74 Togaviridae
Viruses in the family Togaviridae (Latin toga, cloak) are enveloped RNA viruses, approximately 70 nm in diameter, with icosahedral symmetry. The envelope, which contains glycoprotein spikes, is closely bound to an icosahedral capsid. There are two genera, Alphavirus and Rubivirus, in the family. The sole member of the genus Rubivirus is rubella virus, which causes German measles in children and young adults and anomalies in the developing foetus when mothers are not vaccinated.
The genus Alphavirus includes more than 30 species, a number of which are important animal pathogens. Replication of alphaviruses, which contain positive-sense single-stranded RNA, occurs in the cytoplasm. In vertebrates, alphavirus infection results in cytolysis. Viral infection of invertebrate cells is usually non-cytolytic and persistent.
Alphaviruses, in common with certain members of the Flaviviridae, Reoviridae, Rhabdoviridae and Bunyaviridae, are termed arboviruses indicating that they are arthropod-borne and maintained in nature through biological transmission by haematophagous arthropods between vertebrate hosts. This term has no taxonomic significance. Alphaviruses usually have a principal invertebrate vector and an amplifying or reservoir vertebrate host. It is this enzootic cycle of infection that tends to determine the geographical distribution of these viruses.
Clinical infections
Domestic animals and humans are usually considered to be incidental, ‘dead-end’ hosts of alphaviruses because they do not develop a sufficiently high titre of circulating virus to act as reservoir hosts. A number of important equine diseases are caused by infection with members of the genus Alphavirus (Table 74.1).
Table 74.1 Alphaviruses of veterinary significance.
| Virus | Vector | Comments |
| Eastern equine encephalitis virus | Mosquito (Culiseta melanura, Aedes species) | Infection endemic in passerine birds which frequent freshwater swamps of eastern North America, Caribbean islands and parts of South America. Causes disease in horses, humans and pheasants |
| Venezuelan equine encephalitis virus | Mosquito (Culex species) | Infection endemic in small mammals in Central and South America. Causes outbreaks of disease in horses, donkeys and humans in endemic regions, occasionally spreading to southern USA |
| Western equine encephalitis virus | Mosquito (Culex tarsalis and other Culex species, Aedes species) | Infection of passerine birds widespread in the Americas. Causes mild disease in horses and humans |
| Getah virus | Mosquito | Causes sporadic disease in horses in south-east Asia and Australia characterized by fever, urticaria and oedema of the limbs. Subclinical infection occurs in pigs |