Zoonosis infection cycle

Zoonosis: The Chronicles of Massive Deaths

Image Credit: IUBS

As they were in the past, they are now and will be in the not-too-distant future.

Diseases: The Regular and the Icons

Going through a presentation with my classmate, Jesutofunmi, left my head feeling like a bag of cement. It is one of those classes epileptic electricity supply made me miss and apart from the learning gap that missing the class created, it was baffling to read in a slide that “human diseases only exist in relation to people and people live in varied cultural contexts.” Simply put: having a disease is subjective. Yes, can you see why I could not wrap my head around it now? The lecturer however used osteoporosis as an example; a condition that makes bones weak and prone to fracture. It was initially considered a normal sign of ageing in the past but is presently considered a disease that should be managed. Indeed, I reasoned that even ageing in itself, which we know to be a normal part of the cycle of life, is currently being considered a disease that needs to be cured in some scientific alleys. There are researches showing positive results towards “curing” ageing already. If you found that ridiculous, I would have asked you: “why settle for 100 years when you can aim for 1000?” But hey, that is a chat for another day.

Just as Dr ‘Femi made us realise that yes, a disease is a condition that introduces an abnormality in the normal functioning of the body but it depends on whom, when and where. I swallowed the information and drank water; it should serve some good in the exam hall. More so, by now, I have come to realise that just like in life, studying in the College of Medicine will make you unlearn and relearn. I am however of the opinion that some diseases are too massive to be confined to a particular region, culture or time as their impact is global and paradigm-shifting. One particular group of such diseases is called Zoonosis.

Data, taken from Bean et al., (2013) on the emergence of Zoonosis (figure made by Ichiko Sugiyama)

Zoonosis: What Are They?

The word zoonosis as a whole may not ring a bell but a part of it “zoo” definitely will. From that root word, the meaning (or at least a clue) unravels, and it is simply a group of diseases that are passed to humans from animals. Zoonosis differs from vector-borne diseases; while the former is passed to humans by animals like dogs, bats and the likes, the latter is passed by insects or more accurately arthropods like mosquitoes, ticks, houseflies etc. Like most diseases, Zoonosis is caused by viruses, bacteria or a parasite. The significant difference however is that these disease-causing microorganisms are normally picky with the animal they naturally inhabit. Some of these animals have, over the years, got used to living with these microorganisms without coming down with any sickness. Take bats, for example, it has been recorded that the current figure for the number of viruses their bodies house is up to 66, among which are Ebola and most likely COVID-19. Interestingly, these flying mammals seem to live a normal life, with their truckload of extra baggage.

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However, it is usually deadly when these viruses enter our bodies through any contact with these animals because the human body is not equipped to deal with these strange microorganisms. This explains why Ebola and COVID-19 are names that strike apprehension in our minds. Apart from this, another thing zoonoses are infamous for is their ability to start widespread diseases that affect many individuals in a population. Examples of these scenarios abound in the history of mankind, with countless deaths in their trail.

people infected with zoonosis
Credit: History.com

As They Were In The Past


This is caused by a virus through the bite or scratch from an infected animal, mostly a dog but sometimes a bat. Rabies has been around for as long as 4000 years, and it terrorised humans in the past due to its rather strange symptoms which included fear of water, uncontrolled excitement, confusion and lack of consciousness. This aversion to the disease is perfectly captured in the origin of the word “rabies”, which is madness in Latin. What is most disturbing about this disease is that once the symptoms develop, it cannot be cured even with today’s advanced healthcare system. However, both humans and pets can be vaccinated against the disease. It is estimated that 59,000 people still die annually due to rabies. One can only imagine how many must have died back in the ages when all they had were superstitious beliefs and fruitless attempts at curing using dogs’ hair.

The Bubonic Plague

Unlike rabies whose deadliness is still wrapped in Latin, this plague caused by a bacterium earned itself the nickname The Black Death to depict how it ravaged Europe, Asia and Africa. It is reported that the disease must have reduced the world’s population by a quarter with an estimated death of about 75-200 million. It was a disease that changed the course of history as it had many economic and social implications. The disease is transmitted by fleas that lived on black rats who in turn lived rent-free with humans then. This provided the passage of the disease from rats to humans. The disease symptoms do manifest as headache, vomiting, fever and swollen painful nodes. With antibiotics such as doxycycline, this disease can be cured but back in 1346, it was a deadly disease, that infected corpses were even used in what will be one of the first instances of biological warfare.

Other prominent zoonotic diseases that were deadly include the several waves of the Spanish flu, caused by a virus transmitted from birds; and Mad Cow disease from cattle, just to keep this list short.

As It Is Now

Recent manifestations of zoonoses have been from viruses crossing from animals such as bats, chimpanzees, pangolins and other animals that humans either eat or use as livestock. Within a decade, we have seen the emergence of prominent zoonoses (Ebola, Middle East Respiratory Syndrome coronavirus, COVID-19 and more). The major contributor to this is human’s actions that consistently disturb the natural habitat of these animals which makes them move closer to our homes and sometimes, it is our attempt to get meat from wild animals that exposes us to the dangers of these diseases. 

In the Not-too-distant Future

With increased agricultural activities, climate change and deforestation, it is predicted that humans are going to experience more zoonoses. This cannot be prevented, at least not with the dilemma of overpopulation and malnutrition in our hands. Our best shot is to be prepared by making wide researches to identify as many potential zoonotic infections as possible. Armed with this knowledge, we can then prepare ahead before the infection occurs. We can also be consoled with the fact that our current and past exposure to these diseases has provided us with the technical and medical know-how on efficient ways of tackling these diseases. These and the humility to listen to experts in this field are the only weapons we can get to tackle the uncertainties that lie ahead because all it will take could be a scratch from a screeching bat before the stage is set for another cycle of a pandemic.

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