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Emerging Zoonoses: A Human-wildlife Interface?

Outbreaks of emerging zoonotic diseases have increased in the past decade and have affected the population worldwide. Often the blame for such spillover events is put on animals; however, it is humans who have ruined the balance of the ecosystem and exploited flora and fauna for their own gain. The intrusion of humans into wild habitats has increased the human-wild interface thereby increasing the chances of interspecies transmitting diseases in both directions. At the same time, globalisation has increased the likelihood of the rapid dissemination of infection worldwide. Not wildlife but anthropogenic determinants behind the occurrence of such events should be realised and addressed vehemently.

Millions of years ago when life began on Earth, numerous species emerged and co-evolved sharing Earth’s resources and habitat. Homo sapiens, which evolved as the most intelligent species, dominated and exploited the major share of resources pushing the other species towards extinction or (at the very least) struggling to thrive. Humans expanded their habitat, croplands and livestock into the forests, which disrupted the natural ecosystem and thus ruined the harmonious coexistence with other wild species. Meanwhile, broken barriers by close human-animal interfaces enabled the interspecies transmission of pathogens to distant and diverse species. Incidences of novel pathogen emergence by animal-to-human transmissions and the extermination of millions of humans from Earth have been witnessed on various occasions. Outbreaks from emerging infectious diseases have been reported to increase every decade since the 1980s and most of them have been linked to wildlife. Increased human-wildlife interactions brought about the recent pandemics of Human Immunodeficiency Virus, Ebola, swine flu, avian influenza, Severe Acute Respiratory Syndrome, Middle East Respiratory Syndrome, Nipah and many more. At this moment during the COVID-19 pandemic, its causative agent (SARS-CoV-2) is found to be closely related to the SARS-like coronavirus in bats1 and we must examine the anthropogenic determinants behind such circumstances, rather than pointing our fingers towards bats or other wildlife species.

In 2019, when the whole world was busy celebrating New Year’s Eve, China encountered a cluster of cases suffering from pneumonia detected in Wuhan city and linked its emergence to the Huanan wet market. Taking into account the zoonotic emergence of the disease, Chinese authorities closed those animal markets in the city and reported the incident to the World Health Organization (WHO). Within two months, the disease spread globally and caused more than 200,000 casualties out of more than 3 million cases thus far. Considering the spread and severity of the disease, WHO declared the disease as a pandemic on the 11th March 2020. A pandemic is defined as “an epidemic occurring worldwide or over a very wide area, crossing international boundaries and usually affecting a large number of people”2. Simply put, it includes widespread diseases causing large-scale morbidity and mortality. Pandemics not only affect public health but also disturb the sociopolitical structure of the countries. The underlying cause reported for the most recent pandemic is the emergence of a new virus or virus strain/ subtype, due to genetic reassortment. These new viruses are usually highly contagious and after initial transmission from animals readily spread between humans, causing worldwide dissemination. Increased wild-human interface over the past years have increased interspecies transmission of the virus from maintenance hosts to new hosts, seen as a spillover, as well as reverting from spillover hosts to the maintenance hosts, which is known as spillback. These continuous spillover and spillback cycles have expedited the evolution of viruses where wildlife acts as reservoirs together with “living test tubes” facilitating mutation and recombination of the viruses.

Out of more than 1400 documented human pathogens, approximately 61% are considered to be zoonotic. In a study in 2007, Woolhouse and team listed out 87 novel pathogens that were reported to be pathogenic to humans from 1980 to 2005. Two-thirds of these were viruses and 85% had single-stranded RNA (ssRNA) genomes3. Most of the emerging viruses are ssRNA viruses, which lack the proofreading capabilities of DNA polymerase or post-replication mismatch repair, leading to a high rate of error during RNA replication which is around 10 times more than DNA viruses. Most RNA viruses are zoonotic in nature as they are capable of a species jump; they were transmitted, at least initially, to humans from non-human mammals or avian hosts. Examples of RNA viruses retaining the capacity to be directly transmitted from animals to humans include influenza, Nipah, and SARS viruses, but even some viruses commonly transmitted exclusively between humans, such as HIV and hepatitis C, likely have animal origins.