Zoonotic Diseases- The chain of transmission

22 Jun 2021

As COVID-19 continues to rattle the world, discussion around zoonotic diseases has gained prominence. Zoonotic diseases or ‘zoonoses’ are diseases that are transmitted between animals and humans due to viruses, bacteria, parasites, or fungi.

The novel coronavirus has been widely presumed to be transferred from the animal world to humans. The prominent evidence against coronaviruses emerged in 2003 when a particular coronavirus found in palm civets and raccoon dogs in wildlife markets caused Severe Acute Respiratory Syndrome or SARS in Hong Kong.

Coronaviruses are a group of viruses that are covered with protein spikes resembling a crown, which is called the corona. There are hundreds of coronaviruses, but seven to date are known to affect humans. Coronaviruses primarily attack the respiratory and intestinal organs of humans. SARS-CoV caused SARS, MERS-CoV caused Middle East Respiratory Syndrome (MERS) in 2012, and SARS-CoV-2 (the seventh known coronavirus) has resulted in the current pandemic.

The coronaviruses carry their genes in RNA form which is easy to replicate and spread. The viruses, with RNA as the genetic material, have caused Ebola, Zika, and SARS epidemics, including the COVID-19 pandemic.

The reoccurrence of these viruses in different forms, causing epidemics or pandemics, is generally attributed to closer human interface with nature. These global catastrophes could be averted if we understand the source better.

Among the billions of microorganisms that exist, only 1400 pathogens have been known to cause human infections. However, nearly 60 per cent of human infections are zoonotic.  There are various ways for the pathogens to spread-

  • Direct contact through touch, bite, saliva, scratch, etc.
  • Indirect contact through touching, eating, or encountering pathogen-infected areas which might be the habitat of birds, animals, etc.
  • Vector-borne transmission through the bite of a mosquito, flea, or other insects
  • Food or water-borne infection by consuming infected food or water

Throughout the history of human civilization, zoonoses have been a common phenomenon. Plague, tuberculosis, yellow fever, influenza, and rabies, among others have struck the world at regular intervals.

The three biggest plagues, caused by a bacterium called Yersinia pestis and carried by rodents, occurred in 541, 1347, and 1894 CE. The 1918 influenza pandemic (H1N1 virus) was one of the deadliest diseases that took the life of approximately 50 million people across the world.

Here is the timeline of emerging zoonotic diseases (spillovers) in the 21st century:

Ecological changes have a serious impact on the severity of zoonotic diseases. Let’s look at the top three reasons for the possible pandemics in the future:

  1. Overpopulation

The physical contact/dependability amongst humans is much more than any other species, thus making them more susceptible to zoonotic diseases. Increased human traveling in the 21st century is also one of the reasons for diseases to spread quickly.

2. Deforestation, Animal Extinction, & Climate Change

Uncontrolled urbanization, industrialization leading to deforestation, air and water pollution, poaching, among others, have created a huge ecological imbalance and increased the chance of contact with pathogens. A 2017 paper attributed 27 Ebola virus disease (EVD) outbreaks to massive deforestation. Increased deforestation leads to increased contact between hosts and humans.

Animal extinction caused by deforestation and poaching is another reason for zoonotic spillovers. Animals, like humans, are host to several bacteria and viruses. In case of their extinction, zoonotic viruses because of their generalist behaviour (robust to hosts) tend to spillover to other species, including humans.

Climate change can either increase or decrease the change of vector-borne transmission. A multiyear outbreak of Rift Valley fever (RVF) in Africa from 2008 to 2011 was attributed to rainfall and other environmental and geographical factors.

3. Unchecked livestock farming

Unchecked livestock farming is one of the primary reasons behind the regular occurrence of swine flu, avian flu, and other influenza viruses induced flu. Livestock across the world is often kept in low hygienic conditions with little to no attention given to disease control or waste management. More than 50 percent of the zoonotic diseases in humans have been caused because of the intensification and expansion of animal husbandry, resulting in massive deforestation and increased contact with the hosts of zoonotic pathogens.

We have made tremendous strides in microbial genetics and genomics, but there is a lack of surveillance of viruses and other pathogens in wildlife. This can only be done if funding for scientific studies by zoologists, wildlife biologists, and microbiologists is scaled up. If we manage to study certain classes of pathogens, we will be ready with the basic tools to respond to the ‘new’ or ‘unknown’ pathogens. It is also important to have more national Biosafety Level (BSL) 3 and Level 4 laboratories for a better response system.

It will require a concrete 360-degree effort and coordination amongst all countries to control zoonotic diseases. We must understand what we are dealing with to create a future world that is ready to respond and break the transmission chain.

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