Did You Know About Zoonotic Diseases?

Did You Know About Zoonotic Diseases?
Exotic food markets are a prime culprit Photo Credit: Shutterstock

COVID-19 is the newest addition to the growing list of zoonotic infectious diseases across the world

Labanya Maitra
April 28 , 2020
08 Min Read

You’re probably reading this at home where you’ve been for the past few weeks. We are obviously in the middle of a pandemic that’s showing no signs of stopping any time soon. But how did we get here? And have we been here before? Let’s travel through history to find out.

Humans aren’t the only organisms in the world that travel, and we are certainly not the pioneers. Birds and mammals migrate, marine life and reptiles are constantly on the move, and this movement defines the ecosystem that these creatures inhabit. On a galactic level, we are aware of the Earth’s rotations and revolutions around the sun, the moon’s around the Earth, and even of Andromeda and the Milky Way hurtling towards each other for an inevitable collision. And on a microscopic level, it’s much the same.


Through the centuries of movement of humans and animals—where the former has encroached on the latter’s habitat, driving it both into and away from human civilisation—we have reached a stage where people are in closer proximity to animals than ever before. And this has led to an unprecedented rate of migration of infectious disease-causing bacteria and viruses from one to the other. These diseases, transferred from animals to humans through direct and indirect contact, are called zoonotic diseases, or zoonoses. And COVID-19 is one of them.

According to the Centers for Disease Control and Prevention (CDC), these diseases are “caused by harmful germs like viruses, bacteria, parasites, and fungi.” Zoonotic diseases are extremely common and “scientists estimate that more than six out of every 10 known infectious diseases in people can be spread from animals, and three out of every four new or emerging infectious diseases in people come from animals.”

Here’s a quick factsheet on some of the more recent zoonotic disease outbreaks.

Severe Acute Respiratory Syndrome (SARS)

Where did it originate: Guangdong Province, China

When did the outbreak begin: 2002

How many infected: 8,098 (WHO)

Deaths worldwide: 774

Asian Palm Civet

While the scientific community is still not absolutely certain, it is likely that SARS—also caused by a coronavirus (SARS-CoV)—spread from wild bats to civet cats, and was then transferred to humans. According to the CDC, “the illness spread to more than two dozen countries in North America, South America, Europe, and Asia,” before the outbreak was contained.

The most common symptom of SARS was a high fever, along with possible headaches, discomfort and body ache. The World Health Organisation (WHO) reported, “symptoms are influenza-like and include fever, malaise, myalgia, headache, diarrhoea, and shivering (rigors). No individual symptom or cluster of symptoms has proved to be specific for a diagnosis of SARS.”

There were also reports of mild respiratory symptoms and a possible dry cough. “Most patients develop pneumonia,” said the CDC.

The aftermath of the outbreak revealed that SARS spread through close contact and respiratory droplets from coughing and sneezing. The most common points of entry were through the nose, eyes and mouth.

Middle East Respiratory Syndrome (MERS)

Where did it originate: Saudi Arabia

When did the outbreak begin: 2012

How many infected: 2,494 (WHO)

Deaths worldwide: 858

Dromedary or Arabian camel

Also from the coronavirus family, MERS is believed to have been transmitted from bats to camels, and then to humans. Since the initial outbreak in September 2012, 27 countries reported cases of MERS-CoV.

As is typical of the coronavirus, MERS symptoms largely include fever, cough and shortness of breath. WHO also included symptoms like diarrhoea and pneumonia, and there are also asymptomatic cases, which tested positive after aggressive contact tracing. “Approximately 35% of reported patients with MERS-CoV infection have died,” said WHO.

Human to human transmission is difficult and occurs only in cases of close contact, majorly in healthcare settings. Dromedary camels play a major reservoir host for the MERS virus, and the disease is spread through unprotected contact with these camels or an infected person. About 80 per cent of MERS cases have been reported from Saudi Arabia.

Zika Virus

Where did it originate: Uganda  

When did the outbreak begin: 2007 (Island of Yap, Federated States of Micronesia)

Aedes Mosquito

Of the flavivirus family, Zika was first identified in Uganda in 1947, while conducting a yellow fever survey in the Zika forest. The virus was detected in a rhesus monkey. In 1948, it was recovered from an Aedes mosquito, and four years later, the first human cases were detected in Uganda and Tanzania.

It seemed as though the virus lay dormant for a few decades—although sporadic cases appeared across Africa and Asia—before the first large-scale outbreak in the Island of Yap in the Pacific Ocean.

Symptoms of Zika include fever, headaches, rashes, joint and muscle pain, and red eyes. The virus can be transmitted most commonly through mosquito bites, from an infected pregnant woman to her child, through sexual contact, and also likely through blood transfusions.

While Zika is easily treatable and is hardly ever fatal, it can prove risky for pregnant women, causing birth and brain defects, miscarriages and stillbirths.

H1N1 Influenza (Swine Flu)

Where did it originate: United States of America

When did the outbreak begin: 2009

How many infected in the first year: 60.8 million (in the US)

Deaths: 12,469 (USA); 1,51,700 – 5,75,400 (Worldwide)

Pigs in a farm

The (H1N1)pdm09 virus was a new strain of a previously identified H1N1 virus. The virus most likely originated from re-assortment of genetic information “between influenza viruses circulating in North American pig herds and among Eurasian pig herds,” said the CDC.

While this was a completely new strain of the H1N1 virus, it was interesting to note that about a third of adults over the age of 60 showed some form of immunity against it, probably from a previous strain of H1N1. According to the CDC, “80 per cent of (H1N1)pdm09 virus-related deaths were estimated to have occurred in people younger than 65 years of age. This differs greatly from typical seasonal influenza epidemics, during which about 70 per cent to 90 per cent of deaths are estimated to occur in people 65 years and older.”

Ebola Virus

Where did it originate: Central Africa (1976)

When did the outbreak begin: 2014 (West Africa)

How many infected: 28,616 (WHO)

Deaths worldwide: 11,310

Fruit Bats

The first two outbreaks of Ebola virus disease (EVD) happened in 1976 in South Sudan and Democratic Republic of Congo, but the 2014 outbreak was the biggest since the virus was first discovered. The 2014 outbreak began in Guinea with a two-year-old boy, who possibly contracted the disease from a fruit bat. His death was followed by his mother, sister, grandmother and two medical aides’ deaths, and the virus quickly spread through West Africa into Liberia and Sierra Leone. According to WHO, “the average EVD case fatality rate is around 50%. Case fatality rates have varied from 25% to 90% in past outbreaks.”

EVD symptoms overlap with those of influenza like fever, aches and pains, fatigue, diarrhoea and vomiting, haemorrhaging, bleeding, bruising, red eyes and rashes, among others. It can be transmitted through close contact with “blood, secretions, organs or other bodily fluids of infected animals such as fruit bats, chimpanzees, gorillas, monkeys, forest antelope or porcupines found ill or dead or in the rainforest,” said WHO.

In humans, it can spread through close contact, blood, and bodily fluids including breast milk and semen.


Where did it originate: Democratic Republic of Congo (1920)

When did the outbreak begin: 1981

How many infected: 75 million (WHO)

Deaths worldwide: 32 million

Young Chimpanzee

What was thought to be a disease exclusive to gay men, heroin addicts and Haitians in the early 1980s, AIDS was found to be caused by HIV only in 1983. The HIV/AIDS epidemic still rages on today with “37 million worldwide living with HIV, of whom 22 million are on treatment,” according to WHO. The most common HIV subtype was likely passed on to humans by infected chimpanzees that were hunted or eaten in the early 20th century.

HIV/AIDS can be transmitted between humans through unprotected sex, needles, blood transfusions, other exchange of bodily fluids, and from pregnant mothers to their babies. The disease attacks the immune system and disproportionally affects marginalised sections of the population. The only way to detect HIV is to be tested for it and “about 75% of new HIV infections outside sub-Saharan Africa are in men who have sex with men, people who inject drugs, people in prisons, sex workers, or transgender people, or the sexual partners of these individuals,” according to WHO. These are often the most stigmatised groups in society.

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