Togaviridae

Togaviridae
Name	Togaviridae
Caption	Schematic of a togavirus virion
Taxon	Togaviridae
Subdivision ranks	Genera
Subdivision	Alphavirus, Rubenivirus

Togaviridae. The family **Togaviridae** comprises a group of enveloped, positive-sense, single-stranded **RNA viruses** that are primarily transmitted by arthropod vectors. It contains the medically significant genus **Alphavirus**, responsible for numerous human and animal diseases, and the more recently established genus **Rubenivirus**. These viruses are characterized by their spherical virions and a genome that is translated into polyproteins, which are subsequently cleaved into functional components.

Taxonomy and classification

The family **Togaviridae** is classified within the realm **Riboviria** and the order **Martellivirales**. Historically, the family included the genus **Rubivirus**, which contained the causative agent of **rubella**; however, based on significant genomic and structural differences, **Rubella virus** was reclassified into its own family, **Matonaviridae**. The current genera are **Alphavirus** and **Rubenivirus**. The **Alphavirus** genus is further subdivided into complexes, such as the **Semliki Forest virus** complex and the **Eastern equine encephalitis virus** complex, based on antigenic relationships and genetic sequences. This taxonomic organization is maintained by the **International Committee on Taxonomy of Viruses**.

Structure and genome

Virions of the **Togaviridae** family are roughly spherical, approximately 70 nm in diameter, and possess a host-derived lipid envelope. This envelope is studded with glycoprotein spikes, composed of heterodimers of the **E1** and **E2** proteins, which facilitate attachment and entry into host cells. Beneath the envelope lies an icosahedral nucleocapsid core. The genome is a single linear molecule of positive-sense **RNA**, typically 11-12 kilobases in length, featuring a 5' cap and a 3' polyadenylated tail. It encodes two open reading frames: the 5' region is translated into non-structural proteins involved in replication, while the 3' region is translated into the structural polyprotein, which is cleaved to form the capsid and envelope glycoproteins.

Replication cycle

The replication cycle begins with virion attachment to host cell receptors, such as the **LDLR** family of proteins, via the **E2** glycoprotein. This is followed by clathrin-mediated endocytosis and subsequent fusion of the viral envelope with the endosomal membrane, mediated by the **E1** glycoprotein, which releases the nucleocapsid into the cytoplasm. The genomic **RNA** is directly translated to produce the non-structural polyprotein, which is cleaved by the viral protease **nsP2** to form the replication complex. This complex synthesizes a full-length negative-strand **RNA**, which serves as a template for new genomic **RNA** and a subgenomic **RNA**. This subgenomic **RNA** is translated into the structural polyprotein. Assembly occurs at the **plasma membrane**, where nucleocapsids bud through membranes containing the viral glycoproteins, acquiring their envelope.

Pathogenesis and associated diseases

Pathogenesis varies significantly between the genera. **Alphaviruses** are notable for causing diseases ranging from febrile arthralgic illnesses to severe encephalitis. Key human pathogens include **Chikungunya virus**, which causes debilitating joint pain and is associated with outbreaks in **Africa**, **Asia**, and the **Americas**; **Eastern equine encephalitis virus** and **Western equine encephalitis virus**, which can cause fatal encephalitis in North America; and **Ross River virus**, endemic to **Australia** and the **Pacific Islands**. **Rubenivirus** contains **Salmon pancreas disease virus**, which causes significant morbidity in **Atlantic salmon** aquaculture. The former member, **Rubella virus**, causes **German measles**, with congenital infection leading to **congenital rubella syndrome**.

Epidemiology and transmission

The epidemiology of **Alphavirus** members is defined by their transmission cycles, which typically involve arthropod vectors and vertebrate reservoir hosts. Most are maintained in enzootic cycles between wild birds or rodents and specific mosquito vectors, such as **Culiseta melanura** for **Eastern equine encephalitis virus**. Humans and domestic animals are usually dead-end hosts. Epidemic transmission occurs when bridge vectors, like **Aedes aegypti** for **Chikungunya virus**, spread the virus in urban cycles. Geographic distribution is often specific; for example, **Mayaro virus** is found in **South America**, while **O'nyong'nyong virus** is endemic to **East Africa**. **Rubenivirus** is transmitted horizontally among fish in aquatic environments.

Prevention and control

Prevention strategies primarily focus on vector control and personal protective measures against mosquito bites, such as using **DEET** repellents and bed nets. Surveillance of mosquito populations and animal reservoirs is critical for early warning of outbreaks. For **Chikungunya virus**, several vaccine candidates are in advanced clinical trials. There are licensed veterinary vaccines for economically important alphaviruses like **Venezuelan equine encephalitis virus**. For **Rubella virus**, now in **Matonaviridae**, highly effective live-attenuated vaccines like the **MMR vaccine** have led to its elimination in the **Americas** and other regions through campaigns led by the **World Health Organization**. Control in aquaculture for **Rubenivirus** involves biosecurity and management practices.

Category:Virus families