Togaviruses are single-stranded RNA viruses. They are spheri-cal, 17 nm in diameter, and have an icosahedral nucleocapsid surrounded by an envelope.
They replicate in cytoplasm of the infected host cell, and after assembly in the host cell they show budding through the cell membrane. All viruses are serologi-cally related. Togaviruses are divided into two families: alpha-viruses and rubiviruses.
The major togaviruses are chikungunya, Venezuelan, and Western equine encephalitis viruses, Eastern encephalitis viruses, Mayaro, O nyong-nyong, Semliki Forest, and Sindbis viruses. Most of these viruses are transmitted by mosquito arthropods to humans.
Flaviviruses are similar to togaviruses in that they are also single-stranded RNA viruses and have an icosahedral nucleo-capsid surrounded by an envelope. They differ from togavi-ruses in being small, only 40–50 nm in diameter; in contrast, the togaviruses are relatively large and have a diameter of 70 nm. The flaviviruses replicate in the cytoplasm and assembly of viruses occurs within endoplasmic reticulum. All flaviviruses are serologically related.
Bunyaviruses are large, spherical (80–120 nm in diameter), and have a triple-segmented and a single-stranded RNA. They have a helical nucleocapsid surrounded by an envelope. They replicate in the cytoplasm, and the assembly of the virion occurs by budding on smooth membrane of the Golgi system. The members of the family Bunyaviridae are classi- fied into four genera: Bunyavirus, Hantavirus, Nairovirus, and Phlebovirus.
Reoviruses are spherical viruses, measure 16–18 nm in diam-eter, and are nonenveloped viruses. The genome consists of a double-stranded RNA of 10–12 segments. Replication and assembly of the virus occurs in the cytoplasm of the host cell. Reoviruses are divided into two genera: Coltivirus and Orbivirus. Coltiviruses include Colorado tick fever virus, transmitted by ticks. Orbiviruses include African horse sickness virus and blue tongue virus.
Rhabdoviruses include the vesiculoviruses, which are transmit-ted by either mosquitoes or sandflies. Vesicular stomatitis virus and Chandipura viruses are the common examples.
The life cycle of the arboviruses is characterized by their abil-ity to multiply in both the vertebrate hosts and blood-sucking arthropods. Nevertheless, for effective transmission to vectors, the virus must be present in sufficiently higher number in the blood stream of the vertebrate hosts, which can be taken up in small volume of blood ingested during bite of the arthropod. After ingestion, the virus replicates in the gut of arthropods and then spreads to other organs including the salivary gland.
Only the females of the species serve as the vector of the virus, because female species require blood meal for production of the progeny. Most of these viruses in their life cycle show an extrinsic incubation period in the vectors, during which viruses are replicated sufficiently in the vector and are present in suf-ficient numbers in the saliva to transmit infection to the verte-brate host. The extrinsic period for most of the viruses ranges from 7 to 14 days. Some arboviruses are transmitted by vertical transmission through transovarial passage from a mother tick to baby ticks.
Disease caused by arboviruses occurs primarily in dead-end host, but neither in the arthropod vector nor in the vertebrate animal that act as the natural host. For example, after infec-tion yellow fever virus causes disease in humans, who are a dead-end host, but causes a harmless infection among the jun- gle monkeys in South Africa, which serve as the natural host for yellow fever virus.
Arboviruses cause diseases in humans, which may be one of the following clinical syndromes:
a) Fever with or without maculopapular rash.
b) Encephalitis, often with high mortality.
c) Hemorrhagic fever, also frequently severe and fatal.
However, there are some arboviruses that can cause more than one syndrome in the infected human host, e.g., dengue virus. Recovery from the disease usually confers a lifelong immunity.
Arbovirus infection occurs in distinct geographical distribu-tion and vector patterns. The viruses occur in tropics as well as in temperate countries. They show a tendency to cause sudden outbreak of disease that usually occurs in the adjoining areas between human dwellings in jungle or forest areas.
Control methods essentially include vector controls and vac-cination. The vaccination has been effective only in yellow fever and a few other diseases but not in most of arboviral diseases. Hence, vector control continues to be a mainstay in prevention of arboviral diseases.