Arbovirus

# Overview
Arbovirus is a term used to refer to a group of viruses that are transmitted by arthropod vectors. The word arbovirus is an acronym (ARthropod-BOrne virus). The word tibovirus is sometimes used to describe viruses transmitted by ticks (TIck-BOrne virus).  Symptoms of arbovirus infection generally occur 315 days after exposure to the virus and last 3 or 4 days. The most common clinical features of infection are fever, headache and malaise, but encephalitis and hemorrhagic fever may also occur.

# History
Arboviruses have existed throughout human history, but were not known to exist until fairly recently. The connection between arthropods and disease was not postulated until 1881 when Cuban doctor and scientist Carlos Finlay proposed that Yellow fever may be transmitted by mosquitoes instead of human contact, a reality that was verified by Major Walter Reed in 1901. The primary vector, Aedes aegypti, had spread globally from the 15th to the 19th centuries as a result of globalization and the slave trade. This geographic spreading caused Dengue fever epidemics throughout the 18th century, and later, in 1906, transmission by the Aedes mosquitoes was confirmed, making Yellow fever and Dengue fever the first two diseases known to be caused by viruses. The discovery of the West Nile virus came in 1937, and has since been found in Culex populations and causing epidemics throughout Africa, the Middle East and Europe. In 1999, the virus was introduced into the Western Hemisphere, sparking a series of epidemics. During the latter half of the 20th century, Dengue fever reemerged as a global disease, with the virus spreading geographically due to urbanization, population growth, increased international travel and global warming.  Yellow fever, alongside malaria, was a major obstacle in the construction of the Panama Canal. French supervision of the project in the 1880s was unsuccessful because of these diseases, forcing the abandonment of the project in 1889. During the American effort to construct the canal in the early 1900s, William C. Gorgas, the Chief Sanitary Officer of Havana, was tasked with overseeing the health of the workers. He had past success in eradicating the disease in Florida and Havana by reducing mosquito populations through draining nearby pools of water, cutting grass, applying oil to the edges of ponds and swamps to kill larvae, and capturing adult mosquitoes that remained indoors during the daytime. Joseph Augustin LePrince, the Chief Sanitary Inspector of the Canal Zone, invented the first commercial larvicide, a mixture of carbolic acid, resin and caustic soda, to be used throughout the Canal Zone. The combined implementation of these sanitation measures led to a dramatic decline in the number of workers dying and the eventual eradication of Yellow fever in the Canal Zone as well as the containment of malaria during the 10-year construction period. Because of the success of these methods at preventing disease, they were adopted and improved upon in other regions of the world.

# Transmission
Arboviruses maintain themselves in nature by going through a cycle between a host, an organism that carries the virus, and a vector, an organism that carries and transmits the virus to other organisms. For arboviruses, vectors are commonly mosquitoes, ticks, sandflies and other arthropods that consume the blood of vertebrates for nutritious or developmental purposes. Vertebrates which have their blood consumed act as the hosts, with each vector generally having an affinity for the blood of specific species, making those species the hosts.
Transmission between the vector and the host occurs when the vector feeds on the blood of the vertebrate, wherein the virus that has established an infection in the salivary glands of the vector comes into contact with the host's blood. While the virus is inside the host, it undergoes a process called amplification, where the virus replicates at sufficient levels to induce viremia, a condition in which there are large numbers of viruses present in the blood. The abundance of viruses in the host's blood allows the host to transmit the virus to other organisms if its blood is consumed by them. When uninfected vectors become infected from feeding, they are then capable of transmitting the virus to uninfected hosts, resuming amplification of virus populations. If viremia is not achieved in a vertebrate, the species can be called a "dead-end host", as the virus cannot be transmitted back to the vector.
An example of this vector-host relationship can be observed in the transmission of the West Nile virus. Female mosquitoes of the genus Culex prefer to consume the blood of Passerine birds, making them the hosts of the virus. When these birds are infected, the virus amplifies, potentially infecting multiple mosquitoes that feed on its blood.  These infected mosquitoes may go on to further transmit the virus to more birds. If the mosquito is unable to find its preferred food source, it will choose another. Human blood is sometimes consumed, but since the West Nile virus does not replicate that well in mammals, humans are considered a dead-end host.

## In humans
Person-to-person transmission of arboviruses is not common, but can occur. Blood transfusions, organ transplantation and the use of blood products can transmit arboviruses if the virus is present in the donor's blood or organs. Because of this, blood and organs are often screened for viruses before being administered. Rarely, vertical transmission, or mother-to-child transmission, has been observed in infected pregnant and breastfeeding women. Exposure to used needles may also transmit arboviruses if they have been used by an infected person or animal. This puts intravenous drug users and healthcare workers at risk for infection in regions where the arbovirus may be spreading in human populations.

# Structure and genome
The majority of the arboviruses are spherical in shape although a few are rod-shaped. They are 17-150 nm in diameter and most have an RNA genome (the single exception is African swine fever virus, which has a DNA genome).

# Classification
In the past, arboviruses were organized into one of four groups: A, B, C and D. Group A denoted members of the genus Alphavirus, Group B were members of the genus Flavivirus, and Group C remains as the Group C serogroup of the genus Orthobunyavirus. Group D was renamed in the mid-1950s to the Guama group and is currently the Guama serogroup in the genus Orthobunyavirus. This renaming of the group was because the number of groups would eventually exceed the length of the alphabet. Since then, the organization of arboviruses into these groups has fallen out of usage as the standard biological classification system became more preferred for classifying viruses.
- Family Togaviridae
Genus Alphavirus
Eastern equine encephalitis virus (EEE)
Ross River virus (RRV)
Venezuelan equine encephalitis virus (VEE)
Western equine encephalitis virus (WEE)
- Genus Alphavirus
Eastern equine encephalitis virus (EEE)
Ross River virus (RRV)
Venezuelan equine encephalitis virus (VEE)
Western equine encephalitis virus (WEE)
- Eastern equine encephalitis virus (EEE)
- Ross River virus (RRV)
- Venezuelan equine encephalitis virus (VEE)
- Western equine encephalitis virus (WEE)
- Family Flaviviridae
Genus Flavivirus
Tick-borne viruses
Mammalian tick-borne virus group
Kyasanur forest disease virus (KFDV)
Tick-borne encephalitis virus (TBEV)
Mosquito-borne viruses
Dengue virus group
Dengue virus (DENV)
Japanese encephalitis virus group
Japanese encephalitis virus (JEV)
Murray Valley encephalitis virus (MVEV)
St. Louis encephalitis virus (SLEV)
West Nile virus (WNV)
Yellow fever virus group
Yellow fever virus (YFV)
- Genus Flavivirus
Tick-borne viruses
Mammalian tick-borne virus group
Kyasanur forest disease virus (KFDV)
Tick-borne encephalitis virus (TBEV)
Mosquito-borne viruses
Dengue virus group
Dengue virus (DENV)
Japanese encephalitis virus group
Japanese encephalitis virus (JEV)
Murray Valley encephalitis virus (MVEV)
St. Louis encephalitis virus (SLEV)
West Nile virus (WNV)
Yellow fever virus group
Yellow fever virus (YFV)
- Tick-borne viruses
Mammalian tick-borne virus group
Kyasanur forest disease virus (KFDV)
Tick-borne encephalitis virus (TBEV)
- Mammalian tick-borne virus group
Kyasanur forest disease virus (KFDV)
Tick-borne encephalitis virus (TBEV)
- Kyasanur forest disease virus (KFDV)
- Tick-borne encephalitis virus (TBEV)
- Mosquito-borne viruses
Dengue virus group
Dengue virus (DENV)
Japanese encephalitis virus group
Japanese encephalitis virus (JEV)
Murray Valley encephalitis virus (MVEV)
St. Louis encephalitis virus (SLEV)
West Nile virus (WNV)
Yellow fever virus group
Yellow fever virus (YFV)
- Dengue virus group
Dengue virus (DENV)
- Dengue virus (DENV)
- Japanese encephalitis virus group
Japanese encephalitis virus (JEV)
Murray Valley encephalitis virus (MVEV)
St. Louis encephalitis virus (SLEV)
West Nile virus (WNV)
- Japanese encephalitis virus (JEV)
- Murray Valley encephalitis virus (MVEV)
- St. Louis encephalitis virus (SLEV)
- West Nile virus (WNV)
- Yellow fever virus group
Yellow fever virus (YFV)
- Yellow fever virus (YFV)
- Family Bunyaviridae
Genus Nairovirus
CrimeanCongo hemorrhagic fever virus (CCHF)
Genus Orthobunyavirus
California encephalitis virus
La Crosse encephalitis virus (LACV)
Bunyamwera virus
Genus Phlebovirus
Rift Valley fever virus (RVFV)
Toscana virus (TOSV)
- Genus Nairovirus
CrimeanCongo hemorrhagic fever virus (CCHF)
- CrimeanCongo hemorrhagic fever virus (CCHF)
- Genus Orthobunyavirus
California encephalitis virus
La Crosse encephalitis virus (LACV)
Bunyamwera virus
- California encephalitis virus
- La Crosse encephalitis virus (LACV)
- Bunyamwera virus
- Genus Phlebovirus
Rift Valley fever virus (RVFV)
Toscana virus (TOSV)
- Rift Valley fever virus (RVFV)
- Toscana virus (TOSV)
- Family Reoviridae
Subfamily Sedoreovirinae
Genus Orbivirus
African horse sickness virus (AHSV)
Bluetongue disease virus (BTV)
Equine encephalosis virus (EEV)
Genus Seadornavirus
Banna virus (BAV)
Subfamily Spinareovirinae
Genus Coltivirus
Colorado tick fever virus (CTFV)
- Subfamily Sedoreovirinae
Genus Orbivirus
African horse sickness virus (AHSV)
Bluetongue disease virus (BTV)
Equine encephalosis virus (EEV)
Genus Seadornavirus
Banna virus (BAV)
- Genus Orbivirus
African horse sickness virus (AHSV)
Bluetongue disease virus (BTV)
Equine encephalosis virus (EEV)
- African horse sickness virus (AHSV)
- Bluetongue disease virus (BTV)
- Equine encephalosis virus (EEV)
- Genus Seadornavirus
Banna virus (BAV)
- Banna virus (BAV)
- Subfamily Spinareovirinae
Genus Coltivirus
Colorado tick fever virus (CTFV)
- Genus Coltivirus
Colorado tick fever virus (CTFV)
- Colorado tick fever virus (CTFV)
- Family Picornaviridae
Genus Parechovirus
Sebokele virus 1
- Genus Parechovirus
Sebokele virus 1
- Sebokele virus 1

# Signs and symptoms
Many arboviruses (such as African Swine Fever virus) do not normally infect humans or if so, cause either no symptoms or mild and transient infections characterized by fever, headache and rash. Others of this group however can cause epidemic disease and severe infections such as fulminant meningitis, encephalitis, meningoencephalitis, or viral hemorrhagic fever that can be fatal.

## Immune response to infection
The immune system plays a crucial role in defense against infection. Arboviruses are generally good inducers of the production of interferons, which may partially explain why acute infection is often similar to influenza (fever, headache, fatigue, myalgia). Antibodies can be important in controlling viremia and limiting the severity of infection. Recovery typically involves the cell-mediated immune system.

# Diagnosis
Preliminary diagnosis of arbovirus infection is usually based on clinical presentations of symptoms, places and dates of travel, activities and epidemiological history of the location where infection occurred. Definitive diagnosis is typically made in a laboratory by employing some combination of blood tests, particularly immunologic, serologic and/or virologic techniques such as ELISA, complement fixation, polymerase chain reaction, neutralization test and hemagglutination-inhibition test.

# Prevention
Vector control measures, especially mosquito control, are essential to reducing the transmission of disease by arboviruses. Habitat control involves draining swamps and removal of other pools of stagnant water (such as old tires, large outdoor potted plants, empty cans, etc.) that often serve as breeding grounds for mosquitoes. Insecticides can be applied in rural and urban areas, inside houses and other buildings or in outdoor environments. They are often quite effective for controlling arthropod populations, though use of some of these chemicals is controversial, and some organophosphates and organochlorides (such as DDT) have been banned in many countries. Infertile male mosquitoes have been introduced in some areas in order to reduce the breeding rate of relevant mosquito species. Larvicides are also used worldwide in mosquito abatement programs. Temefos is a common mosquito larvicide.
People can also reduce the risk of getting bitten by arthropods by employing personal protective measures such as sleeping under mosquito nets, wearing protective clothing, applying insect repellents such as permethrin and DEET to clothing and exposed skin, and (where possible) avoiding areas known to harbor high arthropod populations. Arboviral encephalitis can be prevented in two major ways: personal protective measures and public health measures to reduce the population of infected mosquitoes. Personal measures include reducing time outdoors particularly in early evening hours, wearing long pants and long sleeved shirts and applying mosquito repellent to exposed skin areas. Public health measures often require spraying of insecticides to kill juvenile (larvae) and adult mosquitoes.

## Vaccination
Vaccines are available for the following arboviral diseases:
- Japanese encephalitis
- Yellow fever
Vaccines are in development for the following arboviral diseases:
- Dengue fever
- Eastern Equine encephalitis
- West Nile

# Treatment
Because the arboviral encephalitides are viral diseases, antibiotics are not an effective form of treatment and no effective antiviral drugs have yet been discovered. Treatment is supportive, attempting to deal with problems such as swelling of the brain, loss of the automatic breathing activity of the brain and other treatable complications like bacterial pneumonia.

# List of common arboviruses
Common arboviruses include:

# List of arboviruses (not complete)
- Chikungunya
- Dengue fever
- Ross River virus
- Vesicular stomatitis virus
- West Nile virus
- Yellow fever
- Tick-borne Encephalitis
- Japanese Encephalitis
- La Crosse Encephalitis
- Murray Valley encephalitis virus
- St. Louis Encephalitis
- Equine Encephalitis
- Epizootic Hemorrhagic Disease
- O'nyong'nyong virus