Spillover: encroachment into forests increases risk of contracting diseases from animals

  • Forest loss has been associated with increased risk of Kyasanur Forest Disease, which is spread by infected ticks and affects people typically living and working in the Western Ghats, presumably due to greater human exposure to infected animals and ticks.
  • The incidence of Lyme disease in the United States and Scrub typhus in South Korea (the former is spread by black-legged ticks while the latter by mites) have also been associated with fragmented forests and deforestation, respectively.
  • Spillover of the Ebola virus disease from wildlife into humans is more likely to occur in fragmented forests. Human activities favour the presence of some bat species—believed to be reservoirs of the Ebola virus—in areas where Ebola outbreaks are linked to deforestation.
  • Tackling zoonotic diseases requires a coordinated and multidisciplinary ‘One Health’ approach by countries, which includes early surveillance of diseases in wildlife, particularly in deforested areas.

Over the past two decades, scientists have been alarmed by the rapid spread of an infectious disease transmitted by tick bites that afflict forest-dwellers in the verdant, biodiverse tropical forests of the Western Ghats running parallel to  India’s west coast. Caused by a virus, Kyasanur Forest Disease, or ‘monkey fever’ as it is also known because it infects and sometimes kills monkeys too, sickens about 400 people in the region each year, although cases vary each year widely.

A vaccine exists, but it is weak; new infections continue and a death was reported just last month, prompting the Karnataka government to launch a research centre. The disease was first identified in the state of Karnataka in 1957, but post-2000, scientists have been puzzled by its expansion northward to the states of Goa and Maharashtra and southward to Tamil Nadu and Kerala. And cases from the northern states have spiked in 2016 and 2017.

According to data released by Global Forest Watch in 2019, the rate of forest loss in the Western Ghats has intensified from 2012 to 2017. Many villagers have been displaced from their original lands into deforested areas, often at the periphery of new agricultural plantations. Scientists have long-suspected that forest loss might have played a role in the rise of Kyasanur Forest Disease (KFD). Now, a team of scientists has found concrete evidence for this association.

Using models, a team of researchers examined the relationship between the landscape suitability of KFD and forest loss and mammalian species richness. They mapped the total outbreaks between 2012 and 2019 and gathered satellite data on forest loss.

Their analysis found that an increase in both forest loss and mammalian species richness was associated with an increased risk of KFD outbreak. Generally speaking, forest loss reflects a more significant human presence in that landscape, Michael Walsh, an infectious diseases epidemiologist and lead author of the study explained. “With that greater human presence comes more opportunities for exposure to novel species of animals, including reservoirs for disease and their ticks.”

But the reasons, he said, are complex as deforestation can alter the “composition of communities of populations in an ecosystem, which in turn can affect the way individuals of a species interact with each other and other species including humans.”

Cases of Kyasanur Forest disease in India depicted by (a) year of the first case in each district (n = 16), (b) number of human cases (n = 9594), and (c) and all seroprevalence antibodies discovered outside of this study’s region of interest (n = 6). Map and description from Chakraborty et al. 2019.

Curiously, another study published early in 2019, reported that in 1983-84, there was a massive outbreak of KFD with 2,589 cases. Coincidentally, they noted—in a 1983 news article—that 400 hectares of virgin forests in the Western Ghats were cleared around that time—a huge amount compared to previous years—to establish cashew plantations.

“However, the data is not strong enough to say that the large KFD outbreak was caused solely by the increase in deforestation,” said Sulagna Chakraborty, a doctoral student at the University of Illinois and lead author of the study. “Deforestation may have been a factor given the likely increased interaction of laborers and workers with ticks in this time period.”

Read more: To find out monkey fever’s next destination, scientists follow the ticks that carry it.

These findings may not be surprising. While shrinking forests are widely known to affect carbon storage, there is another often-overlooked consequence, which scientists are coming to grips with: increasing cases of zoonotic diseases—those that spillover from animals to humans. Three out of every four emerging infectious diseases are zoonotic and with our widespread global connectivity, they could rapidly spiral into a pandemic, scientists fear.

Tick and mite-borne diseases in temperate regions

Tick-borne diseases are also found in temperate forests. One familiar example is Lyme disease, which occurs in Canada, North America, Europe and Asia, and is caused by bacteria spread through the bites of infected black-legged ticks. In the US, cases have been climbing from 2004 to 2016 with the Centers for Disease Control and Prevention (CDC) estimating that 300,000 people may be diagnosed each year. While most cases have been typically concentrated in states in the Northeast and upper Midwest, more have been cropping up from neighboring states.

According to a US-based 2018-study, as forests become fragmented due to increased human settlements, the incidence of Lyme disease rises. Disease ecologist and lead author of the study, Andrew MacDonald at the University of California, Santa Barbara, said that “host species that do well in fragmented forests also are good hosts for ticks and the bacteria that cause Lyme disease.” Deer and mice both thrive in fragmented forests, he explained with the former allowing ticks to reproduce and the latter transmitting bacteria between ticks. “So those two hosts do well and lead to the higher abundance and infection in ticks.”

But he pointed out that it was human interaction with fragmented forests and not fragmentation alone that increases the risk.

Other drivers have also been implicated for the upward trend in Lyme disease, notably rising winter temperatures, which are thought to push tick habitats northward. Overall though, the effects of climate change on insect vectors are challenging to predict, cautioned MacDonald, because the responses to temperature may exacerbate a disease until a point. Still, beyond a certain level, the transmission will decline.

A black-legged tick. Bites of infected ticks can transmit bacteria that cause Lyme disease to humans. Photo by Kaldari/Wikimedia Commons.

Similarly, scientists have found that the incidence of scrub typhus, a bacterial disease transmitted to people by the bites of infected larval mites (chiggers), has also been found to be linked to deforestation in South Korea. Scrub typhus, a neglected infectious disease, is a serious health public health concern in the Asia Pacific region, including South Asia, causing illness in one million people per year.

There are two possible explanations for this association, says lead author Kyung-Duk Min, a research assistant professor at Seoul National University. First, “deforestation could cause secondary growth of scrub vegetation which is favorable for vector mites and their natural host (rodents),” he explained. Second, he surmised that higher levels of deforestation could lead to a higher contact rate between humans and mites.

The number of cases in South Korea has shown an upward trend from 2003 until 2017 due to several reasons. In their study, the researchers used district-level data for 2006-2017, which included the annual number of scrub typhus cases, deforestation levels (from satellite data) and other factors.

The number of cases of scrub typhus in South Korea during 2001-2013. Chart by Wikisanchez/Wikimedia Commons.

Interestingly, early in 2019, Indian doctors reported the rampant resurgence of scrub typhus in India—especially in the north—over the past two decades with frequent outbreaks in many parts of the country, including the South, echoing earlier studies. “Infected mites are found particularly in areas like forest clearings, river banks and grassy regions during the rainy season when the mites lay eggs,” the authors noted. But apart from deforestation, changes in human behaviour and unplanned urbanization could lead to the displacement of mites and rodents from one place to another, stated a 2017 study that flagged its resurgence.

Ebola: the forest connection

But it is not just diseases caused by insects that are rising due to forest loss. The emergence of some diseases such as Nipah virus infection, spread by bats among other animals, has been partly attributed to deforestation. The cutting of trees, apart from bringing bats closer to humans, is also believed to stress them out, increasing their viral load that sheds through urine and saliva. Bats have been found to naturally harbour many viruses and are the suspected reservoir hosts of viruses associated with SARS, MERS, and Ebola (probably also Covid-19). And the deadly Ebola outbreak that ravaged Africa between 2013 to 2016, killing more than 11,000 people, was also reported in 2017 to be related to shrinking forests in West and Central Africa.

To examine the role of forests in Ebola, researchers used fine-resolution satellite forest data to map changes in land use cover from 2004 to 2014 and they marked eleven independent first reported human infection (index) cases that had spilled over from wildlife. Spillover of the Ebola virus from wildlife to humans, they found, was more likely to occur in hotspots of forest fragmentation—but not in clear-cut areas. Specifically, 8 out of the 11 infection events took place in highly fragmented areas.

Forest fragmentation in Central (panels a, and b) and West Africa (Panels c and d) in 2000 (top panels) and 2014 (bottom panels). Fragmentation is measured by the number of patch, edge, perforated and smaller forest areas (<200 ha). Most of the centers of first infection (yellow triangular markers) are located in areas affected by increasing forest fragmentation. Map and description from Rulli et al. 2017.

It is unclear why but many fruit bats outside of Africa were shown to be drawn to new agricultural food sources. Last year, a separate group of researchers probed if the distribution of 20 fruit bat species were affected by human activities such as deforestation and agriculture within Ebola-affected areas in West and Central Africa.

Indeed, the range of some bat species was linked to human activities in regions where the Ebola virus occurs. More importantly, they found an overlap in the areas where human influence favoured the presence of four species of bats linked to the Ebola virus with the areas where Ebola outbreaks were favoured by deforestation.

As more humans venture into forests that have been opened up to plant fruit tree crops, there is a greater food supply for bats, leading to increased interactions between bats and humans. This, in turn, could lead to higher possibilities for infection; John Fa, a professor at Manchester Metropolitan University and co-author of the study, has said in a press release. But he added that “we need to gain clarity over whether increasing rates of deforestation can change the natural circulation of viruses and lead to a greater risk of the spread of Ebola.”

Embracing ‘One Health’ to prevent spillovers

Experts have been advocating the adoption of a coordinated multidisciplinary ‘One Health’ approach to effectively and efficiently tackle the global emergence and re-emergence of zoonotic diseases. This holistic view recognizes that the health of humans and animals is connected to the health of our shared environment. Apart from the US, nearly 50 countries have signed on to the Global Health Security Agenda (GHSA) that aims to promote One Health among countries to prevent, detect and respond to diseases threats, states a new review published last month.

But challenges remain in the implementation of One Health, especially in developing countries. Historically, most government departments—human health, veterinary/agricultural, and environmental agencies—work in isolation with separate mandates and budgets, said Jonathan Epstein, a veterinarian and disease ecologist with the New York-based non-profit EcoHealth Alliance.

“One Health asks them to work on common issues that impact each of their sectors, but there’s often a significant disparity in resources allocated to each agency, and different levels of technical training around health, so it makes it challenging for everyone to work together on a level playing field.”

Launched in 2009 by the US Agency for International Development (USAID), the Emerging Pandemic Threats (EPT) Program’s PREDICT Project uses the One Health framework to detect zoonotic viral threats before they emerge in people. One of their initiatives in Bolivia, mentioned in the review, is to monitor for viruses in wild animals as an early detection tool primarily focusing on deforested landscapes where the “breakdown of natural barriers leads to increased contact between wildlife and people.”

The fruit bat Rousettus aegyptiacus is one of the five species whose presence was favoured by human activities and has been found to be serologically positive for the Ebola virus. Photo by Вых Пыхманн/Wikimedia Commons.

“Policy decisions should be evidence-based,” stresses Epstein. Although “deforestation has been shown to be an important driver of epidemics, but we still need more studies that show how,” he cautioned.

“Understanding the underlying mechanism … can help strengthen arguments for policy change.”


Chakraborty, S., Andrade, F. C. D., Ghosh, S., Uelmen, J., & Ruiz, M. O. (2019). Historical expansion of Kyasanur forest disease in India from 1957 to 2017: a retrospective analysis. GeoHealth, 3(2), 44-55.

Kelly, T. R., Machalaba, C., Karesh, W. B., Crook, P. Z., Gilardi, K., Nziza, J., … & Monagin, C. (2020). Implementing One Health approaches to confront emerging and re-emerging zoonotic disease threats: lessons from PREDICT. One Health Outlook, 2(1), 1-7.

MacDonald, A. J., Larsen, A. E., & Plantinga, A. J. (2019). Missing the people for the trees: Identifying coupled natural–human system feedbacks driving the ecology of Lyme disease. Journal of Applied Ecology, 56(2), 354-364.

Min, K. D., Lee, J. Y., So, Y., & Cho, S. I. (2019). Deforestation increases the risk of scrub typhus in Korea. International journal of environmental research and public health, 16(9), 1518.

Olivero, J., Fa, J. E., Farfán, M. Á., Márquez, A. L., Real, R., Juste, F. J., … & Nasi, R. (2020). Human activities link fruit bat presence to Ebola virus disease outbreaks. Mammal Review, 50(1), 1-10.

Rulli, M. C., Santini, M., Hayman, D. T., & D’Odorico, P. (2017). The nexus between forest fragmentation in Africa and Ebola virus disease outbreaks. Scientific reports, 7, 41613.

Walsh, M. G., Mor, S. M., Maity, H., & Hossain, S. (2019). Forest loss shapes the landscape suitability of Kyasanur Forest disease in the biodiversity hotspots of the Western Ghats, India. International Journal of Epidemiology, 48(6), 1804-1814.


Banner image: Fragmentation of forests due to plantations at Rajamalai in Eravikulam National Park. Photo by Harkrishnan S/ Wikimedia Commons.

Exit mobile version