Arbovirus

Arbovirus is an informal name for any virus that is transmitted by arthropod vectors. The term arbovirus is a portmanteau word. Tibovirus is sometimes used to more specifically describe viruses transmitted by ticks, a superorder within the arthropods. Arboviruses can affect both animals 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	Incubation period	Symptoms	Duration of symptoms	Complications	Case fatality rate	Vector	Primary host	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	Varies
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	Yes
Zika virus	Zika fever	3–14 days	Fever, rash, joint pain, nausea and vomiting	2–7 days	Neurological complications	<1%	Aedes mosquitoes	Primates	Africa, the Americas, Asia and the Pacific	Yes

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	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 genotypes
Chikungunya virus	+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		-
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	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 cells
Zika virus	+ssRNA	10794 nucleobases	40 nm	Icosahedral	Yes	Membrane fusion	Cell cytoplasm	Budding