Arbovirus is an informal name for any virus that is transmitted by arthropod vectors. The term arbovirus is a portmanteau word (arthropod-borne virus). Tibovirus (tick-borne virus) is sometimes used to more specifically describe viruses transmitted by ticks, a superorder within the arthropods. Arboviruses can affect both animals (including humans) and plants. In humans, symptoms of arbovirus infection usually occur 3–15 days after exposure to the virus and last three or four days. The most common clinical features of infection are fever, headache, and malaise, but other features of viral hemorrhagic fever syndrome and encephalitis may also occur.
Signs and symptoms The incubation period – the time between when infection occurs and when symptoms appear – varies from virus to virus, but is usually limited between 2 and 15 days for arboviruses. The majority of infections, however, are asymptomatic. Among cases in which symptoms do appear, symptoms tend to be non-specific, resembling a flu-like illness, and are not indicative of a specific causative agent. These symptoms include fever, headache, malaise, rash and fatigue. Rarely, vomiting and hemorrhagic fever may occur. The central nervous system can also be affected by infection, as encephalitis and meningitis are sometimes observed. Prognosis is good for most people, but is poor in those who develop severe symptoms, with up to a 20% mortality rate in this population depending on the virus. The very young, elderly, pregnant women, and people with immune deficiencies are more likely to develop severe symptoms.
Arbovirus Disease(s) Incubation period Symptoms Duration of symptoms Complications Case fatality rate Vector(s) Primary host(s) Geographic distribution Does infection provide lifelong immunity? Dengue virus Dengue fever 3–14 days Asymptomatic in most cases; fever, headache, rash, muscle, and joint pains 7–10 days Shock, internal bleeding, and organ damage <1% with treatment, 1–5% without; about 25% in severe cases Aedes mosquitoes, especially Aedes aegypti Humans Near the equator globally VariesInfection provides lifelong immunity to the specific serotype causing illness, but temporary immunity to other serotypes. Japanese encephalitis virus Japanese encephalitis 5–15 days Asymptomatic in most cases; fever, headache, fatigue, nausea, and vomiting Encephalitis, seizures, paralysis, coma, and long-term brain damage 20–30% in encephalitis cases Culex mosquitoes, especially Culex tritaeniorhynchus Domestic pigs and wading birds Southeast and East Asia Yes Rift Valley fever virus Rift Valley fever 2–6 days Fever, headache, myalgia and liver abnormalities 4–7 days Hemorrhagic fever, meningoencephalitis 1% in humans; in pregnant livestock, 100% fatality rate for fetuses Culex tritaeniorhynchus and Aedes vexans Micropteropus pusillus and Hipposideros abae Eastern, Southern, and Western Africa Yes Tick-borne encephalitis virus Tick-borne encephalitis 7–14 days Fever, headache, muscle pain, nausea, vomiting, meningitis, and encephalitis Paralysis and long-term brain damage 1–2% Ixodes scapularis, Ixodes ricinus, and Ixodes persulcatus Small rodents Eastern Europe and Southern Russia Yes West Nile virus West Nile fever, encephalitis 2–15 days Asymptomatic in most cases; fever, headache, fatigue, nausea, vomiting, rash 3–6 days Swollen lymph nodes, meningitis, encephalitis, acute flaccid paralysis 3–15% in severe cases Culex mosquitoes Passerine birds North America, Europe, West and Central Asia, Oceania, and Africa Yes Yellow fever virus Yellow fever 3–6 days Fever, headache, back pain, loss of appetite, nausea, and vomiting 3–4 days Jaundice, liver damage, gastrointestinal bleeding, recurring fever 3% in general; 20% in cases with severe complications Aedes mosquitoes, especially Aedes aegypti Primates Tropical and subtropical regions of South America and Africa YesZika virusZika fever3–14 daysFever, rash, joint pain, nausea and vomiting2–7 daysNeurological complications (meningoencephalitis, microcephaly, Gullain-Barré Syndrome)<1%Aedes mosquitoesPrimatesAfrica, the Americas, Asia and the PacificYes
Cause 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 virions present in the blood. The abundance of virions 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.
Virology
Arboviruses are a polyphyletic group, belonging to various viral genera and therefore exhibiting different virologic characteristics.
Arbovirus Genome type Genome length Diameter Capsid shape Enveloped? Viral entry Replication site Viral shedding Infected cell(s) Genetic variability African swine fever virus dsDNA 170-190 kilobases ~200 nm Icosahedral Yes Endocytosis Nucleus Budding Endothelial cells and red and white blood cells 22 genotypesChikungunya virus (CHIKV) +ssRNA 11.6 kilobases 60 - 70 nm Icosahedral Yes Membrane fusion Cell cytoplasm Budding Epithelial cells, endothelial cells, primary fibroblasts and macrophages Three genotypes Dengue virus +ssRNA ~11,000 nucleobases ~50 nm Icosahedral Yes Membrane fusion Cell cytoplasm Budding Langerhans and white blood cells Four serotypes Japanese encephalitis virus +ssRNA ~11,000 nucleobases ~50 nm Icosahedral Yes Membrane fusion Cell cytoplasm Budding Five genotypes Rift Valley fever virus -ssRNA Spherical Yes Cell cytoplasm Budding NoneNo significant distinct genetic populations exist due to the species having recent common ancestry. Tick-borne encephalitis virus +ssRNA ~11,000 nucleobases 40-50 nm Icosahedral Yes Membrane fusion Cell cytoplasm Budding Neural cells Five genotypes West Nile virus +ssRNA ~11,000 nucleobases (11-12 kilo bases) 45-50 nm Icosahedral Yes Membrane fusion Cell cytoplasm Budding Yellow fever virus +ssRNA ~11,000 nucleobases 40-60 nm Icosahedral Yes Membrane fusion Cell cytoplasm Budding Hepatocytes and white blood cellsZika virus +ssRNA 10794 nucleobases 40 nm Icosahedral Yes Membrane fusion Cell cytoplasm Budding
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.
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. Currently, viruses are jointly classified according to Baltimore classification and a virus-specific system based on standard biological classification. With the exception of the African swine fever virus, which belongs to the Asfarviridae family of viruses, all major clinically important arboviruses belong to one of the following four groups:
Order Bunyavirales (Baltimore class V) Genus Orthobunyavirus Bunyamwera virus California encephalitis virus Jamestown Canyon virus La Crosse encephalitis virus Genus Orthonairovirus Crimean–Congo hemorrhagic fever virus Genus Bandavirus Dabie bandavirus Heartland virus Genus Phlebovirus Rift Valley fever virus Toscana virus
Family Flaviviridae (Baltimore class IV) Genus Flavivirus Mosquito-borne viruses Dengue virus group Dengue virus Japanese encephalitis virus group Japanese encephalitis virus Murray Valley encephalitis virus St. Louis encephalitis virus West Nile virus Spondweni virus group Spondweni virus Zika virus Yellow fever virus group Yellow fever virus Tick-borne viruses Mammalian tick-borne virus group Kyasanur forest disease virus Tick-borne encephalitis virus
Family Reoviridae (Baltimore class III) Subfamily Sedoreovirinae Genus Orbivirus African horse sickness virus Bluetongue disease virus Epizootic hemorrhagic disease virus Equine encephalosis virus Genus Seadornavirus Banna virus Subfamily Spinareovirinae Genus Coltivirus Colorado tick fever virus
Family Togaviridae (Baltimore class IV) Genus Alphavirus Chikungunya virus Eastern equine encephalitis virus Ross River virus Venezuelan equine encephalitis virus Western equine encephalitis virus'''
Prevention