Deforestation and disease: How natural habitat destruction can fuel zoonotic diseases

The novel coronavirus disease (COVID-2019) pandemic, believed to have been triggered by the transmission of the virus from animals to humans, has brought into sharp focus zoonotic diseases that are spread by animals forced to move out of their natural habitats that are increasingly being destroyed, say experts.

Destruction of forests for growing crops, urban expansion and building road networks and a parallel intensification of wildlife trade has resulted in ecological conditions and movement of wild animals, which are reservoirs of some viruses or bacteria, towards human settlements. This, in turn, results in the emergence of new pathogens, they say.

The COVID-19 pandemic “is likely a global effect of natural habitat destruction combined with the effects of globalisation,” says Maria Cristina Rulli, professor at the department of civil and environmental engineering at Politecnico di Milano, who has worked extensively on the links between Ebola virus disease outbreaks and forest destruction in Africa.

The pandemic shows how vulnerable humanity is to major environmental and human health emergencies and how a local event may soon turn into a global crisis,” she told Mongabay-India.

“This is something scientists have been predicting could have happened,” Rulli says, adding that one of the key lessons pandemics teach us is “the existence of a nexus between human health and the environment.”

While scientists have not yet identified the “index case” or where exactly the spillover to humans took place, “the province of Hubei and the surrounding provinces in China have indeed been affected by landuse change and habitat destruction,” says Rulli. “But it might (also) be that the virus arrived from another region through illegal importation of bushmeat.”

Ramesh Dhiman, former scientist with the National Institute of Malaria Research, New Delhi, agrees that there are strong links between destruction of natural habitats of animals, and a rise in man-vector contact, leading to an upsurge in zoonotic diseases. “India is among the top geographical hotspots” for zoonotic diseases, and “in recent past, India has seen emergence and re-emergence of high priority and neglected zoonoses,” Dhiman had earlier cautioned in a report in Medical Reports and Case Studies in 2018.

Read more: Taking One Health approach to tackle zoonoses crucial for India

In recent years, the intrusion of human beings into habitats of rodents and mites, particularly for cutting grass, has resulted in a wider distribution of scrub typhus disease in India, he says. A similar link between increased risk of scrub typhus and deforestation was recently reported in South Korea too, he pointed out.

Similarly, local deforestation has been linked to the emergence of Kyasanur Forest Disease (KFD) in and around Shimoga in Karnataka state, while the emergence of Nipah Virus is being linked to changes in ecological conditions that have led to fruit bats coming out of their natural habitat and feeding on agricultural produce.

This is in addition to insect-borne infections such as dengue, chikungunya and Japanese encephalitis, which have re-emerged in the Indian sub-continent, Dhiman says.

Rajan Patil, associate professor of epidemiology at SRM University, Chennai, says growing proximity between human settlements and wildlife is increasing the rate of disease transmission between domestic animals and wildlife. “Irrespective of whether humans are going into forest areas or animals are coming to human settlements due to deforestation, viruses are being exchanged,” he says.

“Usually in undisturbed habitats, viruses keep circulating in mild forms in animals. It is when this equilibrium is disturbed and they come in contact with humans, some cross the species barrier due to a mutation, and human infections start taking place,” Patil explains. He cites the example of swine flu (H1N1) virus that remains a very mild infection in pigs, but becomes “deadly” when it mutates and crosses over to humans.”

Patil’s team had previously attributed Nipah and Hendra virus outbreaks to “destruction of forests that have adversely affected the roosting site for fruit bat species which is the reservoir for these pathogens.” As a result, fruit bats have shifted to the fruit trees in human settlements, increasing contact between human and bats.

Other examples cited by Patil are Lyme disease transmitted by ticks through white-footed mice; West Nile Disease, a mosquito-borne disease whose primary reservoir is wild birds; and an outbreak of anthrax in Chhattisgarh state which he attributes to loss of biodiversity.

Global problem

Globally too, experts are linking deforestation with recent outbreaks. Tropical deforestation in southeast Asia, and in South America, has been increasing at alarming rates, points out Rulli.

Rulli’s work on Ebola has shown how the index cases of about 10 Ebola epidemics in Africa over the last two decades occurred in regions affected by deforestation and forest fragmentation. According to her, “the spillover from wildlife or reservoir species into humans is favored by landuse change such as deforestation and forest fragmentation for a variety of reasons.” One is that as humans encroach wildlife habitats, there are increased chances of human contact with infected species. A second is that natural habitat destruction alters animal community dynamics, and sometimes increases the numbers of some ‘generalist’ pathogens that can reside in a range of hosts, and reduces the numbers of ‘specialist’ species that thrive in limited hosts in the previous “undisturbed” conditions.

Thomas Gillespie, associate professor at the department of environmental sciences at Emory University, U.S. says that when extractive industries, such as logging, oil exploration and mining, are implemented in largely uninhabited wilderness areas, they provide the opportunity for human exposure to novel pathogens.

“Everyone is talking about the problem of the wet (illegal black market) market in Wuhan, but what about the effects of the nearby Three Gorges Dam project?” asks Gillespie. “It is the world’s largest hydroelectric power station, built on the Yangtze river in an area that was previously a mix of secondary forest and agricultural land. Many of the animals that used to live in that area likely died when their habitat was destroyed, but bats can fly.”

While scientists do not have the necessary data yet on where exactly the dislodged bats went or how did they adapt, “but there are plausible linkages,” he says.

“Whenever you have novel interactions with a diverse range of species in one place — whether that’s in a natural environment like a tropical forest or in an artificially created environment like a wet market — you can have spillover events,” says Gillespie.

The wet markets really represent the minority of opportunities for spillover to occur, he says. Close to a third of diseases that emerge are linked to large-scale land-use change like deforestation and well over half of diseases that emerge are coming from wildlife in forests – including such well-known examples as HIV and Ebola.

Gillespie says there are many examples of pathogen spillover related to deforestation for agricultural monocultures including palm oil, for example Nipah and Lassa viruses; sugar cane and soybean in the case of hantavirus.

He traces the emergence of Nipah virus, for example, to deforestation for palm oil production and an El Niño-driven drought that led to large-scale burning of rainforests in Indonesia. Forced to find food elsewhere, fruit bats, which are long-distance flyers made their way to Malaysia, where industrial-scale pig farming was expanding rapidly – to the scale of about 500-1000-hectare farms with 25,000-50,000 pigs.

“The fruit bats found food from the fruit trees on these pig farms, but the pigs became sick after eating partially eaten fruit that fell from these trees into their enclosures. Pig farmers then became infected by the pigs,” and led to the first of a series of recurrent outbreaks of Nipah in people throughout southeast Asia including India, explains Gillespie.

Like the Nipah virus, the global demand for palm oil was the driver for the large-scale land-use change that led to the spillover of Lassa virus, which causes a hemorrhagic fever like Ebola in humans and can kill 30 percent of the infected, in West Africa, says Gillespie. From Sierra Leone to Nigeria, native forest-living rodents were forced to seek out food when forests were cleared to establish palm oil plantations.

In 2013 Gillespie’s team reported the probable emergence of Saint Louis encephalitis in association with the Conquistadors hauling plundered gold from Peru across the isthmus of Panama to ship back to Spain.

“The AIDS pandemic traces its roots back to the butchering of chimpanzees in forested areas of central Africa,” he points out.

Banner image: Deforestation around Pakke tiger reserve. Photo by Nandini Velho/Wikimedia Commons.