Zoonotic diseases are on the rise, are we curbing them?

alyan Ray
Last Updated : 25 June 2020, 01:58 IST
Last Updated : 25 June 2020, 01:58 IST
Last Updated : 25 June 2020, 01:58 IST
Last Updated : 25 June 2020, 01:58 IST

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Eighteen years ago, the world was struck by a new disease, Sars (Severe Acute Respiratory Syndrome) that came out of nowhere but spread rapidly throughout the world infecting 8,422 individuals with a case fatality of nearly 11%. In countries like Malaysia (40%), Thailand (22%), Taiwan (21%) and Hong Kong (17%), the death rate was much more. Years later, the scientists traced the virus to cave dwelling horseshoe bats in China’s Yunnan province while Asian civet cats were the pathogen’s intermediary. Till now, there is no treatment for Sars though fortunately, the disease has not appeared on the earth for a long time.

Cut to December 2019 when mankind encountered a related strain Sars-CoV-2 (Covid-19) that has a lower fatality rate of around 3% but turned into a pandemic within months infecting more than 85 lakh people and killing in excess of 4.5 lakh. With no signs of slowing down, the pandemic has already pushed the world economy down through a deep abyss. Preliminary research suggests this time too, the virus originated in a bat species and jumped to the humans through an intermediary animal with the index case reported from a wet market in the Wuhan province of China.

The conclusion is obvious. With people increasingly encroaching upon nature, the jungle germs found an easy way to strike at those who violate the barrier, leading to a dramatic rise in the new zoonotic diseases that originate in animals but are transmitted to people. Over the last 30 years, some 60-70% of the new diseases that emerged in humans had a zoonotic origin.

Animal pathogens can infect humans directly through contact with the wild animals that are natural carriers of these diseases or indirectly by transmission through intermediate hosts, such as livestock and domestic or peri-domestic animals that live in proximity to humans. These intermediate hosts act as “mixing vessels” that can lead to the genetic variation of diseases, enabling them to infect humans.

When natural ecosystems like forests remain intact, interactions between major human population groups and wild host species are limited. As a result, viruses circulate in natural ecosystems without crossing over into humans. Similarly, wild host species have fewer interactions with domesticated animals and livestock, which generally live in close proximity to humans. It is, therefore, less likely for domestic animals and livestock to become intermediate hosts of these diseases.

Some studies also suggest that greater biodiversity of species in a natural ecosystem like a forest may hinder disease transmission. This may be attributable to what scientists call the “dilution effect,” which makes it more difficult for a single pathogen to spread rapidly or to dominate. Evidence is not fully conclusive that this effect applies broadly across diseases, although one study found significant evidence of this effect in parasite systems and plant-herbivore systems. One study reviewing over 200 assessments found significant evidence of the dilution effect weakening transmission in parasite systems and plant-herbivore systems.

However, over the last century, there has been an alarming increase in the number and frequency of new zoonotic disease outbreaks. The frequency of such outbreaks, caused by a spillover of pathogens from animal hosts to people, may have more than tripled in the last decade. The diversity of these pathogens has also increased, with the number of new zoonotic diseases infecting people quadrupling over the same time period.

“These increases are driven by more frequent contact between humans and dangerous animal pathogens as well as by contact with a wider variety of species, resulting in the emergence of new forms of diseases in humans. These new zoonotic diseases have posed a grave threat to human health around the world,” says a new report prepared by the World Wide Fund for Nature (WWF) released last week.

Covid-19 and Sars aren’t the only examples of such species jump. There are MERS, Hendra, Zika, Nipah and Avian flu to name a few other deadly diseases that came from the wilderness. With no cure available, it was a matter of luck that none of the outbreaks assumed pandemic proportion in the last two decades. But the luck finally ran out with Covid-19.

In modern times, the risk of zoonotic diseases is accelerating, driven by two key factors: the trade of high-risk live wild animals and their meat, and unsustainable food systems driving the large-scale conversion of land for agriculture.

The global sale of wild meat and other animal products is increasingly on the rise. Some of the restaurants in Vietnam sell pangolin meat at a price of $300 (nearly Rs 23,000) per kg. A survey of wildlife consumption in three provinces in China found that high-grade restaurants and hotels accounted for 41% and 34%, respectively, of places where wild meat was consumed.

Domestic and foreign tourists are also driving demand, with local tourism suppliers often promoting the consumption of wild animals in travel destinations as a unique experience based on local traditions. Every year hundreds of thousands of wild animals are internationally traded for commercial purposes.

Land conversion

Land conversion for food and livestock production also destroys and fragments natural habitats around the world. The amount of land converted for food and livestock production is increasing at a rapid rate in order to feed a growing global population. In the last three decades, 178 million hectares of forest have been cleared, which is equivalent to the size of Libya, the 18th largest country in the world.

The loss of primary forest and grasslands has continued to grow in recent years, mainly driven by commodity production and shifting agriculture. Most habitat loss associated with agriculture is attributed to just three commodities: beef, soy and palm oil. As a result of extensive land conversion, about 70% of forests globally are now within one km of a forest edge and exposed to further fragmentation.

“India too is highly susceptible to zoonotic diseases as we have higher possibilities of humans coming in contact with animals, both due to increasing populations and because of reverence to animals that renders them to have more contacts,” commented K N Ganeshaiah, former professor and head, Department of Forestry and Environmental Sciences, School of Ecology and Conservation at the University of Agriculture Sciences, Bengaluru.

For years, India (and Karnataka) dealt with such an illness called Kyasanur Forest Disease - a tick borne viral haemorrhagic fever. Fortunately, KFD has not spread to other parts of the country because the host (monkey) and parasite (virus) combination has a restricted distribution. For it to spread there are gaps in the distribution landscape - an issue that has come to India’s advantage.

The Covid-19 pandemic has demonstrated the power of global responses to drive forward systemic changes, with unprecedented shifts in the way that people behave. With the outbreak revealing the strong interconnectedness between environmental issues and health crisis, the WWF in its report suggests to adopt the same approach in an effort to heal nature and stop entry of any future pandemic to the human enclosure.

This requires a number of steps to be taken by the governments, business enterprises and people at large. If the governments are serious, nature can be put on a path of recovery by 2030 for the benefit of all people and the planet, notes the report.

Published 24 June 2020, 18:27 IST

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