Zoonoses can be transmitted from animals to humans or from humans to animals
This term covers all situations where a pathogen – a bacterium, virus or parasite – can be transmitted from animals to humans and/or vice versa.
Zoonoses do not always cause disease in both humans and animals
Some animals carry pathogens that cause disease in humans without showing any symptoms themselves. This is the case with poultry and Campylobacter bacteria and sometimes Salmonella, or with bats and viruses of the coronavirus family. In contrast, foot and mouth disease is most often asymptomatic in humans, while it affects cattle, goats, sheep and pigs.
Animals are reservoirs of pathogens
The great diversity of animal species goes hand in hand with a great diversity of pathogens, to which humans are not usually exposed and which could cause diseases of varying severity if transmitted. It is estimated that 75% of emerging infectious diseases in humans come from animals.
The transmission of a pathogen from one species to another does not always result in the emergence of a new disease
Pathogens are frequently transmitted between species. For a disease to emerge, the transmitted pathogen must adapt to the new host and then spread among individuals of the newly infected species. It may also be a re-emergence if the disease reappears in a region from which it had been eradicated.
Not all emerging threats lead to a pandemic
Not all emerging diseases go on to cause an epidemic or pandemic. An epidemic is an infection that spreads in a specific area, while a pandemic is a global one. The transition of an outbreak to a pandemic is most often declared by the WHO. The spread of an epidemic depends on many factors related to the pathogen, the infected hosts and environmental conditions. The means of transmission plays a major role: a respiratory disease is more likely to spread than a disease with another means of transmission. International travel and trade in animals and products are also important factors. In the case of purely animal pathogens, we use the terms epizootic and panzootic.
For a pathogen to cross the species barrier, it must first mutate
Viruses, especially RNA viruses, which use ribonucleic acid (RNA) as their genetic material, mutate more frequently than bacteria or parasites. They are therefore more likely to acquire a mutation that allows them to adapt to a new species. This explains why most of the zoonoses that have emerged in recent years, such as avian influenza, COVID-19 and Ebola, are RNA viruses.
Zoonoses are not only transmitted through direct contact between animals and humans
Other means of transmission are possible, such as via the environment (water and soil), food or vectors.
Vectors are one of the means by which diseases spread
A vector is an arthropod – a member of a group including insects and arachnids (mosquitoes, ticks, etc.) – that transmits a pathogen (a virus, bacterium or parasite). The vector acquires this pathogen by feeding on an infected host, and then transmits it to other individuals.
Genomic tools help identify the spread of pathogens
Genetic sequencing of pathogens detected through epidemiological surveillance enables better identification of the spread of new pathogens or variants and adaptation of control measures.
One Health, One Welfare and Ecohealth, three related concepts
The concept of One Health has been put forward with growing awareness of the close links between human health, animal health and the state of the global ecosystem. It encourages consideration of all the factors of disease emergence, through both disease research and management.
The Ecohealth concept places more importance on the role of the environment and a healthy ecosystem in animal and human health.
Lastly, the One Welfare concept adds welfare to the health dimension: it emphasises that animal welfare and human well-being are linked, and interact with the physical and mental health of both.