Re-Emergence of the unexpected: Kyasanur Forest Disease (Monkey fever)

It was not one of the common infections that we see every day. It seemed less dangerous due to the impact it had on the population. Last reported in the late 1950s, this virus took one of the scariest forms in recent times with more versatility. This viral infection has become one of the leading causes of death among the workers whose only means of livelihood is based on forests. It is remembered in history as one of the major infections associated with deforestation.

Amplifying host of KFD virus: Red-faced bonnet monkey (Macaca radiata) and the Black-faced langur (Semnopithecus entellus) (Source:;

Increasing deaths among the monkeys- the black-faced langur (Semnopithecus entellus) and the red-faced bonnet monkey (Macaca radiata) led to the discovery of this virus. Kyasanur Forest Disease (also referred to as Monkey Fever) was first described during the outbreak of 1957 in Shimoga, India. It was so named, after an outbreak causing monkey deaths in Kyasanur forest that led to another outbreak of febrile illness in humans. Kyasanur forest disease was first reported in 1956 among monkeys in the forests of Shimoga. This virus on emergence had poor host reliability and thus fewer affected cases were seen during its early times. Most cases went unnoticed due to lower detection of this new type of virus. In the early 1950s, this disease was all about extensive deforestation in that particular area.

However, during the year 1957, the virus swept across 10 villages and most adult males who were collecting firewood were worst affected. It was the death of the monkeys that brought this disease into attention. Grazing cattle at the forest horizons also carried the disease into human settlements. For almost half a century the disease was restricted to Karnataka; however, the disease took a major leap and is now spreading all over India. It is said that cutting forests not only becomes a hotspot for the disease; but also changes the dynamics of the vectors and associated fauna nearby.

The Kyasanur forest disease “is truly a bhumi dosha, a trouble of the land”.
Haemaphysalis spinigera, the tick that carries the KFD virus (Source:

Kyasanur Forest Disease is caused by Kyasanur Forest Disease Virus (KFDV) belongs to the family Flaviviridae which also consists of Dengue and Tick-borne encephalitis viruses etc. It is a highly pathogenic virus with a mortality rate of about 2–10%. This virus harbors in monkeys and is transmitted to human via an arthropod vector- tick. It is known to cause severe disease manifestations in primate species red-faced bonnet monkey (Macaca radiata) and black-faced langurs (Semnopithecus entellus) etc. Here, the monkey is the amplifying host; it becomes infected by the bite of tick; the monkey dies and ticks drop from its body or ticks in nature take a blood meal (as a part of their life cycle) from dead infected monkeys and then transmit the disease to human. It is transmitted by tick species Haemophysalis spinigera and Haemophysalis kyasunarensis etc.

With an incubation period of 3–8 days, infection begins with sudden chills, fever and headache, severe muscle pain with vomiting, gastrointestinal symptoms and bleeding problem. The patient may recover within 1–2 weeks. It is a biphasic illness; once the fever comes down, that’s when the neurologic symptoms pitch in. The late stages of the symptomatic disease show fever, neurological manifestations, severe headache, mental disturbances, tremors, and vision deficits and sometimes it may even lead to death.

Kyasanur Forest Disease Virus transmission cycle (Source:

The factors responsible for the emergence and re-emergence of such a viral disease can be due to many reasons. Earlier, humans lived in small groups; the first epidemiological transition began with population explosion which led to the conversion of natural habitat into concrete jungles for human living. With this, humans interfered in a million different ways in the ecology of life. Changes in land use for agricultural expansion, climate change, deforestation, urbanization and human interface with domestic and wild species triggered zoonotic jump from vectors to humans. Re-emergence of such infections also depends on the host, vector and pathogen evolution, host adaptation, immunity and different genetic factors of the host etc. The virus has evolved in such a way that a known virus then which was not a risk for a particular host has now re-emerged and is known to cause severe manifestations in another host.

KFD virus has become a matter of concern due to its multiple re-emergence events in a very short period of time as compared to the expected hypothetical evolutionary time. KFD has re-emerged with known outbreaks in present times. The virus once confined to the five villages of Karnataka is now being reported in five states with around 348 cases and 9 deaths.

The exact cause of how the virus emerged in India is still unknown due to lack of information about the disease occurrence before the year 1950. One interesting fact about this virus was its diversity with respect to host range: discovery among the monkeys first and then its detection in humans, cattle and other non-primate hosts simultaneously. Non- primate hosts are not effective in transmitting the virus to other hosts, however, they play a role in maintaining the virus. Due to this reason, the virus must have disappeared for almost half a decade. But that’s the thing about zoonotic viruses; in order to survive, it must find a way go back and forth between a vector and a reservoir organism and sustain itself. The whole emergence of this virus can be referred to as spill-over event (phenomena of transition of the pathogen from one species to a whole new another species).

The only need for the virus to survive is by depending on a living cell. It can infect any plant or animal species with/ without causing the disease. They may kill some animals or birds in the wild and the carcasses thus produced is quickly absorbed by the forest. And we humans barely notice things happening in the wild!

Re-emerging infectious diseases are real and the challenges that we face due to such viruses are apparent. None can ignore the fact that newer pathogens will continue to emerge. Thus, it is important to control such infections before it becomes a global threat. Proper surveillance should be developed for the detection of multiple viruses not only in humans but also in the wild.

It is our duty to not only protect mankind from the onslaught of such emerging infectious diseases but also promote animal welfare.