The largest ever Monkeypox outbreak outside African countries has brought a renewed attention on zoonotic diseases. Global public health experts are a bit puzzled about how this outbreak began in multiple countries — something not seen in the past — but they are not completely surprised. We must remember that viruses have been on earth for millions of years before the origin of human species. They are far greater in numbers and only a minuscule fraction of an estimated 1031 viruses on earth or around 10,000 virus species have the capacity to infect human beings. Fortunately, a majority circulate and are found in wild animals.
However, things are changing slowly. In the last 50 years, scientists have identified around 1,500 disease-causing agents (the pathogens), most of which have jumped from animals to humans. Such infectious diseases, which are caused by pathogens jumping from animal to humans, are called zoonotic diseases.
In today’s world, zoonotic diseases are becoming a major public health problem. Between 1940 and 2004, nearly 330 diseases had emerged, of which more than 200 were zoonotic. Of these, 70 per cent pathogens were from wildlife. The ongoing Covid-19 pandemic, the H1N1 swine flu pandemic and the Ebola virus were zoonotic in origin.
The reality is that with every passing day, the possibility of emergence (of a new pathogen and disease) and re-emergence (of older pathogens) of zoonotic diseases is increasing. The reasons are multiple. First, deforestation and increasing human intervention in forests mean new microbes are coming into human contact. A study by the researchers at Aix-Marseille University and Montpellier University had documented a link between reduced global forest cover between 1990 and 2016 and an increase in reported outbreaks and epidemics. Second, global warming and rising temperatures which result in microbes adapting to and surviving in new conditions and places. Third, a rapid and unplanned urbanization, the dense settlements and overcrowding in cities contributes to faster spread of infections. Fourth, rapid, widespread and increased air travel connectivity ensures that a human being (and a pathogen) can travel from one part of the world to another in less than 24 hours and well within the incubation period of disease.
Fifth, the single animal livestock or “intensive” farming increases these risks by allowing the multiplication and mutations of the pathogen. In factory farms, tens of thousands of animals are cramped together in indoor settings, creating an ideal environment for rapid multiplication and spread of viruses and bacteria to many animals. This provides an opportunity for a greater chance of the viruses mutating and, therefore, increasing the probability of emergence of variants or strains which can be more harmful and have higher zoonotic potential. These are not hypothetical situations. The 2009 swine flu (H1N1) pandemic virus spread from a factory pig farm 8 km away from a locality in Mexico. Sixth, the indiscriminate use of antimicrobials, especially in poultry farms and the agriculture industry, means pathogens become resistant and can survive longer to cause severe disease in humans.
Seventh, the increased consumption and wildlife trade are other contributors of zoonotic diseases through “wet” markets. These provide a perfect environment for the spread of undocumented pathogens that exist in some wild animal populations. The initial cases of COVID-19 were identified from people who had come in contact with a wet market in Wuhan, China. In 2003, the first large outbreak of Monkeypox outside endemic countries in Africa, was the outcome of wildlife trade in the US. The virus spread through imported African wild mammals, housed alongside Prairie dogs, which became infected and were then adopted as pets across multiple states.
The evidence on greater probability of zoonotic diseases is increasing. A study by researchers at the University of Georgetown in the USA and published in the journal Nature in April 2022, has estimated that if the Earth’s temperature rises by 2 degrees in the next 50 years, it would mean that wild animal species will be forced to settle in new areas and closer to human settlements. If that happens, it may facilitate a “zoonotic spillover” or disease-carrying germs coming into contact with humans. In that scenario, by 2070, we could be exposed to about 10,000 to 15,000 new pathogens (bacteria and viruses) previously confined to wild animals and forests. This would result in a 4,000 times likelihood of cross-species transmission of the virus. They have also projected that the countries in Africa and Asia are most likely to be impacted by this situation. Since the majority of these microbes will be new with no prior immunity in people, it will increase the likelihood of disease spread and epidemics.
These conditions and scenarios may appear scary but also remind us that there is a lot which we can do to avert that scenario. The zoonotic diseases spread through multiple modes and the prevention methods differ for each pathogen and their route of transmission. Every country individually and the world community together need to take action. There is a need for the scientific, public health and medical community to work together. It requires interventions in areas of protecting the environment, preventing deforestation, planning urban settlements better and ensuring rational use of antimicrobials, among others.
These interventions are interlinked. As an example, by preventing global warming, we can avoid diseases and outbreaks as well. It requires different sectors and ministries which deal with animals, humans, forests, agriculture and climate change work together. A beginning has been made and there is global attention on the concept of ‘One-health”, which proposes combined actions for the health of animals, environment, and humans.
The pathogens (viruses and bacteria) have been part of the evolution of the earth. They appeared on earth millions of years ago and long before humans. It is the disturbance in harmony that we are seeing the emergence of zoonotic disease, a situation which is a stark reminder of humans overstepping with nature.
The COVID-19 pandemic has once again reminded us that economic and social costs of pandemics could be in trillions of dollars, enough to halt and reverse the economic growth trajectory. The investment on epidemic preparedness and response is only a fraction (of this cost) and if Governments start investing on such interventions, we can both delay a few and avert a few more of the future epidemics and pandemics. The way things have changed in the last century and for the factors listed earlier, the zoonotic diseases are going to be part of our life. We may not prevent all future zoonotic disease outbreaks and epidemics, but with the right precautions and preparedness, it is possible to stop some epidemics turning into a pandemic.
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(Dr Lahariya specialises in infectious diseases, health systems and epidemiology. He is Co-Founder of the Delhi-based ‘Foundation for People-centric Health Systems’. He Tweets at @DrLahariya)
