Kyasanur Forest disease
Tick-borne viral haemorrhagic fever endemic to southwestern India.
Kyasanur forest disease (KFD) is a tick-borne viral haemorrhagic fever endemic to southwestern India. It is caused by a virus of the family Flaviviridae transmitted to humans through the bite of infected hard ticks (Haemaphysalis spinigera). The disease is notable for its biphasic course and long convalescent period.
Quick Facts
- Field
- Infectious disease
Facts from the source article.
Signs and symptoms
The illness begins with high fever, frontal headaches, chills, severe muscle pain, vomiting, and other gastrointestinal symptoms. Bleeding problems may appear 3–4 days after onset. Patients often develop low blood pressure and reduced platelet, red blood cell, and white blood cell counts. Some recover after 1–2 weeks without complications, but 10–20% experience a second wave in the third week featuring fever and neurological signs such as severe headache, mental disturbances, tremors, and vision deficits. Convalescence lasts several months with persistent muscle aches and weakness.
Cause
KFD virus is a typical flavivirus with a single-stranded, positive-sense RNA genome of 10,774 nucleotides encoding a polyprotein cleaved into three structural (C, prM/M, E) and seven non-structural (NS1, NS2a, NS2b, NS3, NS4a, NS4b, NS5) proteins. Its genome is over 92% homologous to Alkhurma Hemorrhagic Fever Virus found in Saudi Arabia; the two diverged over 700 years ago. Reservoir hosts include porcupines, rats, squirrels, mice, and shrews. Monkeys, especially Presbytis entellus and bonnet macaques, are amplifying hosts that develop high viremia and infect ticks. The vector is Haemaphysalis spinigera; humans are infected by nymph bites and are terminal hosts, with no human-to-human transmission.
Diagnosis
Early diagnosis involved inoculating serum into suckling mice and observing death, or using hemagglutination inhibition, complement fixation, and neutralization tests. Modern molecular methods include RT-PCR, nested RT-PCR, TaqMan-based real-time RT-PCR, and detection of Immunoglobin M and Immunoglobin G by ELISA. RT-PCR methods target the conserved NS-5 gene and are positive only 8–10 days from symptom onset. ELISA detects anti-KFDV antibodies from about the fifth day of symptoms up to three months.
History
The disease was first reported in March 1957 from Kattinakere village forest in the Kyasanur forest range of Karnataka, India, named after a signboard there. It initially appeared as an epizootic among monkeys, killing many, leading to the local name "monkey disease" or "monkey fever". British neurovirologist Hubert Webb noted similarity to Russian spring-summer encephalitis and raised the possibility of migratory bird involvement. Subsequent studies found no bird involvement, though their role in initial establishment was not ruled out. The virus was found distinct from Russian strains but antigenically close to Omsk hemorrhagic fever; birds from Siberia showed antigenic response to KFDV. Sequencing later revealed close relation to Alkhurma virus in Saudi Arabia. In 1989 a patient in Nanjianin, China had fever symptoms; in 2009 the viral gene sequence exactly matched the 1957 KFD reference virus, a finding questioned because Indian virus sequences vary over time. Immune response tests suggested birds and humans in that region had been exposed. One study dated the nearest common ancestor of KFDV and related viruses to around 1942, while another indicated divergence 700 years ago.
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