The World Health Organisation, in its March 2020 paper ‘Origin of SARS-CoV-2’, traced the initial amplification of the Covid-19 outbreak to the Hunan Wholesale Seafood Market in Wuhan city of China. It suspects that the SARS-CoV-2 virus, originally found in bats, jumped species and infected humans through an intermediary animal host. Covid-19 is neither the first nor will be the last epidemic to be caused by pathogens jumping species. The intense and profoundly disruptive consequences of anthropogenic activities are both the cause and consequence of pandemics. The number of disease outbreaks has tripled every decade since the 1980s.
Evolutionary biologists believe that though communicable diseases existed during the early hunter-gatherer days, their spread and impact were localised because of the small and nomadic nature of these bands. The spiralling of these disease outbreaks into epidemics and pandemics was largely a consequence of human evolution and resultant developments. The emergence of large settlements some 11,000 years ago was a consequence of man shifting to an agrarian way of life. These settlements entailed proximity and frequent interactions, which proved to be fertile ground for man-to-man transmission of infectious diseases and made epidemics possible.
The National Geographic Society says that around the same time, “people in Mesopotamia began to tame animals for meat, milk, and hides. Goats, sheep, and chicken were domesticated for consumption, while larger animals like oxen and horses for purposes of agriculture and transportation. Animal husbandry has today expanded manifold to cater to the dairy, meat, fish and poultry requirements of the world.”
Close man-animal interaction has paved the way for pathogens to jump species and cause disease outbreaks in humans. Measles is believed to have originated from the cattle disease, Rinderpest. It infected more than 30 million people annually before vaccination contained it. The spread of Nipah virus in Malaysia in 1997 is attributed to the intense pig husbandry introduced there. Mango trees planted alongside pig enclosures attracted fruit bats in large numbers. Fruits contaminated with bat saliva were eaten by pigs. When this pork was eaten by humans, it led to disease outbreaks.
The Severe Acute Respiratory Syndrome (SARS) outbreak in 2003 has been traced to a virus found in the cave-dwelling horseshoe bats in Xiyang Yi town of the Chinese province of Yunnan. It spread to humans through the intermediate host, palm civets. The Middle Eastern Respiratory Syndrome (MERS) outbreak of 2012 in Saudi Arabia occurred when a virus normally found in dromedary camels jumped species to infect man. The dromedary camels are domesticated for their meat and milk. They are also used for riding and as a beast of burden. Diphtheria, Avian Influenza, Rotavirus and Mumps also have zoonotic origins.
Rodent infestation of stored food grains led to the emergence of diseases like the Plague and Typhus. Contamination of food grains by rodent faeces or urine causes Salmonellosis, Leptospirosis and Hantavirus pulmonary syndrome. Some 10% of rat bites result in Rat-bite fever, which is common among people who work in rodent-infested grain storages or live in rat-infested areas. In India’s North-Eastern states, bamboo flowering and fruiting results in an exponential increase in the number of rodents and outbreak of rodent-borne diseases.
Travel has been a crucial factor in the spread of diseases like SARS, MERS, Ebola, Influenza, Dengue, Chikungunya, Zika virus and Covid-19. The largest epidemic of Ebola started in a small village in Guinea when an 18-month-old boy was infected by bats. It spread to neighbouring Sierra Leone and Liberia within days through travellers. International air travel then carried the disease to the US, Italy, United Kingdom and many other countries. In 2009, the H1N1 Influenza emerged on a pig farm in Mexico, spread to other countries and resulted in more than two lakh deaths. The 2007 Chikungunya epidemic in Italy was started by a traveller returning from India.
Climate change-induced extreme weather events like droughts and floods are a major cause of disease outbreaks. Water stagnation and mosquito breeding bring in their wake increased incidence of Malaria, Dengue, Japanese Encephalitis, Kala Azar, Chikungunya and Filariasis. Drought-induced food shortages drove people in Africa to increased consumption of bushmeat, which led to Ebola outbreaks. On the other hand, heavy rainfall led to an abundance of fruit, which attracted bats and apes, increasing the chances of Ebola outbreaks. As global warming melts ice and permafrost, ancient infectious agents long frozen are re-emerging. The anthrax outbreak in Siberia was a result of a heatwave that melted permafrost and the thawing of a reindeer that had died 75 years ago.
Wildlife habitat destruction due to agriculture, urbanisation, mining and industrialisation has increased the exposure of wild viruses to humans and increased the chances of virus transmission to humans. More than two thirds of the diseases in the last hundred years were directly transmitted from wildlife to people. In 1957, Kyasanur Forest Disease (KFD), a febrile haemorrhagic illness, was caused when the virus jumped species to infect people when local forests were destroyed. Human Immunodeficiency Virus (HIV) crossed over to humans in the 1920s when chimpanzees were hunted and eaten by people in the Democratic Republic of Congo. Covid-19 is just the most recent example of zoonotic viruses and pandemics.
Environmental degradation, wildlife habitat destruction and loss of biodiversity have contributed to a surge in disease outbreaks and deadly pandemics like Covid-19. These depredations will be both a cause and a consequence of the many pandemics that the future portends for us.
(The writer is an IAS officer with the Government of Tamil Nadu)