Wildlife & Biodiversity

Understanding zoonotic diseases: Here’s a video primer

While the origins of COVID-19 are still unknown, epidemics like SARS and MERS happened after transfer of pathogens from animals to humans

Published: Wednesday 06 May 2020

The novel coronavirus disease (COVID-19) pandemic has brought the world to a near halt. It’s not just trade and commerce or travel and tourism that have been suspended, but regular, mundane activities such as visits to the supermarket have also been curtailed.

But where did the SARS-CoV-2 virus come from?

It has been previously speculated that bats transferred the virus to a human being at a busy wet market in China. Another theory about the virus jumping to a pangolin from an infected bat, and then to humans, has also done a few rounds.

We may not be absolutely certain about the origin of COVID-19, but we know that most disease-causing pathogens, over 60 per cent of them, came from animals.

The transfer of a virus from an animal to human beings is called zoonoses. Diseases caused by this process are called zoonotic diseases.

All recent epidemics have had their origin in animals. The extremely fatal Ebola virus, for example, which killed 11,323 people between 2014 and 2016 in West Africa, was transmitted either from bats or non-human primates.

While the Severe Acute Respiratory Syndrome (SARS) epidemic of 2002-03 was reportedly caused by civet cats, the Middle Eastern Respiratory Syndrome (MERS) came from camels. Even the roots of influenza or the common flu can be found in birds and pigs. Human immunodeficiency virus (HIV) came from monkeys. And the list goes on.

Yet zoonotic diseases differ in the way the viruses are transmitted. For example, dog owners are conscious that their pets need vaccination against rabies. Rabies is spread among humans through a bite of an infected dog. This is called direct transmission.

Similarly, influenza, which can be transmitted through air, or SARS, which may have been acquired by meat handlers directly from the animal are examples of direct transmission. Direct transmission is when a human gets the disease directly from an infected animal. 

In contrast, viruses can also be transferred to humans through an intermediary or a vector. Here, the vector, like a mosquito, simply transports the pathogen. The mosquito is not infected — it simply transfers the pathogen to a human. 

The Zika fever is one such example, where mosquitoes may have first transported the virus to humans from monkeys.  Another example is the African sleeping sickness, wherein the microbe was transported the tsetse fly from wild and domesticated animals — and eventually to humans.

Very little is known why these diseases are so rampant. Deforestation can be a propellant. Scientists suspected that a two-year-old child playing under bat-infested trees triggered the 2014 Ebola outbreak in West Africa.

Similarly, the spread of Lyme disease, which infects around 30,000 people in the United States annually, has spiralled due to the dwindling population of animals such as opossums and chipmunks that control the population of ticks, the main carriers of the disease-causing bacteria.

Bad sanitation policies have led to an increase in vector-borne zoonotic diseases such as dengue and chikungunya. COVID-19 and SARS are the two examples of viruses spreading through this route. 

Commercial farming of animals has made people more susceptible to diseases like the bird flu and swine flu. Overexposure of antibiotics on farm animals has increased antibiotic resistance in disease-causing microbes, creating a silent pandemic across the world. 

Over 700,000 people die every year because of antimicrobial resistance, according to World Health Organisation. This includes multi-drug resistant tuberculosis, which was once a disease of zoonotic nature.

COVID-19 is only one of the over 200 zoonotic diseases we know about. Many more will emerge because of the severe degradation we have subjected the environment to. 

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