Understanding zoonotic diseases: ‘Humans are changing disease ecosystem’

The global scientific community has time and again speculated the origins of the novel coronavirus SARS-CoV-2. That it may have jumped from animals to humans has been the most talked about explanation so far. The probable zoonotic origin of the virus has made it imperative to about the burden zoonoses put on global health and economy. 

Down to Earth spoke to Bethan Purse, ecologist, UK Centre for Ecology and Hydrology, to understand the ecologically dynamic system of zoonotic diseases. Edited excerpts:

Ishan Kukreti: How big is the burden of zoonoses and how does it compare with other non-communicable diseases?

BP: The burden of zoonoses is enormous, evolving and concentrated in resource-poor tropical areas.

Zoonoses account for around a quarter of the years lost due to ill-health, disability or early death in tropical areas, according to recent study that looked at worldwide impacts of diseases.

The burdens of many neglected zoonotic diseases are vastly under-reported worldwide, but are likely to cause hundreds of thousands of cases and hamper livelihoods.

Leptospirosis, for example, infected about 1.03 million people and caused 60,000 deaths annually across 34 countries. In 2000, the World Health Organisation estimated that over a billion people were at risk of scrub typhus; more than one million cases occurred annually.

South Asian countries with good surveillance have shown a rising incidence of scrub typhus over the last 8-10 years.

IK: What are the global responses in terms of research and funding to zoonoses. Are those enough?

BP: Several organisations work on zoonotic diseases to understand the complex systems, often in challenging circumstances. It is important that efforts to understand zoonotic diseases are cross-disciplinary and involve both decision-makers and local communities in concentrated tropical areas where the rate of emergence is high.

Zoonotic diseases can only be understood and dealt with through cross-disciplinary collaboration because they are ecologically complex, dynamic systems, involving animals and sometimes arthropod vectors in transmission. Each species involved may respond differently to habitats and ecosystems.

Exposure to zoonotic diseases is often linked to ways people use ecosystems. We are gradually changing the burden of zoonotic diseases through habitat encroachment, human population pressure and settlement, intensive agriculture, forest loss and degradation, global trade and travel and climate change.

Managing disease outbreaks, therefore, needs a combination of several kinds of expertise and active exchange of knowledge and tools between researchers, practitioners, local communities and decision-makers.

IK: Do you think the world learnt any lessons from the SARS and MERS outbreaks. Do you think we’ll learn any from the present pandemic? 

BP: I am not an expert in health systems and these situations are obviously complex. However, I think we can see evidence of such learning in the ways South Asian countries implemented effective contact tracing, provision of personal protective equipment and lockdown measures.

We will undoubtedly learn lessons from the present pandemic, but it is too soon to say what they would be — the medical, social, political, ecological and environmental evidence is still accumulating.

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IK: Why do you think zoonoses continue to be neglected?

BP: Their burdens are concentrated in resource-poor tropical areas where less research is conducted and health systems are weaker. Many neglected zoonotic diseases affect economically and socially marginalised communities that may be remote from both healthcare and political systems.

Zoonotic diseases that are endemic and occur in countries every year are neglected as compared to exotic new threats that arrive from elsewhere. Examples include scrub typhus and Kyasanur Forest Disease, which can hamper the livelihoods of forest-dependent communities in south India. Such endemic zoonotic diseases are vastly under-reported — the actual number of cases could be a hundred or thousand times more than reported numbers.

It, therefore, becomes difficult to understand how the risk of disease is changing and to know who is at risk, where and why. When policy makers try to choose the diseases that should be targeted for interventions, some important zoonotic diseases are missed off the list due to scarce resources.

IK: Once the COVID-19 pandemic is over, what steps would be needed to better address threats posed by zoonoses outbreaks? 

BP: Humans are changing zoonotic disease systems and increasing their impacts in many different inter-linked ways. We need to understand how these changes bring people into closer contact with animals, arthropod vectors and pathogens.

We can only understand how human modification of ecosystems increases risk from zoonotic diseases if we examine both the complex ecology and social factors involved.  

We also need to study pathogens in source areas to understand which reservoir wildlife hosts and arthropod vectors are involved in transmission and how their population dynamics and interactions with each other, and with people, are affected by habitats and ecosystems and ecosystem change.

At the same time, we have to understand how people come into contact with zoonotic infections as they use ecosystems for their livelihoods, what are their priorities and means of coping are.

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