A study published in Ecology Letters finds that human disturbance affects the disease dynamics of a place through its effect on the prevalence of host and parasite species in the geography, as reported by Mongabay-India.

Researchers from the Indian Institute of Science Education and Research (IISER)  Tirupati and the University of Georgia, South Carolina, USA, carried out the study in southern 600 km of the Indian Western Ghats, a treasure trove of biodiversity and one of the world’s eight ‘hottest hotspots’ of biological diversity.

The Western Ghats regions have been subject to severe human disturbances. They 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.

Research findings highlight that 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. This altered parasite community structure may result in increased occurrence of infectious disease outbreaks, many of which are emerging and re-emerging.

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.

Authors of the study state that these disturbances could be anything resulting from human presence or their activities, for instance, how the 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,” Mongabay-India reports. According to the researchers, humans continue to alter the environment and encroach wild spaces, potentially contributing to increase in 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 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.

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 analyzing effects of human disturbance in otherwise isolated areas.

The authors elucidate how climate, habitat and human disturbance affects parasite prevalence both directly and indirectly via their effects on host diversity. They emphasize 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.

The Mongabay-India report states that 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 acclimatize 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.

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.