Kyasanur Forest disease virus (KFDV) is a tick-borne flavivirus responsible for Kyasanur Forest disease, a severe febrile and haemorrhagic illness first recognised in 1957 in Karnataka, India. The virus has a positive-sense RNA genome of roughly 11 kilobases that encodes a single open reading frame translated into a polyprotein, which is cleaved into structural components—capsid, precursor membrane and envelope—and nonstructural proteins essential for replication. The virion is enveloped and exhibits icosahedral symmetry with glycosylated E proteins on its surface.
Genome and Virology
KFDV shares genomic organisation and replication strategies with other tick-borne flaviviruses but is phylogenetically distinct, closely related to the Alkhurma haemorrhagic fever virus isolated in Saudi Arabia. After entry into host cells via receptor-mediated endocytosis, low pH in endosomes induces rearrangement of the E protein, promoting fusion between viral and cellular membranes and releasing the nucleocapsid. The genomic RNA, bearing a 5′ cap and lacking a poly(A) tail, is translated at endoplasmic reticulum membranes. Host signal peptidases and the viral NS3 protease process the polyprotein into structural and nonstructural proteins. Replication occurs in virus-induced membrane vesicles where the NS5 polymerase synthesises complementary negative-strand RNA and progeny genomes. Virions assemble by budding into the endoplasmic reticulum and undergo maturation in the Golgi where prM is cleaved. KFDV replicates efficiently in haemaphysalid ticks, monkeys and small rodents, and can infect human monocytes and endothelial cells, leading to cytokine release and vascular damage.
Epidemiology and Public Health
KFDV is endemic to the Western Ghats region of southern India. The virus is maintained in a sylvatic cycle involving Haemaphysalis spinigera ticks and small mammals; epizootic transmission occurs in monkeys, which serve as amplifying hosts and alert to virus circulation. Humans acquire infection through bites of infected nymphal ticks or handling infected animal carcasses. Annual cases number in the hundreds; outbreaks have expanded into neighbouring states such as Kerala, Tamil Nadu and Goa. The illness typically presents with abrupt onset of high fever, headache, myalgia and haemorrhagic manifestations; a second phase marked by neurological symptoms may occur. Case fatality rates range from 3 to 5%. There is no specific antiviral therapy. A formalin-inactivated vaccine derived from chick embryo passages is used in endemic districts, requiring two doses and periodic boosters, but uptake and coverage remain suboptimal. Prevention focuses on forest worker education, personal protective measures to reduce tick exposure and surveillance using monkey deaths as sentinels.
KFDV underscores the public health significance of tick-borne flaviviruses outside Europe and highlights the need for sustained surveillance and vaccination in endemic areas.
Related Terms: Alkhurma hemorrhagic fever virus, Omsk hemorrhagic fever virus, Tick-borne encephalitis virus, Flavivirus, Dengue virus