Forest virus resurfaces in Western Ghats, kills more than 100

A tick-borne virus related to Zika and dengue has resurfaced in the forests of Western Ghats^{1, 2, 3} killing over 120 people in the last two years, about a decade after it was thought to have been contained with vaccination. The Kyasanur Forest Disease (KFD) is being reported for the first time from Kerala, Goa and Maharashtra states — not far from its original hub in Karnataka — prompting scientists to call for a fresh probe into the return of the deadly virus.

KFD first emerged in Karnataka’s Shimoga district in 1957 marked by a spate of monkey deaths in the Kyasanur forest⁴, giving the disease its name. Soon, around 70 people from neighbouring villages died with symptoms such as body ache, high fever and internal haemorrhage. The disease spread to four neighbouring districts in the 1970s and flared up in the early eighties, infecting more than 1000 people and killing about 100.

Over the last 60 years, more than 500 people have died in Karnataka⁵ of KFD — a fatality rate of about 5%. KFD has no cure. Patients have to be given supportive care to treat the symptoms.

In 2004, having used a vaccine developed against KFD in the 1960s for years, the National Institute of Virology (NIV) declared a “complete success story from detection of virus to the development of killed vaccine”⁶. But a decade later, scientists reported infections from Karnataka saying “human infections have reached an alarming level in spite of the availability of a vaccine”⁷. People in the endemic areas have been vaccinated since 1990. However, between 2005 and 2010, half of the target population did not receive the vaccine. And some who did receive were still infected⁸.

Extensive field work in the 1960s by the Virus Research Centre, Pune (now NIV), supported by the Rockefeller Foundation, found that deforestation was the main reason behind the emergence of KFD in Shimoga. The virus was carried by several species of ticks. The one biting humans was Haemaphysalis spinigera. The population of adolescent (or nymphal) ticks, which transmit the virus, peaked during February-March triggering monkey deaths. Humans, who ventured into the forests got bitten by the ticks. Small mammals, such as shrews and rodents, hosted the ticks and acted as reservoirs of the KFD virus⁹.

But how did the virus break out of this enzootic cycle? “There were peculiar ecological conditions,” says Payyalore Krishnaier Rajagopalan, a former director of the Vector Control Research Centre, Pondicherry, who lived in a field station in Shimoga for more than a decade. Along with colleagues, Rajagopalan found that domestic cattle amplified and distributed the tick population. Both small mammals and monkeys, dwelling amongst the invasive weeds between the forest and cultivated areas, facilitated the exchange of ticks and the virus.

There are many unknowns about the virus, including what it does once inside the body and how it regulates the immune system. Serological surveys in 1952 had found antibodies to the KFD virus in animals and humans in places far away from Shimoga¹⁰.

Since 2012, KFD has been reported from Tamil Nadu, Kerala, Goa and Maharashtra – all in the Western Ghats. G Arunkumar of the Manipal Centre for Virus Research believes this is because modern diagnostic tests and active surveillance have allowed KFD, with dengue-like symptoms, to be diagnosed readily. However, a recent study says tribals in Kerala, though bitten by ticks before, did not get infected in previous years¹.

“Wherever KFD has appeared, our scientists must pounce upon it and find out why it has resurfaced,” says Rajagopalan.