There are certain striking common threads in each outbreak, with the most deadly ones being zoonotic.
Over the last century, scientists have been quietly acknowledging increasing outbreaks of novel infectious diseases at regular intervals. Sixty per cent of these emerging infectious diseases are zoonotic and strikingly 71.8% originate in wildlife. A careful analysis has shown that the increasing frequency of these outbreaks is not merely due to improved surveillance or our technological prowess of identifying new viruses.
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The Spanish flu which swept the world in 1918-19 affected 20-30% of the world’s population and killed up to 40 million people. Genetic testing and phylogenetic analysis of the influenza viruses from the fixed and frozen lung tissue of the 1918 epidemic victims have revealed that it originated from avian influenza viruses. The natural reservoir of these influenza viruses are wild waterfowl. Episodic reassortment of the genetic material of avian and human influenza viruses result periodically in a new virus to which humans may be non-immune. Since pigs can be infected with both avian and human viruses, they may sometimes be a carrier for these reassortments.
Since 1994, there have been several outbreaks reported from Hong Kong, Vietnam, Thailand, Canada and the U.S. of influenza originating mainly from poultry farms. Influenza epidemics tell us that it is inevitable that we will be repeatedly besieged by new viruses episodically as viruses evolve and our immunity to them wanes.
In the mid-1990s, bats were identified as the reservoir of a deadly outbreak of Hendra virus in Australia. The virus spread to horses and then to humans. Since then, bats have been repeatedly identified as the reservoir for a series of zoonosis including the Nipah virus, Ebola, SARS-CoV-1 and now the SARS-CoV-2. Bats often live in large colonies, going up to even a million members. They are a much-maligned but vital part of complex ecosystems. Their unsung duties include controlling insect populations, reseeding cut forests, pollinating plants and scattering nutritious guano to enrich the soil. For various reasons, they are secret reservoirs of viruses which intermittently emerge in history to wreak havoc on unheeding creatures who tamper with the fragile, tightly intertwined biological tapestry of what can loosely be called life.
They have what is called a “fission-fusion” social structure. They collect in large breeding grounds and split up into variable subgroups. This gregarious behaviour encourages viral spread to large numbers. Another peculiar behaviour is their habit of daily torpor and prolonged hibernation during the cold season. It is hypothesised that their low metabolic rates and temperatures in this period suppresses their immune response to viruses. They were also some of the earliest mammals, and chiropterologists wonder if their innate and acquired immune response is perhaps not as well developed as those who came later such as rodents and primates.
In September 1998, there was an outbreak of acute febrile encephalitis among pig farmers of Malaysia with a mortality rate of 40%. It was noted that it had been preceded by an epidemic of barking cough and encephalitis in the pigs. Finally, the virus was identified in the cerebrospinal fluid of these patients and named as Nipah virus. Further studies determined that the natural reservoir of the virus was the fruit bat widely prevalent in the forests of Malaysia. The chain of events which led to the epidemic is worth understanding.
Due to the El Nino Southern Oscillation effect in 1997, there was an excessively dry season which exacerbated anthropogenic fires traditionally used in slash and burn cultivation in Indonesia. More than 5 million hectares of forest was destroyed resulting in a severe haze never experienced before. Forest trees failed to flower and fruit. Fruit bats were compelled to seek food in orchards. Many of these were adjacent to pig farms which are a huge industry in Malaysia. It is surmised that the bat to pig transmission occurred there by contamination of pig feed with bat excrement. The pigs were instrumental in passing it on to the pig farmers and were an intermediary amplifying host. The disease jumped countries when pigs imported from Malaysia infected abattoir workers in Singapore with subsequent deaths there.
When the whole chain of events was decoded, abattoirs in Singapore and Malaysia were shut down, one million pigs were culled in Malaysia, rest of the pigs were kept in close surveillance and finally the outbreak was quelled with international and multidisciplinary inputs.
Later outbreaks of Nipah in Bangladesh revealed that bats whose natural habitats had been disturbed had started frequenting forest fringes to eat date palm sap. The contamination of the date palm sap which is collected by the village people to make fermented date palm juice led to transmission to humans without any other animal intermediary.
The most recent outbreak of Nipah virus encephalitis in Kerala infected 18 people with an 88.8% mortality. The virus was again isolated in bats of neighbouring forests.
Towards the end of 2002, there emerged a new disease termed SARS (severe acute respiratory syndrome) which was labelled the first pandemic of the 21st century. It infected 8,422 patients in 29 countries with 916 deaths. This was one disease where the transmission was meticulously traced. The first patients were in the Guangdong province of China. On February 22, 2003, one patient went to Hong Kong and stayed in the Metropole Hotel. Here he infected 10 more people living in the hotel by aerosols generated by vomiting in the corridor. Guests who got infected here were the beginning of a chain which led to international spread as they flew back home to Canada, Singapore and Hanoi. The origin of this coronavirus was again traced to bats which transmitted to humans probably via an intermediary host — either the Himalayan palm civet or racoon dogs linked to a live animal market in Shenzhen, China.
The details of the current SARS-CoV-2 epidemic is still evolving. By all counts, it originated in a wet market in Wuhan, China and has since spread malignantly all over the world. The natural reservoir has again been traced to bats with a possible animal intermediary being the pangolin.
What do all these outbreaks tell us? There are certain striking common threads in each story. Most deadly emerging infectious disease outbreaks are zoonotic. Because organisms are constantly evolving, it is just a roll of dice as to when a new destructive organism will manifest. Viral zoonotic diseases have a potentially immense mortality as we have not yet developed effective antivirals. RNA viruses are more prone to spillover into humans. This is because RNA viruses multiply more than DNA viruses and are more prone to replication errors or mutations.
Many wild animals harbour viruses which are essentially not pathogenic to them. Degradation of their wild habitats due to climate change, excessive deforestation due to intensified agricultural practices and human settlements result in their migration to new habitats often near human habitation. Intensive animal husbandry such as poultry farming and pig farming increase the risk of spillover of new viruses from wild animals. Wet markets where animals are slaughtered in unhygienic conditions are also a high-risk zone for transmission of zoonotic diseases. Hunting and eating of wildlife has also been implicated in various zoonosis such as Ebola. Another driver of rapid spread across countries is the large scale movements of people, livestock, food and goods as well as ubiquitous air travel. The human population has also increased from 1 billion in the beginning of the 20th century to 6 billion by the end of the century. It is predicted to cross 10 billion by 2050. This dense clustering in urban spaces encourages outbreaks to spread like wildfire.
So what are the possible actions that mankind can take to reduce the risk of further inevitable outbreaks. It needs a concerted effort by ecologists, scientists, economists and sociologists and all of mankind. We need to relook at conservation of our forests, wildlife and other complex ecosystems. We need to put limits on our insatiable appetite for more, which is depleting the earth’s resources. We need to monitor emerging infectious diseases in both wild animals and livestock with systems such as the Global Early Warning System ( GLEWS) developed by the FAO-OIE-WHO. Human greed and hubris need containment. It’s time for a complete transformation of the way we live and the way we let live.