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Wildlife, a reservoir of pathogens

Published on 21/04/2021

Keywords : Wildlife

Some pathogens are specific to the wild animals that carry them. However, they can be transmitted to humans or domestic animals through contact. ANSES's work, which focuses on both wild and domestic animals, helps improve understanding of how these pathogens jump from one species to another and attempts to combat these transmissions.

Because they harbour a great diversity of animal species, natural environments are a reservoir of pathogens to which domestic animals and humans are not accustomed. So in the event of direct or indirect contact with wildlife, transmission can occur. This type of contamination is a major problem, both economically – as shown by the avian influenza epidemics, which can sometimes only be controlled through the mass culling of domestic poultry – and in terms of public health, with the risk of emergence of new diseases for humans, such as the COVID-19 pandemic.

Not all transmissions result in the emergence of a disease

Cases of pathogen transmission from wild to domestic animals are common. It is only if the pathogen adapts to the new host and spreads among individuals of the newly infected species that there is the emergence of a disease, or a re-emergence if the disease reappears in a region from which it had been eradicated. Some of these emerging or re-emerging diseases only affect animals, such as African swine fever. Others, such as tick-borne encephalitis or rabies, can be transmitted to humans and are therefore zoonoses.

Did you know?
Because RNA viruses (i.e. which use ribonucleic acid (RNA) as their genetic material) have a higher mutation rate than DNA-based viruses, bacteria or parasites, they are more likely to acquire a mutation that allows them to adapt from one species to another. This explains why most of the zoonoses that have emerged in recent years (influenza, coronavirus, Ebola, etc.) are RNA viruses. 

Ecosystem disruption, a risk factor for transmission

The places with the greatest biodiversity are also those where pathogen diversity is highest. Environmental disruption is often behind disease emergence, as it allows contact between species that are not accustomed to living alongside each other. The Nipah virus in South-east Asia is a typical example of this: the virus, from the same family as measles, was transmitted from fruit bats to pigs, which then passed it on to humans. These flying mammals had found refuge in pig farms after colossal fires, a consequence of climate change, destroyed their habitat in the Malaysian rainforest. The Nipah virus has a specific mortality rate of over 40% in humans. The outbreak was contained through the slaughter of practically a million pigs. 

Closer to home, the development of green belts to facilitate the movement of wild animals can increase the risk of spread of diseases such as tick-borne encephalitis, which is transmitted by these mites from rodents to humans. In other situations, biodiversity can, on the contrary, have a buffering effect, by diluting pathogens. Some species are not as competent as others in allowing replication of a pathogen; they are known as dead-end hosts. The pathogen will then be transmitted less than if the community only consisted of a few susceptible species. In systems where this dilution effect is present, therefore, the greater the diversity of species, the less likely humans or domestic animals are to encounter a contaminating species.

ANSES's work on wildlife

One of ANSES's laboratories specialises in wildlife, while the Agency's other laboratories working on animal diseases also study the risks of disease transmission from wild animals to domestic animals, or from domestic animals to humans.

Understanding how pathogens jump from wildlife to domestic animals

One area of ANSES's work is exploring the origin of diseases that appear in livestock and the risk of transmission when a new disease is detected in wildlife. This activity may be carried out in partnership with the SAGIR network, coordinated by the French Biodiversity Agency (OFB), which is responsible for monitoring wildlife mortality. In the event of abnormal mortality, ANSES's laboratories may participate in investigations into these events. 

ANSES's laboratories conduct research to understand how pathogens evolve and what factors allow them to jump from one species to another. This includes work on coronaviruses, influenza virus, various bacterial zoonotic agents (such as the one responsible for tuberculosis) and certain parasites. 

Developing new control measures

The Nancy Laboratory for Rabies and Wildlife in particular has contributed to the development of rabies vaccine baits. This vaccine has made it possible to eradicate rabies in foxes in France. Other control measures are being developed, such as a vaccine for badgers against bovine TB. Another example comes from the Maisons-Alfort Laboratory for Animal Health. It is developing ways to control ticks, which are vectors of diseases that can be transmitted from wildlife to humans. It applies global approaches to identify pathogens transmitted by ticks and mosquitoes. It develops tools for characterising the pathogenicity of various bacteria and parasites transmitted by wildlife to humans. 

Understanding risk factors to help develop management measures

Understanding how a disease is transmitted, its geographical range, which species carry the pathogen and which are most likely to transmit it, the conditions most conducive to contamination and the practices posing the greatest risk... knowledge of all these different factors is used to develop recommendations in order to limit the risks of transmission and manage epizootics caused by pathogens transmitted by wild animals. ANSES regularly issues opinions on the risks of transmission of pathogens from wild species to domestic animals, for example on avian influenza or bovine tuberculosis

Improving surveillance and diagnosis

ANSES's laboratories hold numerous national, European and international reference mandates for diseases that can potentially be transmitted by or to wildlife (bovine tuberculosis, African swine fever, rabies, avian and swine influenza, foot and mouth disease, etc.). In this capacity, they improve the methods for detecting these pathogens and organise training sessions for the official laboratories monitoring these diseases in animals. This enables them to guarantee the reliability of the analyses performed.

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