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Chapter: Microbiology and Immunology: Virology, Virus: Arboviruses

Togaviruses: Alphaviruses, Rubivirus

Togaviruses are the largest family of arboviruses and contains two genera: Alphavirus and Rubivirus.


Togaviruses are the largest family of arboviruses and contains two genera: Alphavirus and Rubivirus. The genus Alphavirus includes 32 different viruses of which at least 13 viruses are associated with human diseases. They are transmitted mainly by mosquitoes. The genus Rubivirus contains rubella virus that causes rubella, which is not arthropod-borne. Togaviruses are classified as follows:

a)       Alphaviruses

a.        Encephalitis group

i.               Western equine encephalitis

ii.               Eastern equine encephalitis

iii.               Venezuelan equine encephalitis

b.        Febrile illness group

i.               Chikungunya virus

ii.               O’nyong-nyong virus

iii.               Semliki forest virus

iv.               Sindbis virus

v.               Ross river virus

b)       Rubivirus

a.        Rubella virus


Alphaviruses are mosquito-borne arboviruses, which can be classified as febrile illness group and encephalitis group. The febrile group includes chikungunya virus, O’nyong-nyong virus, Semliki Forest virus, Sindbis virus, and Ross River virus. The encephalitis group includes Western equine encephalitis virus, Eastern equine encephalitis virus, and Venezuelan equine encephalitis virus.

 Encephalitis group

Encephalitis is an acute inflammation of the brain parenchyma often with secondary meningeal involvement. Western equine encephalitis, eastern equine encephalitis, and Venezuelan enceph-alitis are the alphaviruses that are associated with encephalitis.

Western equine encephalitis virus: This virus causes diseasemore frequently than the eastern equine encephalitis virus, but the infection is less severe. It is a neuropathic Alphavirus,which causes encephalitis and viral symptoms with an associated rash. The virus is spread primarily by the vector mosquito Culextarsalis. The condition is most commonly seen in the UnitedStates and is subclinical. The isolation of the virus from the cerebrospinal fluid (CSF) or demonstration of rise in specific antibody titer in the serum confirms the diagnosis. No antivi-ral therapy is available. A killed vaccine is available to protect horses but not humans.

Eastern equine encephalitis virus: Eastern equine encepha-litis virus is one of the alphaviruses that causes most severe disease and is associated with highest fatality. The virus is transmitted primarily by Culiseta mosquito. Wild boars are the reservoir host, and horses and humans are the dead-end hosts. The diagnosis is made by isolating the virus or demonstrating a rise in antibody titer in the serum. There is no antiviral therapy. There is a killed virus vaccine for horses, but not for humans.

Venezuelan equine encephalitis: This virus is transmittedprimarily by Culex and Culiseta mosquitoes. The condition that manifests as a mild systemic infection and may lead to enceph-alitis is prevalent in North and South America. Birds are the reservoir hosts.

 Febrile illness group

Chikungunya virus: Chikungunya virus was first describedin humans and Aedes aegypti mosquito from Tanzania in 1954. The name chikungunya means “doubled up”, which describes the clinical manifestation in which patient lies doubled up due to severe joint pains. The disease is transmitted primarily by A. aegypti mosquitoes. Humans are dead-end hosts and no ani-mal reservoirs are seen. The virus causes a severe disease with a sudden onset of fever, frequent joint pains, lymphadenopathy, and conjunctivitis. Fever typically is biphasic with a period of revision after 1–6 days. The condition is often associated with maculopapular rash. Hemorrhagic lesions were also observed in the patients in Kolkata during 1963.

Chikungunya virus was first demonstrated in India in 1963 during extensive epidemic caused by the virus in Kolkata, Chennai, and other areas. Outbreaks of Chikungunya have also been documented at regular intervals along the east cost of India and in Maharashtra till 1960s. In 2006, an outbreak occurred in India with 2500 cases reported. About 70% cases were from Andhra Pradesh and Maharashtra. But often there are no outbreaks. No antiviral therapy or vaccine is available.

O’nyong-nyong virus: This virus is restricted to Africa andwas first reported from Uganda. The virus, closely related to Chikungunya virus antigenically, is transmitted by Anophelesmosquito.

Semliki forest virus: This virus is yet to be associated withany human disease, although serum antibodies against this virus have been demonstrated in African population.

Sindbis virus: Sindbis virus was first reported fromCulexmosquito in Egypt in 1952. Since then the virus has been reported from other parts of Africa, Philippines, and Australia. Serological evidence of Sindbis virus by demonstration of anti-bodies in the serum has been demonstrated in India.

Ross river virus: The virus has been associated with epidemicpolyarthritis in Australia.


 Rubella virus

Rubella is not transmitted by any arthropod vector.

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