The earth we abuse and the living things we kill will, in the end, take their revenge; for in exploiting their presence we are diminishing our future. ~ Marya Mannes

Each species represents a thread in the closely woven fabric of Nature. For centuries, we humans have prided ourselves on being the most ‘evolved’ species. Superior intelligence and technological capability have bred this arrogance.

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But man has unravelled a very small portion of Nature’s secrets; yet he nourishes the wrong notion that he can subjugate Nature. He considers all non-human species as ‘inferior’ beings and exploits them as a captive resource. In the words of E F Schumacher: ‘Modern man does not experience himself as a part of Nature but as an outside force destined to dominate and conquer it’.

He is trying to change the rules of game with Nature, forgetting that Nature has its own powerful arsenal of weapons to take revenge for such domination. Its arsenal bristles with extreme weather events including drought, floods, hurricanes, wildfires and so on. It also wields biological weapons in the form of Zoonoses like yellow fever, plague, Ebola, SARS, MERS and most recently Covid-19.

Man’s daily life is interwoven inextricably with microorganisms (bacteria, viruses, fungi, protozoa, algae ). They abound in the soil, in the seas, and in the air. Everywhere abundant, although usually unnoticed, they live in highly competitive surroundings and determine their survival and genetic persistence either from the co-evolution of mutually supportive species or from what Richard Dawkins called the ‘biological arms race’.

Humans, livestock and wildlife also share large pools of microbes and parasites. We humans are mostly microbes. All of us have a microbiome of microbes (around 100 trillion) that outnumber the cells of our bodies by a ratio of ten to one. The microbiome is essential for human development, immunity and nutrition. Animals have long been recognised as agents of human diseases. Indeed, it has been discovered that many invertebrate animals are capable of transmitting causative agents of diseases from man to man or from other vertebrates to man.

Such animals, which act as hosts, agents and carriers of disease, can cause and perpetuate human illness. For example, bats are known to host around 140 viruses, including Ebola and Nipah. Of these, about 60 can be transmitted to humans. Different rodents also carry about 180 types of viruses of which about 79 could be transmitted to humans.

There has been a stark increase in the incidence of emerging infectious diseases (EIDs) among both humans and domesticated animals in the recent past. Over the past 70 years, more than 300 zoonotic diseases (or Zoonoses) have been detected. Notably, between 60 per cent of all human diseases and 75 per cent of all EIDs among humans originate in animals. According to a 2012 report of the Department for International Development, UK, Zoonoses are responsible for over 2.7 million deaths and over 2.5 billion cases of human illness every year.

Indeed, zoonoses are increasingly being considered as a potent threat to the stability of society. Little is known about the reasons behind the spread of zoonoses because there is hardly any research on the subject. But it is well established that deforestation could be the primary reason because changes in forest cover and land use increase human contact with domesticated and wild animals.

Consequently, the possibility of crossing over of new viruses from animals to humans also gets increased. Indeed, a close scrutiny reveals that frequency of such virus transfer has increased 2 to 3 times in the past four decades, in a cycle of every three years. Moreover, ‘climate change is the biggest global health threat of the 21st century’, according to a report published in The Lancet in 2009. Indeed, climate change is disrupting the natural ecosystems in a way that is making life vulnerable to infectious diseases. Mosquitoes are highly sensitive to temperature.

They pick up viruses faster in higher temperatures, and the hotter it is outside, the more likely that a mosquito will bite. Certain disease-carrying insects have unwittingly moved from the mid-latitude to other regions; the shifting of conditions are contributing to the spread of diseases like dengue hemorrhagic fever, yellow fever and the West Nile fever. The viral hemorrhagic fever, Kyasanur forest disease (KYD), occurs in south-western India during the dry months, from January to May.

Monkeys are the common host of the virus of disease. Ticks that live on monkeys harbour the pathogen. And the virus is transmitted to humans through the bite of a tick or when humans come in contact with infected animals. There is evidence that global warming that increases the population of insects could also lead to the spread of the disease. Acquired Immunodeficiency Syndrome (AIDS) is a zoonosis of the immune system caused by the human immunodeficiency virus (HIV).

Humans and lower vertebrate animals share it. According to the World Health Organization, an estimated 39 million people have died worldwide of AIDS-related illnesses between 1981 and 2013. According to the National AIDS Control Organisation, more than 2 million people in India were suffering from HIV/AIDS in 2014. A 2010 research paper published in Nature mentions the host species in which pathogens multiply rapidly to high levels providing an important source of infection for vectors.

These are generalist species that invest less on immunity and more on adaptability to a wide variety of habitats and food sources. By contrast, specialist species, which act as buffers against pathogen proliferation, are highly adapted to only one specific habitat and type of food. But they invest heavily in their immune system. A loss of biodiversity due to a change in the habitat often results in a simplification of the environment through elimination of specialist species and over-population of generalist species.

The link between the outbreak of pathogens and loss of biodiversity and habitat can also be explained. Hantavirus causes the hemorrhagic fever with renal syndrome in many parts of Asia and the Hantavirus pulmonary syndrome (HPS) in America. The infection spreads through bite, scratch or faecal aerosols of Murid rodents that carry the virus. Here Murid rodent is the generalist species and therefore can adapt to varied and changing ecosystems.

Habitat changes caused by forest fragmentation in the US and Latin America has thus led to the emergence of HPS. Similarly, in areas with compromised biodiversity, the prevalence of pathogen in the blood of reservoir species was found to have increased threefold as compared to undisturbed habitats. Habitat fragmentation and loss of biodiversity also explains the emergence and transmission of novel diseases, such as Lyme disease in the US and Europe, to humans.

The disease is caused by a bacterial pathogen, called Borrelia burgdorferi. White footed mice and white tailed deer have been identified as the natural reservoir species for the bacterium, which is transmitted by tick bites. Diminishing biodiversity and the lack of large predators led to the vector borne disease in the US.

Nipah virus is a zoonotic virus. Habitat destruction due to deforestation forced species to venture from forest to urban areas in search of food. The first outbreak of encephalitis in Malaysia was reported in 1998 when fruit bats, which act as vectors for the Nipah virus, were displaced due to deforestation. The bats relocated to nearby pig farms where they fed on fruit trees. Persons who were affected by the disease were closely associated with pigs, which were infected through contact with bats.

The Nipah virus outbreaks observed so far in South and South-east Asia have been relatively shortlived and contained in scale. Migratory impacts on proliferation and adaption for pathogens can be much more severe. Avian influenza, commonly known as bird flu, is caused by diverse variants of the H5N1 virus. The primary mode of transmission is bird to bird, although bird flu has adapted to humans with increased human-bird contact since the advent of largescale poultry farming. Since birds do not obey geographical borders, the virus has caused flu globally.

Over the past 70 years, more than 300 zoonotic diseases have been observed. They are increasingly being considered a threat to the stability of human society. The outbreak of Covid-19 is an example of Nature’s proactive response to the suicidal behaviour of humans. What is needed is called ‘behavioural distancing’. We should change our lifestyle and our attitude towards Nature to heal our planet. In the words of Jimmy Carter, the former US President and Nobel Peace laureate, ‘As individuals we can act to reduce our risk of exposure to disease and extend our care to others.’

(The writer is a retired IAS officer)