Arboviruses: Types, Symptoms, Causes, Treatment

INTRODUCTION


Arboviruses (arthropod-borne viruses) are RNA viruses that are transmitted by blood-sucking arthropods from one vertebrate host to another. Inclusion in this group is based on ecological and epidemiological Considerations and hence it contains viruses of diverse physical and chemical properties. Despite the fact that systematically unsatisfactory, the name arbovirus is a valuable natural Concept.

CLASSIFICATION Of ARVOVIRUSES


Arboviruses belong to six families: Togaviridae, Flaviviridae, Bunyaviridae, Reoviridae, Rhabdoviridae and Orthomycoviridae.


Arboviruses cause the following clinical syndromes such as febrile illness with or without rash and arthralgia, encephalitis, haemorrhagic fever, and the characteristic systemic disease, yellow fever.

LABORATORY DIAGNOSIS OF ARBOVIRUSES


Diagnosis may be established by the virus by isolation or serology.

Specimens

  • Blood, CSF and brain may be used for isolation of the virus.
  • As all arboviruses are viraemic, blood is collected during the acute phase of the disease.
  • CSF is useful in encephalitis cases but the best specimen is the brain.

Virus Isolation For Arbovirus


Suckling Mice
  • Specimens are inoculated intracerebrally into suckling mice.
  • The animal develops fatal encephalitis.
  • This is the most sensitive method for isolation of arboviruses.


Infection Isolation from Insect Vectors and Reservoir Animal

Such isolations are important to know the identification of arbovirus activity in the area.

Serology For Arbovirus

  • The detection of a four-fold rise or more in antibody titre by ELISA, complement fixation, haemagglutination inhibition or neutralisation test also provide good evidence of infection.

FAMILIES OF ARBOVIRUSES

  1. Togaviridae
  2. Flaviviridae
1. Togaviridae

Morphology Of Togaviridae


Viruses of the family Togaviridae (from Toga, meaning mantle) are spherical, 60-70 nm in diameter with icosahedral capsid Surrounded by lipoprotein envelope and contain single-stranded positive-sense RNA.

Classification Of  Togaviridae


  • Togaviridae is the largest family of arboviruses.
  • It contains two genera: Alphavirus and Rubivirus.
  • Viruses belonging to this family are transmitted principally by mosquitoes.
  • Genus Rubivirus, which contains the rubella virus, is not arthropod-borne.

Viruses of Togaviridae


(i) Alphavirus (Mosquito-borne)

Encephalitis group

  1. Western equine encephalitis (WEE)
  2. Eastern equine encephalitis (EEE)
  3. Venezuelan equine encephalitis (VEE)
Febrile illness group

  1. Chikungunya virus
  2. O’nyong-nyong virus
  3. Semiliki Forest
  4. Sindbis
  5. Ross River virus
Rubivirus 
  1. Rubella virus

Alphavirus

Chikungunya Virus

The virus is transmitted by Aedes aegypti. The disease is characterised by fever, crippling joint pains, lymphadenopathy, conjunctivitis and rash. Haemorrhagic manifestations are seen in some patients. The fever is typically biphasic.

Chikungunya is the native word for the disease in which the patient lies doubled up due to severe joint pains. No vaccine is available.

2. Flaviviridae


Generic name of Flavivirus has beer assigned to members of the family Flaviviridae. Some members of the family are mosquito-borne while others are tick-born Hepatitis C Virus is neither mosquito nor tick-borne.

Morphology Of  Flaviviridae


  • The viruses of the family Flaviviridae are spherical 40-50 nm in diameter.
  • They contain a single-stranded positive-sense RNA.

Mosquito-borne Flaviviruses


Encephalitis Viruses

  1. St. Louis encephalitis virus
  2. Zeus virus
  3. West Nile virus
  4. Murray Valley encephalitis virus
  5. Japanese B encephalitis virus

Japanese B Encephalitis Virus


The virus was named Japanese B encephalitis to distinguish it from Japanese encephalitis virus which was prevalent at that time. This virus is responsible for the most serious illness among five viruses of this group.

Pathogenesis


Natural infections of Japanese encephalitis occur in Android birds (herons and egrets) and the virus spread from bird to bird through Culex tritaeniorhynchus. Other birds (ducks, pigeons and sparrows) may also be involved. Vertebrate hosts may include cattle and buffaloes, besides pigs. The high cattle-pig ratio in India has been Suggested as a factor limiting human infection.

Epidemiology


It is widely distributed in China, Japan Epidemiology, Korea, Philippines. and Southeast Asia Several outbreaks have occurred in Bankura and Burdwan in West Bengal, Dibrugarh in Assam, Gorakhpur in U.P, Goa, Kolar in Karnataka, South Arcot in Tamil Nadu, Puducherry, and various areas in Andhra Pradesh.

Sporadic cases have been reported from different parts of the country. The disease has become a major public health problem of national importance.

Prophylaxis


Preventive measures include mosquito control and the establishment of piggeries
away from residential areas. A formalin-inactivated mouse brain the vaccine has been employed for human immunization in Japan and in India also.

Two doses of vaccine are administered at two weeks interval, followed by a booster 6-12 months later. The immunity produced is short-lived.

Yellow Fever


Yellow fever is a mosquito-borne acute febrile illness. America. It does not exist in India. Virus infection occurs in two forms: the urban cycle and forest or sylvatic cycle.

In the urban cycle, man serves both as the reservoir and as a definitive cause, the virus being transmitted by the Aedes aegypti mosquito. In the forest or sylvatic cycle, wild monkeys act as reservoirs and several species of forest mosquitoes are vectors. 

Prophylaxis

The French neurotropic Vaccine (Dakar) produced from infected mouse brain carries a high risk of producing encephalitis in the vaccinees. A safe and equally effective vaccine is the 17D vaccine. The 17D vaccine is administered by subcutaneous inoculation.

Vaccination is mandatory for travel to or from an endemic area and is valid for ten years beginning ten days after administration of the vaccine. In India, the 17D vaccine is manufactured at the Central Research Institute (CRI), Kasauli.

India has a receptive area for yellow fever, with a large population of Aedes aegypti and non-immune individuals, but still, yellow fever does not exist in India. The reason could be that the stray virus may not be able to get established in the vectors due to the prevalence of the Aedes aegypti of the Dengue fever virus.

Dengue


Dengue is characterized by the fever of sudden onset, headache, retrobulbar pain, conjunctival infection, pain in the back and joints (also called break-bone fever), lymphadenopathy, and maculopapular rash.

It is endemic and often epidemic in the tropics and subtropics, particularly in Asia, the Caribbean, the Pacific and some areas of West Africa. In India, dengue is common in the East Coast.

Dengue virus has four serotypes: Dengue 1 (DEN 1), dengue 2 (DEN 2), dengue 3(DEN 3) and dengue 4 (DEN4). It is transmitted by Aedes aegypti.

Clinical Features


The disease may occur in two forms:
  • classical dengue fever
  • dengue in more serious forms with haemorrhagic manifestation

Classical dengue fever has a relatively benign course with fever, headache and pain in muscles and bones. The fever is typically biphasic (saddleback). These are known as dengue haemorrhagic fever (DHF) and dengue shock syndrome (DSS).

DHF/DSS is seen in patients previously
infected with dengue virus. On reinfection with a different serotype of dengue virus, the antibody formed against the first virus reacts with the second serotype virus forming immune complexes (virus- antibody complex).


Initial symptoms in DHF/DSS are like those of dengue fever but associated with haemorrhagic rash, thrombocytopenia and shock. The mortality rate is 5-10%.

Laboratory Diagnosis


Isolation of the virus is difficult but serology plays a major role in diagnosis. Demonstration of IgM antibody in serum provides an early diagnosis. Detection of a four-fold rise in IgG titre in paired sera taken at an interval of ten days or more is confirmatory. ELISA is used for the detection of IgG antibody.

A strip immunochromatographic test for IgM is also available for rapid diagnosis. Detection of NS1 antigen can be done by immunochromatographic test. It is a rapid test and detects antigen on the first day of fever before antibodies appear. Viral RNA can be detected in clinical specimens by reverse transcriptase
polymerase chain reaction (RT PCR).

Prophylaxis


  • Control measures include the elimination of mosquitoes.
  • No effective vaccine is available.

Tick-borne Flariviruses


Tick-borne encephalitis viruses

  1. Russian spring-summer encephalitis
  2. Powassan virus

Tick-borne haemorrhagic fevers


  1. Kyasanur forest disease (KFD)

  2. Omsk haemorrhagic fever

Hepatitis C Virus

It has been described in Chapter 62 on hepatitis viruses.


Eyasanur Forest Disease (KFD)


This is an Indian haemorrhagic disease that appeared in Kyasanur Forest of Karnataka in 1957. Birds and small mammals are believed to be the reservoirs of the virus. It is transmitted by the bite of tick (Haemophysalis spinigera). Monkeys act as amplifier hosts.

For many years after the discovery of KFD, it remained confined to the areas contiguous to its original focus in Sagar, Sorab and Shikarpur taluks of Shimoga district in Karnataka. Between 1972 and 1975, a few other foci developed in the adjacent areas in North Kanara. The first human case was seen in seen December 1982.


Control

  • Control of ticks.
  • Vaccination-the population at risk should be vaccinated with killed KFD vaccine.

Bunyaviridae


Morphology

Viruses belonging to family Bunyaviridae are spherical 90-100 nm in diameter. They contain a single-stranded RNA genome and possess a lipid envelope with glycoprotein peplomers.

Classification Of Bunyaviridae


Sunyaviridae contains four genera:
  • Bunyavirus – Mosquito-borne.
  • Phlebovirus – Phlebotomus or mosquito-borne.
  • Nairovirus – Tick-borne.
  • Hantavirus- Non-arthropod-borne.

Viruses of Bunyaviridae

Bunyavirus 
  1. California encephalitis virus
  2. La Crosse virus
  3. Chittor virus

Phlebovirus
  1. Sandfiy fever (Phlebotomus fever) 
  2. Rift valley fever

Nairovirus
  1. Crimean Congo haemorrhagic 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


Family Reoviridae contains four genera-Orthoreovirus, Coltivirus, Orbivirus and Rotavirus. Only Orbivirus causes arthropod-bome infections. These viruses differ from other arboviruses in having double-stranded RNA genomes.

Colorado tick-borne virus is the only recognised pathogen in this genus (orbivirus). It causes Colorado tick fever and is spread by wood tick Dermacentor andersoni.


Rhabdoviridae

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 role of Chandipura virus is not yet fully established.

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