Zoonoses: how animals are getting us sick and why these diseases have been multiplying

According to the World Organisation for Animal Health, 60 per cent of human infectious diseases are zoonotic. Some such as salmonellosis affect the digestive system. Others attack the respiratory system, such as avian and swine flu as well as the coronavirus. Some – for example, rabies – are related to the nervous system.

Bats are a source of many viruses that affect humans. Some have been known for a long time, such as rabies, but many have emerged in recent decades, such as Ebola, the Sars coronavirus, Sars-CoV-2 (which causes Covid-19) or the Nipah virus, which appeared in Asia in 1998.

Badgers, ferrets, mink and weasels are often the source of viral zoonoses and in particular those caused by coronaviruses. Other mammals, such as cattle, pigs, dogs, foxes, camels and rodents, are often where parasites live and grow until they are ready to infect their primary hosts.

All the viruses responsible for major influenza pandemics came from birds, either directly or indirectly. Insects are also vectors of many viral diseases that affect humans – for example, ticks spread the Powassan virus.

The severity of these diseases depends on the type of pathogen and its virulence, or how easily it spreads from one person to another. It can also be more dangerous for certain people who may have a particular sensitivity to the pathogen.

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Having first appeared thousands of years ago, zoonoses have multiplied over the past 20 or 30 years. Above all, the expansion of human activities and increased interactions with wildlife increase the risk that viruses capable of infecting humans will find their host.

The growth of international travel has allowed them to spread more quickly. Humans have also been occupying increasingly large areas of the planet, which is disrupting the ecosystem and promoting the transmission of viruses.

Deforestation increases the risk of contact between wildlife, domestic animals and humans. Climate change is also pushing many animals to flee their homes for more liveable lands.

By mixing more, species will transmit their viruses more often, which could promote the start of new diseases that are potentially transmissible to humans.

Large-scale industrial farming increases the risk of pathogens spreading between animals. Capturing wild animals to sell them also increases human exposure to the microbes carried by the animals.

“Without preventive strategies, pandemics will emerge more often, spread more rapidly, kill more people, and affect the global economy with more devastating impact than ever before,” the UN Biodiversity Expert Group warned in October 2020.

According to estimates published in the journal Science in 2018, there are 1.7 million unknown viruses in mammals and birds; 540,000 to 850,000 of them could infect humans.

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Some signs and symptoms of zoonoses include diarrhoea, vomiting, nausea and poor appetite. Individuals might also experience flu-like symptoms such as fever, body aches, headaches and fatigue. In some cases, those who are infected might suffer from skin lesions, scratches or bite marks.

Here are ways to minimise the chances of contracting zoonoses:

• Wash your hands thoroughly with soap and clean water after being around animals or cleaning their waste.

• Apply insect repellent when outdoors.

• Handle food safely.

• Avoid bites and scratches from animals.