Deforestation drives epidemics

Source: By Jagmohan Sharma: Deccan Herald

Reports suggest Covid-19 pandemic to be an outcome of coronavirus spillover from bats to pangolins to humans. Should that be it, the question arises, why has such spillover occurred? The answer lies in our slighting attitude towards forests.

The disease-causative agents behind majority of the outbreaks in the past including yellow fever (Africa, South America), dengue (pan tropical), Chikungunya (Africa, south and southeast Asia), Ebola (Africa), HIV (worldwide), Nipah (South Asia), SARS (Southeast Asia), Malaria (Africa, Southeast Asia, South America) and Lyme disease (worldwide) are forest denizens, and at least the initial disease outbreak is attributable to deforestation and forest fragmentation.

According to the World Health Organisation (WHO), “two thirds of known human infectious diseases are shared with animals, and the majority of recently emerging diseases are associated with wildlife”.

Let us consider how deforestation results in the emergence of infectious diseases. Forest is a natural ecosystem hosting a community of trees of different types and sizes with vegetative undergrowth and thriving populations of microorganisms (such as bacteria and viruses), insects, reptiles, birds and other animals.

All these life forms exist in a dynamic balance in a forest ecosystem and constitute its biodiversity. Disease causative agents (virus, bacteria or parasites) that have co-evolved with their reservoir hosts (mosquitos, ticks, rodents, bats and other wild animals) are all part of this dynamic balance of biodiversity.

When a forest habitat is cleared for the development of infrastructure like roads, power lines, dams and canals, or for human habitations and industrial purposes, or for extension of agriculture and grazing livestock, the outcome is loss of tree cover and creation of fresh forest-edges. Such changes alter the balance of biodiversity and modify the locality factors of temperature, sunshine, moisture and wind. This has implications for the outbreak of infectious diseases.

Firstly, under the stress of disturbance, the reservoir host may shed higher pathogen load through saliva, urine and faecal matter. This can increase the chances of infection among humans and their livestock interacting with such disturbed forest habitat. Second, the populations of host species and their predators can undergo change resulting in increased/decreased abundance of hosts, and thereby the scope for pathogen spillover.

Third, fresh habitat opportunities can arise that may favour breeding and result in increased abundance of host species locally. For example, sunlit puddles of water on forest edges are favoured habitats for breeding mosquitoes. Fourth, reservoir host species can disperse from the disturbed forest habitat along with the pathogen load-carrying disease to new areas.

Fifth, the disease-causing agents can find alternative hosts in humans and their livestock that use such disturbed forest habitat. Sixth, enhanced interaction of livestock with the host wild animals can facilitate spill over of disease-causing agents to humans.

Nipah virus, first isolated in 1999, caused more than 100 human deaths in 1998 in Malaysia. Nipah virus had infected humans through reared pigs.  Pigs had nibbled on the fruits eaten by fruit bats (reservoir host). Fruit bats were displaced by deforestation carried out for cultivation of palm oil plantations and pig farming. In May 2018, Nipah virus claimed 21 lives in Kerala.

Also, a study published in the International Journal of Epidemiology in December 2019 on the Kyasanur Forest Disease (KFD) in the Western Ghats in Karnataka provides clear evidence for increasing forest loss and disease spread. KFD (also known as monkey fever) is endemic to the Ghats and was first reported in 1957 from Kyasanur Forest of Shivamogga district in Karnataka.

A tick - Haemaphysalis spinigera - found in forest margins transmits KFD virus to humans. During the past five years, KFD has expanded its range throughout the Ghats and beyond. This indicates its potential as an emerging infectious disease (EID) in India.

Thus, it is seen that loss of forests and biodiversity increases the chances of forest-linked disease outbreaks. Experts opine that in a high biodiversity forest, availability of a number of other potential victims of the disease agent creates a “dilution effect” reducing the chances of infection to humans.

Protective factor

According to a review of the state of knowledge published by WHO and Convention on Biological Diversity (CBD) on “biodiversity and human health” in 2015, rich “biodiversity may serve as a protective factor for preventing transmission, and maintaining ecosystems may help to reduce exposure to infectious agents.” Precluding the emergence of infectious diseases through forest conservation is a recognised novel ecosystem service of forests.

Further, the impending climate change has added a whole new dimension to forest-linked disease outbreak as changes in ambient temperature and humidity, drought frequency and rainfall distribution are likely to modify the disease rate, range, seasonality, distribution of disease agents and their hosts, and human-disease host interactions.

Risk of infection intensifies when people venture into forest habitats for collection of forest produce, cattle grazing, bush-meat hunting and tourism. Such risk is further compounded due to migration of more-vulnerable poor people into open up forest areas without adequate housing, water, medical and sanitation facilities.

Creation of buffer area zones between forests and human-cultured landscape and stringent measures against bush-meat hunting and trade in wild animals are necessary to safeguard the larger common interest of preventing zoonotic disease outbreak. It is pertinent to take note in this context that the government notifies eco-sensitive zones around national parks and sanctuaries that host rich biodiversity.

The outbreak of Covid shows that society ends up paying extremely high economic and social costs for violating the natural environment. However, in our pursuit for economic development, we continue to neglect and erode its environmental basis.

Uncaring the development process is likely to intensify the possibility for outbreak of forest-linked infectious diseases. Contextually, halting deforestation and defragmenting forests, strengthens the environmental basis of economic development and insures against the outbreak of diseases.

There lurks a real risk of the next pandemic emerging from forests. Epidemiologists suggest preservation of the health of our forest ecosystems to secure our own.