- Human disturbance has led to ecological crisis of a global scale, one of the paramount impacts being loss of biodiversity and alteration of community structure of many species.
- A recent study finds that altered parasite community structure may result in increased occurrence of infectious disease outbreaks, many of which are emerging and re-emerging.
- Experts call for a One Health approach to deal with the problem on the policy front, along with stringent biodiversity conservation measures and appropriate health systems.
Human disturbance affects the disease dynamics of a place through its effect on the prevalence of host and parasite species in the geography, a study finds. The study published in Ecology Letters attributes this to persistence of parasite communities which tend to inhabit multiple host species, thereby increasing the risk of emergence of infectious diseases.
Anthropocene, the current geological period, is marked by dominant human influence on the environment with climate change, pollution, deforestation, habitat destruction, over-exploitation of resources and ecological degradation ruling the planet. With COVID-19 setting a recent and profound example of the scale of the impact of an infectious disease, direct health-associated impacts have topped the list. An IPBES expert guest article published in 2020 states that pandemics are triggered by activities such as rampant deforestation, uncontrolled expansion of agriculture, intensive farming, infrastructure development etc. that bring increasing numbers of people into direct contact with animals carrying disease-causing pathogens.
Biodiversity loss is accelerating at both global and local scales with about one million species threatened with extinction, according to the 2019 Global Assessment Report on Biodiversity and Ecosystem Service, backed by United Nations. Extinctions, however, are not random. Some species are extremely vulnerable to extinction while others are not. In free-living species, or species that are not dependent on other organisms for survival, specialists are more sensitive to environmental perturbations than generalists. Specialists are organisms thriving on narrow diversity of resources or habitats, while those that acclimatise to a broad diversity of resources are referred as generalists. Similarly, there are specialist parasites which infect only one or few related host species and generalist parasites that prefer inhabiting broad host diversity. However, whether human disturbances and other environmental perturbations affect parasites similar to free-living species, was not known until recently.
Ecological significance of parasites
Parasites are important components of ecological networks and biodiversity. Known as ecosystem engineers, they contribute to shaping the community structure by influencing host populations in a multitude of ways, and regulating stability of food web or ‘what-eats-what in a community.’ Thus, loss of parasite species may result in biodiversity crisis with dramatic impacts on ecosystem health, and services including nutrient and energy cycling, and disease dynamics.
The research teams at University of Georgia, South Carolina, USA and IISER Tirupati, India therefore sought out to understand the impact of human-mediated disturbances on parasitic communities and diseases. V. V. Robin, Assistant Professor at IISER Tirupati and joint senior author on the study, who spent most of his time in the wild, observes that these disturbances could be anything resulting from human presence or their activities, for instance, how do humans use the land they inhabit. “We found that human-mediated disturbance and host community structure affects parasite community structure after controlling for the effects of environment i.e., climate, terrain,” underlines Guha Dharmarajan, disease ecologist at the University of Georgia and corresponding author of the study. According to the researchers, humans continue to alter the environment and encroach wild spaces, potentially contributing to increased emerging infectious diseases such as COVID-19 and other zoonotic diseases that spread from wildlife to humans through alteration of parasite communities and their host preferences.
The scientists decided to carry out their study in southern 600 km of Western Ghats, a treasure trove of biodiversity and one of the world’s eight ‘hottest hotspots’ of biological diversity. The sampling was done across four major geographical regions separated by three biogeographic barriers which are mainly isolated and present no records of disease prevalence and transmission patterns. Large parts of these areas are under the protected area network, however, fragmented human settlements are found setting the stage for analysing effects of human disturbance in otherwise isolated areas.
The scientists sampled almost the entire community of birds during the pre-monsoon season between 2011-13, as it is the time when birds breed and are susceptible to parasitic infections. To do so, they set up mist-nets at 42 sites to capture the birds that were harmlessly released ten minutes later after blood samples were drawn for study. The scientists then extracted DNA from the blood samples to detect the parasites infecting each bird.
They identified a total of 47 parasite lineages, and chose two main genera of avian Haemosporidian parasites for their study, namely Plasmodium and Haemoproteus. Out of these, they could explicitly specify 30 species as either generalists or specialists – Plasmodium being host generalists and Haemoproteus host specialists. The researchers calculated the infection risk by each of the two genera in conjunction with human disturbance. The latter was measured as distance from the nearest protected area boundary, and by studying the effect of both distance and human population size together such that lowest levels of disturbance were attributed to locations that were far from low density human populations, and the highest values for locations that were close to high density human populations. The environmental effects like climate, terrain etc. were controlled using a statistical approach.
“We found that anthropogenic pressure had opposite effects on infection risk associated with the two parasite genera. It reduced the prevalence of specialist parasites i.e. Haemoproteus and favoured the generalists i.e. Plasmodium, thereby contributing to increased infection risk by generalist parasites,” said study co-author Pooja Gupta. In line with this, reports have shown that unlike Haemoproteus infections, Plasmodium infections have been responsible for large-scale deaths of wild birds on some islands because Plasmodium species, being generalists are more likely to emerge in novel host communities. “Additionally, human disturbance results in a loss of the diversity of host communities, thereby reducing the diversity of resources available for specialist parasites to thrive. On the contrary, generalists, that can jump from one host to another tend to find a new host to flourish. This suggests that specialist parasites are more sensitive to environmental perturbations compared to generalist parasites and specialist parasites are also subject to risk of co-extinction due to the loss of specific host species on which they depend,” explains Robin.
Western Ghats regions under human influence are known for harbouring host communities that have become less diverse over time. In many well-characterized host-parasite systems, the low diversity communities tend to comprise host populations that are most competent to harbor and transmit parasites. The parasites retained in disturbed ecosystems are mostly robust and can infect multiple species, a characteristic which also increases their ability to cause emerging infectious diseases. “Our work provides a mechanistic explanation of why we see increased disease emergence in areas undergoing rapid human-mediated landscape modifications,” notes Guha adding that it has critical implications for human and animal health.
Biodiversity loss and disease dynamics
Human-forest relationships have changed over time, quite not for good. The linkage of increasing infectious disease outbreaks and loss in biodiversity has been around for a while, being attributed to persistence and multiplication of generalists. Moreover, human proximity to wildlife increases the propensity of transmission of pathogens occurring in the wild. At times, the pathogen may jump to humans from a non-human animal that it may inhabit causing zoonosis. More than half of the disease-causing organisms in humans are zoonotic, with about 13% of total organisms emerging and re-emerging. A steady rise in zoonotic diseases has been observed in recent times, with India being one of the four high-burden countries. While COVID-19 pandemic being the classic example of all times, other notable outbreaks were SARS (Severe Acute Respiratory Syndrome) from 2002-2003, Nipah Virus in 2018, KFD (Kyasanur Forest Disease) in 2002-03 and 2014-17, the latter traced to have originated and spread at an alarming rate in Western Ghats.
Policy landscape in India
The Wildlife (Protection) Act, 1972 was enacted to provide for the protection of wild animals, birds and plants with a view to ensuring the ecological and environmental security of the country. Fifty years later, the pandemic is an eye opener in terms of the devastation the persisting ecological crisis can bring and India’s preparedness to deal with it. By and large, existing Indian policies in the health and environment domain do not address human and animal health in terms of biodiversity and ecosystem health. India enacted the Biological Diversity Act back in 2002 with an aim to preserve the country’s biodiversity and manage sustainable use of natural resources, two decades down the line implementation of which is still in its infancy. While India’s National Biodiversity Action Plans, first made in 1999 with two subsequent revisions in 2008 and 2014 respectively, call for adaptation of existing health systems to the impacts of climate change at the local and national levels, little seems to have been done in this direction. The National Health Policy 2017 recognises the need of controlling zoonotic diseases at the national front, however the focus is limited to rabies. National health programmes under the National Health Mission have also limited their scope to control of vector-borne diseases with no addressal efforts targeted at restoring the ecosystem balance. Disease surveillance for epidemic-prone diseases under the National Integrated Disease Surveillance Program covers entomological surveillance only upon an outbreak, with no surveillance being carried out in host populations especially of animal origin.
In 2018, the government of India launched the One Health and Zoonoses programme with an aim to study the spread and control of zoonotic diseases based on the framework recommended by the World Health Organization in 2008. The framework calls for bringing together experts from human health, animal health, plant health, and environment sectors to address the challenges posed by ecological imbalances on health and disease. “India’s One Health and Zoonoses Programme is a part of the National Mission on Biodiversity and Human Well-Being (NMBH) launched by the Prime Minister’s Science, Technology and Innovation Advisory Council in 2019 to be a proactive surveillance and capacity building endeavour,” informs Abi T. Vanak, Senior Fellow at the Ashoka Trust for Research in Ecology and Environment, who leads the program. However, various bottlenecks are in play towards its successful implementation. The National Environment Policy 2006 outlines the impact of anthropogenic climate change on increase in disease vectors but has no mention of a multitude of other human actions leading to biodiversity loss and consequently, increased disease outbreaks.
What can be done?
Guha and team emphasise that there is a critical need for experts in conservation and public health policy to work together to ensure healthy ecosystems conducive to the health of human and wildlife populations. “This can be achieved by reducing the overlap between human and wildlife populations, and thus reducing disease transmission risk at the human-wildlife interface,” they suggest. Meghna Krishnadas, Senior Scientist at the CSIR-Centre for Cellular and Molecular Biology, who was not associated with the study, reiterates that it could further be achieved by maintaining the integrity of host communities which are specialists or those that help maintain pathogen diversity. “Promotion of hosts that are less likely to come in contact with humans or pathogens that are less likely to emerge in human communities can go a long way,” echoes Guha.
Experts around the globe suggest a cross-sectoral approach (One Health) to mitigate the challenge posed by increasing biodiversity loss, especially in case of zoonotic diseases. “One Health allows us to look at the bigger picture and frame the biodiversity crisis not just in terms of saving the natural world and the ecosystem processes that sustain life on Earth, but in terms that politicians can better understand i.e. the deleterious impacts on human health and well-being,” Vanak told Mongabay-India. “The major policy shift that needs to take place is to provide a mechanism for better inter-sectoral collaboration amongst the human health, animal health, biodiversity and human welfare sectors in India,” he notes.
Preventive approaches which include early warning systems, delay and prevention of outbreaks are need of the hour, suggests a review published last year. Robust routine surveillance systems concentrating on people, host, pathogen, and their interactions can prove to be instrumental in detection of predictable disease patterns before and during outbreaks. In case of an outbreak, India’s preparedness with appropriate health programmes, strengthened existing programs, trained workforce, and containment strategies cannot be undermined. “More de-centralisation is also needed, for instance, development of diagnostic and surveillance capacity at the state level so that the reaction time is reduced,” describes Vanak.
Read more: [Interview] Taking One Health approach to tackle zoonoses crucial for India
Banner image: Nilgiri blue robin (Nilgiri sholakili), endangered bird species restricted to Nilgiri hills is declining rapidly due to human disturbance. Photo by Prasenjeet Yadav.