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Chapter: Microbiology and Immunology: Virology, Virus: Arboviruses

Tick-Borne Flaviviruses

Tick-borne encephalitis viruses, Tick-borne hemorrhagic fever, Hepatitis C virus.

Tick-Borne Flaviviruses

 Tick-borne encephalitis viruses

Russian spring–summer encephalitis: Russian spring–summer encephalitis caused by Russian spring–summer encephalitis virus is transmitted by the bite of ixodid ticks. This condition, prevalent in Central Europe, Eastern Europe, and Russia, is transmitted mainly by the bite of ticks. Transovarial transmission of the virus takes place in ticks and makes it an important reservoir host. Wild rodents and birds are the other reservoirs. Ingestion of infected meat obtained from infected goats may also transmit the infection to humans. Avoidance of tick bite is an important control measure. A formalin-treated Russian spring–summer encephalitis vaccine has also been found to be useful.

Powassan virus: Powassan virus is another tick-borne viruscausing Powassan encephalitis. This has been reported from Canada and northern United States.

 Tick-borne hemorrhagic fever

Kyasanur forest disease: KFD is an example of tick-bornehemorrhagic fever. This disease is called so, because this was first reported in Kyasanur forest of Karnataka in 1957. The virus was associated with a fatal epizootic, affecting monkeys along with severe illness in some of the villages in the area. The causative virus was isolated from the patients and from dead monkeys.

After an incubation period of 3–7 days, the KFD manifests with a sudden onset of fever associated with headache, vom-iting, conjunctivitis, myalgia, and severe prostration. In some of the cases, bleeding through gastrointestinal tract and chest cavity may occur. The condition is associated with a case fatal-ity of 5%.

The infection is mostly transmitted by the bite of the tick Haemaphysalis spinigera. The tick also acts as the reservoir hostbecause the virus is transmitted transovarially. The monkeys appear to act as amplifier host, but not reservoir host because monkeys die of the infection.

The KFD is mostly localized in distribution and confined to Shimoga district of Karnataka. The disease remained restricted to north Karnataka between 1972 and 1975. Subsequently, the outbreaks of the disease were reported in Velthabelthangadi in South Karnataka between December 1982 and May 1983. During this outbreak, a total of 1142 human cases were reported with 104 deaths.

No specific antiviral treatment is available. Prevention of the disease is based primarily on control of ticks and personal protection by using adequate clothings and insect repellants. A killed KFD vaccine is also available for use against the KFD.

Omsk hemorrhagic fever: This fever is caused by a virusrelated to KFD. The condition is transmitted by bites of Dermacentor ticks and it occurs in Russia and Romania. Clinicalmanifestations of the disease are similar to that of KFD.

 Hepatitis C virus

The virus is not an arbovirus.

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