Arboviruses - Morphology, Classification, Cultivation, Pathogenesis, LD...

ARBOVIRUSES – MORPHOLOGY, CLASSIFICATION, CHARACTERISTICS, CULTIVATION, PATHOGENESIS & LABORATORY DIAGNOSIS

INTRODUCTION TO ARBOVIRUSES

⇒ Arboviruses are the RNA viruses that are transmitted by blood-sucking arthropods from one vertebrate host to another

Arboviruses are worldwide in distribution but are more numerous in the tropical than in temperate zones.

Arboviruses are very wide host range including many species of animals and birds. The most important arbovirus vectors are mosquitoes, followed by the Ticks.

ARBOVIRUSES - MOSQUITO BORNE DISEASES - ARBOVIRUS VECTOR

CLASSIFICATION OF ARBOVIRUSES

FAMILYGENUS
TogaviridaeAlphavirus, Rubivirus
FlaviviridaeFlavivirus
BunyaviridaeOrthobunya virus, Phlebovirus, Nairovirus
ReoviridaeOrbivirus
RhabdoviridaeVesiculovirus
OrthomyxoviridaeThogotovirus




CHARACTERS OF ARBOVIRUS FAMILIES

Togaviridae

  1. Morphology
    • Viruses of Togaviridae family are Spherical, Enveloped virus with icosahedral
    • Size: 60 – 70 nm in diameter.
    • Genetic Material :- (+) ssRNA.
  2. Classification
    • Togaviridae contains two Genera: Alphavirus & Rubivirus.
    • Viruses of Togaviridae are transmitted principally by Mosquitoes.
    • Genus Rubivirus contains Rubella virus, is not Arthropod borne.
    • The genus Alphaviruses includes:-
      1. Encephalitis Group
        1. Western equine encephalitis (WEE)
        2. Eastern equine encephalitis (EEE)
        3. Venezuelan equine encephalitis (VEE)
      2. Febrile Illness Group
        1. Chikungunya virus
        2. O’nyong-nyong virus
        3. Sindbis Virus
        4. Ross river virus

Flaviviridae

    1. Morphology
      • Viruses of Flaviviridae family are Spherical viruses.
      • Size: 40 – 50 nm in diameter.
      • Genetic Material :- (+) ssRNA.
  1. Classification
    • Some members of Flaviviridae are Mosquito-borne while others are Tick-borne
    • Hepatitis C virus is neither mosquito nor tick-borne.
    • Mosquito-borne Flaviviruses
      1. Encephalitis viruses
        1. St. Louis Encephalitis Virus
        2. Ilheus Virus
        3. West Nile Virus
        4. Murray Valley Encephalitis Virus
        5. Japanese B Encephalitis Virus
      2. Yellow Fever
      3. Dengue
    • Tick-borne Flaviviruses
      1. Tick-borne Encephalitis viruses
        1. Russian Spring-Summer Encephalitis
        2. Powassan Virus
      2. Tick-borne Hemorrhagic Fevers
        1. Kyasanur Forest disease (KFD)
        2. Omsk Hemorrhagic fever

Bunyaviridae

  1. Morphology
    • Viruses of Bunyaviridae family are Spherical, enveloped viruses with glycoprotein peplomers.
    • Size: 90 – 100 nm in diameter.
    • Genetic Material :- (-)ssRNA.
  2. Classification
    • Bunyaviridae contains four genera :
      1. Bunyavirus – Mosquito-borne
      2. Phlebovirus – Phlebotomus or Mosquito-borne
      3. Nairovirus – Tick-borne
      4. Hantavirus – Non-Arthropod-borne
  3. Viruses of Bunyaviridae
    • Bunyavirus
      1. California encephalitis virus
      2. La Crosse virus
      3. Chittor virus
    • Phlebovirus
      1. Sandfly fever (Phlebotomus fever)
      2. Rift valley fever
    • Nairovirus
      1. Crimean Congo hemorrhagic fever virus
      2. Ganjam virus
    • Hantavirus
      1. Hantaan virus
      2. Belgrade virus
      3. Seoul virus
      4. Puumala virus
      5. Muerto Canyon virus
      6. Sin Nombre virus

Reoviridae

  1. Morphology
    • Viruses of Reoviridae family are cubic shaped.
    • Size: 60 – 80 nm.
    • Genetic Material:- dsRNA.
  2. Classification
    • Reoviridae contains four genera – Orthoreovirus, Coltivirus, Orbivirus, and Rotavirus. Of which the only Orbivirus causes Arthropod-borne infections.
    • Colorado tick-borne virus is the only recognized pathogen in Orbivirus, causes Colorado tick fever & is spread by the wood tick.

Rhabdoviridae

  1. Morphology
    • Viruses of Rhabdoviridae family are Bullet-shaped.
    • Size :- 170*100 nm.
    • Genetic Material :- (-)ssRNA.
  2. Classification
    • The chandipura virus, belonging to the genus Vesiculovirus of family Rhabdoviridae was isolated in Nagpur (INDIA) in 1967.
    • The vectors are sandflies and Aedes mosquitoes, in which the virus multiplies.
    • The pathogenic significance of this virus has not been established.

CULTIVATION OF ARBOVIRUSES

In the laboratory, mice are commonly used for growing Arboviruses, intracerebral inoculation in suckling mice being the most sensitive method for the isolation of Arboviruses.

They can be grown in the Yolk sac or Chorioallantoic membrane of the chick embryo, in tissue cultures of primary cells like chick embryo Fibroblasts or continuous cell lines like Vero or HeLa.

PATHOGENESIS OF ARBOVIRUSES

The virus enters the blood circulation via

the bite of an infected vector

the virus comes into contact with susceptible target cells

such as endothelial cells of capillaries, monocytes,

macrophages & cells of RES.

After replication in endothelial cells & RE cells, a secondary

Viremia results leading to infection of target organs

such as Brain, Skin, musculature, and liver

virus reaches the brain by infecting small blood vessels

of the brain or choroid plexus.




LABORATORY DIAGNOSIS OF ARBOVIRUSES

Diagnosis may be established by virus isolation or serology:-

  • Specimen: Blood, CSF (Cerebrospinal fluid) and Brain may be used for isolation of virus. All Arboviruses are viremia – blood is collected during the acute phase of the disease. CSF is useful in encephalitis cases but the best specimen is the brain.
  • Isolation of the virus :
    1. Suckling mice – specimens are inoculated intracerebrally into suckling mice. The animal may develop fatal encephalitis.
    2. Tissue culture – Arboviruses may also be isolated in tissue cultures – Vero, BHK-21 & mosquito cell lines are inoculated with specimens. The growth of virus in cell cultures is identified by immunofluorescence, haemagglutination inhibition, complement fixation, ELISA or neutralization tests.
  • Serology: Using ELISA, serotype-specific IgM antibody may be detected in patient serum within 1-3 days after the onset of illness.

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ARBOVIRUSES – MORPHOLOGY, CLASSIFICATION, CHARACTERISTICS, CULTIVATION, PATHOGENESIS & LABORATORY DIAGNOSIS

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