News anchor: As the novel coronavirus continues to spread from its starting point in Wuhan, China
News anchor: An outbreak of Ebola.
News anchor: The galloping rise of SARS
News anchor: The latest of the swine flu epidemic
News anchor: The growing monkeypox outbreak
Narration: Infectious diseases have been at the forefront of public health concerns forefront of public health concerns,
news headlines, and of course, pandemics, for the last few years.
Although each outbreak arises independently, many of these diseases have something
in common; they're what's known as zoonotic diseases. In this video,
we will discuss what a zoonotic disease is, how they arise, and why the world has been
seeing them more and more often. First, let's discuss exactly what a zoonotic disease is.
Zoonotic diseases are diseases that can be transmitted naturally between animals and humans.
Essentially, it's a disease that comes from an animal, but can infect humans too.
61% of human infectious diseases are zoonotic, which is a large proportion.
However, there are many diseases humans contract which do not come from animals.
There are many classifications of zoonotic diseases - bacterial, parasitic, and fungal, to name a few.
However, one type of zoonotic disease is the culprit for many illnesses: viral.
Viral zoonoses are diseases transmitted by viruses. Viruses are non-living infectious agents
that can only replicate within a host organism, which is an organism that it lives in or on.
Some diseases you may have heard of before that are actually viral zoonoses are rabies, Ebola, HIV, monkeypox, and COVID-19.
Now, let's discuss how these diseases can jump from animal to human.
There are many ways this can happen. Zoonotic diseases can be transmitted through direct and indirect contact.
They can also be vector-borne, food-borne, and water-borne.
Starting with direct contact: this means that humans come into contact with the saliva, blood, urine,
Mucous, feces, or other bodily fluids of an infected animal. This could happen from
touching infected animals, or from getting bites or scratches. Indirect contact occurs when
humans come into contact with areas where animals reside and roam, or objects and surfaces
that have been contaminated with the virus. This might include aquarium tank water, pet habitats, chicken coops,
barns, plants, and soil, and even pet food and water dishes. Vector-borne zoonotic
diseases are transmitted by being bitten by a tick, or an insect like a mosquito or a flea.
Next, food-borne zoonotic diseases are transmitted by eating or drinking a contaminated product.
This usually happens with unpasteurized (raw) milk, or undercooked meat or eggs as they can contain parasites.
This can also happen with consumption of raw fruits and vegetables that are contaminated with feces from an infected animal.
Lastly, water-borne zoonotic diseases are transmitted by drinking or coming in contact with water that has been contaminated with feces from an infected animal.
Now that we know the methods through which an animal disease can infect a human, let’s
discuss the more technical side of how this happens on a molecular level.
Now you might be wondering
how these zoonotic diseases essentially gain the ability to become a human virus and affect populations at large!
Most viruses only infect the nervous system accidently when the immune system is compromised.
But some “neurotropic” viruses have evolved to target neurons as part of their normal infectious cycle.
Let's take a look at rabies as an example.
Concerningly, rabies have begun to resurface as a virus and in efforts to prevent the spread of rabies,
the Canadian government has begun banning imports of dogs from approximately 100 countries as of September 28th, 2022.
The virus begins its transmission in the saliva of an infected animal. The rabies virus travels
along nerves to the spinal cord and then to the brain, where it multiplies. Essentially, the virus can spread throughout the central nervous system
and into the salivary glands, where it can be readily transmitted to other hosts through saliva. Rabies is almost always lethal if it reaches the spinal cord and brain.
However, depending on where the bite was, it normally takes the virus at least 10 days and more often, 30 to 50 days to reach the brain.
During that time, steps can be taken to stop the virus from reaching the brain and causing death.
You may have started to wonder why we’ve been seeing more and more zoonotic diseases in recent years.
Despite zoonotic diseases originating in animals, the answer to this has a lot to do with humans. First, land use change has been linked to more than 30% of new diseases reported since 1960.
Land use changes increase human contact with wildlife and potential new diseases. Humans invade natural habitats
for things like mining, logging, and building settlements. The lack of natural habitat left drives wild
animals into human-made landscapes, carrying diseases with the potential to infect humans along with them.
Next, the wildlife trade and wild meat market are also key contributors to this issue. Recently, wildlife trade has expanded dramatically.
This includes trading of wild meat, known as bushmeat, as well as ivory, fur, live animals, and other wild animal products.
Zoonotic pathogens are spread through the consumption of bushmeat, increased human-wildlife contact, and poor biosafety measures.
Through the wildlife trade industry, wildlife species are coming in contact with one another, allowing new zoonotic diseases to potentially jump from species to species
before eventually landing on humans. Also, the intensification of livestock production has increased the risk of zoonotic disease.
The increasing size of farms, combined with the increased amount of animals in those farms allows for pathogens to circulate through and infect more animals,
thus increasing the risk of humans being infected. For example,
several variants of the swine flu are carried among livestock and can be transmitted to humans.
Finally, Climate change allows pathogen-carrying species to inhabit new areas that were previously uninhabitable,
leading to increased spread of existing zoonoses. For example, the warming climate of North America has allowed multiple species of ticks
to inhabit new areas that were previously too cold for them to survive in.
Thus, climate change has allowed ticks to infect more and more humans with Lyme disease each year.
In this video, we went over the basics of zoonotic diseases. We discussed what they are, how
they are transmitted, and why we’ve been seeing them more often. We hope that this video helps to
demystify zoonotic diseases and provide some insight into the zoonotic viruses responsible for diseases like COVID-19, Monkeypox, and Rabies.