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Animal-to-human Transmission of Diseases is on the rise, why?

April 17, 2023

From Covid-19 to monkeypox, Mers, Ebola, avian influenza, Zika, and HIV, the exponential increase in diseases transmitted from animals to humans in recent years has raised fears of new pandemics.


What are zoonotic diseases?


A zoonotic disease (plural zoonoses) is a disease or infection that is transmitted from vertebrate animals to humans and vice versa. The pathogens involved can be bacteria, viruses, or parasites.


These diseases are either transmitted directly during animal-human contact, or indirectly through food or through vectors such as insects, spiders or mites.


Some diseases eventually become unique to humans, such as Covid-19. According to the World Organization for Animal Health, 60% of human infectious diseases are zoonotic.


What types of diseases are involved?


The term "zoonotic diseases" includes a wide variety of diseases. Some affect the digestive system, such as salmonellosis; others are respiratory, such as avian influenza, swine flu and neocoronavirus; or neurological, such as rabies.


The severity of these diseases in humans varies widely depending on the disease and the virulence of the agent, but also on the infected person, who may be particularly susceptible to the pathogen.


What animals are involved?


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


Badgers, ferrets, mink and weasels are often associated with viral zoonoses, especially those caused by coronaviruses. Other mammals, such as cattle, pigs, dogs, foxes, camels, and rodents, also often play the role of intermediate hosts.


All viruses responsible for major influenza pandemics have a direct or indirect avian origin. Finally, insects such as ticks are vectors of many viral diseases that affect humans.


Why has the frequency of zoonotic diseases increased?


Zoonotic diseases emerged thousands of years ago, but have increased exponentially over the past 20 or 30 years. The increase in international travel has caused them to spread even faster.


Humans are occupying larger and larger areas of the planet, also disrupting ecosystems and facilitating the spread of viruses. At the same time, industrial farming has increased the risk of spreading pathogens between animals. The wildlife trade has also increased human contact with the microbes they may carry. Elsewhere, deforestation has increased the risk of contact between wildlife, domesticated animals and humans.


Should we worry about another pandemic?


A study published last year in the scientific journal Nature warned that climate change will force many animals to flee their ecosystems for more livable lands. With increased mixing, species will spread their viruses more, which will facilitate the emergence of new diseases that could be transmitted to humans.


"Without prevention strategies, pandemics will emerge more frequently, spread faster, kill more people, and affect the global economy with more devastating impacts than ever before." But most importantly, the expansion of human activity and interaction with wildlife increases the risk of viruses infecting humans, and these viruses have the potential to "find" hosts.