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Behind Deforestation, Another Threat Looms – a Kyasanur Forest Disease Outbreak

Behind Deforestation, Another Threat Looms – a Kyasanur Forest Disease Outbreak

People have chopped down trees in the Western Ghats and replaced them with, among other things, tea plantations. Photo: Aayushmaan Sharma/Unsplash

  • A new study has found reason to believe both species diversity and forest cover are important factors in limiting KFD outbreaks.
  • The ticks that carry the virus responsible for the disease are common in the Western Ghats.
  • The study found that outbreaks appear to become more common even when the diversity of animal species is low if a region has been deforested.

Bengaluru: The Kyasanur forest disease (KFD) is caused by a virus transmitted by many species of ticks found in the forests of the Western Ghats. And according to a new study, the diversity of animal species in the forest and the loss of forest cover both contribute to outbreaks of this illness in a region.

Specifically, the risk of a KFD outbreak has been known to increase in areas that have many kinds of animals and whose forest cover is largely intact. At the same time, outbreaks appear to become more common even when the diversity of animal species is low if the region has been deforested.

This is the first study to establish these connections – between the number of animal species and forest cover. It was conducted by researchers at the University of Sydney and the Manipal Academy of Higher Education.

KFD has been known to have a significant forest connection – as its name itself indicates. A 2018 review article, for example, said based on a 2017 study: “Destruction of forest cover in the Western Ghats caused significant changes in annual precipitation levels in the region that will possibly affect patterns of this wildlife disease via survival of vectors or by influencing host-parasite relationships.”

“This study highlights the importance of understanding what is happening to our forests and host species in those habitats,” Kiran S.K., deputy director (KFD nodal officer) of the Virus Diagnostic Laboratory, Shivamogga. He wasn’t involved in the study.

A map of Indian states (orange) and regions (red) where KFD has been reported. Image: DOI: 10.3389/fcimb.2018.00149

KFD’s primary vector is the forest tick Haemaphysalis spinigera. It is common in the Western Ghats, and attaches itself to many animal species, including humans. People are usually exposed to the ticks at the fringes of forests.

When primates get KFD, their bodies accumulate a large quantity of the viral particles and, more often than not, they die.

According to the US Centres for Disease Control, the symptoms of KFD kick in among humans about a week after infection. They include, among many others, fever, headache, vomiting and low BP, and disappear a week or two later. Some symptoms tend to recur in 10-20% of patients in another week, and these include the risk of neurological damage. Over all, 3-5% of humans who get KFD die.

The new study also reported that the associations between the number of animal species and forest cover extend to the risk of KFD outbreaks at the sub-taluka level, up to that of a taluka[footnote]An administrative division typically composed of a few villages[/footnote].

The study also said the population of two monkey species – the bonnet macaque and the gray langur – could be outbreak-indicators, but Kiran said this association will need more study to confirm.

“We actually estimated the total number of species across the Western Ghats directly … to be able to assess this relationship between the species and KFD risk,” said Michael G. Walsh, senior lecturer at the Marie Bashir Institute for Infectious Diseases and Biosecurity, University of Sydney, and the study’s lead author.

At least one study, published in April 2020, has shown that forest loss could be strongly correlated with KFD outbreaks over larger areas. A previous study led by Walsh himself linked forest loss with KFD outbreaks – and found that places with higher animal diversity posed a greater risk of an outbreak as well. But the animal diversity data he used at the time wasn’t good enough, which meant the study’s results couldn’t be conclusive.

Also read: The Seven-Decade Transnational Hunt for the Origins of the Kyasanur Forest Disease

This time, Walsh and his peers tried to establish links between the number of animals and KDF outbreaks, and in what way forest cover mattered.

To do this, the researchers focused on 24 mammal species that occur in the Western Ghats. They analysed the distribution of each species and arrived at a hi-res diversity map of animals across the Western Ghats. They then correlated this to forest loss data (available from older studies), and details of KFD outbreaks in different areas since 2012.

The results threw up a significant correlation between species diversity and forest loss. The team also found that even though the population of many species was correlated with KDF outbreaks at the sub-taluka level, the link vanished in most cases at the taluka level itself. There were only four exceptions: bonnet macaque, langur, leopard and spotted deer.

While neither leopards nor spotted deer are known to be susceptible to KFD, they both carry the ticks that transmit the pathogen, and can spread them to other animals. “The association of leopards with KFD risk may explain the movement of KFD from one geographical area to another, and of KFD outbreaks in far-off areas,” according to Kiran.

Illustration: Ita Mehrotra

He also said a KFD outbreak risk increases when the animal diversity is greater in densely forested areas with little human activity, because of spillovers from animals to humans. On the other hand, deforested landscapes “are host to generalist species, which are more resilient to anthropogenic pressures and often host zoonotic pathogens, which explains the endemicity of KDF in those areas,” Kiran said.

That is, animals that adapt to anthropogenic changes to the environment are likely to contribute to greater spillover of the KFD virus in a given region.

In many parts of the Western Ghats, bonnet macaques and langurs are commensal with humans[footnote]That is, they get their food and other benefits through human activity, without being symbiotic.[/footnote]. Spotted deer can be found in semi-urban areas and leopards have been known to move around urban areas.

Also read: It’s Not Coexistence Just Because People and Animals Are Around Each Other

“The most important takeaway of the study is recognising the fact that we are yet to identify the full range of hosts of KFD virus, their relative competence and efficiency in transmitting the disease,” Kiran said. “This knowledge can be used to reduce the host human interface to prevent KFD outbreaks.”

According to Walsh, real-life applications of his team’s work include the development of better disease-surveillance programmes.

“Because the relationship between the number of animals and KFD outbreak risk was consistent at a [sub-taluka] level and at the scale of the taluk, it suggests not only that the relationship was robust but also that new targeted surveillance could be optimally implemented at the municipal level of the taluk,” Walsh said.

“Meaning there is potential for effective cooperation between animal and human health infrastructure.”

Vrushal Pendharkar is a freelance science writer.

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