Reoviridae

The term reovirus is an acronym derived from respiratory, enteric and orphan viruses. Original isolations of these viruses were from respiratory and enteric samples but without associated clinical disease, hence the term orphan. Members of the Reoviridae are icosahedral in structure, 60–80 nm in diameter, non-enveloped and possess a one-, two- or three-layered capsid. Two morphological forms are recognized, reflected in the division of the family into two subfamilies: Spinareovirinae (Fig. 55.1) and Sedoreovirinae (Fig. 55.2). Viruses in the subfamily Spinareovirinae have spikes or turrets at the vertices of the icosahedral virus while members of the subfamily Sedoreovirinae lack such projections. The development of infectivity by some reoviruses requires the action of a protease on the outer capsid to produce infectious or intermediate subviral particles (ISVPs). The genome of the virion comprises 10–12 segments of linear, double-stranded RNA. Genetic reassortment occurs readily between members of the same genus. Replication occurs in the cytoplasm of host cells and frequently gives rise to intracytoplasmic inclusion bodies. The family contains 15 genera, of which members of the genera Orthoreovirus, Rotavirus and Orbivirus infect animals and man (Fig. 55.3). Members of the genus Coltivirus are associated with infections of arthropods, rodents and man while viruses belonging to the genus Aquareovirus infect fish. The genera Fijivirus, Phytoreovirus, Seadornavirus, Idnoreovirus, Cypovirus, Dinovernavirus, Cardoreovirus, and Oryzavirus contain viruses of arthropods and plants, while the genera Mycoreovirus and Mimoreovirus contain viruses of fungi and algae respectively. The viruses are moderately resistant to heat, organic solvents and non-ionic detergents. Reoviruses and rotaviruses are stable over a wide pH range while orbiviruses lose infectivity at low pH.

Figure 55.1 Electron micrograph of orthoreoviruses.

Figure 55.2 Electron micrograph of rotaviruses.

Figure 55.3 Classification of reoviruses of veterinary significance.

Orthoreoviruses are widespread in nature and have been isolated from many mammalian and avian species (Table 55.1). Infections are generally considered nonpathogenic except in poultry and rodents. Mammalian and avian orthoreoviruses have distinct group antigens. Orthreoviruses may also be divided into subgroups on the basis of their ability to induce cell–cell fusion from within, termed fusogenic and non-fusogenic orthoreoviruses. Typically, avian isolates are fusogenic while mammalian isolates are non-fusogenic. Atypical, fusogenic orthoreoviruses have been isolated from baboons, flying foxes and snakes. Phylogenetic analysis based on the nucleotide sequences of the three S-class genome segments encoding the major sigma class core, outer capsid, and non-structural proteins indicates at least four genogroups or species of orthoreoviruses (Duncan 1999), five species are currently recognized. Rotaviruses are a significant worldwide cause of human infantile gastroenteritis and of acute diarrhoea in intensively reared farm animals. Transmission of orthoreoviruses and rotaviruses is by direct and indirect contact with contaminated faeces. A number of important animal diseases are caused by orbiviruses. Twenty-two serogroups or species of orbiviruses are currently recognized, as well as a number of unclassified viruses. The groups include serotypes and antigenic complexes. Both African horse sickness and bluetongue are categorized as listed diseases by the Office International des Epizooties (OIE). Epizootic haemorrhagic disease virus and Ibaraki virus of deer and cattle respectively are similar to bluetongue virus in infection and disease patterns. Infection with equine encephalosis virus has only been recognized in southern Africa and Israel. Serological evidence suggests widespread infection with this virus but acute disease is sporadic in occurrence. Orbivirus transmission involves arthropod vectors, primarily Culicoides species.

Table 55.1

Members of Reoviridae of veterinary significance

Virus	Natural host	Significance of infection
Mammalian orthoreovirus	Wide range of species	Four serotypes. Associated with mild enteric and respiratory disease. Significance often dependent on presence of secondary infectious agents
Avian orthoreovirus	Chickens, turkeys and other avian species	Multiple serotypes. Important cause of viral arthritis/tenosynovitis in chickens and possibly turkeys
Rotavirus A to G	Neonates of many intensively reared domestic species including calves, lambs, piglets, foals and poultry	Mild to severe diarrhoea dependent on age, colostral intake, accommodation and concurrent infections. Rotavirus E is associated with pigs while rotaviruses D, F and G are associated with birds
African horse sickness virus	Equidae	Non-contagious arthropod-borne infection. Endemic in Africa south of the Sahara. Mortality rate often high in susceptible horses. Listed disease
Bluetongue virus	Sheep, cattle, goats, deer, antelopes	Arthropod-borne infection of ruminants. Clinical disease highly variable in severity. Severe disease seen in fine wool and mutton breeds of sheep and in certain deer species. Teratogenic virus. Clinical disease is uncommon in cattle. Listed disease
Epizootic haemorrhagic disease virus (EHDV)	Deer, cattle, buffalo	Arthropod-borne, principally Culicoides species. Important disease of deer in North America. Clinically similar to bluetongue. Infection of cattle is usually subclinical or mild. Seven serotypes
Ibaraki virus	Cattle	Acute febrile disease similar to bluetongue. Described in southeast Asia. Member of the EHDV group (EHDV-2)
Equine encephalosis virus	Horse	Seven serotypes. Only reported in South Africa and Israel. Majority of infections are subclinical with sporadic cases of an acute fatal, disease characterized by brain oedema, fatty liver degeneration and enteritis
Colorado tick fever virus	Arthropod-borne, primarily ticks but also mosquitoes. Prolonged viraemia in rodent species (reservoir host)	Humans and other mammals may be dead-end hosts. May cause encephalitis in children
Palyam virus	Cattle	Large serogroup of viruses. Arthropod-borne. Associated with abortions and teratogenicity. Recorded in southern Africa, southeast Asia and Australia