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Pathogens find new pathways as agriculture, deforestation intensify

  • The changing patterns of how infectious diseases spread in India underscore the impact of agricultural expansion and deforestation on disease transmission.
  • Zoonotic diseases, associated with forest ecosystems, often maintain a silent cycle but are increasingly affecting human populations due to forest habitat disruption.
  • Experts advocate for One Health approach as a means to mitigate these diseases and express optimism in initiatives such as the National One Health Mission.

The 2019 outbreak of Kyasanur Forest Disease (KFD) that puts Shivamogga district of Karnataka in the spotlight underscored a significant shift in the transmission pattern of the tick-borne zoonotic disease. Most of those affected in that outbreak were primarily plantation workers. Parsvanath, Somavathi, Ramamma, Raghavendra, Manjunath — each of these workers shared a common link: their association with arecanut plantations, which dominate the landscape of Shivamogga alongside forested areas.

Dr. Gudadappa Kasabi, the district’s vector-borne disease control officer, noted this transition, stating, “Until then, KFD infection was predominantly observed among forest-dependent communities — those whose livelihoods involved forest and forest produce. However, in 2019, plantation workers bore the brunt.” He further mentioned that this year, when the viral disease broke out in the state again, certain villages in Uttara Kannada district, marked as KFD hotspots, also witnessed the virus spreading among plantation workers.

Arecanut plantation surrounded by forest patches in Shivamogga, Karnataka.  Photo by Abhishek N. Chinnappa/Mongabay.
Arecanut plantation surrounded by forest patches in Shivamogga, Karnataka. Photo by Abhishek N. Chinnappa/Mongabay.

Kasabi and team also noticed a similar shift in the spread of dengue virus. In 2023, intrigued by it, they conducted a survey across various blocks in Shivamogga district to investigate why there’s been a surge in dengue cases in rural areas, contrary to the past trend of higher incidence in urban centers. Their findings revealed a crucial link between arecanut plantations and the proliferation of Aedes aegypti mosquitoes, vectors for the dengue virus. The stagnant water collected in dry arecanut leaves on the ground provided breeding grounds for mosquito larvae. Notably, plantations equipped with sprinkler irrigation systems were more prone to this issue compared to those with drip irrigation systems.

The role of land use and land cover change

Numerous studies and observations point to the role of land use and land cover change (LULCC) in the form of infrastructural development, plantations, agricultural land expansion, leading to large-scale deforestation, in driving tropical infectious diseases. In hotspots of zoonoses like KFD, plantations like arecanut, cashew and rubber have been found to play a significant part in spillovers. Several studies have already attested that deforestation has caused malaria epidemics in South America.

A study conducted by the Indian Institute of Science (IISc) in Bengaluru in 2020 revealed significant transformations in the forests of the Western Ghats over the past three decades, marked by the fragmentation of contiguous native forests into smaller parcels of land. Analyzing forest cover loss from 1973 to 2018, the study found a net decrease of 10% in forest cover, declining from 43.83% in 1973 to 34.02% in 2018. This loss was primarily attributed to the expansion of agriculture, horticulture, and forest plantations . Forest fragmentation has also escalated during this period, as evidenced by an 11% decline in interior forest cover, shrinking from 26% in 1973 to 2018.

A pathway in Shivamogga forest in Karnataka. Forest loss primarily attributed to the expansion of agriculture, horticulture, and forest plantations. Photo by Abhishek N. Chinnappa/Mongabay.

The paper highlights that forest fragmentation has constrained species movement, thereby limiting their potential for dispersal and colonization. Additionally, another study examining the correlation between mammalian species richness and the abundance of the KFD virus, as well as disease outbreaks, found that in deforested areas, lower species richness was associated with increased risk of disease outbreaks.

Darshan Narayan, a scientist affiliated with ATREE, who has previously collaborated with institutions like the Indian Council of Medical Research (ICMR) and the state health department on KFD research, underscores the adverse impact of forest cover loss on the natural disease cycles maintained by wild predators. “It can also lead to the emergence of new diseases in displaced animals,” he emphasizes.

According to emeritus scientist, ICMR-Vector Control Research Center (VCRC), SL Hoti, infectious diseases like KFD, leptospirosis, etc. are sylvatic diseases, inherently associated with the forest ecosystem. Typically, these diseases follow a silent cycle devoid of human involvement, as long as humans maintain a distance from the natural cycle. “The animals or hosts of these pathogens rely on undisturbed forests; thus, if forests remain intact, the pathogens remain confined within the forest,” Hoti explains. However, anthropogenic activities such as urbanization and agricultural expansion disrupt forest ecosystems, disturbing the silent pathogen cycle.

From forests to farms

Hoti draws attention to bamboo cultivation in northeast India, where, following each flowering cycle, the plants shed copious amounts of seeds, attracting rodents in large numbers. After consuming these seeds, the rodents migrate to nearby paddy fields, becoming vectors for diseases like leptospirosis and plague. “Rodents serve as significant reservoirs of pathogens, and wherever their population surges, the likelihood of disease outbreaks escalates,” he asserts, citing similar occurrences in bamboo plantations across countries such as Madagascar, Chile, Brazil, and Japan.

The impact of livestock intensification in villages bordering forests is only compounded by the burgeoning rodent population resulting from land-use changes, as pointed out by Narayan. This dual effect can heighten the risk of livestock transmitting diseases to humans.

A field rat infesting a rice field.  Photo by the International Rice Research Institute (IRRI)/Flickr.
A field rat infesting a rice field. Photo by the International Rice Research Institute (IRRI)/Flickr.

In certain Southeast Asian countries experiencing outbreaks of diseases like Nipah, Hoti says that deforestation driven by palm oil and sugarcane cultivation has displaced bats from their natural habitats, bringing them into close proximity to commercial pig farms. Abi T. Vanak, Director of the Center for Policy Designs at ATREE, emphasizes that displacement induces stress in bats, leading to increased shedding of pathogens.

Vanak also points out certain socio-behavioral factors among humans that facilitate the seamless spread of pathogens. In parts of Karnataka, for instance, farmers collect nuts from areca and cashew trees after bats have consumed the pulpy fruit. While this interaction with animals is viewed positively, he advises caution, urging individuals to take personal safety measures to avoid contact with bat saliva.

Certain traditional practices within farming communities can render them oblivious to the risks posed by ticks in their surroundings, emphasizes Darshan. “Farmers who have grown up with ticks around their houses and plantations are unaware of the health risk associated with them from ecological changes that are recent,” he says. Limited disease awareness, reluctance to report outbreaks, and restricted access to veterinary services all contribute to the spread of diseases. Moreover, practices such as using ponds for washing clothes and cattle, cleaning houses with cow dung and urine, entering cattle sheds barefoot out of reverence, and consuming food grains damaged by small mammals, all constitute risky behaviors, according to him.

Urgent call for One Health approach

In recent decades, numerous infectious disease outbreaks such as Nipah, H5N1 avian influenza, and SARS-CoV-2 have escalated into global public health crises. The emergence of COVID-19 has raised global attention towards averting future pandemics. While efforts have been concentrated on identifying and treating these diseases, there remains a glaring lack of focus on prevention and mitigation strategies on a global scale.

India, grappling with a significant burden of zoonotic diseases including rabies, Nipah virus, and KFD, underscores the urgent need for a One Health approach. This approach advocates for a collaborative, multisectoral, and transdisciplinary strategy aimed at achieving optimal health outcomes by recognizing the interconnectedness between humans, animals, plants, and their shared environment.

The impact of livestock intensification in villages bordering forests is compounded by the burgeoning rodent population resulting from land-use changes.  Photo by Arathi Menon/Mongabay.
The impact of livestock intensification in villages bordering forests is compounded by the burgeoning rodent population resulting from land-use changes. Photo by Arathi Menon/Mongabay.

While One Health may be a relatively recent terminology, India has a longstanding history of addressing infectious vector-borne diseases through what was previously termed as inter-sectoral collaboration, notes Hoti. He recalls a successful initiative led by the Vector Control Research Center in Pondicherry (now Puducherry) to combat Lymphatic Filariasis, a significant health burden in the late 1970s. The five-year program (1980-85), which brought together various government and medical departments, effectively curtailed the disease’s spread through community involvement. Similar success stories were witnessed later in Kerala’s Alleppey district in combating Filariasis, he adds.

Despite past successes in collaborative disease mitigation efforts, experts highlight India’s sluggish adoption of One Health, primarily due to institutional barriers, inadequate resources, and lack of awareness. Vanak, however, suggests a shift in perspective, advocating for the integration of One Health into every facet of healthcare rather than viewing it as a specialized field. They express optimism with the recent launch of the National One Health Mission on January 1 of this year, which institutionalizes One Health, signaling India’s progress towards integrated disease control and pandemic preparedness.

Banner image: A rubber tapper in a rubber plantation in Shivamogga, Karnataka. As agriculture intensifies leading to deforestation, pathogens are finding it easy to jump hosts to reach humans. Photo by Abhishek N. Chinnappa/Mongabay.