy options

There seems to be a consensus that the links between biodiversity and zoonotic diseases vary, depending, among other things, on the mode of transmission, the likelihood of interaction between hosts, pathogens and/or vectors. Each case would require particular attention to assess the best way to reduce the risk.

One way to assess the risk of zoonotic outbreaks is to assess the diversity of hosts and viruses. Virus monitoring alone may not be sufficient and needs to be complemented by an assessment of human-animal interactions. This may include monitoring vectors such as mosquitoes and ticks, and assessing the relationships between fragmented habitats, agricultural land and human habitation in hotspots.

Wilkinson et al. (2020) developed a model that uses the relationship between species and their habitat to predict the risk of new zoonotic diseases that might infect humans depending on the size of the human population. Their results support studies that show that the risk of novel zoonotic diseases increases inversely with biodiversity loss until a medium level of biodiversity loss is reached.

In some cases, such as wildlife close to European urban areas, the management (e.g. to reduce the risk of interaction with carnivores or wildlife) or vaccination of reservoir hosts may be a solution to avoid zoonotic diseases.

Policy options for areas with limited human influence

Buffer zones around natural areas with a diversity of pathogens and hosts can reduce animal-human contact and the likelihood of new pathogens emerging. The link between deforestation and the emergence of (new) pathogens suggests that significant efforts are needed to preserve intact forest cover in tropical countries.

Ongoing policy legislation is linked to the EU's sustainable trade policy and the European Parliament's initiative to provide legal instruments for deforestation-free trade.

International cooperation, alongside ambitious action to reduce the risk of zoonotic diseases, can provide developing countries with the technical capacity to transform their economies to meet international and EU environmental and social standards.

Policy options for areas under anthropogenic influence

The poor health status of livestock and deficiencies in sanitary controls can increase the risk of zoonotic diseases, especially when domestic animals are in close proximity to wild animals. The EU Biodiversity Strategy stresses the need to 'greening' urban areas. The use of 'baits' with vaccines also helps to prevent the transmission of viral pathogens between reservoirs, similar to the vaccination of foxes against rabies.

Policy options connected to wildlife hunting and wildlife trade

For measures to prevent future pandemics, the UN has proposed a trade ban on live wild animals at wet markets. Border controls should also include testing for known zoonotic pathogens to reduce the spread of zoonotic diseases. International conventions such as the Convention on International Trade in Endangered Species of Fauna and Flora (CITES) address only a part of the problem. Adequate regulatory and enforcement mechanisms could be put in place to combat the hunting and trade of primates, bats, pangolins, civets and rodents. Another way forward could be to review the action on illegal wildlife trade within the EU and its borders. A complete restriction on hunting wild animals is not an option, as many indigenous people depend on the consumption and sale of wild animals for their livelihoods. Improving indigenous peoples' traditional knowledge and providing other sources of income can ensure that wildlife hunting and consumption remains local. Where necessary, governments should support education and awareness on animal handling, sanitation, disease transmission and sustainable wildlife management. General awareness of the risks of using wild animals as food and the adoption of alternative food sources should also be promoted.