Chapter 59

The family name is taken from the Latin word toga meaning cloak or mantle and is a reference to the viral envelope. Members of the family are 60–70 nm in diameter. The envelope contains glycoprotein spikes and is tightly adherent around an icosahedral capsid (Fig. 59.1). The family is composed of two genera: Alphavirus and Rubivirus (Fig. 59.2). The genus Alphavirus contains more than 25 species of which a number are important veterinary pathogens. Alphaviruses are thought to have arisen from an insect-borne plant virus, probably in the New World. They can be divided into several groupings or complexes on the basis of antigenic and genetic studies; Venezuelan equine encephalitis complex, Eastern equine encephalitis complex, Semliki Forest complex and Western equine encephalitis/Sindbis complex (Powers et al. 2001). Western equine encephalitis virus is believed to have arisen by recombination between Eastern equine encephalitis and Sindbis-like viruses (Strauss & Strauss 1994, Weaver et al. 1997). Relatively recent discoveries include an alphavirus of salmonids, the cause of salmon pancreas disease and sleeping disease of rainbow trout, as well as southern elephant seal virus. These alphaviruses are unusual in that they do not seem to require an invertebrate vector. Rubella virus, which causes German measles in humans, does not require a vector and is the sole member of the genus Rubivirus.

Replication occurs in the cytoplasm of infected cells. The nucleic acid is positive-sense single-stranded RNA. The viral nucleocapsids are assembled in the cytosol. Alphavirus infection of vertebrate cells involves the shutdown of macromolecular synthesis and is cytolytic. Release from infected cells is effected by budding through the virus protein-modified plasma membrane with the consequent acquisition of a lipid envelope with glycoprotein spikes. In invertebrate cells the infection is usually non-cytolytic with host cells surviving and becoming persistently infected. In this case, the assembled nucleocapsids bud into cytoplasmic vesicles. The mature virions are not very stable in the environment, being sensitive to pH, heat, detergents and disinfectants. Togaviruses agglutinate goose and chick erythrocytes.

Almost all members of the genus Alphavirus are arboviruses (arthropod-borne). Arboviruses are defined as viruses maintained in nature through biological transmission between vertebrate hosts by haematophagous arthropods. The viruses multiply in the tissues of the arthropod vector. The term arbovirus has no taxonomic status, being a heterogeneous group of viruses belonging to several viral families including Togaviridae, Flaviviridae, Reoviridae, Rhabdoviridae, Arenaviridae and Bunyaviridae. Most arboviruses are mantained in complex sylvatic life cycles involving a primary vertebrate host and a primary arthropod host. Such cycles usually remain undetected unless domestic animals and humans encroach or the virus escapes its primary cycle by means of a secondary vector or vertebrate host due to ecological change thus bringing the virus into the peridomestic environment. Domestic animals and humans are generally ‘dead-end’ hosts as they do not develop sufficient viraemia to contribute to the transmission of the virus. Almost all arbovirus infections are zoonotic. The majority of arboviruses are found in tropical developing countries and have a distinct geographical distribution. Ecological factors limiting the distribution of particular arboviruses include temperature, rainfall and distribution of both vertebrate reservoir host and of the arthropod vector. The most important arthropod vectors are mosquitoes, ticks, sandflies and midges. The vector remains infected for life.

A number of important equine diseases are associated with infections by members of the genus Alphavirus (Table 59.1). The three equine encephalitis viruses (Venezuelan, Eastern and Western) are confined to the western hemisphere and are transmitted by mosquitoes. Getah virus is confined to southeast Asia and Australia. A number of outbreaks of disease have occurred in Japan.