REOVIRIDAE

N J MACLACHLAN - Distinguished Professor Emeritus, BVSc, PhD, Dip ACVP, Department of Pathology, Microbiology and Immunology, VetMed 3A, School of Veterinary Medicine, University of California, One Shlelds Ave, Davis, California, 95616, USA / Department of Veterinary Tropical Diseases, Faculty of Veterinary Science, University of Pretoria, South Africa

The family Reoviridae is comprised of genetically diverse viruses that include the agents of several clinically and economically important diseases. Members of the family infect mammals and birds, as well as fish, molluscs, reptiles, amphibians, crustaceans, insects, plants and fungi. The viruses that belong to this family share a number of characteristics that were, at the time of discovery at least, quite unique in the biological world. The most important of these is the possession of a double-stranded RNA (dsRNA) genome. When Gomatos and his colleagues discovered in 1962 that the RNA in reovirus is double-stranded, it created quite a stir among virologists and a wider biological audience. At that time, viruses were the only living organisms known to have an RNA genome and in every case until then the RNA was single-stranded.  All viruses with genomes of 9, 10, 11 or 12 distinct segments of dsRNA are now classified within the family Reoviridae, whereasthose with 2 segments of dsRNA are included in the families Birnaviridae or Picobirnaviridae.

The name reovirus, an acronym for “respiratory enteric orphan virus”, was coined in 1959 to distinguish a group of human virus isolates originally lumped together with others in a group called echoviruses (“enteric cytopathic human orphan viruses”), most of which are now classified as enteroviruses (genus Enterovirus in the family Picornaviridae, order Picornavirales)¹. Distinguishing features of the reovirus group were their larger size and distinctive cytopathology in cell cultures. As indicated by the term ‘orphan’, these viruses were not associated with any particular disease.  Despite the large genomic differences the family Reoviridae is currently organized into two subfamilies and 15 officially recognized genera ¹ (Table 1) including Rotavirus and Orbivirus. The two subfamilies, called Spinareovirinae and Sedoreovirinae, are distinguished by the presence or absence of so-called “turrets” or spikes (from Latin spina=spike) in the capsid structures of their member viruses. Rotaviruses (genus Rotavirus) are important pathogens of humans and animals, typically neonates. The genus Orbivirus is by far the most important as far as animal health is concerned; diseases such as bluetongue, African horse sickness and equine encephalosis are caused by members of this group.² The Orbivirus genus is large and includes many viruses without known veterinary or medical significance. The relationship between the viruses in this group is complex. At present 22 species (Table 2) are recognized.¹ Each species is further subdivided into serotypes (Table 2), based largely on plaque neutralization assays and cross-protection studies. However, with the advent of whole genome sequencing, genetically-based typing is increasingly used.

Table 1 Taxonomic organization of the family Reoviridae¹

* group includes viruses that infect humans

Table 2 Species in genus Orbivirus and number of serotypes of important viruses pathogenic to livestock

References

1. INTERNATIONAL COMMITTEE FOR TAXONOMY OF VIRUSES. https://talk.ictvonline.org/taxonomy
2. PALMARINI, M., 2017. Reoviridae. In: MACLACHLAN, N.J. & DUBOVI, E.J., (eds). Fenner’s Veterinary Virology, 5th Edition, Amsterdam: Academic Press.