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Insights into Editorial: Understanding zoonotic diseases: How viruses break the nature-human divide





In the first 20 years of the 21st century, the world has seen outbreaks of avian influenza, Ebola virus disease, Zika virus disease, Nipah virus disease, Severe Acute Respiratory Syndrome (SARS), Middle East Respiratory Disease (MERS) and now, the novel coronavirus disease (COVID-19).

Of these, three — SARS, MERS and COVID-19 are all caused by beta coronaviruses of the coronavirus family. Despite the phylogenetic similarity among the viruses, all three have created very different outcomes.

Severe Acute Respiratory Syndrome (SARS), Middle East Respiratory Disease (MERS):

Between 2002-03, SARS caused by SARS CoV virus infected 8,422 people and killed 914. The MERS outbreak caused by MERS CoV virus which came a decade later, infected 1,791 and killed 640 people between 2012 and 2016.

The SARS-COV-2 virus, in comparison, is much more contagious, with over 3 million cases in five months, but is less deadly with a mortality rate of 2-5 per cent as compared to SARS (9.5 per cent) and MERS (34 per cent).

In fact, experts say SARS-CoV-2 is more contagious because it is less deadly.

But one thing is common among all these diseases: They are all caused by zoonotic viruses that made the jump to humans.

Various contact routes for disease transmission are possible:

  1. Direct contact with an animal’s bodily fluids (e.g. saliva, faeces and blood), via, for example:
    1. Touching an infected animal’s skin;
    2. Being bitten by an infected animal.
  2. Indirect contact within areas where infected animals live and roam, including:
    1. Breathing in dust particles and small droplets of saliva;
    2. Consuming contaminated food products;
    3. Contact with contaminated water, soil, objects or clothing.
  3. Disease vectors: organisms that transmit infectious disease between animals, and between animals and humans. These are typically insects that feed on the blood of humans and animals, and in so doing, transmit pathogens between them.
  4. Direct contact and food consumption are the most common modes of transmission for diseases associated with food systems. Airborne transmission of viruses are also common routes for infection.
  5. The risk of transmission to humans is higher for those with high level of occupational exposure to livestock and livestock products, such as those working with livestock, in abattoirs, and in the meat processing industry.
  6. While the type of contact is what directly enables disease transmission to take place in each case, many other factors may promote or reduce the likelihood that a contact event occurs (i.e. degree of exposure to an infection), and that if it does, this then leads to a human infection (i.e. degree of susceptibility to infection).
  7. These include natural and human induced changes in ecosystems, changes in food and agriculture systems, and changes in human living environments and consumption practices.

Sylvatic cycle / Enzootic cycle: How and When Viruses mutate to effect our immune system?

  1. Viruses are everywhere and exposure to them is always there. But only a few are able to mutate and infect us.
  2. In most cases, either the virus hasn’t evolved enough to infect or the immune system recognises it and protects us against it.
  3. The infections did not usually happen in the first exposure. Prolonged exposure or a long period of incubation was required for that.
  4. For any infectious disease, be it an emerging or an established one, there are three major requirements, often referred to as epidemiological triad:
  5. The causative agent, the host and an environment in which the host and agent are brought together.
  6. In case of most of the recent spill-over events, the causative viruses or their precursor strains already existed in the system through the “sylvatic cycle / enzootic cycle”, a natural transmission cycle of a viral pathogen within its natural animal host (bats for rabies and Nipah; macaques and rodents for Kyasanur Forest Disease, etc).
  7. For instance, Ebola was not new to Africa, and outbreaks had been confirmed as far back as 1976.
  8. However, the initial outbreaks were restricted to one or two countries during individual episodes and outbreaks gradually faded out.
  9. However, the 2013-2014 outbreak in West Africa was the largest-ever recorded and differed dramatically from prior outbreaks in its duration, number of people affected, and geographic extent.
  10. More than 5,000 laboratory-confirmed human cases were recorded, with 50 per cent plus mortality.

How are diseases transmitted between animal species, and then on to humans?

If pathogens are biologically able to infect more than one type of animal species (including humans), when contact is made between such animals, the potential for the pathogen to be transferred to a new host species exists. Occasions where this takes place are known as ‘spill over’ events.

Environmental factors like temperature, ultraviolet radiation, relative and absolute humidity, and air ventilation or air movement are important drivers influencing virus viability in the air, the study found.

Factors like temperature and humidity impact the size of droplets, which in term, affect the viability of the virus.

What are the common factors in animal and human disease?

In both animals and humans, infectious diseases are caused by microorganisms known as pathogens.

Types of pathogen include:

  • Fungi
  • Viruses
  • Parasites
  • Bacteria

Many pathogens are intrinsically able or have developed the ability to infect both animals and humans, and so may be transmitted between them.

These are collectively known as ‘zoonotic’ diseases and make up more than 60% of all known human infectious diseases.

Through the biology and ecology of their shared pathogens, human and animal health are interconnected.

In turn, humans, animals and pathogens are also influenced by wider changes in ecosystems and the natural environment in which they exist.

In recent years, a growing trend has been the adoption of more interdisciplinary and transdisciplinary approaches to managing health risks.

These approaches include One Health, Planetary Health, and Eco-health, which look beyond human medicine, and also incorporate the biology and health of non-human species, ecosystems functioning, and the effects of environmental change.

Population growth areas are having direct correlation with spreading of zoonotic diseases:

There was a positive correlation between the outbreak and population and urban growth between the 1970s and 2013 in these countries.

Population growth has been dramatic in the region, with population densities increasing by 223 per cent, 178 per cent, and 275 per cent in Guinea (1960-2012), Sierra Leone, and Liberia, respectively (1961-2013).

Rural-to-urban migration and growth in the affected countries has significantly increased here; the proportion of the population that is now urbanised has increased significantly in Guinea (248 per cent, 1960-2013), Sierra Leone, and Liberia (130 per cent and 163 per cent respectively, 1960-2013).

All the major Ebola outbreaks were in such urbanised set ups with high human densities.


Zoonotic diseases are so dangerous because they are novel and highly unpredictable.

Because they jump from animals to humans, our immune system is unable to fight them. And because they are unpredictable, no one knows when a localised outbreak can turn into a pandemic.

Unlike the old diseases like cholera, pneumonia, which we know how to deal with, these diseases are highly unpredictable.

With factors like climate change, zoonoses are emerging as the single biggest threat to human health and humanity is not prepared, as is evident from the COVID-19 pandemic.


Insights Current Affairs Analysis (ICAN) by IAS Topper