Wildlife & Biodiversity

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

Their novel nature and unpredictability make diseases that jump from animals to humans extremely dangerous

By Ishan Kukreti
Published: Wednesday 06 May 2020

Small Indian Civet,  in Silchar, Assam. The 2003 SARS epidemic spread from civets to humans. Photo: Wikimedia CommonsIn 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.

READ OUR COVER STORY Bats spread viruses, so do humans  

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.

“Viruses are everywhere and exposure to them is always there. But only a few are able to mutate and infect us. In most cases, either the virus hasn’t evolved enough to infect or the immune system recognises it and protects us against it,” Abi T Vanak, disease ecologist with ATREE, said.

He added that infections did not usually happen in the first exposure. Prolonged exposure or a long period of incubation was required for that.

For any infectious disease, be it an emerging or an established one, there are three major requirements, often referred to as epidemiological triad consisting of the causative agent, the host and an environment in which the host and agent are brought together, according to Sanath Muliya, scientist with the Wildlife Institute of India.

“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),” he said.

For instance, Ebola was not new to Africa, and outbreaks had been confirmed as far back as 1976. However, the initial outbreaks were restricted to one or two countries during individual episodes and outbreaks gradually faded out.

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. More than 5,000 laboratory-confirmed human cases were recorded, with 50 per cent plus mortality.

Muliya said 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),” he said.

“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,” he added.

A lot depends on the mode of transmission too. The ability of the virus to transmit from human to human is what makes COVID-19 so contagious as opposed to SARS or MARS viruses which could not.

Studies have found that the pandemic potential of a virus drastically increases if it mutates to acquire the ability to become airborne ie transit through respiratory droplets and aerosol particles.

A study titled Identification, Characterization, and Natural Selection of Mutations Driving Airborne Transmission of A/H5N1 Virus in the journal Cell in 2014, found that it only took five mutations for the avian influenza virus to be able to transmit through aerosol particles between ferrets.

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. 

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,” S Abdul Rahman, executive director, Commonwealth Veterinary Association Graham and former dean of Bangalore Veterinary College, said.

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, he added.

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