Arboviruses


            Arboviruses are arthropod-borne viruses, viruses of vertebrates biologically transmitted by blood-sucking insects. They multiply in them, transmitted by bite to vertebrate hosts. Arboviruses  have a very wide host range including many species of animals and birds. The ability to multiply in arthropods is their special characteristic. The most important arbovirus vectors are mosquitoes and ticks. In the laboratory, mice are commonly used for growing arboviruses. They can be grown in the yolk sac or chorioallantoic membrane of chick embryo, in tissue cultures of primary cells like chick embryo fibroblasts, HeLa cells. Most arboviruses agglutinate the red cells of one-day old chicks. Hemagglutination is influenced by pH , temperature etc. Arboviruses are labile, inactivated at room temperature, bile salts, ether.

Antigenic Structure: Three antigens are important in serological studies- hemagglutinins, complement fixing antigens and neutralizing agents.
Pathogenicity: the virus enters the body through the bite of the insect vector. After multiplication in the reticuloendothelial system, virus transported to the target organs such as the central nervous system. The clinical syndromes are fever with or without rash, encephalitis, characteristic yellow fever.
Laboratory Diagnosis: by virus isolation or serology. Blood collected during the acute phase of the disease may yield virus. Isolation may also be made from CSF, but the best specimen is the brain. Isolates may be identified by complement fixation, immunofluorescence, ELISA etc. Prophylaxis include vector control and immunisation.

Togaviruses  are spherical enveloped viruses with a diameter of 50-70 nm. The genome is a single stranded positive sense, RNA 11-12kb in length. The virus replicates in the host cell cytoplasm and is released by budding through host cell membranes.

            It causes encephalitis, an inflammation of the brain. Mainly three types based on the location ie.,Eastern equine encephalitis, Western equine encephalitis, Venezuelan equine encephalitis. The equine varieties infect horses more often than humans. The life cycles of these viruses involve transmission from a mosquito to a bird, back to amosquito, then to a horse, human or other mammal and finally back to mosquito. The viruses which are introduced into the body through bites of infected mosquitoes, first multiply in the skin and spread to lymph nodes. In some cases they invade central nervous system where they cause shrinkage and lysis of neurons. Western equine appears every summer. Fever and headache are common symptoms. Eastern equine is a much more seriuos disease, causes a severe necrotizing infection of the brain. Venezuelan encephalitis is mainly a disease of horses, when it occurs in humans resembles influenza.

Laboratory Diagnosis- the causative virus can be isolated from blood, CSF or brain biopsy or autopsy by intracerebral inoculation of mice, inoculation of cultured cells such as Vero. The isolate is identified by immunofluorescence, complement fixation or ELISA.

 

Flaviviruses  are smaller than togaviruses, being 40nm in diameter. They possess a single stranded plus RNA. Inner viral core is surrounded by a lipid bilayer envelope covered with glycoprotein and a matrix of membrane protein. They are of two sections : the mosquito-borne and the tick-borne viruses.


Mosquito-Borne group

  • Japanese Encephalitis- virus occurs along the Korea, Japan, India, Malayasia, first recognized in Japan. Culex tritaeniorhynchus which breeds in rice fields is the principal vector. The disease has an abrupt onset with fever, headache and vomiting. After 1-6 days coma stage occurs. The fever is high and continuous. There is neutrophil leucocytosis in the peripheral blood and raised sugar, slightly raised protein in CSF. The mortality rate may be upto 50%. Convalescence or recovery may take many weeks. Preventive measures include mosquito control and locating piggeries away from human dwellings. A formalin inactivated mouse brain vaccine using a specific strain has been employed successfully for human immunization in Japan. Two doses at 2 weeks’ interval followed by a booster 6-12 months later constitute a full course.

  • Yellow Fever – the fever is a native of Africa and was transported along the trade routes to Europe & America. The fever is spread by Aedes aegyptii mosquitoes. The mosquitoes are infected by feeding on human patients during the early viremic phase and become infective after an incubation period of 12 days. The disease starts as a fever, headache, nausea, vomiting. Jaundice, albuminuria, hemorrhages occur and the patient may die of hepatic or renal failure. On tissue studies, the liver shows cloudy and fatty degeneration and necrosis. Acidophilic intranuclear inclusion bodies seen in the infected liver cells in the early stages. It can be controlled by eradicating the vector mosquito. The vaccine(Dakar) produced from infected mouse brain is thermostable. Later another vaccine, 17D, deceloped which is thermolabile, more safer and administered by subcutaneous inoculation.

  • Dengue Fever – also been called breakbone fever  because of the severe bone and joint pain it causes. Dengue virus is transmitted from person to person by Aedes aegyptii. The extrinsic incubation period is 8-10 days. Fever of sudden onset with headache, conjunctival infection, pain in the back and limbs, rashes are the symptoms. The fever lasts for 5-7 days. Dengue may also occur in serious forms with hemorrhages or with shock. The hemorrhage occurs while the virus is replicating in circulating lymphocytes. Other symptoms are rapid breathing and low blood pressure which leads to shock. The shock is reversible if treatment is started promptly. Demonstration of circulating IgM antibody provides early diagnosis, as it appears within 2-5 days of the onset of illness and persists for 1-3 months. A vaccine against one immunological type of the virus appears to confer immunity. Infection with one serotype does not provide cross-protection against the other serotypes, so one person can have dengue fever 4 times during his life time. Control of dengue is limited to vector control as vaccine are not available for all the serotypes.

Tick- Borne group

·         Encephalitis- infection is transmitted by the bite of Ixodid ticks. Wild rodent and migrating birds are the main reservoirs. A formalin inactivated vaccine is available.
Laboratory Diagnosis- virus can be isolated by intracerebral inoculation of newborn mice or by inoculation of cultured vertebrate cells. The sample, blood is collected as early as possible during the course of dengue and yellow fever and from the vectors. The viral isolates can be identified by immunofluorescence of the cultured cells, neutralisation tests.

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