In the past, coronaviruses that circulated in humans caused only mild infections.
This changed in 2002, when the SARS-CoV virus presented itself in humans as the disease we now know as SARS. In 2020, SARS-CoV-2–a relative of the same virus–has presented itself in humans in the disease we now call COVID-19.
Many emerging diseases have been reported in the last 10 years, but we have not seen anything like COVID-19, which has affected nearly every country in the world. What is surprising is the rapid spread of this virus across the world.
A review of the global trends of emerging infectious diseases since 1940 confirms that their outbreaks have been increasing with time. About 60 per cent of these diseases are zoonotic and slightly over 70 per cent of the zoonotic infections are caused by pathogens that originate in wildlife. Some of the reasons why the frequency of the diseases is increasing is because of
1. increased close contacts between wildlife and
2. humans, encroachments of lands,
3. urbanization and socio-economic development.
As the human population increases and economies develop, the demand for food and other goods increases. Industries like agriculture are intensified. Land use, climate change, economic development, population growth and people living in densely populated areas are all contributors for zoonotic emergence, making it easy for diseases to spill over from animals to humans. Emerging diseases occur due to changes in the biological structure of a pathogen, the ability of hosts to resist infection and/or the ability of environment to control outbreaks.
A virus that might be harmless today could evolve into a much more pathogenic form, either through genetic mutations or recombination with other organisms that carry critical pathogenic traits. Small mutations often occur as viruses multiply, or as a mechanism to enable these viruses to survive better in their hosts.
For humans, we think of changes in livelihood practices—mainly socio-economic development, which typically promotes intensive agriculture where many animals are raised in a small, dense environment. Animals and birds that are often raised in such farms are of a narrow genetic diversity.
Genetically homogenous host populations are more susceptible to diseases. Land use and climate changes degrade the ability of an environment to control diseases. They destabilize pathogen-hosts interactions that occur in pristine environments, therefore increasing opportunities for zoonotic spillovers. Increased utilization of protected areas for example for horticulture, illegal grazing of livestock or bushmeat hunting bring humans and domestic animals into direct contact with potentially infected environments. When people degrade wildlife habitats or establish their own settlements in some areas, then they become part of the ecosystem. They become part the sylvatic cycle–the cycle of virus transmission that happens between animals in forests. If you tie these three things together, you have an ideal environment for zoonotic diseases.
Urbanization influences disease occurrence in peculiar ways. Poor drainage and waste disposal systems provide conditions for arthropod vectors, rodents and birds that can transmit pathogens to thrive. The high human population densities in peri-urban settlements is another critical factor that increases effective contacts that would enhance infectious disease transmission. There is a great link between climate change and disease transmission. Climate change is driven by the rising mean global temperatures. Its effects on disease transmission is complex. In general, a rise in temperature increases the development rates of vectors and pathogens up to a point (around
40°C), where a further increase in temperature would kill most vectors. Climate change also increases the incidence of extreme events, such as high rainfall and flooding, that have substantial impacts on disease transmission. Flooding, for example, boosts vector abundance, leading to an upsurge in arthropod-borne infections.
Nature takes care of itself so the best way to manage zoonotic diseases is to conserve nature and protect biodiversity. Interventions for emerging infectious diseases should be implemented through multi-agency collaborations espoused under the One Health framework. Zoonotic diseases affect humans, livestock, and wildlife at the same time. Medical experts must come together with veterinary and environmental experts and stakeholders to find solutions to these diseases. Social scientists should also be part of these interventions because we need to better understand behaviour to implement any changes in communities working in hotspots for zoonoses.
Finally, we need to diversify our focus–look at the economy, look at livelihoods. Because at the end of the day when we want to recover from the epidemics or pandemics, we will go back to our livelihoods. It would be good for governments to think of multi-pronged approaches to managing COVID-19.