Why spill-over events could cause the emergence of future pandemics
Spill-over refers to transmission of pathogens from vertebrate animals to humans.
This means the host’s immune system is naïve to the pathogen and can suffer serious pathology. The process of spill-over might be direct from a single animal host or through an intermediate host and may take years to take hold and adapt to the new hosts, finally leading to genetic evolution, enabling a secondary epidemiological cycle in humans and, in some cases, persistence.
This is a chance event rather than a normal part of a zoonotic cycle. The human population expanding into wildlife habitats, increasing human interaction with domestic and wild/exotic animals, increased travel and movements of humans and animals, globalization of food supply, and climate change are some of the factors associated with emergence of spill-over events.
In the last two decades, there have been a number of spill-over events related to the novel corona viruses. SARS-CoV-1, which originated from bats, passed into humans through intermediate racoon dog or civet cat. Strong and rapid public health measures and massive culling of civet cats abruptly stopped the spread.
SARS affected 29 countries, with more than 8,000 cases, 774 deaths, and the economic impact was estimated to be around $30-$140 billion. MERS (Middle Eastern respiratory syndrome), yet another disease cause by another novel coronavirus, was first identified in Saudi Arabia in 2012. It is believed the virus spread from bats to camels and then from camels to humans. Although this remains a hypothesis.
From 2012, there have been >2,000 cases and 858 deaths in 27 countries, but mostly from the Middle East.
The current Covid-19 pandemic, which is also caused by infection with another novel coronavirus (SARS-COV-2), was first detected in the Wuhan city of Hubei province in China. Epidemiological investigation identified that about 100 of the initial Covid-19 patients had exposure to the Hunan seafood wholesale market at Wuhan city in China. The SARS-CoV-2 virus is 96% identical at the whole-genome level to a coronavirus isolated from horseshoe bat (Rhinolophus affinis) in Yunan province in China and its evolution suggests common ancestry around 50 years ago.
However, coronaviruses are common across multiple vertebrate species and other hosts cannot be ruled out for SARS-CoV-2 at this stage. Malayan Pangolin also shared 91% identical sequence at full genome level, but further phylogenetic analysis suggests that the human SARS-COV-2 responsible for the current pandemic was unlikely to have come directly from pangolins. Subsequently, the SARS-COV-2 has been detected in a number of animals, including three dogs (two in Hong Kong, and one in North Carolina), four cats (one in each of Hong Kong, France, Belgium, and the US), three tigers and four lions in the Bronx Zoo in New York, and 10 mink farms in Netherlands and Denmark.
However, there is not enough evidence to suggest transmission of infection from animals to humans.
Spill-over in the context of Bangladesh
We have had multiple spill-over events in the past in Bangladesh. Nevertheless, many spill-over events in Bangladesh never got enough airtime. Nipah is a classic example of a zoonotic disease that continues to cause recurrent spill-over events in Bangladesh. Nipah is an emerging zoonotic disease cause by the Nipah virus.
Fruit bats (Pteropus giganteus) are the only natural reservoir for Nipah, which is transmitted to humans through spill-over events. It was first identified in 1998 in the Nipah village of Malaysia (hence the name), where the virus jumped from bats to pigs and then from pigs to humans.
Nipah since then has not been identified in Malaysia. In 2001, Nipah was identified in Bangladesh for the first time.
In Bangladesh, it is believed that the virus spreads from fruit bats to humans through drinking of raw date palm juice. Date palm juice is a delicacy in Bangladesh and research has demonstrated that there is a clear link between human Nipah infection and drinking date palm juice contaminated with bat urine and faecal content in Bangladesh.
Nipah continues to cause small outbreaks, often once every one to two years in Bangladesh, but the fatality rate is very high, sometimes reaching up to>90%. Most recently, in 2018, there was one in Kerala, India which caused 17 deaths.
We have also had spill-over events associated with the highly pathogenic novel H5N1 avian influenza (commonly called bird flu) and the pandemic (H1N1) 2009 influenza (swine flu) viruses in Bangladesh. These are both caused by influenza viruses, and have resulted from spill-overs between wildlife to domestic animals and then from domestic animals to humans.
This is not the first pandemic and will not be the last one.
Spill-over events remain a significant cause for the emergence of future pandemics. Our ability to predict future spill-over events will be dependent on sustained international collaboration and multi-sectoral coordination, a high level of political commitment, and strong public infrastructure, including enhanced surveillance on human-animal interfacing.
Dr Najmul Haider is Post-doctoral Researcher, Royal Veterinary College, University of London, UK. Dr Nusrat Homaira is Senior Lecturer (Respiratory Epidemiology), University of New South Wales, Sydney, Australia. Dr Rumi Ahmed Khan is Associate Professor, UCF College of Medicine, USA.